Control valves are made up of different styles which are usually determined by experience and its application. Valve selection can be a difficult task and be sizing them can even be more complicated and tricky at the same time.
The question is “how do you size a control valve?”
To size a control valve, the rule of thumb is often applied so that it performs between 20-80 percent open at maximum flow rate, and a little bit less than 20 percent at a minimum flow rate. This is to make sure a wider range of valve’s control is utilized in maintaining a reasonable factor of safety.
Why is it important to size your control valve?
A control valve is considered undersized when it doesn’t have the capacity to pass the required flow. An oversized valve can be very sensitive to operating conditions, even the slightest change in position of the valve will bring about a significant change in flow. This makes it crucial to size your valve.
According to the Valve Magazine “in order to achieve the maximum degree of process control for the liquid or gas, it is important to select an appropriately sized valve”
For accurate control valve selection and sizing, the following guidelines can be followed:
• The valve’s flow characteristic is wrong if a set of loop turning parameters only works on one end of the control range.
• When the pressure difference across the valve decrease as flow increase an equal percentage of monitoring valve should be used. And should also be used where the flow rate increases as the process gain increases. If none of the above conditions apply, use a linear control valve.
• Size the valve to function at 20-80 percent open at the maximum flow, and a little than 20 percent at the minimum flow so as achieve best control of flow.
• A control that is less than half the line size or lager is often considered as weak piping practice.