Control Valve Rating

[Barmmer]

I have a system on a district CHW system that operates at a fairly high pressure ~130 psi static supply and ~95 psi static return. my question is in sizing control valves. I have been installing valves with a 125 psi close off rating based on the high operating pressure. Though several people have told me I need only to match the close off to the differential pressure about 40 psi. Which is the best method to size the valve?

 

[TD2K]

If the valve will always see 130 psig on the upstream side and 95 psi on the downstream side, you only need to size the actuator for the actual dP, in this case 35 psi (or 40 to give you a margin).

However, if you 'could' be in a position (startup?) where you have 130 psi on the upstream side and a much lower pressure on the downstream side you need to size for the potential dP to be able to ensure the valve can operate.

Undersizing an actuator for an on-off valve will mean that with higher dPs across the valve, you might not be able to open the valve. If it's a control valve and depending if it's 'flow to open' or 'flow to close', you might not be able to close or open the valve, depending on the flow path.

 

[hacksaw]

where you have a system that never requires shut off against the maximum system pressure, you might be be able economize on some of the valves.

In so doing, however, you create a wharehouse fiasco, and will have to carefully document the actuator sizing conditions of all your valves.

I have never been a case where an actuator has been over sized, but countless cases where they have been undersized.

Some of the vendors make it a practice to standardize on the worst case actuator for the valve anyway.