Control Valve Sizing Step 1:
Define the System Variables System Variables
Units
Location/Equation*
System fluid
-
P&ID
Specific gravity of fluid
-
Fluids Text
Fluid temperature
°F
Heat Balance / Vendor P&ID
Vapor pressure, P v
psig
Fluids Text
Flowrate
gpm
Heat Balance / Vendor P&ID
Pressure @ Valve Inlet, P1
psig
See Below**
Pressure @ Valve Outlet, P2
psig
See Below***
Pressure Drop, Δ Drop, ΔP P
psi
ΔP = P1 - P2
Valve Recovery Factor, Km
0.80
-
Constant
Cavatation Index Factor, Kc
0.95
-
Constant
Critical Pressure Ratio, rc
0.92
-
Constant
* Indicates where or how the particular system variable can can be found or calculated. ** P1 = Pump Discharge Head - Friction Losses - Flow Element Loss - Static Head *** P2 = Drum Pressure + Friction Losses + Flow Element Loss + Static Head
Step Step 2: 2:
Defin Define e the the Inle Inlett and and Outle Outlett Pipe Pipe/Li /Line ne Con Condit ditio ions ns Line Condtions
Inlet
Units
Outlet
Units
Location* Isometric
Pipe size
in
in
Pipe wall**
Sch / in
Sch / in Isometric
Connection type***
-
Pressure class
lb
lb
Isometric Isometric
* Indicates where the particular line condition can be found. ** Refers to the the pipe pipe schedule schedule or minimum wall wall thickness. thickness. *** Refers to how the valve will will be connected to the pipe. Ex. Butt weld, Flanged, Flanged, etc. Step Step 3: 3:
Defin Define e the the Allo Allowab wable le Pre Press ssur ure e Drop Drop for for the the Val Valve ve ΔP ΔPallow = Km(P1 - rc*Pv) Allowable DeltaP, ΔP DeltaP, ΔPallow
Step Step 4:
psi
Deter Determi mine ne if Cavit Cavitat ation ion Contr Control ol is Need Needed ed If ΔP > ΔPallow Cavitation Control is needed. If ΔP < ΔPallow Cavitation Control is not needed. not needed.
Step Step 5:
Predic Predictt the Pressu Pressure re Drop Drop at which which the the Valve Valve will begin begin Cavi Cavitati tation on ΔP ΔPc = Kc(P1 - Pv) ΔPc
Step Step 6:
psi
Calc Calcula ulate te the Valve Valve Flow Flow Coef Coeffic ficie ient, nt, Cv
C v
Q
where, Cv is the valve flow coefficient
G P
Q is the flow rate (maximum, normal, minimum), gpm G is the specific gravity of fluid ΔP is the pressure drop (psi) across the valve for corresponding flowrate Cv
Step 7:
Repeat Steps 1 through 6 for all other operating conditions. (ie maximum, minimum and normal conditions)
Step 8: 8:
Select th the Va Valve Ty Type Decide which type valve is best for your application. The valve types are listed on following pages.
Step 9: 9:
Select th the Va Valve Siz Size e Use the calculated Cv's and manufacturer supplied chart to choose the proper valve size. Below is an example of a manufacturer supplied chart.
Useful Tips: Manufacturer supplied charts can be found in a valve vendor catalog. Try not to use a valve that is less than half the pipe size. A valve is much easier to control in the 30-70% travel range. Step Step 10: 10:
Conta Contact ct the the Val Valve ve Ven Vendo dor( r(s) s) At this point contact the valve vendor(s) and present the above information. The vendor may be useful in the f inal selection of the valve that is best for your application.
Valve Types Gate Valve Type Service:
On-Off
Fluids:
Steam Feedwater Condensate
Systems:
Isolation
Recommended Us Uses:
Non-throttling Infrequent operation
Advantages:
Low cost High capacity Tight shutoff Low pressure drop
Disadvantages:
Poor flow control Prone to cavitate at low pressure drops
Ball Valve Type Service:
On-Off
Fluids:
Steam Condensate Natural Gas
Systems:
Balance of Plant
Recomme mmended Us Uses:
Fully op open/closed or or lilimit mited th throttl ttling High temperature fluids (steam drains) Back pressure control
Advantages:
High capacity Low leakage Tight sealing with low torque
Disadvantages:
Poor throttling characteristics Prone to cavitation
Butterfly Valve Type Service:
On-Off Flow Control
Fluids:
Condensate Circulating Water Raw Water
Systems:
Balance of Plant
Recommended Uses:
Fully open/closed or throttling Frequent operation
Advantages:
Low cost High capacity Good flow control Low pressure drop
Disadvantages:
High torque required for control Prone to cavitation at lower flows Flow obstruction due to center post
Globe Valve Type Service:
Pressure control Flow control Temperature control
Fluids:
Steam Feedwater Condensate Natural Gas
Systems:
Critical Service Bypass Balance of Plant Natural Gas
Recommended Uses:
Throttling Frequent operation
Advantages:
Efficient throttling Accurate flow control Available in multiple cages*
Disadvantages:
High pressure drop More expensive than other valves
* Characterized Cages Cages for Globe-Style Valve Bodies Bodies Equal Percentage:
Equal increments of valve travel produce an equal percentage in flow change. Used in processes where large changes in pressure drop are expected. Used in processes where a small percentage of the total pressure drop is permitted permit ted by the valve. Used in temperature and pressure control loops.
Linear:
Valve travel is directly proportional to the flow change. Used in liquid level or flow loops. Used in systems where the pressure drop across the valve is expected to remain fairly constant. Used in steady state systems.
Quick Opening:
Large increase in flow with a small change in valve travel. Used for frequent on-off service. Used for processes where "instantly" large flow is needed. Used in safety systems or cooling water systems.
Other Valves Check Valve
Designed to restrict the flow to one direction. If the flow reverses direction, the check valve will close.
Relief Valve
Designed to regulate the operating pressure of incompressible flow. Used to release excess pressure in gases or compressible fluids.
Plug Valve
Designed for on-off service and some throttling services. Controls flow by means of a cylindrical or tapered plug with a hole in the center that lines up with the flow path of the valve to permit flow. A quarter turn in either direction blocks the flow path.
Pinch Valve
Designed particularly for ap applications of of sl slurries or liliquids with large amounts of suspended solids. It seals by means of one or more flexible elements, such as a rubber tube, that can be pinched to shut off flow.
Diaphragm Va Valve
Designed to to cl close by by means of of a flexible di diaphragm attached to a compressor. When the compressor is lowered by the valve stem onto a weir, t he diaphragm seals and cuts off flow. Used to handle corrosive, erosive and dirty services.
