Reciprocating Pumps

Pumping Capacity Example Question

BASIC PRINCIPLES I. The pumping action in any reciprocating pump is dependent upon the positive displacement or the fluid pumped by a piston or plunger. The capacity of the pump is, therefore, determined by the area of the piston and its rate of travel. In order to obtain a practical machine, some method of reversing the direction of the pistons is required. In the direct-acting steam pump, this is accomplished by the steam valves and valve gear; in power pumps, this is accomplished by use of crank and connecting rods. 2. The ability of the pump to produce pressure is dependent upon the ratio of total steam force (steam pressure per unit area x area of steam piston) to total liquid force (pump head x area of liquid piston). In order that pumping may occur, it is necessary that the steam force exceed the liquid force by an amount which slightly exceeds the various mechanical and hydraulic losses encountered. The basic principles for steam pump operation are shown below.

TYPES OF PUMPS I. Direct-acting reciprocating pumps are classed as follows: A. Horizontal or vertical. B. Single or duplex. A single pump has one liquid piston or its equivalent single or double-acting plunger; a duplex pump has two liquid pistons or their equivalent single or double-acting plungers. C. Single or double-acting. A single-acting unit pumps on one direction of piston travel only whereas double-acting units pump on both strokes. Direct-acting steam pumps are usually double acting.

2. Direct-acting steam pumps are conventionally described by stating the steam cylinder diameter, the liquid cylinder diameter, the length of stroke, horizontal or vertical (H or V), single or duplex (S or D), and single or double-acting (SA or DA). Thus a pump identified as 11 x 8 x 18 VSDA has an 11 inch steam cylinder, 8 inch water cylinder, 18 inch stroke, and is a vertical single double-acting pump.

General Characteristics Reciprocating pumps move water or other liquids by a plunger or piston that travels back and forth inside a cylinder. Positive displacement, often used for small capacities and when needed to avoid churning of centrifugal pumps. Can pump foaming liquids and high viscosity liquids. Can control flow by regulating speed of drive with no head loss by throttling as in a centrifugal pump. Used often at high or very high pressures. Also often used as metering pumps because of constancy of flow rate. The flow rate can be easily changed by adjusting the RPM of the driver. Pumps ideally will produce any head that is impressed on them. The maximum head is determined by the power available and the strength of the pump parts. An automatic relief valve set at a safe pressure is used on the discharge side of all positive displacement pumps. Never throttle on the discharge side to reduce the flow rate of a positive displacement pump. The fluid has no place to go and something will break. Can throttle on the steam driver or regulate the RPM of the electric motor to change the flow rate. Unlike centrifugal pumps, positive displacement pumps are self priming.

USAGE IN MARITIME SERVICE On ships a great number of applications are still served by steam reciprocating pumps, including: A. Auxiliary feed. B. Standby fuel oil service. C. Fuel oil transfer. D. Auxiliary circulating and condensate.

E. Fire and bilge. F. Ballast. G. High pressure evaporator. H. Lubricating oil transfer. I. Cargo stripping. J. General service. Direct-acting steam reciprocating pumps are not obsolete. If the steam conditions are not too severe in pressure, temperature, or superheat, they have many features of simplicity, reliability, and economy of operation and maintenance that still warrant serious consideration for many services. Operation PREPARING PUMP FOR OPERATION The following steps .should be taken before putting a pump into operation for the first time, after an overhaul, or after the ship has been drydocked: 1. Check alinement and correct if necessary. If pump is operated out of line, scoring of rods and liners will result. 2. Steam and liquid lines should be free from scale and foreign matter. 3. Check all packing and repack if necessary. 4. Move steam pilot valve rods by hand to be sure pilot valve moves easily. 5. Check all connections and fittings to ensure they are tightly in place.

STARTING PROCEDURE To start a reciprocating pump proceed as follows: 1. Oil the pins of the steam valve operating gear and set up on all grease cups. 2. Open the liquid end valves: a. Suction. b. Discharge. 3. Open the cutout (or root) valves in the: a. Exhaust line. b. Steam line.

4. Open steam cylinder drains: a. Top. b. Bottom. c. Valve chest. 5. Open exhaust valve at pump. 6. Crack the throttle valve and open slowly so as to ad- mit steam and warm up gradually. 7. Close the steam cylinder drains after the pump makes a few strokes and the steam cylinder is clear of water. 8. Bring the pump up to the proper speed by sufficiently opening the throttle valve. If pump is controlled by a pres- sure governor, open throttle gradually until governor takes control of pump and then open the throttle valve fully. 9. Adjust the cushioning valves, if fitted, until an adjust- ment is obtained that permits silent and smooth working of the pump, i.e., sufficient pump speed at the end of the stroke without knocking. After best point of operation is obtained, cushion valve should be set and not changed. When a reciprocating pump is operating at maximum speed, the cushioning valves should be almost completely closed. STOPPING AND SECURING To stop and to secure, proceed as follows: 1. Close the throttle valve. 2. Close the exhaust. 3. Open cylinder drains. a. Top. b. Bottom. c. Valve chest. 4. Close the water end suction valve. 5. Close the water end discharge valve. 6. Close the steam and exhaust cutout valves (root valves).

7. After steam cylinder is drained, close the valve chest drains leaving the steam cylinder drain valves open to prevent hydraulic action.

Pumping Capacity

Simplex single acting pumps discharge the cylinder volume for each 2 strokes. The forward stroke discharges the cylinder and the back stroke or reverse stroke fills the cylinder.

Simplex double acting pumps discharge the cylinder volume for each pump stroke. The forward stroke discharges the cylinder in front of the piston while filling the cylinder behind the piston. The back or reverse stroke discharges the cylinder behind the piston while filling the cylinder forward of the piston.

Duplex double acting pumps use 2 double-acting cylinders in parallel, and pump two cylinder volumes for each pump stroke. Duplex single acting pumps use 2 single-acting cylinders in parallel, and pump one cylinder volume for each pump stroke.

Capacity Pump Capacity in GPM = volume discharged in gallons per pump stroke multiplied by strokes per minute. To determine the volume of the cylinder, multiply the area of the circle by the height of the cylinder. Volume of a Cylinder is equal to: = (area of the circle) * (height) = ( R2) * (height) = R2 H

Reciprocating Piston Pumps The basic principle of the reciprocating single piston pump is shown below. The piston expels liquid through a one-way valve (check valve). The pumping rate is usually adjusted by controlling the distance the piston retracts, thus limiting the amount of liquid pushed out by each stroke, or by the cam rotating speed.

Schematic of the reciprocating single piston pump. CAM is pushing a sapphire piston back and force. When the piston is moving backwards it sucks the eluent through the inlet check valve (on the bottom). The sapphire ball is lifted and opens the path for the eluent. When the piston moves forward, the liquid pushes the inlet ball down and closes the path, but the outlet ball is lifted and opens the outlet valve (upper).

The main disadvantage of this type of pump is sinusoidal pressure pulsations which lead to the necessity of using pulse dampers.

Dual Piston Pumps A more efficient way to provide a constant and almost pulse free flow is the use of dualheaded reciprocating pumps. Both pump chambers are driven by the same motor through a common eccentric cam; this common drive allows one piston to pump while the other is refilling. As a result, the two flow-profiles overlap each other significantly reducing the pulsation downstream of the pump; this is visualized below. Since the acceleration/deceleration profile is somewhat non-linear, the more efficient types of these pumps use eccentricity-shaped cams to obtain the best overlapping of the pressure curves and to obtain smooth flow.

Schematic of a dual-head reciprocating pumps. The advantages of this pump are the unlimited solvent reservoir allowing long-term unattended use and quick changeover and clean out capability. However, unless special care has been exercised in manufacture, these pumps may have several disadvantages. There is a tendency for the incompletely compensated pulsations to be observable at high refractive index detector sensitivities, especially at low flow rates where piston cycles are widely spread. Furthermore, since each head has two check valves, pump reliability depends on the cleanliness of the mobile phase and continued sealing capability of four check valves on each cycle, with cycles normally occurring several times per minute. Recent improvements to this popular pumping system include: A computer-designed camshaft is used to achieve maximum overlap of pump strokes, resulting in virtually undetectable pulsation or ripple. Staggered inlet/outlet lines are employed to allow complete flushing when liquids are

changed or if air is inadvertently drawn through the pump. Small-volume check valves are used to allow the pumps to function reliably at flow rates as low as 0.001 mL/min. This has the added benefit of providing excellent gradient reproducibility especially when programs start from extremely low concentrations. There are fewer moving parts, with all maintenance-requiring components (pump seals, check valves) readily accessible from the front of the instrument. A wide flow rate range (0.01 to 10 ml/min) is provided without gear change.

Check valves on the reciprocating pump are the most weak part. It may be easily contaminated or clogged which leads to the pump misfunction. Most of the recent HPLC instruments use improved dual piston pumps which have three or even two check valves.

The schematic of this pump is shown above. The first piston, called low pressure, is sucks the liquid from the reservoir while the second (high pressure piston) is supplying the eluent to the system. Then the first piston refills the second piston very fast, during 1/100 of the whole pump cycle. This scheme allows the use of only 3 check valve, one of which is working under low pressure. There are no cavitation effects. Because the piston volumes are small (~100 l), pressure pulastions are small and sharp and easy to damp.

Another type of dual piston pump uses only two check valves, but piston volumes are different. While the smaller piston dispenses an eluent in the HPLC system, the bigger piston is sucking an eluent. When pistons change their direction, the bigger piston simultaneously refill the smaller chamber and dispenses eluent into the system.

This set-up allows only two check valves for the dual piston pump.

CONTRUCTION DETAILS OF A RECIPROCATIN PUMP: Components of reciprocating pumps:a) Piston or plunger: a piston or plunger that reciprocates in a closely fitted cylinder. b) Crank and Connecting rod: crank and connecting rod mechanism operated by a power source. Power source gives rotary motion to crank. With the help of connecting rod we translate reciprocating motion to piston in the cylinder. c) Suction pipe: one end of suction pipe remains dip in the liquid and other end attached to the inlet of the cylinder. d) Delivery pipe: one end of delivery pipe attached with delivery part and other end at discharge point. e) Suction and Delivery value: suction and delivery values are provided at the suction end and delivery end respectively. These values are non-return values. WORKING OF RECIPROCATING PUMP Operation of reciprocating motion is done by the power source (i.e. electric motor or i.c engine, etc). Power source gives rotary motion to crank; with the help of connecting rod we translate reciprocating motion to piston in the cylinder (i.e. intermediate link between connecting rod and piston). When crank moves from inner dead centre to outer dead centre vacuum will create in the cylinder. When piston moves outer dead centre to inner dead centre and piston force the water at outlet or delivery value. EXPRESSION FOR DISCHARGE OF THE PUMP:-

Where: Q: discharge in m3/sec A: cross-section of piston or cylinder in m2 L: length of stroke in meter N: speed of crank in r.p.m