Sizing & Selection of PRVs

Finding the correct pressure regulating valve, or PRV, for your system depends on a number of factors.

Nature of the Controlled Fluid
Choosing the materials of construction for the PRV will depend on the corrosive nature of the fluid being controlled. The orientation of the valve and valve seat will depend on whether the fluid being controlled is a gas or liquid. A liquid valve's direction of opening usually goes with the flow of fluid, whereas a gas valve will open against the flow of fluid. Also, a regulator for liquids and non-steam gases will typically have a soft seat or valve that can be shut off tightly to prevent leaking. A steam valve would most likely be metal to metal seating.

PRV Application
The type of valve for any particular system will be determined by how the valve will be used. Capacity may dictate that a piloted valve be used rather than a direct-acting valve. Services where the valve will be dormant for long periods of time may require a valve that will not seize when turned off. Heating systems that may be turned off for six months are a good example of this scenario.

Inlet Pressure Maximum
Inlet pressure will tell us the body material or connection type required for the valve and will help determine the maximum capacity for the valve (capacity is based on pressure differential).

Inlet Pressure Minimum
This will be the worst case scenario of pressure differential compared to set point pressure. The valve must be able to pass the maximum flow at this minimum pressure differential and not be oversized at the maximum pressure differential.

Fluid Flow Maximum
This is the maximum flow the valve must pass at the minimum pressure differential.

Fluid Flow Minimum
By comparing fluid flow minimum with the maximum flow, we can determine if the demands of the system are greater than the flow rangeability and controllability of a single valve. This may require a parallel valve installation, with each valve staged to handle a portion of the maximum flow (typically with a 1/3 to 2/3 split of the maximum flow required).

Desired Outlet Set Point
Outlet set point is needed to compare to the minimum and maximum pressure inlets to make sure the valve can pass the minimum and maximum pressure differentials. Outlet set point will also help determine the range spring or air-loading pressure required to obtain the set point. When compared to the inlet pressure, the pressure differential may dictate the need for a two-stage pressure reduction (two PRVs in series due to the large reduction of pressure required).

"Dead End Service" or "Tight Shut Off" Rating
Some applications will require the valve to leak no more fluid than 0.01 percent of the maximum rated capacity of the valve when it is closed. To leak more than this amount could cause the system to over-pressurize, causing safety valves to pop or damaging equipment not designed to handle higher pressures. Most valves are rated for "dead end service" or ANSI class-IV leak rate, depending on the type of fluid on which they are used.

External Pressure Pilot Two Stage Reduction

This piping application is used when the pressure turndown ratio is greater than that of a single valve. Pressure reduction is accomplished by using two valves in series to reduce the pressure in stages. Depending on the volume of fluid required and pressure reduction, the second stage valve typically will be larger in size than the first stage valve. Unless a specific intermediate pressure of the fluid is required, this intermediate pressure is typically selected so as to keep the pressure turndown ratios of both valves as similar as possible. This will help equalize and maximize the service life of both valves.

External Pressure Pilot One-Third to Two-Third Reduction Station

This piping application is used when the flow rangeability is greater than that of a single valve. Better control is achieved by piping two valves in parallel and sizing one to handle 1/3 the maximum load and the other 2/3 the maximum load. These two valves are staged by offsetting their pressure set points by 2-3 psig (.1 - .2 bar). The smaller valve is usually the lead valve and would have a pressure set point at the desired pressure. The larger valve is usually the lag valve and would have a pressure set point of 2-3 psig (.1 - .2 bar)below the lead valve. This offset of set points will stage the valves so that the lag valve will remain closed until the lead valve can no longer pass the required flow and is wide open. This lack of flow will cause the set pressure to drop slightly until the lag valve opens and regulates at the higher demands of flow.