Typically, a design engineer sizing a control valve will follow a set of steps like the following:
- Size for maximum flow
- Estimate the maximum required flow
- Calculate system pressure drop without the valve
- Choose a valve that will pass the maximum flow when about 90% open
- Check for cavitation, flashing, etc.
- Size for minimum flow
- Choose a valve that will pass the minimum when about 10% open
- Size for “normal” flow
- Choose a valve that will pass the normal flow when about 60- 70% open
You want to maintain valve turndown and operability across the full range of anticipated conditions. A valve that is closed or wide open cannot modulate the flow, so you should avoid any plans to operate with the valve “pegged”.
You also should note that automatic control valves are not designed to produce “tight shutoff”, and will likely be prone to small amounts of leakage when closed. Consequently, most control valve installations include block valves , manual valves which can be turned when complete shutoff is needed.