valve-sizing-selection-1231875721684103-3

Valve Sizing & Selection

Ranjeet KumarM.Tech – Chemical

08 Jan 09 Ranjeet Kr. 2

Steps in Sizing

Science with many rules of thumb Define the System. Maximum Pressure Drop for the Valve. Calculate the valve Characteristics (Cv). Preliminary valve selection. Check the Cv and stroke percentage at minimum flow. Check the gain across applicable flow rates.


Define the System – Key Variables

Total pressure drop, Design flow, Operating flow, Minimum flow, Pipe diameter, Specific gravity


Maximum Allowable Pressure Drop

The usual rule of thumb is that a valve should be designed to use 10-15% of the total pressure drop or 10 psi, whichever is greater.

Investigate the Pump & its maximum available head. MAPD = NPHSA – NPSHR. Trade off-

larger pressure drops increase the pumping cost (operating) & smaller pressure drops increase the valve cost because a

larger valve is required (capital cost).


Valve Characteristics – Cv

Note – Check thumb rules before referring valve chart or characteristic curve.


Preliminary Valve Selection

Thumb Rules – Never use a valve that is less than half the pipe size. Avoid using the lower 10% and upper 20% of the valve stroke.

The valve is much easier to control in the 10-80% stroke range.

Select the type of valve & use its corresponding valve chart.

Valve chart is supplied by manufacturer. Check Cv for minimum flow with selected size of valve &

check for minimum flow should not fall below 10% of valve stroke.


Valve Chart / Characteristics Curve

Table 1 – Chart for Equal Percentage Globe Valve


Gain across applicable flow rates

Gain #1 = 85/38 = 2.2Gain #2 = 40/12 = 3.3

Acceptance criteria :– Gain should never be less than 0.5. Gain#2 – Gain#1 < 50% of (max of Gain#1 or Gain#2)

Here 0.5 (3.3) = 1.65and 3.3 - 2.2 = 1.10. Since 1.10 is less than 1.65 so it can be a choice

Flow (GPM)

Stroke (%)

Change in Flow(GPM)

Change in stroke (%)

25 35 110 – 25 = 85

73 – 35 = 38

110 73

150 85 150-110 = 40

85 – 73 = 12


Choke Flow FL

At max restriction – flow rate is max & pressure is minimum.

Vapor bubbles flashes if liquid pressure falls below Vapor Pressure of liquid.

Bubbles has (a) no effect on flow, (b) increases pressure drop across valve, (c) cavitations

Decreased efficiency Noise Vibration Material loss – sand blasted surface

FL checking is recommended when difference in Max & Min

flow exceed 90% of Max flow.


Basic Valve Types – mechanical characteristics

Ball valves; Diaphragm valves; Gate valves; Globe valves; Butterfly valves; Plug valves; Check valve; Safety/relief valve


Basic Valve Types – control / openness

Equal Percentage: equal increments of valve travel produce an equal percentage in flow change. Large changes in pressure drop are expected Minimum pressure drop due to valve In temperature & pressure control loop

Linear: valve travel is directly proportional to the valve stoke In liquid level or flow loops Pressure drop across valve is almost constant.

Quick opening: large increase in flow with a small change in valve stroke For frequent on-off services Instantly large flow is needed.


Gate Valve

Best Suited Control: Quick Opening Recommended Uses:

Fully open/closed, non-throttling Infrequent operation Minimal fluid trapping in line

Advantages: High capacity Tight shut off, Low cost, Little resistance to flow

Disadvantages: Poor control Cavitate at low pressure drops Cannot be used for throttling

Applications: Oil, Gas, Air, Slurries, Heavy liquids, Steam, Non-condensing gases, and Corrosive liquids


Globe valve Best Suited Control: Linear and Equal percentage Recommended use-

Throtteling services/flow regulation Frequent operation

Advantages: Efficient throttling Accurate flow control valves Available in multiple ports

Disadvantages: High pressure drop More expensive than other

Applications: Liquids, vapors, gases, corrosive substances, slurries


Ball valve

Best suited control – Quick opening linear. Recommended uses –

Fully open/closed limited throttling Higher temperature fluids

Advantages – Low cost High capacity Low leakage & maintenance Tight sealing with low torque

Disadvantages – Poor throttling characteristics Prone to cavitation

Applications – Most Liquids, high temperatures, slurries


Butterfly valve

Best Suited Control: Linear, Equal percentage Recommended Uses:

Fully open/closed or throttling services Frequent operation Minimal fluid trapping in line

Advantages: Low cost and maint. High capacity Good flow control Low pressure drop

Disadvantages – High torque required to control Prone to cavitation at lower flows

Applications: Liquids, gases, slurries, liquids with suspended solids

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