Compressor Packing PET TCL WEEE Lubrication TTT tT) . [RESOURCES COMPANTTables of Content Literature Mechanical Packing ‘Compressor Packing Breaker Rings. Packing Ring BT. Packing Ring BD. Ring Materials Lubricated, Semilubricated, Nonlubricated Packings Packing Ring TU. ‘Thermal Effects Garter Spring Breakage Undersize and Oversize Rods Tapered Rods Dirt or Foreign Matter. Rate of Leakage. Break-in Procedure Nonlubricated Packing il Wiper Packing Material Selection for Compression Packing Lubrication ‘Type of Lubricant Packing Leakage Control Problems Associated with Low Suction Pressure Problems Associated with Low Leakage Requirements Affect of Ring Type on Leakage Leakage Control with Distance Piece Pressure Static Compressor Packing Compressor Barrier Fluid System for Fugitive Emmision Control Barrier Systems, Cooling Reciprocating Compressor Packing Applying Packings to Liquids (Pumps) High Pressure Packings Packing Ring Arrangements ee eee oe 12 12 12 44 15 16 7 19 20 21 22 23 24 26 27 27 28 35 36 36 Illustration Packing Nomenclature Pressure Drop Across Rings Packing Ring Type P & PA Packing Ring Type BT. Packing Ring Type BD. Packing Ring Type BTR Packing Ring Type TR. Packing Ring Type C Packing Ring ‘Type CR Packing Ring Type TU Packing Ring Type TUU. Packing Ring Type WB. Packing Ring Type E. ‘Typical Compressor Cycle Effect of Backflow. Misfit Rings Effect of Rod Taper. Causes of Ring Leakage ‘Typical Oll Wiper Packing Material Selection Chart Rod Specifications Friction Force on Ring ‘Axially Loaded Rings. Venting Buffered Packings Rings Used at Low Pressure Distance Piece Buffering Static Pac Barrier System Seals Nomenclature ‘Typical Cooling Methods Pressure on Ring Faces. Friction Loads. Coefficient of Friction Friction Foree—BTU/Min. Cooling Requirement Examples Pump Packings High Pressure Packings Packing Ring Arrangements Seemmyioonneenne u 14 14 16 18 19 20 21 22 23 24 27 27 28 29 29 30 31 32 35 35 36Mechanical Packing Over a century ago, Charles Lee Cook designed the first packing to restrict the leakage of ‘steam around the rods in steam locomotives. Today, Cook sealing devices are used in compressors, engines, pumps, valves—almost any device with a rotating or reciprocating shaft that has the potential to leak liquid or gas. A “mechanical” packing is a seal around a shaft passing ‘through a cylinder or pressure vessel head. It consists of one or more rings contained within a case that is typically bolted to that head. Packing rings may be used for a variety of purposes, from simply wiping oll off a shaff, to sealing an aggressive fluid at high pressure. Packing may be as basic as one ring in ‘a case, or it may be an assembly consisting of a number of different type rings in a case that might have provision for lubrication, venting, purging, cooling or heating. How well a packing seals is determined by how closely its shape conforms to the rod. and the dimensional accuracy of rings and sealing surfaces of the packing case. Packing life fs primarily a function of the materials with which it is produced, all other factors being equal. Cook packing is precision manufactured with surface characteristics measured in millionths of an inch. Materials are evaluated in Cook's in-house testing facility ‘Mechanical Packi for their ability to resist wear and corrosion under the most severe conditions. Only those with a demonstrated superiority are offered as Cook materials and many formulations are proprietary, having been developed for specific applications, ‘The Cook Advantage is essen- tially the ability to design and manufacture packing compo- nents to meet modern seal requirements, and to provide materials for optimum packing life. With over a century of experience, C. Lee Cook is the recognized leader for engineered sealing devices, and countless customers have learned to... “Count on Cook When the Pressure’s On!” Figure 1 Packing NomenclatureCompressor Packing Compressor Packing A typical compressor packing consists of a series of seal rings in which each ring is meant to stop or restrict the flow of gas Into the distance piece. The rings are held in separate grooves, or cups within a packing case. Each ring seals against the piston rod and also against the face of the packing cup at a right angle to the rod axis. The rings are free to move radially, Iterally “floating” within the grooves. A basic packing arrangement consists of: 1) a pressure breaker that functions as a flow restricter (rather than a true sealing ring), 2) several seal rings that are meant to stop flow or leakage into the vent or distance piece, and 3) a vent control ring that prevents gas from leaking into the distance piece from the vent. Even though seal rings do not provide a direct leak path, ‘a vent to carry incidental leak- ‘age away from the packing is necessary in most applications since misalignment of the ring with the case or rod, or slight imperfections in the materials in the mating components, may allow some small volume of gas to blow-by, Gas flow through a restriction creates a pressure drop. When there is a series of restrictions, the resultant total pressure drop can be significant. This is called the “labyrinth” effect. Gas flow past a series of rings also creates a pressure differ- ential, but because of the rapid rise and fall of pressure within the compressor cylinder, and because of low leakage past the rings, the “labyrinth” effect is usually negligible. ‘Typical pressure levels in a 5 ring case are illustrated in Figure 2. The curves represent calculated values for a new packing (which should have very small leak paths) compared to a “used” packing set. The number on each curve represents the pressure upstream of that particular ring. ‘The two sets of curves illustrate how the pressure drop is highest across rings nearest the pressure when the set is new. As the packing wears, the downstream rings see more and more pres- sure drop as the leak paths of the rings increase with age. Figure 2 Pressure Drop Across Rings 1000 New Rings: Pressure Breakers QE Zero End Clearance End Clearance‘The “used” packing curves are also typical of the pressure drop across a leaky new packing set. It is apparent that a “rev- erse” drop exists across some rings during the suction stroke: that is, gas will flow back out of the case toward the cylinder during that portion of the stroke. The pressure drop through a series of rings is highest across the one ring that forms the best seal. In packing sets con- taining well-made rings which fit the rod and grooves properly, the ring nearest the cylinder will, initially, carry almost the full pressure drop. As this ring wears, or changes for any reason, the load will shift to one of the other rings, usually the next ring in line. The pressure drop may distribute across a series of rings in almost Individual rings in a packing set wear one-at-a-time, so the num- ber of rings in the set influences packing life but has little effect on leakage. In a comparison of piston rings and packing rings, it should be noted that piston rings in a compressor cylinder are usually not true seal rings; that is, they have definite “open” Jeak paths by design. This means the pressure drop will be spread over all of the rings (they act as. a labyrinth). This division of the oad, plus a lower total pressure differential across the piston ring set, results in a lighter loading for piston rings when compared to packing rings. Breaker Ring Breaker Rings Breaker rings, the simplest form of packing rings, are designed to restrict or control flow rather than effect a tight seal. In type P, the flow controlling orifice is the gap that is formed at the segment ends. In a type PA Breaker, the orifice is formed by clearance between the ring bore and rod. ‘The performance of both rings is very similar. However, a type PA Breaker provides lower rod loading than a P, at the expense of a more difficult-to-control orifice area. ‘The most important function of Breakers is to retard rapid expansion of gas from within the packing case back into the cylinder during the suction stroke. On the intake portion of the stroke, gas contained Figure & Packing Ring Type BT Side Clearance Pressure Side Radial Cut Packing Ring Type BD \_Butertangent Cut ‘Butt/Tangent cutwithin the packing case tends to reverse direction and flow toward the cylinder where the pressure is dropping rapidly to suction pressure levels. Without some restriction to this flow an “exploding” action of the rings may occur causing premature ring failure and damage (Refer to Figure 15) Pressure Breakers are not required in all packing. They are generally not needed at pressures below 300 psi and are not required when the seal rings themselves act to restrict backflow. Pressure Breakers may be constructed with any currently available material, but those materials with a low thermal coefficient of expansion and high stiffness are preferable for a stable orifice area. Metal Js usually superior to TFE in this respect: however, breakers can be made satisfactorily from some of the high strength plastics. Packing Ring Type BT ‘The BT packing ring is a true sealing ring set which is made up ofa radially cut ring (facing the high pressure side) and a second, butt/tangent cut ring. ‘The individual rings are doweled together so that the inner butt cuts of the butt/tangent cut ring are offset, or staggered, ‘Common Packing Ring Features/Packing Ring “BT” & “BD” to be between the radial cuts of the first ring in the pair. In this manner, gas passage by or through the ring is prevented, In order for these rings to operate satisfactorily, it is necessary that both fit the rod perfectly (“X* in Fig. 4) ‘They must contact each other at their mating faces and the tangent ring must lie flat against the groove surface (‘Y" in Fig, 4). In addition to this, all sealing edges must be sharp and square (BY in Fig. 4). If these edges are beveled or rounded, they form a path for gas flow from the radial cut in the first ring to the radial portion of the cut in the butt/tangent ring. If the tangential edges are not square, they form a passage from the outside toward the radial cut at the bore. All other edges (such as point “A” in Fig. 4) are gen- erally given small bevels to assist lubrication between the contact faces. Under ordinary circumstances, lubrication serves to help the seal function because it fills up minute crevices. This type of ring is single acting, or directional, {in that it seals pressure from one side only. Packing Ring Type BD ‘The BD packing ring set is “double acting”, meaning it will seal pressure from either direction. It consists of two butt/tangent cut rings doweled so that the radial portion of the cuts are staggered, blocking any flow path for leakage. All of the features that are important for successful operation of the “single acting” BT ring, such as square sealing edges and proper face contact between rings, rod and cup, are also important for successful ‘operation of the BD ring Common Packing Ring Features ‘The P, BT, and BD are the three most common types of packing rings in general use. Numerous variations of these three exist. but the principles involved are generally the same and conditions affecting proper operation are common to all types. Packing rings are made in segments for two reasons: 1) for installation over the rod, 2) to allow free radial move- ment down against the rod. Free radial movement allows for slight size variations and also provides a means to acco- modate ring wear. Both spring and gas pressure loading cause rings to contract, or move radial- ly inward, toward the rod.All types of packing rings are manufactured with an initial “end clearance” of sufficient size that no adjustment is required throughout their useful life. When they have worn to a point where the ends butt, they should be discarded. In most rings end clearance could be adjusted to maintain the proper opening. However, It is seldom practical to rework such a ring after it has operated in a butted condition. The bore will be worn out of round, and merely recreating end clearance will not correct poor rod contact. Ring Materials Lubrication and gas pressure are two of the factors that influence selection of ring material. With respect to lubri- cation, packing may operate at conditions that vary from full or normal lubrication all the way to dry or nonlube service. ‘Common materials used in packing fall into two categories: 1) metallic - (typically bronze, cast iron, or to a diminish- ing extent, babbitt) which require some lubrication but may be used at very high pressure, 2) nonmetallic - (typically carbon-graphite, TFE, or other plastics). ‘The important advantage of nonmetallics is their ability to run in poorly lubricated applications. Carbon-graphites and ‘TFE blends work well in Ring Material the complete absence of lubrica- tion, but are somewhat limited insofar as the pressure at which they can be used, due to thelr low strength (compared to metallies) Filled TFE blends are good to approximately 800 psi; but, with the addition of a backup. or anti-extrusion ring, can be used at much higher pressures. Carbon-graphites are moderate- ly strong, but somewhat brittle, and therefore limited to approx- imately 1500 psi. Plastic is the material of choice for rings which require a high degree of conformability. All rings must conform at their Joints and around the rod in order to establish a good seal, but some types will begin to Jeak slightly as they wear or as, they change Figure 6 Packing Ring Type BTR Figure 7 Packing Ring Tyne TR ‘Both Rings have Zero End Gapsize due to thermal expansion, With regard to sealing, a good ring material is one that is not excessively stiff relative to the pressure acting on it. Nonmetallic materials, includ- ing the new plastics and blends with performance improving fillers, are the most commonly used materials in the manufac- ture of modern packing rings. Metallics are principally useful at high pressure, or as anti- extrusion rings, where strength or rigidity is an important requirement. Lubricated, Semilubricated & Nonlubricated Packing Packing rings made with TFE Lubricated, Semilubricated & Nonlubricated Packing “filled” or blended with other materials (including other plas- tics) can now be used in many applications including those with less than full lubrication. By definition, LUBRICATED PACKING receives just the amount of lubrication required for long life. SEMILUBRICATED PACKING is that which receives either reduced lubrication through a lubricator or, in the event there is a short distance plece, minimal lubricant carry- over from the crankcase. Packing installed under these conditions Is sometimes operated initially (the first 24 hours or so) with normal “force feed” lubrication. NONLUBRICATED PACKING is packing installed in systems where double distance pieces, rod oil slingers, or some other means is used to prevent any lubrication from reaching the rod rings. ‘TFE is the material used most often for packing installed under conditions where lubrication is impaired or less than normal. Generally, this would be in the presence of corrosives, diluting liquids, or high temperatures. ‘The sealing principles of all ‘TFE ring types are virtually the same as those previously described. Due to their inherent flexibility, TFE rings have the added advantage of immediate sealing without requiring break-in. Ring types TR and BTR are designs that include nonmetallic rings plus a rigid back-up ring Figure 8 Packing Ring Type C End Gap Zero End ‘Clearance Packing Ring Type CR RSS Zero End Gap‘The back-up ring is made slightly larger than the rod It has butted ends and does not grip (or grips lightly) the rod under pressure loading Its function is to prevent, extrusion of the softer nonmetal- lic ring and also, when condi- tions warrant, to conduct heat away from the rod surface through the light contact. Usually back-up rings are made of metal, but they can also ‘be manufactured from strong plastics for some applications. ‘The pressure limitation of plastic rings with metal back up has not ‘been determined. This configura- tion has been used successfully up to 50,000 pst. At pressures above 800 psi, type TR rings are the preferred alternative to the standard BT Lubricated, Semilubricated & Nonlubricated Packin: sealing ring, To obtain a better seal where there is insufficient pressure to properly actuate the TR ring, type BTR rings may be used. The BTR is limited to relatively low pressure of about 2500 psi. Above this pressure, deformation may occur intruding into the gap of the butt/tangent cut ring, To overcome sealing problems with the TR and deformation of the BTR, a tangent cut may be combined with a radial cut ring. ‘This is the type C, or when a backup is added, the type CR. ‘These styles are most useful when applied above 2500 psi. ‘The basic TFE ring set for pressures below 800 psi consists of BT rings to seal cylinder pressure with a double- acting BD ring downstream of the vent. This arrangement is the same for nonlubricated or fully lubricated cases. When pressure exceeds 800 psi and the application fs lubricated or semilubricated, the arrange- ment should be as follows: 1) one breaker ring type P, 2) BIR or TR rings in intermediate grooves, 3) a BD ring beyond the atmospheric vent. Unless some special condition exists, the P and BD rings should be metal, and the BTR or TR rings a combination of metal-TFE. ‘The type P ring in this case will have an added function. Along with preventing back- flow, garter spring breakage, Figure 10 Packing Ring Type TU Figure 11 Packing Ring Tyne TUU y IRSPacking Ring Type TU etc., the fact that it is in contact with the rod around the entire circumference (without heavy pressure loading) allows it to function as a means of dis- sipating heat. BD rings may be made of metal for this Packing Ring Type TU ‘The TU ring set is essentially the same as the TR ring set, except the radially cut ring in the TR has been replaced with an uncut ring, which can be either plastic or metal. When the stiffness of the uncut U ring is low compared to the pressure acting against the ring, it will collapse inwardly, main- taining a seal even as it wears. With the proper choice of material, the TU ring set will seal low pressure (suction), with the tangentially cut ring and high pressure (discharge) with the uncut ring, The TR ring set will function in the same manner but lacks the almost perfect seal charac- teristics of the uncut ring in the TU set. ‘An added benefit with an uncut ring is reduced contact pressure between the ring bore and the rod. Compressive stress in the ring acts counter to gas pressure against the ring's outside diameter, so there is ‘lower ring-to-rod loading than there would be with the tangen- tially cut ring, To prevent extrusion of the soft plastic sometimes used in the U ring, an additional back- up ring can be added to form the TUU style. Typically the second U ring would be metal, or a very rigid plastic. With proper choice of materials for the three rings in the TUU, each ring can be made to seal only over a particular pressure range. Thus wear and heat generation can be divided over individual rings to give better performance and longer life. ‘The difficulty of installation somewhat offsets the advantages, lower leakage and reduced wear, of the TU or TUU ring set. Uncut rings can only be installed over the rod end, and with a thread protector that is no larger than the rod diameter. Packing Ring Type WB Figure 13 Packing Ring Type E TRISThermal Effects When rings expand circumfer- entially due to a temperature increase, there are usually leak paths created at the joints. The effect is similar whether this occurs in a true tangent-cut T ring such as the TR, or in a butt/tangent ring such as the BT. As it expands, a T ring is more likely to leak when the Joint is not covered by another upstream ring (as in the type ©). Contraction due to lower temperature rarely occurs, but if it does. it is much like having an undersized ring or oversized rod. All these temperature effects can be minimized by using an upstream radially cut ring to block leak paths through the adjoining ring cut. Thermal Effects and Garter Spring and /or Ring Breakag: This addition is found in the Cor CR ring arrangement. These ring styles are free of unsup- ported areas where extrusion can occur, so they're also suitable for higher pressures and temp- erature than a BT or BTR. One method to minimize the tendency toward breakage and lack of conformability of both butt/tangent and true tangent, or T rings, is to use “bridge” Joints as illustrated in figures 12 and 13. The shorter, sturdier segments make the WB style suitable for service in situations where elevated temperature and pressure may cause other styles of ring to leak or break. The handling and installation of the WB or E rings may present problems due to the greater number of segments involved. As with other styles of ring, the E and WB may be used with back-up rings to prevent extrusion along the rod. Garter Spring and Ring Breakage ‘Small, sometimes microscopic, leak paths occur in packing rings due to the nature of the material itself. This is the cause of the “labyrinth” effect across the rings and the penetration of gas into the space between ring grooves. Figure 14 shows a typical ‘compressor pressure curve which illustrates the time required to compress gas from suction pressure to discharge pressure ‘Figure 14 Typical Compressor Cycle oeGarter Spring and/or Ring Breakage is considerably longer than the expansion time from discharge to suction. It is during the compression time that gas pressure builds up within the packing assembly. At the start of suction the time required for expansion is considerably shorter than that required for compression. and gas trapped in the case tends to reverse direction and flow toward the cylinder where pressure is dropping rapidly toward suction level. This can cause an “exploding” action, particularly to tangent type rings, sometimes to the extent that they strike the inside of the groove with considerable force Garter spring damage and ring breakage may result. Normally the tangent lip will break at the point indicated in Figure 15. Garter spring breakage, and ring damage due to backflow, {is usually found in rings nearest the pressure side of the packing assembly. Proper application of a breaker ring is an effective preventive measure in that it's built-in orifice slows down or minimizes backflow from the case. Ring breakage may also occur when ring LD. and rod size do not match (Figure 16). Rings will make contact with the rod at the center of each segment on an undersize rod. In this circumstance, pressure tends to force the rings inward at the ‘tangential point and the resultant flexing may cause a fatigue break from the corner where the tangent and radial cuts meet. If undersize rings are installed ona standard rod or standard rings are installed on an oversize rod, the segments will contact the rod first at the segment ends. Pressure pulsations will then flex the entire segment. A fatigue break near the center of the segment, as indicated in Figure 16, may occur: A tapered rod can combine the stresses of both an undersize and an oversize rod with the ring constantly adjusting to the variations. The stress conditions discussed here are most impor- tant for carbon and other materials which have low fatigue strength. Figure 16 Misfit Rings Proper Ft Ring WQ SS) Undersize Rod or Oversize RingUndersize Rods Ifa rod is undersize, but true in circularity and without taper, the rings still form an effective seal because bore contact will be in the center of each segment and at this point the cut in each ring is overlapped by its mate. This doesn't normally cause a problem if the amount of undersizing does not exceed .002" per inch of rod diameter. When a rod is undersize, some additional time is required for break-in before a packing will give the best seal. Oversize Rods When packing rings have a somewhat smaller bore than the rod diameter, the segments touch only at each end, leaving the center away from the rod. This is in line with the cut, or joint, of the mating ring so that direct passage for gas flow occurs along this line. This condition, if not severe, will also be corrected by “wear in’, One problem that may occur with either an undersized or ‘oversized rod is that lubrication may be blown off the rubbing surfaces by the passage of gas. ‘The subsequent “dry contact” may result in high friction, high temperature or rapid wear. Tapered Rods In the presence of lubricating films, which help fill (or block) very small leak paths, a packing can function adequately with some slight rod taper. Generally, taper is found at one, or both, ends of the rod, the rest of which is relatively straight. Excessive Undersized, Oversized and Tapered Rods amounts of taper will destroy the seal as shown in Figure 17. The effect of the rod passing through the packing case from the tapered to the straight por- tion and back, is wear at the ring’s edges. This is due to the ring bearing directly on one edge while over the tapered section of rod and on the other while over the straight section. This dual edge wear leaves a gas passage along the bore from one radial cut to the other. Tapered rods can also cause cyclic flexing of garter springs and ring segments. If this flexing Is excessive, it can lead to spring and segment breakage. Figure 17 Effect of Rod Taper "Dirt or Foreign Matter Packing will not operate satis- factorily when excessive dirt or other foreign matter gets between the faces. This is just as destructive to packing as it would be to a bearing, or any similar rubbing surface under high load. An effective safeguard against foreign material damaging the rings is well-fitting packing that insures low leakage. If packing is not sealing properly, it will act as a filter, catching particles in the packing case where they proceed to damage all of the parts. Under operating condi- tions where dirt cannot be ‘completely removed, hardened rods. above Re 60, should be used to minimize abrasive wear. Special wear-resistant ring materials will help reduce the effect of dirt or foreign matter. ‘The selection of materials to use depends on the nature of the foreign matter and the operating conditions, When dirt is present, lubrication at two or three times the normal rate may help keep the packing clean. Added oil will not only lubricate and cool the packing but assist by flushing dirt from the rings. Matter, Rate of Leakage and Break-in Procedure Rate of Leakage Passage, or flow, of gas through. a packing set depends on: 1) the amount of “misfit” in the rings and rod, 2) any disturbances in the faces, which must be in contact to properly form seals. Under the best of eircum- stances, packing will weep or bleed in small amounts which cannot be detected by ordinary means. Some of this escape is caused by failure to maintain the various contacts as a result of wear or lubrica- ton loss. Some leakage may be due to gas being forced into the rod pores or into the lubrication film by high pressure. This small amount of gas would then escape as the rod enters a reduced pressure area. This type of leakage is quite small, almost negligible, in comparison to that caused by large imper- fections in the sealing surfaces. Itis possible to manufacture packing of extreme flatness and ‘smoothness with lapped, extra true surfaces (at added cost). ‘This is only effective if perfect, alignment and finish of the mating components is achieved and maintained. The rod surface 4s vital to packing performance so it must be equally smooth and true, as well. If packing is allowed to leak excessively for a long period of time, the condition causing leakage may be difficult to correct. New rings cannot be expected to perform satisfac- torlly if the rod is tapered or scratched, or if the case has become damaged in like manner. Under normal operating conditions, a period of time is required for all of the sealing surfaces to “mate” or “wear-in", after which the packing will be virtually leak-tight and remain 0 until normal wear or some outside influence disturbs its condition. Break-in Procedure Because of the great variation in operating conditions, i difficult to outline a universal procedure for break-in. The goal is to allow the working parts to adjust and to come to a satis- factory seat without overheating or overstressing contact faces. Overheating and high temper- ature can result in the failure of both the packing rings and the piston rod surface.Gas pressure tends to increase the true area of contact between mating faces. Accordingly. the best possible break-in would be at full load under the slowest, speed available so that frictional heat is not generated faster than it can be dissipated. ‘The amount of time needed for break-in will vary from packing to packing, depending on the pressure, speed, and lubrication Normally, metal and other rigid materials, require approximately four hours. Some conditions. such as fully lubricated, ultra smooth surfaces may require up to several days to completely break in. Where conditions do not permit slow speeds such as with elec- tric motor drives, packing can be effectively broken in by grad- ual load increases. This can be as little as 10-15 minute inter- vals, with corresponding small pressure increases, For instance, i is better to increase the pres sure through a 1000 psi range using 250 psi increments at 15 minute intervals than to run a full hour with a given load and then increase a full 1000 pst for an additional hour. ‘The amount of lubrication applied during break-in is best determined by considering the specific operating conditions and determining exactly how the packing is reacting to the lubrication and the operating environment, Usual practice is to lubricate at 2 to 3 times the normal rate during break in, The use of special, heavy lubricants during this period {is only advisable if some char- acteristic of the service causes a lowering of oil viscosity. ‘Typically this would be caused by something in the gas stream, or high temperatures. It is sometimes effective to direct a small stream of lubri- cant on the rod immediately outside of the packing case during break-in, This not only ensures better cooling and lubrication, but also serves to carry off wear particles and other debris from the working surfaces, Some wear products or fines are normal during break-in since the idea is to “wear off” the high spots or unevenness, promoting a good, smooth con- tact between mating surfaces. Break-in Procedure At no time should packing, be allowed to heat to such an extent that smoke, indicating high temperature, forms Packing, during break-in, may exhibit alternate periods of low and high leakage. although there should be a gradual over-all reduction in leakage. If packing is allowed to leak excessively. it if not impossible, to maintain a proper lubricating film. becomes extremely difficult, Whenever overheating or other distress signals appear, the unit should be shut down and allowed to cool, after which a restart may be undertaken. ‘Oceasionally several such restarts are necessary. Time spent in proper break-in, however, usually results in longer packing life. ‘TFE and similar plastics do not require a lengthy break-in due to their inherent flexibility, Full pressure can be applied after a 15-30 minute run to make sure there are no mech- anical problems. However, some break-in time may still be necessary for the rod surface against which the plastic Is running.Nonlubricated Packing Nonlubricated Packing Nonlubricated packing usually consists of TFE rings, or, if the operating pressure is high, TFE with back-up rings. The back-up rings, pressure breaker, and vent ring may be metal, but high strength plastics are also applied in many applications. Most ring materials will act as abrasives when no lubrication is present, Therefore, harder rod material and a finer rod finish is required. Chrome- plated, nitrided, or tungsten carbide coated rods are the most common materials used with nonlubricated packing. ‘The choice of rod material. as with packing case material, may also be dictated by corrosive elements in the gas stream. Packing cases may be plated but water or water/anti-freeze for corrosion resistance, but are mixtures are the most effective, more commonly made of corro- Oil offers some advantage as sion resistant cast iron, bronze, far as corrosion resistance or or stainless steel. deposit prevention, but is much, less capable of carrying away One potential problem in nonlubricated conditions is the heat than water. high frictional heat generated ‘Some compressors incorporate by the packing at the ring bore. liquid or gas coolant circulated This heat must be removed by directly against and around the conduction through the case or_—_rod, When liquid is used, the rod. Cases are normally cooled problem of effectively sealing by circulating coolant through —_the coolant is offset by its channels in the individual comparatively better heat packing case cups. The channels _ transfer properties. can be formed internally in the Regardless of how it is cups So no O-ring seals encir- cling the rod are required. or cups can be made two-piece for easier disassembly and cleaning. Almost any “heat transfer fluid” can be used as coolant, achieved, keeping friction low and removing the heat generated is crucial to long life for nonlubricated packing Figure 18 Causes of Ring Leakage (WY (oS / => KF ‘Opening Through Joints and Past Edges A Figure 19 Typical Oi! Wiper Packings FN () sa Misalignment, Lack of Flatness, or Poor Surface Finish “ OW! DrainGil Wiper Packing ‘The increase in nonlubricated applications and the use of incompatible oils in the same machine have created the need for greater effectiveness in crankcase oil control packing. ‘This need has been met by designs based on two important principles which are somewhat opposed to designs commonly in use. The first feature, shown in Figure 19, is that the scraper ring on the dry side of the wiper groove has its drain notches facing away from the oil source. This reduces “valving” of oil into the seal ring groove because there is less surface contacting the back side of the wiper groove. During operation, the lower part of the ring set is generally covered with a significant amount of oil which has been scraped from the piston rod surface. The oil collects on the bottom of the rings before dropping into the groove sump. ‘This “collecting of oil” on the bottom of the rings, coupled with the flat surface of the back ring in some older styles of packing, promotes “valving” action over the division plate. ‘The residual oil can accumulate in the normal ring side clearance where it is squeezed out at each stroke, with some of it being pumped up and over the barrier, and some of it being deposited down into the sump. ‘The second design feature affecting oll wiper packing is the fact that the ring’s flat face is installed toward the wet side of the wiper groove. There are no drain notches which would allow the oll to enter the groove in any great volume. In most old style wiper packing, the first ring is usually ventilated by notches on the front face. In new style packing, the rings are radially cut with minimum end clearance so that oil entering the groove is reduced to that which can enter the small openings defined by the cut and the radial clearance of the case bore around the piston rod. In this manner, the normal oil return holes are not overly taxed by oil pooling or build up, Also, the oil level never reaches point high enough to increase the valving tendency between the back ring and the division plate. ‘The second groove on the compressor side in the design shown in Figure 19 contains a double-acting seal ring which is independent of the wiper group. ‘This rings function is to minimize Oil Wiper Packi passage of gas from the distance piece to the crankcase and also to block the slight “pumping” action of the cross-head which can push ofl past the wiper. In all cases, the condition of the piston rod is an extremely important factor for oil control. Rods which are scratched, out of round, or otherwise damaged, will not perform satisfactorily, regardless of wiper ring design. Wiper ring material needs to be conformable in order to mate to an imperfect rod, but also be of sufficient strength to maintain a sharp wiping edge. Using nonmetallics with the right combination of flexibility and hardness has proven to be the best approach toward solving wiper problems. There are several possible variations of ring configuration and design. The final selection of rings is a question of the specific task to be performed. Some packing is intended to repel or return oil while others are intended to collect of! and direct it to separate drains. Compressors which use incom- patible oils in the cylinder and crankcase should have auxiliary wiper assemblies built into the main compressor pressure packing. Regular distance plece 8Material Selection for Compressor Packing wipers under this circumstance will collect and drain only crank- case oll back into the crankcase. Extending the rod length to prevent overtravel between the crank packing and the main compressor packing is also desirable wherever maxi- mum separation of two oils is required. Material Selection for Compressor Packing ‘Selection of the most suitable ‘material for compressor packing rings depends on the varied conditions in which they may be installed. The parameters are often interdependent, adding a degree of complexity to the decision. For instance, specific pressure limits are partially dependent on temperature, which is influenced by lubrication, which in turn, is influenced by the physical properties of the gas. The chart {n Figure 20 can be used as a general guideline, but some Judgment must be exercised regarding the over-all service for which the rings are intended and the suitability of the materi- al selected. In the Materials Selection Chart, when a material is. recommended for use with @ particular gas, more than Just the corrosive properties of that gas are considered In addition to any material compatibility effect, the gas may also influence lubrication. It may dilute or degrade the oil film, or products of corrosion (formed elsewhere in the system) may enter between the seal surfaces. When either occurs, the effect can be rapid abrasive wear or a rise to destructive temperatures because of high friction. The proper choice of material must be made consid- ering its resistance to attack from components of the gas stream, and, more importantly, its wear properties when there is poor lubrication and possible abrasive solids. Rod materials are not consid- cred in the chart. However, mat- erial selection becomes impor- tant when lubrication is poor since the rod surface then has, more influence on the wear of the ring material. Material Selection Chart Bil ecommendes Lubrication Required al le 3 Ring Material Ter Pressure | & $§/e2|ssiss| = ‘Norm Winimum | Non time | “time” ||, | €| £ |es|88)5 lube Lobe bebe cry | sy | 22/8 /8 6a/82| 5 [Cookroe Laminated Phenolic 350) 1500 Cook Graphite ron 700 | Not timed Cookmet Bronze 500 | Not Limited Carbon-Graphite 750 1500 Baboit 300, "1500 Filled Tie Pius Metal Back-up 500 | Not Limited Filed We 500) 700, Flled Polyimide 600 | Not Limite 8There are a few rod/ring material combinations which are incompatible. However, hardness is generally the most critical rod characteristic. Hardness can be obtained in the base material or it may be achieved as a result of a coating such as one of the metal sprays, or electro- plating. For most applications, especially those at pressures above 1000 psi, through hardening or surface hardening of the base rod {terial is preferred, ‘The table in Figure 21 outlines the optimum rod specifications for several different service conditions. Lubrication Without question, lubrication has more influence on material selection (and packing perforn ance) than any other factor. When full lubrication is supplied to the packing, and nothing in the gas stream affects its operation, then any of the available materials will operate satisfactorily. All are capable of operating with full lubrication. When minimum lubrication is present, the choices are nar rowed: and with nonlubricated service, there are even fewer choices of material Full lubrication, of course, means that oil of the proper viscosity is applied at the proper flow rate. Generally. viscosity for packing above 1000 psi Lubrication should be between 120 and 150 SSU at 210°F, while below this pressure, viscosity down to 60 SSU at 210°F will be satisfactory, ‘There have been various rules or equations developed by compressor manufacturers or oll suppliers which relate the quantity of oil required for packing to the operating conditions. Contact your com- pressor OEM representative for specific recommendations. Some packing, obviously, will require more lubrication than others because of poor operat ing conditions, while others, where the service is not severe, can operate with less than this amount Not Recommended Recommended only if ga is dry w‘Lubrication “Mini” or “minimum” lube Is used to define a level of compressor lubrication which is approximately one-quarter normal. This level may vary slightly because it essentially indicates a level below which metal rings will not operate satisfactorily. The lubrication level then dictates the use of plastic rings with some self: lubricating properties: that is. a material that can operate almost without lubrication. ‘The small amount of of can be supplied from several sources. It can be “accidentally” carried over from the crankcase, con- tained in the gas stream, or it may be due to controlled feed directly to the packing case. For pressures below 2500 psi and a “normal” length packing, ‘one lube inlet per packing case fs sufficient, Above this pressure, with oils of recommended viscosity, and with packing which does not contain a bushing, two lube points are commonly used. Where very light oils are necessary, addi- tional lubrication points may be provided. Ifa bushing is pres- ent and the case is relatively long, a separate inlet may be provided for the bushing itself. Fortunate most large com- pressor cylinders are horizontal. ensuring that if oil is introduced at the top of the rod or cylinder, gravity will take care of distrib- luting it around the circumfer- ence. A point often overlooked is that if the oil inlet is only a few degrees off top center, the upper portion of the rod may receive no oll. Gas flow and the motion of the rod distribute oil in the axial direction. ‘The general method for lubri- cating a rod ts to machine a channel around the off outlet, at the packing case bore to cause oil to drop from the cup bore to the rod. Even with this feature, the oil may cling to, or creep around, the inside of the cup rather than drop onto the rod— the bottom of the rod receiving lubrication while the top remains dry. To overcome this, a small ‘TFE quill may be inserted into the oil passage. The quill is only a few thousandths away from the rod. Instead of forming a drop, the quill provides an oil wick to the rod surface so there will be a continuous Figure 21 Rod Specifications Conditions Rod Materials Lubricated, ‘carbon oF tow alloy steel, Noneorrosive through or surface hardened— ‘Below 1000 ps! Re 25 min. 32 RMS 1000-6000 psi Re 40 min, 16 RMS, [Above 6000 pst Re 55 min, 16 RMS, Poorly Lubricated, Noncorrosive Material as above— ‘Below 1000 psi Re 40 min, 16 RMS 11000-6000 pst Re $5 min,, 10 AMS ‘Above 6000 psi Hard coated Re 65 min. 4-6 FMS Corrosive Hardness and finish as above in corrosion resistant materiat“moving” film applied. This ensures that oil will be deposited directly to the top surface ina reasonably steady flow. In all lube lines leading to packing cases, a check valve should be installed as close to the case as possible. Often gas flow and compression in an open oil passage will create considerable heat. The check valve serves to keep the volume of the open line small and, thus, minimize the heating effect within the passage. Oil channels within the packing case should be kept as small as practical, and with oll of the proper viscosity, capillary action will usually keep the lines down- stream of the check valve full of oll, Type of Lubricant For the most part, lubrication of packing rings takes place in the “boundary” region. This is the area where an extremely thin lubricant film separates the ring bore surface from the rod. The most important factor affecting film thickness is oil viscosity. However, other prop- erties of the oil may also affect film thickness. For example, low refined oils or ones containing oily agents or film strength improvers are best for any application where some condi- tion of the intended service may tend to reduce viscosity. Straight mineral-based oils, are adequate for dry gases at low pressures; but for gases containing liquids (water or condensed gas), compounded oils or oils with proper additives ‘Type of Lubricar offer the best lubrication. Nearly all high speed, high pressure compressor packing will benefit from an oil contain- Ing an oxidation inhibitor and a film strength improver. ‘There are a large number of possible additives for modern oils, and the necessity for using an oil with any particular one 1s, In most cases, determined from actual field tests. Usually the choice of which oll to use is based on its compatibility with the gas stream or with other process equipment, and only rarely, if viscosity is adequate, would the oil be unsuitable for packing lubrication. When mineral oils are unsuit- able and it becomes necessary to consider synthetics, the Figure 22 Friction Force on Ring Gas Pressure ‘Acting on Ring Fiction Force ‘Acting to Push Ring Off Cup FacePacking Leakage Control effect on packing performance is the same as with highly refined oils. Some of the synthetics lack the ability to lubricate well under boundary conditions. Using high viscosity oils becomes doubly important; but even at that, some synthetics will lubricate so poorly with thin films that it is necessary to consider the degree of lubrication as “minimum” and select a ring material accordingly. ‘There are a large number of synthetic oils and some are more effective as packing lubricants than others. Normally, the synthetics are not chosen because of their lubricating properties but because of their other advantages over mineral oils, such as temperature resis- tance or process compatibility. ‘Those which offer the least friction and wear are the poly- alkylene glycol oils, synthetic esters, halogenated hydrocar- bons, and fluorosilicones. With these it is nearly always necessary to depart from the standard rate of application. It {is necessary to increase oil feed rate and, sometimes, number of injection points in a packing case to help offset poor lubricity With very expensive synthetics such as fluorosilicone, it is often desirable to decrease feed rates for economic reasons. Packing Leakage Control Relative to the potential sealing problems that may occur in high pressure compressors, the potential sealing problems in low pressure (50 psi or less) compressors may seem insignif- icant. Wear and packing life may not be major problems, however, obtaining a good seal between packing ring and rod, and espe- cially between the ring and cup face, may be difficult. ‘There are two reasons for this. The first is because of the relative stiffness of conventional seal rings. As pressure differen- tial increases across a ring, some of the leak paths will be decreased as the ring deforms. Obviously, using materials with low modulus of elasticity, such as plastics, will improve the seal. However, even plastics are relatively rigid when sealing against 1 or 2 psi. Figure 23 Axially Loaded Rings We AL oe at SSSA, BAY VRORE REE Nae! Side Loaded ‘ounat oo a Toe XRBT Sey S zesuch as when suction pressure 4s close to atmospheric pressure. ‘The second reason for leakage at low pressure is loss of ring contact with the flat cup face. Figure 22 illustrates the frictional forces that may cause a rod to hold a packing ring away from the cup. The point at which this occurs depends on ring size, the coefficient of friction, and the pressure level. For “normal” size rings, approximately 50 psi differential pressure is required to hold a seal ring against the cup. In a compressor, pressure at the packing vent is normally well under 50 psi, so good practice dictates the use of a double-acting ring to minimize leakage from the vent into the distance piece. This ring func- tions by forming a reasonable seal regardless of which face it rests against. However, there is, an instant during stroke reversal when even the double-acting ring will leak, and in order to improve on this it is necessary to use rings which are spring loaded axially, such as in Figure 23. ‘The advantage of Cook axially loaded ring types such as the the AT or WAT, is the almost equal axial and radial load provided by the spring. If these loads are not equal, or very nearly equal, the ring will cither “hang” in the groove and thus leak, or it will be pulled from one face to the other by the frictional force. Problems Associated with Low Suction Pressu Problems Associated with Low Suction Pressure When compressor suction pressure is below atmospheric pressure (when pressure within the cylinder falls to vacuum), air may pass back through the packing and into the cylinder. Either double-acting rings or those spring loaded against the cup face will reduce leakage, but neither will eliminate it completely due to the minor Inconsistencies in the ring surfaces insufficient gas pres- sure to make the ring conform. One solution is to introduce gas at low pressure at the vent, as in Figure 24A. The gas that enters the cylinder will be drawn from this vent rather than from atmosphere. The gas pressure Figure 24 Venting During Suction a Gas To cylinder During Suction Vent Gas Supply ‘at Pressure Above ‘Atmosphere Vent Gas to AtmosphereProblems Associated with Low Leakage Requirements, in the vent need be only a few psi above atmospheric pressure. Loss of vent gas to the distance piece can be minimized with another lower pressure vent to atmosphere, as in Figure 24B. This more conventional arrangement will control gas leakage into the distance piece to at least the same degree as with a single atmospheric vent. When it is impractical to use a source of gas slightly above suction pressure, then gas at full discharge pressure may be used. This will increase leakage both toward the cylinder as well as out the atmospheric vent. It also puts a higher pressure load on the ring between the vent and the distance piece, or between the two vents, and will cause more frictional heat. ‘This can mean that for part of the stroke two rings will be loaded at high pressure, when normally only one ring will have a high differential pressure across tt. Everything discussed to this point assumes the gas used to exclude air was the same as the gas going through the compressor cylinder. There are circumstances where it is more practical to use a gas other than the process gas being compressed. The usual choice for this buffer gas is an inert gas such as nitrogen. If nitrogen is used and the pressure at the vent is less than discharge pressure, then the gas leaked into the vent or the distance piece will be a mixture of the inert gas and process gas. Of course, gas delivered from the compressor cylinder will also contain a small quantity of the inert gas. Problems Associated with Low Leakage Requirements In addition to low suction compressors, where entry of air should be avoided, there are a number of applications where any loss of gas to the atmosphere must be prevented. This is true for toxic or danger- ous gases, very expensive gases, or in processes where it is desirable to maintain a constant ‘quantity of gas in the system— such as within a refrigeration cycle, for example Figure 25 Buffered Packings Recovery Vent I> (ix of Cylinder Butfer Gas to Gas Plus Butler Gas)‘There are several methods that will accomplish the goal of minimizing leakage. One way fs to use a double-vented pack- ing, figure 25B, with buffering gas introduced at constant pressure to the outer vent instead of the inner vent. The “buffering” gas is usually inert or at least compatible with the atmosphere. Leakage from the main sealing rings flow out the Inner vent in addition to some flow of buffering gas. The loss to atmosphere of the gas being compressed is zero and the mixture of the two gases from the inner vent can be recovered. Another way to prevent leakage is much like the method used to prevent air from entering the cylinder during vacuum condi- tions. As illustrated in Figure 25A, a buffering gas is introduced through the packing vent at a pressure which exceeds the compressor discharge pressure. ‘The result is a gas mixture in the cylinder as opposed to one in a recovery vent. In each of the illustrations, only one ring set is shown between the two vents and downstream of the outboard vent. Depending on the pressure levels, additional rings may be used at either of these two points. Affect of Ring Type on Leakage Control In each of the low pressure vent arrangements previously discussed and illustrated in Figures 24 and 25, double- acting (BD) rings are often used. ‘When the requirements call for ‘Affect of Ring Type on Leakage Contn less leakage, one of the axially loaded type of rings would normally be substituted. Axial loaded rings are used for control of gas flow toward the cylinder, or into the vent or distance piece, as well. Figure 26 shows how the various ring types are installed in order to be effective seals in typical packing arrangements. It is difficult to relate the choice of ring type to discharge pres- sure. However, the choice of ring type can be related to the pres- sure differential at a particular point in a case, and to the amount of leakage which can be tolerated When confronted with low pressures, the simplest control method is to use soft packing Rings Used to Control Leakage At Low Pressure ‘B0 Rings at Discharge L_! Pressure Below 200 psi ‘Single Acting Seal Rings ‘at Pressure Above 200 psi BBD Rings at Discharge Pressure Below 200 psi. Single Acting Seal Ringe at Pressure Above 200 psi. Butter at Low ) Pressure Pressure BD Rings at Discharge Pressure Below 200 psi. Single Acting Seal Rings at Pressure Above 200 ps. ‘Gas Supply j-— or Vent at ‘Single Acting Seal Rings ‘at Pressure Above 200 pl. Low Pressure 180 Rings at Discharge ue Pressure Below 200 psi. | Gas Supply Single Acting Seal Rings at Pressure Above 200 psi. for Vent at Cow PressureLeakage Control with Distance Piece Pressure or a lip seal, as illustrated in Figure 26C or D. The result will be a tighter seal than with cut rings. However, packing life may be somewhat shorter due to a limited tolerance for wear. In many reciprocating machines, lubrication is marginal, and when this is coupled with significant rod “float”, there may be heavy wear in soft plastic seals. This packing arrangement, though not 100% effective, may be chosen for gases such as ammonia, propane, or methane. In general, the escape to atmosphere of these gases may be tolerated to a greater degree compared to gases like vinyl chloride or hydrogen sulfide. ‘The quantity of leakage with any of the ring types or with lip seals is difficult to estimate. ‘The best that can be done is to predict a range in which the leakage quantity will most likely fall. This is normally done based on experience and empirical data for most ring types. Leakage paths through packing rings occur not by design, but due to tolerances, alignment, and various characteristics of the compressor and ring itself. Although it would appear that flow rate would be heavily dependent on pressure differential, this is not the case as rings tend to conform and effect a better seal at high pressure. Under normal conditions and with most gases, leakage from the cylinder or through any of the vents will be on the order of 0.1 to 0.2 scfm (standard cubic feet per minute). As the factors which cause leakage progres- sively get worse (misalignment, dirt, poor finishes, poor lubrica- tion, etc.), or with lighter gases such as hydrogen or helium, leakage rates can be expected to increase by 2 to 4 times. ‘Once again, this is just a guide based on measured flow rates at various conditions. If a more accurate flow rate is required, it must be obtained by test and measurement. Leakage Control with Distance Piece Pressure ‘One method to prevent both air contamination and/or gas leakage is to close, but vent the distance piece rather than Figure 27 Distance Piece Buffering T (il Recoverythe packing case. To accomplish this, the conventional packing arrangement, less the atmos- pheric vent, may be used. It is also necessary to provide a positive seal for the partition packing in double distance piece machines, or for the wiper packing in a single distance piece machine. To reduce air contamination, the distance piece vent is used to supply gas, whereas when it is required to control leakage, the distance piece vent will be used to recover gas. In either case, one type of partition or wiper packing seal arrangement is illustrated in Figure 27A. It consists of side- loaded rings between which a buffering liquid (normally oil), or more commonly buffer gas, is injected. When oil is used, it is generally taken from the crankcase. Part of the leakage past this seal goes back into the crankcase, while part must be recovered from the distance piece. In the event a buffer gas is used, it must be one that is, tolerated in the crankcase and suitable for mixing with the recovered gas. When two distance pieces are used, the second one can be pressurized to serve the same function as the AL ring arrangement shown in Figure 27A. In nearly every instance where as is used in the distance piece to control leakage, the process pressure is very low which usually requires the use of axially loaded rings. Leakage Control with Distance Piece Pressun ‘The methods previously described are the more com- mon ways of controlling leakage. ‘There are several reasons for wanting to do so including economic, ecological, safety or to control odor. There are other methods, basically variations of those previously discussed, used for controlling leakage not only in reciprocating compres sors, but in a wide variety of applications such as rotary seals, pumps, engines, “clean” rooms, and process vessels. ‘The basic principle remains the same in all applications: ‘The prevention of gas flow in one direction along some particular path is accomplished by estab- lishing pressure conditions that cause gas to flow in the opposite direction along that same path. Gas Buffer Inlet Double Distance Piece‘Static Compressor Sealing Static Compressor Sealing In some applications, after a compressor stops, it is desirable to maintain pressure within the cylinder. Rod packing will generally leak slightly more when rod motion is stopped as compared to when it is moving, ‘This is due to a number of factors; loss of oil which filled the leak paths, changes in the ring shape as the ring cools, and changes in rod alignment as the temperature changes. To eliminate leakage through the packing rings, a product called the “Static Pac” is avail- able in "kit" form to adapt to existing cases, or it can be supplied as part of the original design. The Static Pac is essen- tially an uncut, conformable ring which is forced against the rod by a piston when the ‘compressor is stopped. Actuation ‘occurs when pressurized gas is admitted to the piston. The piston stroke is approximately 1/4”, and when actuated, wedges the seal ring inward against the rod. The shape of the seal is such that pressure from within the cylinder will cause the seal to move away from the rod when actuating pressure is lowered. A typical packing case containing the Static Pac is illustrated in Figure 28. The minimum pressure required to actuate the Static Pac seal is one-half cylinder pressure. It is often convenient however, to use up to full cylinder pressure, ‘When the compressor is in operation and after it has stopped, a three way valve would be opened, admitting pressurized gas behind the internal piston. Pressure must be maintained to the Static Pac for as long as it Is desirable to seal the cylinder. Deactuation would take place when the valve is opened to atmosphere, reducing pressure on the pis ton and allowing the springs to push the piston back away from the seal. The seal itself would then lift from the rod surface. ‘Avent to atmosphere (or some other low pressure area) must be located downstream of the Static Pac in order for seal actuation to occur or for seal release. ‘The Static Pac can be adapted for pressures up to approximate- ly 2000 psi. The seal itself is made of relatively soft synthetic rubber for pressures up to 700 psi or Teflon for pressures above that. Leakage from a cylinder may occur through the packing case itself as well as the rings. When the Static Pac is installed, other areas for potential leakage should be checked. It may be necessary to “lap” the case mating faces or install O-rings between the individual cups. Piping, cylinder heads, and any gasketed joint are possible leak points which must be sealed if the gas is to be kept within the cylinder: ‘The Static Pac is excellent for overcoming the loss of gas through valves used to isolate a compressor from the pipeline. Often these valves leak and while the compressor is not operating, gas may flow through the valves, back through the cylinder and packing to atmos- phere. The Static Pac will pre- vent this from occurring. Possible uses for the Static Pac Include any application where it Is desirable to eliminate the loss of an expensive gas or to prevent an objectionable gas from escap- Ing to atmosphere. The Static Pac may be used in conjunction with special vented or buffered packing to provide an overall seal arrangement that can achieve very close to 100% leakage control.Compressor Barrier Fluid Systems for Fugitive Emissions Control and Barrier System Compressor Barrier Barrier Systems Fluid Systems the vent which normally ABCooling Reciprocating Compressor Packing the barrier seal and become fugi- tive emissions outside the packing flange. These may measure as much as 200 ppm but generally are much lower. Gas transported by the oil film or rod surface becomes important only when allowable emissions are near zero. Cooling Reciprocating —_ Compressor Packing One of the critical, if not the most critical, factors in obtaining good service from compressor packing is proper cooling. A primary source of heat is from the work required to overcome the frictional resistance to motion of the seal rings. ‘This is influenced by material selection, the ring dimensions, the operating characteristics of the compressor, and the condi- tions of the intended service. ‘The relation between cooling requirements and the various influencing factors is discussed here. This is intended to serve as guide, indicating when special cooling is required, and to help in sizing the equipment needed to provide the cooling. The solutions set forth are general in nature but based on basic engineering principles and empirical data. Low friction materials, such as TFE, carbon graphite (or compounds composed of plastics filled with TFE and ‘carbon graphite), and other solid lubricants, have made It possible to manufacture pack- ing and piston rings capable of satisfactory operation in a non- lubricated compressor cylinder. ‘The frictional characteristics of these materials are good, but not nearly as good as when lubrica- tion is used, The configuration of the seal rings affects this somewhat, but even with the best designs, considerable heat due to friction is generated. ‘Temperature is a limiting factor in the design of high speed, high Pressure compressors. A point is reached where compressors fail unless the heat is conducted away from the rod or cylinder surface Friction with the packing rings. although not the only source. is the major contributor to excess heat. ‘The primary purpose of cooling the packing is to remove the heat generated due to friction ‘Figure 30 Nomenclature ‘A- Area (Fr) Ps - Compressor Suction Pressure (PSIA) ; wn. eee cosag™ i st vs F- Force required to move Rod Deneeet — Soest Ym ee FPM Avg. Rod Speed (FL/Min) the - Film Coefficient (BTU/HE-FE-"F) ‘k- Thermal Conductivity (BTU-FUHI-Fx2F) 1n- Gas Constant D- Gas Density (LAF) a- Average Pressure (SIA) Pa - Compressar Discharge Pressure (PSIA) 1r- Surface Temperature of Rod (F) Ts - Temperature of Suction Gas (*) Temperature Diterence (°F) Gas Viscosity (Lira V- Gas Velocity (FUN) W- Ring With (a)between the seal rings and the rod. Nearly all the work done to overcome friction converts to heat at the ring and rod mating surface. This heat is transferred to the case, the gas passing through the cylinder, the dis- tance piece, and the crankcase, Factors affecting generation of heat, and heat flow, vary from compressor to compressor making accurate predictions of the quan- tities involved very difficult. A general method of calculation, coupled with certain assump- tions, is a starting point that can be modified by empirical data gathered in actual field installations. This will provide reasonably accurate results for most appli- cations. Estimating the effect of friction is difficult in any event but especially difficult for installations with less than full lubrication. When a compressor is lubricated and pressures are relatively low, friction loads can be estimated fairly accurately. However, at low pressures cooling is frequently not required. At higher pressures the lubricant film separating the ring and rod surfaces is, at best, partially effective and the coeffi- cient of friction is more difficult to determine without actual operating experience and empirical data. Examples of methods to cool cases are shown in Figure 31 Coolants in successful applica- tions range from oll, which is circulated by convection, to Cooling Reciprocating Compressor Packin special fluids chilled and pumped through the case. In ‘some instances gas is blown through the case or over the rod for cooling. Currently, the best available method to affect cooling is to use a case with internal cooling channels, shown in Figure 31C, which circulates water or a water anti-freeze mixture. Oll is not often recommended because it Is ineffective at removing heat compared to water or water and anti-freeze. In a packing case containing several rings, most of the pres- sure drop will be across one ring, For calculation purposes, the heat generated by friction can be considered to be totally from one ring carrying the entire Figure 31 ‘Tynical Cooling Methods Figure 32 Pressure on Ring Faces Fluid Pressure “Metal-To-Metat” Contact Pressure ‘Acting on Rng Side. uid Prossure lid Pressure ‘Acting on Ring 0.0. | fet tata To Meta” —— Contact Pressure Friction Force F Acting At Bore of Ring Rod Motion ~~ a isaCooling Reciprocating Compressor Packing pressure drop. A pressure drop distributed in any manner over a complete set of rings generates essentially the same heat. For purposes of simplifying the calculations it can be assumed that the Coefficient of Friction is independent of load and contact area, Therefore, equation (1) in Figure 33 represents the work required to overcome friction. Obviously, the velocity of the rod {is not constant throughout the stroke, but again the Coefficient of Friction can be estimated on the assumption it is indepen- dent of velocity. As shown in equation (2), the work done can be converted to heat generated. Figure 32 shows a cross section of a packing ring arrangement on a rod, illustrating the forces and pressure loading. Friction force is dependent on the Coefficient of Friction, the ring dimensions and the pressure acting on the ring. Reasonably accurate results can be obtained using the “mean” pressure between suction and discharge. ‘A more accurate calculation of friction force can be made using an average pressure “Pa” for the entire cycle, from equation (3). ‘Typical ring configurations found in packing cases are shown in Figure 34. These can be broken down into four groups based on the load exerted against the rod. ‘Ata given pressure and total width each group exerts a different load due to the type of cut found between the ring segments. The type of cut changes Pressure distribution across the bore of the ring. This applies to normally proportioned rings for the most common ring types in current use. The friction force for the various ring types is shown as related to pressure and ring dimensions, ‘The Coefficient of Friction normally varies between 0.01 to 0.8 in most compressors. The lower figure corresponds to a well-lubricated surface, and the higher for cases where dirt and/or abrasives are present. It may even range higher than 0.3 but, coupled with high pressure, the packing is usually unable to fune- tion and can be totally destroyed. At very high frictional resistance, it becomes nearly impossible to Figure 33 Friction Equations ‘o Fevouren =) 218) minute = LM irom P| ove 2) By (apa ‘Figure 34 Friction Loads Q OOO oO Group B I = (x) (0) (Pa) (W) (1.24 ©O F= tr) (Pa) (W) (7+ 5)get rid of the heat generated or to maintain a reasonable seal. ‘The Coefficient of Friction for compressor packing is shown in Figure 35. The data contained in the table is a result of experience, tests and other empirical data. ‘This information on f, along with ring dimensions, operating pressures, and red speed can be used to calculate BTU/Min. generated by the seal rings. (From Equation 2) In Figure 36, BTU/Min. at various products of Average Pressure times FPM are shown. Common groove widths have been chosen for purposes of illustration. For other widths, rod diameters, and intermediate values of Pa X FPM, interpolation of data in the tables may be made. ‘The heat generated, as men- tioned earlier, is dissipated through several means. For most compressors, the two major paths are: 1) Through the case or case coolant, and 2) Through the gas flowing in the cylinder. Heat Is lost to gas passing through the cylinder when the rod, warmed by friction, moves into the cylinder and releases some of its heat to the inlet gas at suction temperature, and also during part of the stroke occurring during discharge, ‘There are several formulas, or empirical relationships, which could be used to describe the flow of heat from the rod into Cooling Reciprocating Compressor Packit the gas, all dependent on the same factors. One which might be used is equation (4), the formula for calculating the surface coefficient for gas passing over a smooth surface. ‘The total heat flow, Q, into the gas, may be expressed as shown in Figure 37. Obviously, the rod is not a flat surface with gas flowing exactly parallel to it. The gas velocity and direction of flow vary widely from point to point on the surface of the rod. The rod temperature and area of exposure are con- stantly changing as well. Any calculation made without taking all this into consideration is an approximation, although a useful one. OO! QO 000 CO | Iwi F(x) (Pa) (W)(7D+.5) Ft) 0 (Pa) (WN 1.24 +1) Coefficient of Friction Various Products of Pa x (FFM) Operating Conditions ‘Suggested {to use ‘when estimating eTUMin [Lubricated ‘Metal Rings P2000 pst Pa>2000 psi Poorly Lubricated ‘Metal Rings. ‘Nonmetal tH Non Lube Jory inort Gases [TRE Rings. Nonmetalic Rings Fine ee WW DinorSolis Present | 12-7 ‘Carton or TFE Rings Din or Solids Present ‘Carbon Graphite Rings BBR bin keCooling Reciprocating Compressor Packing Relative values for most com- pressors can be calculated after making several approximations. ‘The area of rod exposure can bbe as shown in Figure 37. The change in temperature is the dif- ference between rod temperature and the suction gas temperature. Gas velocity can be set equal to the rod speed, Using these assumptions, the expression for quantity of heat is shown in equation 6. If k, p. and u are obtained for the par- ticular gas being compressed, we can assign values to everything but the rod temperature Tr. Rod temperature is one of the conditions to be controlled with cooling. Using the maximum. value of rod temperature will give the maximum heat flow, both into the gas and the case. If the value ts lower, the heat transfer is lower, and the rod temperature will tend to rise. ‘Therefore, it is logical to base heat flow predictions on maxi- mum allowable rod temperature. In general, nonlubricated machines can run with rod temperatures as high as 250°F. In lubricated machines the limi- tation is approximately 150°F. ‘When heat flow into the gas exceeds the heat generated, no separate liquid coolant is required. Experience with compressors in the field Indicates that unless the total heat to be removed from the case exceeds 20 BTU/minute per inch of rod diameter, it is not necessary to provide coolant, ‘Once it is established that coolant is required, it is necessary to determine required coolant temperature and amount of flow. Usually, one gallon per minute per inch of rod diameter provides sufficient velocity through most cases to ensure ood heat transfer. The increased flow for larger diameter rods absorbs the increased heat generated and also compensates for larger coolant passages. Larger rod diameters generally have larger cases and thus more room for coolant passages, It has been difficult to find a correlation between the calculat- ed thermal resistance of a pack- ing case and the observed heat rejection rates. Because of this, coolant temperatures are deter- mined by first setting the tem- perature of the coolant as it leaves the case at about 90°F Figure 36 Friction Force — BTU/Min. = ae = > Te fsa | Pair Ys ea Hee 2 [aoemes | sarin | | een 3 femmes | canes | nen [essen + some | Sean | sna] a2 5 __heasnen | roam [zara] ssa Tun aoa “GENERATED VARIOUS Pa) (FPN GENERATED @ VARIOUS Pa) EFM) FOR GROUP A RINGS IN 8 WIDE GROOVE TOR GROUP © RINGS IN'38 WIDE GROOVE fe a | ae | | | fos | Pax FP I | Bamewe| “acon | sooo | re | ens] | Samewr| “100000 | sooo | 1 | 2x1 7 eso oot saan] [+ sor aot — soot — 000" 2 get tarot aot saat || 2 far yet Naat oot 5 Sst Yeo! soot aot || 3 fort Geet tarot ‘ for ast tant oot || 8 Bat taser soot 5 Sci enor Seat tonsor | |S Ser eter Sat FOR GROUP A NGS IN 58 WIDE GROOVE FOR GROUP a PINGS IN S8 WIDE GROOVE 1 Bt erst seat soot | | 1 ‘st isso" 2 fei baot aot ever || Bo 2390! 5 Stet boeot Soot rowan | | 3 ear ‘ reat beat tsow!| | ¢ & foot eot ieroot| [8and then calculating the inlet temperature, assuming the coolant temperature will behave this way and absorb the required amount of heat. For example: If 200 BTU’s per minute are to be removed from a particular packing and circulation is two gallons (16.6 pounds) per minute of water, there is 16.6 pounds of water available to absorb the heat. 200 divided by 16.6 yields a temperature rise of 12 degrees. Subtracting this from the 90°F exit temperature, gives a maxi- mum allowable inlet tempera- ture to achieve this of 78°F. A definite temperature difference between the coolant and the rod is required for any given amount of heat to be conducted from the case. Using the method previously outlined, it is apparent that there are instances where rod temper- atures are not equal to the 250° or 150° level for unlubricated and lubricated service. ‘The influence of materials on heat generation is illustrated in a general way in Figure 35. In addition to frictional properties, heat transfer characteristics also affect temperature control. ‘These two parameters are not the only basis for choosing a material to be used for packing. as strength, resistance to the medium, cost, and wear resis- tance are also important. At the extremes of lubrication, choice of material is limited. With full lubrication, metals such as bronze, cast iron, or babbitt are best. Plastics such as phenolic, nylon or TFE may be used due Cooling Reciprocating Compressor Packin, to conditions other than heat transfer characteristics. For non- lube service, filled TFE is usually the first choice. Filled polyimides are also excellent but very costly, and some of the less expensive plastics do not have the frictional properties to allow them to be effective for nonlube service. Between these two extremes, compressors operate in mini or semi-lube service or, as shown in Figure 35, “poorly lubricated”, In this type of service it is more difficult to select the optimum material to provide the lowest operating temperature due to the overlapping performance of metals and non-metals. For example; at a certain level of lubrication, metal rings will operate at a satisfactory temp: erature level. However, with a Figure 37 Heat Transfer Equations su (2) (he) (A) (AT) A 60 (4) he =, wo rea of Rod Exposure AT = Temperature Difference Between Rod and Gas (7) (D) (sy/2Cooling Recipracating Compressor Packing slight change of conditions, non- metallic rings may perform even better. Metals conduct heat more efficiently and can run in condi- tions where the coefficient of friction is a bit higher, whereas nonmetallics require low frietion for optimum results. The operator can enjoy the best of both by combining nonmetallic seal rings with a metallic back-up ring, which not only aets as an anti-extrusion ring but aids the nonmetallic ring by using its minimal contact with the rod to conduct heat away from the ring-rod interface. Itis often difficult to estimate cooling requirements when lubrication is poor. The best approach may be to change ring styles or materials in the event of a problem, rather than changing the existing coolant system, Figure 38 contains two examples of calculated coolant require- ments. Due to the generalizations and assumptions made in arriving at the formulas, the results are only approximations, as stated previously. However, designs often are put into practice in the field without benefit of even these rough calculations. Since ‘many problems experienced with compressor packing stem from inadequate cooling, the approximate method outlined here should eliminate those problems. Many machines operate under conditions that do not exactly match the assumptions or descriptions used. They have features or use materials that could change to some degree the calculated values of either heat generation or heat flow. For example; in a compressor with large clearance volume, the ‘temperature in the cylinder may have an effect not allowed for in the calculations. Units which have short strokes will have a very limited amount of heat flow into the gas and a more concen- trated input of heat to the rod than normal. A good approxima- tion of this is to ignore the heat calculated from formula (6), and plan on removing all the excess heat through the case. ‘There are materials and material- lubrication combinations which provide a different coefficient of friction than found in the “Friction Versus Operating Figure 38 Coolant Requirement Examples Sones od ah a ar Sing eae ome eae carat : : a " as 8 bt 8 — coe |PaX FRM 1.03 X 10" is amma = 2 3 [pear rg mm : 2 estinepsag29 —— a |@ into Gas (trom 64 66 sone Tevoen (RUE srt au 0 14 14 4 ey 2 wei feat : : 7 a | ong cow's [Sara in =e os frecumaleConditions* chart. The area in a compressor. In very high designated as “poorly lubricated” pressure applications however, is actually a broad range, and _breaker rings may be used to to assign one value for the achieve a labyrinth effect and Coefficient of Friction may be thus reduce loading across an oversimplification. These individual rings. oversimplifications can be corrected by obtaining accurate operating results and adjusting the calculations. the TUU, be used. Most pumps Applying Packings 0... ow suction pressure an Liquids (Pumps) eee ‘There is little difference in cases _ rod direction at the start of the or rings for packings applied to discharge stroke, sometimes liquids or gases. Within the cylin- results in slight leakage. This der, pressure rise and fall is more can be minimized by using side rapid with liquids but this has _loaded rings, such as the TULU. ttle effect on the rings. In general, the tolerance for leakage is less with fluids than it is with gases so this dictates that low leakage rings, such as In pumps where the plunger ‘There Is less backflow out attaches directly to the crosshead, of a pump packing case, which __ there is little need for a case means pressure breakers are _bushing, except to serve as an not normally needed as they are initial alignment aid. In those ‘Applying Packings to Liquids (Pumps pumps where the plunger attaches to a yoke attached to drive rods, there can be greater lateral move- ment of the plunger, and thus the need for better bushing guid- ance. Cases typical of these two types of construction are shown in Figure 39. ‘As with compressor packings, lubrication is all important because of its effects on wear and friction. Pump packings can be pressure fed lubricant in the same manner as compressor packings. When the liquid being pumped has lubricating proper- tes, the need for additional lubrication is less. Cooling effect of liquid will be greater than it would be with gas so this usually eliminates the need for separate cooling through the case. Actually, some Pump Packings High Pressure Packings | Packing for Horizontal Pump =} p>High Pressure (Hyper) Packings pump packings require heating rather than cooling to provide better performance with viscous fluids or to overcome the low temperature effects caused by leaked liquid flashing. Pump packings can be purged, or buffered, the same as com- pressor packings, normally by adding an "AL" or “ALW" outside of the vent/drain. The purging fluid can be liquid or gas. Purging at high pressure is also sometimes applied to pump packings to flush across the rod or prevent entry of solids or abrasives into the case. High Pressure (Hyper) Packings “Hyper” Is generally taken to mean over 10,000 psi. Compressor or pump discharge may go up to 100,000 psi. At these pressure levels, fluids are usually acting very much like liquids in that compressibility is low. The type of rings used to seal these pres- sures are similar to those used in lower pressure compressors or pumps, but ring designs and materials must be selected to withstand high “compressive” cyclic pressures. ‘The problems (wear, friction and heat) caused by high contact pres- sure between ring and plunger are normally overcome by using ring sets that act as a laybrinth and thus reduce the pressure drop across each ring. Life of hyper packings is increased also by high lubrication rates plus, in some cases, coolant (oll) flow across the rod downstream of the packing. The other principle problem in hyper packings is containing very high cyclic pressure within the case, which Is essentially a thick- walled pressure vessel. Particular attention has to be paid to stress concentrations, such as holes or notches, that might raise stress beyond acceptable levels. Com- pounding of cups, autofrettaging, or pressure loading the outside of the case are typical ways to ensure high fatigue strength in the case parts. Typical high pressure packing is shown in Figure 40. Packing Ring Arrangements Because of the many different Ting types, there are a large number of combinations into which these can be arranged to Figure 41 Packing Ring Arrangements a o ~<200 PSI a Pressure Pressurerove packings that function as intended. What is shown in Figure 41A are the basic “elements” that can be put into a set. The only necessary clement is the primary seal; all the others would be optional. Figure 4B is a basic low pres- sure arrangement set up to also minimize entry of air into the cylinder. The vent seals and wiper would be optional. Not shown, but other elements that could be added, would include a Static- Pac or rod cooling. The use of all double-acting rings as primary seals should be avoided over 200 psi since these can trap sressuire between them and thus ncrease total friction load. Figure 41C would be the arrangement applied to moderate pressures, It might, in some cases, include a pressure breaker since this is sometimes needed above 200—300 psi. All the options mentioned with respect to a low pressure set would also apply to the moderate pressure range. For higher pressures, options would again be the same, but a pressure breaker is nearly always needed. The different ring types in the primary seal would be chosen on the basis of lubricaton level and pressure ranges. Quantity of rings in the primary seal could vary somewhat from what Is indicated in Figure 41D. ‘The number of rings required for any application is based more on experience that science. ‘The number of rings shown. Packing Ring Arrangement in each arrangement simply represent what Is generally applied to most modern packing. Figure 41 represents what 1s currently applied to pump packings. The most common variations would be to include purging.or rod (plunger) lushing. The number of rings in the primary seal again is somewhat variable in that for all pressure ranges, the quantities would most likely vary from two to four. o o [0000 sr | ae aaa) rm vouwe OS OS : eT Pa fae "1200 - 2000 PSI | Ad (1) Ring war OT) Purge [eG ‘88 (2) ings al a srannene L2200081 _] 45019) Roe rar] ree OE | “parm | #00 0g = 7 a ‘Tuu - rou Oi arovide packings that function as intended. What is shown in Figure 41A are the basic “elements” that can be put into a set. The only necessary element is the primary seal; all the others would be optional. Figure 41B is a basic low pres- sure arrangement set up to also minimize entry of air into the cylinder. The vent seals and wiper would be optional. Not shown, but other elements that could be added, would include a Static Pac or rod cooling. The use of all double-acting rings as primary seals should be avoided over 200 psi since these can trap oressure between them and thus. merease total friction load. Figure 41C would be the arrangement applied to moderate pressures. It might, in some cases, include a pressure breaker since this is sometimes needed above 200—300 psi. All the options mentioned with respect to a low pressure set would also apply to the moderate pressure range. For higher pressures, options would again be the same, but a pressure breaker is nearly always needed. The different ring types in the primary seal would be chosen on the basis of lubricaton level and pressure ranges. Quantity of rings in the primary seal could vary somewhat from what is indicated in Figure 41D. The number of rings required for any application is based more on experience that science. ‘The number of rings shown Packing Ring Arrangement sn each arrangement simply represent what is generally applied to most modern packing, Figure 41£ represents what is currently applied to pump packings. The most common variations would be to include purging,or rod (plunger) flushing, ‘The number of rings in the primary seal again is somewhat variable in that for all pressure ranges, the quantities would most likely vary from two to four. cc) o Drain 7200 — 600 PSI te Pg BR PR PR t ¥ 00-1200 PS] Pressure. & A roe BB A'S .: er ; PA ee 1200 - 2000 PSI | Ada (1) Ring wr ge | af "2000 - 3000 PSi | Add (2) Ringe r—c[E} a au | caf a 32000 Pst] Add (2) Rings wo ff) 000 PS | 00 P51