Sizing and Selection of Control Valves
Globe vs. Ball
The control valve is the most important single element in provide an equal percentage flow characteristic, enabling
any fluid handling system, because it regulates the flow of stable control of fluids. Additionally, there are more cost-
fluid to the process. To properly select a control valve, a effective valve actuators now available for globe valves.
general knowledge of the process and components is Better control and more-competitive pricing now puts globe
usually necessary. This reference section can help you valves on the same playing field as characterized ball
select and size the control valve that most closely matches valves.
the process requirements.
Most Cost-effective by Application
The sizing of a valve is very important if it is to render Lets look at a cost comparison as it relates to the decision to
good service. If it is undersized, it will not have sufficient select ball or globe valves. For terminal unit applications
capacity. If it is oversized, the controlled variable may requiring less than 25 GPM, the globe valve is a more cost-
cycle, and the seat, and disc will be subject to wire effective choice. However, on larger coils the characterized
drawing because of the restricted opening. ball valve is the more cost-effective solution.
Systems are designed for the most adverse conditions From a practical standpoint, many jobs will use mostly one
expected (i.e., coldest weather, greatest load, etc.). In type or the other. If the majority of valves on a project tend to
addition, system components (boiler, chiller, pumps, coils, be terminal unit valves, then globe valves would offer better
etc.) are limited to sizes available and frequently have a control at a lower price. If the majority of the valves are for
greater capacity than system requirements. Correct sizing AHUs (1-1/4or larger) characterized Ball Valves are the
of the control valve for actual expected conditions is preferred solution from a pure cost standpoint.
considered essential for good control.
Different tolerances to temperature, pressure and steam
should also be considered in the selection process.
Technical Comparison Between
Globe and Ball Valves Selection Guidelines
Technically, the globe valve has a stem and plug, which Globe Valve
strokes linearly, commonly referred to as stroke valves. Lower cost
The ball valve has a stem and ball, which turns horizon- Close off of 50 psi or less (typical for most HVAC
tally, commonly referred to as rotational valves. applications)
Early ball valves used a full port opening, allowing large High differential pressure across valve
amounts of water to pass through the valve. This gave Rebuilding of the valve is desired
HVAC controls contractors the ability to select a ball valve Better control performance
two to three pipe sizes smaller than the piping line size. Better low flow (partial load) performance
Compared to traditional globe valves that would be only Use for steam, water or water/glycol media
one pipe size smaller than the line size, this was often a Smaller physical profile than a comparable ball valve
more cost-effective, device-level solution. In addition, the Characterized Ball Valve
ball valve could be actuated by a damper actuator, rather Tight shutoff or high close offs of around 100 psi* are
than expensive box-style modulating motors. required
Isolation or two position control**
Pricing Comparison Cv ranges from 16 to 250
Today, with equivalent pricing between ball and globe (equates to line sizes 1-1/4 to 2)
valves, the full port ball valve is falling out of favor for most Use for water or water/glycol solution only
HVAC control applications. This is also due to its poor
installed flow characteristic that leads to its inability to
maintain proper control. New flow optimized or charac-
terized ball valves, specifically designed for modulating
applications, have been developed. Characterized ball
valves are sized the same way as globe valves. They
* This equates to a pump head pressure of approximately 230 ft. Not very common HVAC applications
** Valve can be line sized to minimize pressure losses; butterfly valves are also used for these applications.
86
Sizing
reference
Pressure Drop for Water Flow Pressure Drop for Steam
A pressure drop must exist across a control valve if flow is The same methodology should be applied for selecting a
to occur. The greater the drop, the greater the flow at any valve for steam where the most important consideration is
fixed opening. The pressure drop across a valve also the pressure drop.
varies with the disc positionfrom minimum when fully
First, the correct maximum capacity of the coil must be
open, to 100% of the system drop when fully closed.
determined. Ideally, there should be no safety factor in this
To size a valve properly, it is necessary to know the full determination and it should be based on the actual BTU
flow pressure drop across it. The pressure drop across a heating requirements. The valve size must be based on
valve is the difference in pressure between the inlet and the actual supply pressure at the valve. When the valve is
outlet under flow conditions. When it is specified by the fully open, the outlet pressure will assume a value such
engineer and the required flow is known, the selection of a that the valve capacity and coil condensing rate are in
valve is simplified. When this pressure drop is not known, balance. If this outlet valve pressure is relatively large
it must be computed or assumed. (small pressure drop), then as the valve closes, there will
be no appreciable reduction in flow until the valve is nearly
If the pressure drop across the valve when fully open is
closed. To achieve better controllability, the smallest valve
not a large enough percentage of the total system drop,
(largest pressure drop) should be selected. With the valve
there will be little change in fluid flow until the valve
outlet pressure much less than the inlet pressure, a large
actually closes, forcing the valves characteristic toward a
pressure drop results. There will now be an immediate
quick opening form.
reduction in capacity as the valve throttles. For steam
Figure 1 shows flow-lift curves for a linear valve with valves, generally the largest possible pressure drop
various percentages of design pressure drop. Note the should be taken, without exceeding the critical pressure
improved characteristic as pressure drop approaches ratio. Therefore, the steam pressure drop should approach
100% of system pressure drop at full flow. 50% of the absolute inlet pressure.
It is important to realize that the flow characteristic for any Examining the pressure drops under Recommended
particular valve, such as the linear characteristic shown in Pressure Drops for Valve Sizing Steam, you might be
Figure 1 is applicable only if the pressure drop remains concerned about the steam entering the coil at 0 psig
nearly constant across the valve for full stem travel. In when a large drop is taken across the control valve. Steam
most systems, however, it is impractical to take 100% of flow through the coil will still drop to vacuum pressures
the system drop across the valve. due to condensation of the steam. Consequently, a
pressure differential will still exist. In this case, proper
A good working rule is, at maximum flow, 25 to 50% of the steam trapping and condensation piping is essential.
total system pressure drop should be absorbed by the
control valve. Although this generally results in larger
pump sizes, it should be pointed out that the initial
equipment cost is offset by a reduction in control valve
size, and results in improved controllability of the system.
Reasonably good control can be accomplished with
pressure drops of 15 to 30% of total system pressures. A
drop of 15% can be used if the variation in flow is small.
Recommended Pressure Drops
for Valve Sizing Water
1. With a differential pressure less than 20 psig, use a
pressure drop equal to 5 psi.
2. With a differential pressure greater than 20 psig, use a
pressure drop equal to 25% of total system pressure
drop (maximum pump head), but not exceeding the
maximum rating of the valve.
Figure 1.
87
Sizing and Selection of Control Valves (continued)
Recommended Pressure Drops for Valve Sizing Steam Valve Sizing and Selection Example
1. With gravity flow condensate removal and inlet pressure Select a valve to control a chilled water coil that must have a
less than 15 psig, use a pressure drop equal to the inlet flow of 35 GPM with a valve differential pressure ( P) of 5
gauge pressure. psi.
2. With vacuum return system up to 7 Hg vacuum and an Determine the valve Cv using the formula for liquids.
inlet pressure less than 2 psig, a pressure drop of 2 psig
should be used. With an inlet pressure of 2 to 15 psig,
Cv = Q = 35 GPM = 15.6
use a pressure drop equal to the inlet gauge pressure.
3. With an inlet pressure greater than 15 psig, use a Select a valve that is suitable for this application and has a
pressure drop equal to 50% of inlet absolute pressure. Cv as close as possible to the calculated value.
Example: Inlet pressure is 20 psig (35 psia). Use a
One choice is 277-03186: a 1-1/4 NC valve with a Cv of 16.
pressure drop of 17.5 psi.
Refer to Flowrite Valves Reference section.
4. When a coil size is selected on the basis that line
pressure and temperature is available in the coil of a Valve Selection Criteria
heating and ventilating application, a very minimum 1. Flow characteristicModified Equal Percentage which
pressure drop is desired. In this case, use the following provides good control for a water coil.
pressure drop: 2. Body rating and materialSuitable for water plus a metal
disc which provides tight shut-off.
3. Valve type and actionA single seat NC valve with an
Initial Pressure Pressure Drop adjustable spring range which can be sequenced with a
15 psi 5 psi NO valve used for heating.
50 psi 7.5 psi 4. Valve actuatorActuator close-off rating is higher than
the system P.
100 psi 10 psi
Over 100 psi 10% of line pressure 5. Valve line sizeIts Cv is close to and slightly larger than
the calculated Cv (15.6).
6. For Ball ValvesSelect a full port valve the same size as
the line size for isolation.
The Most Important Variables to Consider
When Sizing a Valve:
1. What medium will the valve control? Water? Air? Steam?
What effects will specific gravity and viscosity have on the
valve size?
2. What will the inlet pressure be under maximum load
demand? What is the inlet temperature?
3. What pressure drop (differential) will exist across the
valve under maximum load demand?
4. What maximum capacity should the valve handle?
5. What is the maximum pressure differential the valve top
must close against?
When these are known, a valve can be selected by formula
(Cv method) or water and steam capacity tables which can
be found in the Valves section of the Master HVAC Products
Catalog. The valve size should not exceed the line size,
and after proper sizing should preferably be one to two
sizes smaller.
88
reference
Full-Port (no flow optimizer) Ball Valve Part Nos. and Flow Coefficients
Valve Valve Effective (Installed) Cv (Kvs)
Line Part Supply Line Size in Inches (mm)
Size No. 1/2 3/4 1 1-1/4 1-1/2 2 2-1/2 3 4 5 6
in. (mm) (13) (20) (25) (32) (38) (51) (63) (76) (102) (127) (152)
1/2 (15) 599-10208 10.0 7.44 6.54
(8.62) (6.41) (5.64)
3/4 (20) 599-10210 25.00 20.02 16.08
(21.55) (17.26) (13.86)
1 (25) 599-10214 63.00 37.25 32.01
(54.31) (32.11) (27.59)
1-1/4 (30) 599-10217 100.00 69.84 51.72
(86.21) (60.21) (44.59)
1-1/2 (40) 599-10219 63.00 62.29 56.29
(54.31) (53.70) (48.53)
1-1/2 (40) 599-10221 160.00 95.87 77.45
(137.93) (82.65) (66.77)
2 (50) 599-10223 100.00 100.00 91.07
(86.21) (86.21) (78.51)
2 (50) 599-10225 250.00 193.94 142.91
(215.52) (167.19) (123.20)
Key Valve may be oversized.
Optimal valve size.
Valve may be undersized.
The temperature-pressure ratings for ANSI Classes 125 and 250 valve bodies made of bronze or cast iron
are shown below.
Pressure
Description Temperature ANSI Class 125 ANSI Class 250
Bronze Screwed Bodies -20 to + 150F (-30 to + 66C) 200 psig (1378 kPa) 400 psig (2758 kPa)
Specification #B16.15-1978 -20 to + 200F (-30 to + 93C) 190 psig (1310 kPa) 385 psig (2655 kPa)
ANSI Amer. Std.;
USA; ASME -20 to + 250F (-30 to + 121C) 180 psig (1241 kPa) 265 psig (2586 kPa)
-20 to + 300F (-30 to + 149C) 165 psig (1138 kPa) 335 psig (2300 kPa)
-20 to + 350F (-30 to + 177C) 150 psig (1034 kPa) 300 psig (2068 kPa)
-20 to + 400F (-30 to + 204C) 125 psig (862 kPa) 250 psig (1724 kPa)
Cast Iron Flanged Bodies -20 to + 150F (-30 to + 66C) 175 psig (1206 kPa) 400 psig (2758 kPa)
Class A-sizes 1 to 12 -20 to + 200F (-30 to + 93C) 165 psig (1138 kPa) 370 psig (2551 kPa)
Specification #B16.1 1975
-20 to + 225F (-30 to + 106C) 155 psig (1069kPa) 355 psig (2448 kPa)
ANSI Amer. Std.;
USA; ASME -20 to + 250F (-30 to + 121C) 150 psig (1034 kPa) 340 psig (2344 kPa)
-20 to + 275F (-30 to + 135C) 145 psig (1000 kPa) 325 psig (2241 kPa)
-20 to + 300F (-30 to + 149C) 140 psig (965 kPa) 310 psig (2137 kPa)
-20 to + 325F (-30 to + 163C) 130 psig (896 kPa) 295 psig (2034 kPa)
-20 to + 350F (-30 to + 177C) 125 psig (862 kPa) 280 psig (1931 kPa)
-20 to + 375F (-30 to + 191C) 265 psig (1827 kPa)
-20 to + 400F (-30 to + 204C) 250 psig (1734 kPa)
89
Sizing and Selection of Control Valves (continued)
Valve Sizing Formulas 1. For liquids (water, oil, etc.): Remarks:
The following definitions apply in the following formulas: Specific gravity correction is
Cv=Q
negligible for water below
Cv Valve flow coefficient, U.S. GPM with P = 1 psi
200F (use S=1.0). Use
P1 Inlet pressure at maximum flow, psia (abs.) actual specific gravity S of
other liquids at actual flow
P2 Outlet pressure at maximum flow, psia (abs.)
temperature.
P P1 P2 at maximum flow, psi Use this for fluids with
Q Fluid flow, U.S. GPM viscosity correction fact.
Cv=KrQ Use actual specific gravity
Qa Air or gas flow, standard cubic feet per hour (SCFH) S for fluids at actual flow
at 14.7 psig and 60F temperature.
W Steam flow, pounds per hour (lb./hr.) 2. For gases (air, natural gas,
propane, etc.):
S Specific gravity of fluid relative to water @ 60F Cv= Qa G(T+460) Use this when P2 is greater
G Specific gravity of gas relative to air at 14.7 psig and 60F 1360 P(P2) than 1/2P1.
T Flowing air or gas temperature (F) Cv= Qa G(T+460) Use this when P2 is less
660 P1 than or equal to 1/2P1.
K 1 + (0.0007 x F superheat), for steam
3. For steam (saturated or
V2 Specific volume, cubic feet per pound, at outlet
superheated):
pressure P2 and absolute temperature (T + 460) Use this when P2 is greater
Cv= WK
than 1/2P1.
Kr Viscosity correction factor for fluids (See Page I-4) 2.1 P (P1 + P2)
Use this when P2 is less
Cv= WK
than or equal to 1/2P1.
1.82 P1
Viscosity Factors 4. For vapors other than When P2 is less than or
The relationship between kinematic and absolute viscosity: steam: equal to 1/2P1, use the
Cv= WK value of 1/2P1 in place of P
Centistoke = Centipoise and use P2 corresponding
63.4
Specific Gravity to 1/2P1 when determining
specific volume V2.
Sizing Formulas and Tables
Process Formulas For Heating Air with Steam Coils:
For Heating or Cooling Water: lbs. steam/hr. = 1.08 x (F air temp. rise) x CFM
GPM = Btu/hr. 1000
(F water temp. rise or drop x 500)
For Heating Air with Water Coils:
GPM = CFM x .009 x H GPM = 2.16 x CFM x (F air temp. rise)
formulas F water temperature change
(H = change in enthalpy of air expressed in Btu/lb. of air)
For Heating Water with Steam:
lbs. steam/hr. = 0.50 x GPM x (F water temp. rise)
1000 x (F water1 temp. drop)
For Radiation:
lbs. steam/hr. = 0.24 x ft.2 EDR (Low pressure steam)
EDR = Equivalent Direct Radiation
1 EDR (steam) = 240 BTU/Hr. (Coil Temp. = 215F)
For Heating or Cooling Water:
& tables
GPM1 = GPM2 x (F water2 temp. rise or drop)
F water1 temp. drop
1 EDR (water) = 200 BTU/Hr. (Coil Temp. = 197F)
GPM = ft.2 EDR
50 (Assume 20F water TD)
90
reference
Cast Iron Flanges
2-1/2 to 8-inch Cast Iron Flange Dimensions (as defined by ANSI standard B16.1)
ANSI Class 125. ANSI Class 250.
ANSI Class 125
Flanges Drilling Bolting
Nominal Length of
Pipe Flange Flange Diameter of Diameter of Number of Diameter of Machine
Size Diameter Thickness Bolt Circle Bolt Holes Bolts Bolts Bolts
A B D E F
2-1/2 7 11/16 5-1/2 3/4 4 5/8 2-1/2
3 7-1/2 3/4 6 3/4 4 5/8 2-1/2
4 9 15/16 7-1/2 3/4 8 5/8 3
5 10 15/16 8-1/2 7/8 8 3/4 3
6 11 1 9-1/2 7/8 8 3/4 3-1/4
8 13-1/2 1-1/8 11-3/4 7/8 8 7/8 3-1/2
ANSI Class 250
Flanges Drilling Bolting
Nominal Length of
Pipe Flange Flange Diameter of Diameter of Diameter of Number of Diameter of Machine
Size Diameter Thickness Raised Face Bolt Circle Bolt Holes Bolts Bolts Bolts
A B C D E F
2-1/2 7-1/2 1 4-15/16 5-7/8 7/8 8 3/4 3-1/4
3 8-1/4 1-1/8 5-11/16 6-5/8 7/8 8 3/4 3-1/5
4 10 1-1/4 6-15/16 7-7/8 7/8 8 3/4 3-3/4
5 11 1-3/8 8-5/16 9-1/4 7/8 8 3/4 4
6 12-1/2 1-7/16 9-11/16 10-5/8 7/8 12 3/4 4
8 15 1-5/8 11-15/16 13 1 12 7/8 4-1/2
91
