Rules of Thumbs for Sizing Control Valves ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Rules of Thumbs for Sizing Control Valves

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In many applications where control valves are applied, sizing and selection can be quite challenging. In the majority of applications, it is either the control valve is under sized or over-sized. In an undersized control valve, the valve is unable to deliver the required flow for each stage of the valve lift creating control problems.

Over-sized control valves will under all normal operations be confined to small openings of the valve with great risk of variable sensitivity and aggravation of any uneven movement of the valve and actuator combination. Poor accuracy and unstable control are often the result of over-sized control valves.

Given the problems that could arise because of failure to size control valves accurately, the following rules of thumb are presented as a guide for the selection and sizing of control valves:


Rule 1
Set the design flow rate as either:


Design Flow Rate = 1.3 x Normal Flow Rate


Or


Design Flow Rate = 1.1 x Maximum Required Flow Rate


Rule 2
Set the control pressure drop to 50 - 60% of the frictional pressure loss of the piping system.

Rule 3
Limit the maximum flow rate through the valve: Minimum flow rate turndown should be 5:1 for linear trim valves. 10:1 for equal percentage valve trims.

Rule 4
Avoid using the lower 10% and upper 20% of the valve stroke. The valve is much easier to control in the 10–80% range. Consequently, The valve should be sized so as to control the required range of flow rates between 10% and 80% of valve opening. This is illustrated below:

Rule 5
Ideally select a valve that has a body size one (1) pipe size smaller than the pipe in which it is to be installed e.g select a size 2’’ valve for a 3’’ pipe). Generally, control-valve bodies are one size less than the line size. However, if this causes the valve body to be significantly less than the line size, which would reduce the valve’s effective Cv. then do not apply this rule of thumb.

Rule 6
Never select a valve larger than the pipe in which it is to be installed.