Control Valve in Nutshell – Valve Design and Selection Factors

In this article, you will learn the important factors for control valve selection and process design parameters in a nutshell.

Control Valve in Nutshell

Control Valve in Nutshell - Valve Design and Selection Factors
Control Valve in Nutshell
Valve TypeValve StrengthValve Weakness
Globe ValveBest suitable for accurate control/throttling service  Permanent pressure drop reasonably high  
Butterfly ValveBest choice for larger pipe size, low permanent pressure drop, Cv is quite highCavitation may happen at low-pressure drop Metal to metal seated type does not provide tight shutoff  
Ball ValveBest choice for tight shutoff, low permanent pressure drop, Cv is quite higher. Best choice for Shutdown application Low costPoor in throttling application Temp limited by seating material for soft seated  
Gate ValveHigh capacity, tight shutoff  in low costNot suitable for throttling application, cavitation at low-pressure drop, slow acting, Bulky
Angle ValveBest suit for high-pressure drop applicationPiping routing is a major concern i.e. 90° offset between inlet & outlet

Valve Design Factors

Valve Sizing ConcernsValve Countermeasures

Anti-cavitation valves are Low-pressure recovery design
Globe or angle with hardened trim, specially designed multi-hole/path cavitation trim, or noise abatement trim) shall be used (17-4 pH, 440C, 420SST HT, COLMONOY 6,  ALLOY 6).

Select a valve with a higher liquid pressure recovery factor (FL) to install a restriction orifice directly downstream of the control valve install two (or more) control valves in series
FlashingUse Hard faced trim use harden trim (stellite, 440C, or Colmonoy® 6).
Use erosion-resistance body material.
Increase valve body size.
Use angle valve.

The major reason for high noise in case of gas service is uncontrolled velocity.

Noise in liquid application is mainly due to Cavitation/ flashing.
Source Treatment: Special trim; Larger valve size.

Path Treatment: pipe Insulation, Silencers/diffusers

Inputs and Outputs for Valve Sizing

Inputs and Outputs for Valve Sizing

Valve Standards

Standards of Valve

Valve Materials

Valve Body Material example
Carbon Steels – LCB, A216 Gr WCB, WCC, LCC.
Alloy Steels – WC9, C5 & WC6.
Stainless Steels – 316 SS, 316L SS, 304 SS.
Monel is used in oxygen service
Valve Trim Material ExampleSS316, Stellited, SS410, SS420, SS 440,
Hardening Treated material

Valve Hardened Trim Applications

Valve Hardened Trim Applications

Valve Severe Service Classification

High TurndownQmax/Qmin >10
High Power Conversion
Pa = kg⋅m−1⋅s−2
Flow = m3⋅s−1
Watt= kg⋅m2⋅s−3
dP(Pa) x Q(m3/sec) >100 kW
High Noise    >85 dB
Flashing – Only for liquidP2<Pv
Cavitation – Only for liquiddp/(p1-pv) > 0.38
High-pressure drop ratio (choked flow)dp/p1 > 0.6

Body Outlet Velocity Criteria for Valve Sizing

Gas, vapor, and steam< 0.3 Mach
emergency depressurizing services Services for which the control valve is closed for more than 4 hours during an 8-hour shift,< 0.4 Mach
water service< 5 m/s
liquid services other than water< 10 m/s
erosive fluid< 5 m/s
FL: Rated liquid pressure recovery factor (dimensionless)

Globe-style valves generally have low-pressure recovery capability (means FL is higher).
The chance of cavitation is low.

Butterfly valves generally have high-pressure recovery capability (means FL is lower).

The chance of cavitation is high.
xT Rated pressure drop ratio factor (dimensionless)

Globe valve has a higher xT value

Ball and Butterfly have lower xT Value

Valves with a high xT value can cope with greater differential pressures than ball or butterfly valves.
FP: Piping geometry factor (dimensionless), should be >0.5. Fp equals unity for the case of valve size equal to line size.

The pipe size is, for different reasons, frequently bigger than the nominal size of the control valve.

The control valve is installed with reducers and expanders.

The reducer will increase pressure drop.

If DP increases, we can have less Cv which is due to the reducer effect. To compensate for this, Cv values are further increased by the Fp factor.
Expansion factor Y: Expansion factor (ratio of flow coefficient for gas to that for a liquid at the same Reynolds number).

This factor accounts for the reduction in flow rate that occurs when the Differential pressure ratio (∆p/p1) increases.

The typical value is between 0.67 to 1
Fd Valve style modifier (dimensionless): depends exclusively on the trim type and the shape of the throttling area.

Fd value of 1.0 can be used for valves with one single flow passage.

Fd value of 0.7 can be used for valves with two flow passages, such as double port globe valves and butterfly valves.

Valve Selection Factors

For liquid sizing calculations with given process data and calculated Cv, if the viscosity of the liquid is increased (keeping other parameters constant), the Cv will increase.

For a given sizing calculations with given process data and calculated Cv, if now DP across the valve is increased (keeping other parameters constant), the Cv will decrease.

The angle type is best suited for the very high-pressure drop.

Ball valves are generally used for tight shutoff.

Control valve leakage classes are defined as per ANSI/FCI70.2.

During the “Chocked flow” condition, the further increase in pressure drop will not result in an increase in flow.

Flashing can NOT be eliminated by suitable trim design. Cavitation can be isolated/eliminated

Anti-cavitation valves are Low-pressure recovery designs.

The higher value of the “Liquid Pressure Recovery factor” means Low-pressure recovery.

Valves with noise-reducing trims are inherently having the linear characteristic.

Equal percentage plugs shall normally be used when there are large variations in valve pressure drop whereas Linear characteristic plugs are selected for small variations in valve pressure drop.

Piston actuators are preferred for fast stroking.

Fail Locked (FL) position of control valve with double acting actuator does not require volume tank.

Fail Open / Fail Closed position of control valve with double acting actuator requires volume tank.

17-4 PH material is stainless steel.

Stellite Hard Facing (HF) is a Cobalt-Chromium alloy.

In general, during minimum operating conditions flow is low and DP is high whereas under max operating conditions flow is high and DP is low.

Valve trim seating to meet the classification “TSO” shall be class V or VI

Toxic / Volatile Organic Compounds (VOCs), H2S, and H2 service valve requires special low emission valve packing (Class AH) and/or bellows sealing. Class AH packing is more stringent than Class CH packing.

Minimum acceptable hardness in case of stellited or hardened trim shall be of the order of RC38 (BHN 352) to RC45 (BHN 429).

Generally, valve packing shall be Teflon for temperatures below 200 deg C and graphite for steam services and temperatures above 200 deg C. Graphite-based packing system shall be used for fire-safe applications.

Valve Size shall be limited to no less than half the piping line size.

The use of globe design of valve for valve size more than 12” is not economical. it is preferred to have butterfly valve size of 4” as a minimum. 

Kv =0.865 Cv or Cv = 1.156 Kv

Valve Cost Calculator Points

Fireproofing requirement
Partial Stroke testing requirements
Local panels for field operation
Minimum stroke time requirement e.g. anti-surge
Mechanical stopper
Fugitive emission packing requirement
Special accessories like volume booster, airlock, Volume tank, Safety valve
Special Valve Testing requirements such as

Bend test,
Magnetic particle test,
Positive material inspection,
Helium leak test,
Impact testing,
Dye penetration testing,
Dead-band / Hysteresis testing Cryogenic Test,
Seismic test,
Proof of design/type test
Special Valve Painting requirements
Special Valve Services such as

Anti-surge control (Compressor)
Turbine Bypass (Power)
Slide Valves, diverter valves (FCC)
Double Guillotine Valves / Goggle Valve (FCC)
Hot gas bypass (SRU)
Choke Valves of well fluids
CEP, BFP Recirculation Flow (Power)
Speed control – Turbine governing valve (Power)
HP/LP Bypass Spray (Power)
Oxygen service, Vacuum, low-temperature Cryogenic valves, HF, H2S, Steam, H2 services

Disclaimer: The information shared is intended only for informational & educational purposes. All content mentioned does not constitute professional advice and is not guaranteed to be accurate, complete, reliable, up-to-date, or error-free.

Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address

1 thought on “Control Valve in Nutshell – Valve Design and Selection Factors”

Leave a Comment