Two pressure-reducing valves (PRVs) in series (Two-step pressure reduction)
Volume 3/ Issue 1/ March 2016
Reduce a steady or varying high inlet pressure to a constant adjustable low delivery pressure.
Two pressure-reducing valves in series are generally used when the total reduction in pressure is greater than that attainable with one valve. Here’s the rule of thumb:
- Use a two-step reduction whenever the supply pressure and desired downstream pressure differ by more than a factor of 10, or
- When the total pressure drop is more than 100 psig.
This arrangement may also be used in some cases where more accuracy is desired.
- As in a single-stage reduction, the main valves are operated and controlled by incoming pressure acting through the spring pilots.
- As delivery pressure nears the spring settings of the pilots, the pilots modulate the main valves to maintain the set pressures.
2 Main Valves
2 Spring Pilots
2 Hardware Kits for spring pressure pilots
For product selection of the Main Valve and Spring Pilot, use Series 2000 Selector on the Hoffman Specialty
website: https://rcwapp.xyleminc.com/esponline/hoffman/SteamMain.asp or see Hoffman Specialty Catalog HS-900F.
Remember that the desired pressure drop will be made in two steps. (See System Pressure Drop.)
Two-Step Pressure Reduction Station
Use HS-900 for capacity at a given Δ P. The application capacity should be between 50 percent and 100 percent of the selected valves’ capacity in HS-900. The capacity of the system will equal the lower of the two valves’ capacities, if they differ.
Do not oversize the valves. Oversized valves will “hunt” and also wear prematurely. Resist the temptation to size the valve to the surrounding pipe size. The correct valve will normally be smaller.
System Pressure Drop
This application is made up of two complete PRV stations piped in series. The total pressure drop is divided between the two main valves.
- The first PRV will reduce the high-pressure steam to an intermediate pressure.
- The second PRV will reduce the intermediate pressure to the final desired pressure.
- Select PRV station as follows:
– Pressure drop (Δ P) no greater than 150 psig.
– A maximum Δ P of 100 psig per valve is recommended, as higher pressure drops shorten plug and seat life.
Dead-end flow applications must allow for leakage of 0.01% of the rated capacity of the second PRV, downstream of the PRV station. Applications with more leakage may experience relief valve popping. Be particularly careful of this when the valve station connects to the load through a single soft-seated valve with tight shutoff. Solutions may include adding a length of pipe or removing a section of insulation so condensation can occur.
Relief Valve Size
Size the relief valve for the steam capacity of the full port (FP) valve, regardless of the application’s port size. This is required by ASME code, in case the seat is replaced in the field with one of a different size. Use the maximum steam capacity at the selected Δ P.
Relief Valve Set Point
Select the relief valve opening set point pressure based on the downstream pressure from the regulator, which is controlled by the spring pressure pilot setting. The actual relief valve set point pressure depends on the relationship of the flow (load) to the main valve rated capacity.
Determine the relief valve set point pressure as follows:
Accuracy of Regulation
The Series 2000 valve’s accuracy in regulating and holding pressures is not absolute. It depends on:
- accurate sizing of the main valve
- up and downstream pipe sizes
- speed of system requirement changes
- several other factors
According to ASME and FCI standards, an individual valve capacity is based on 10 percent accuracy of regulation with the set point at minimum controllable flow, usually 2 percent of maximum flow.
Assuming correct selection and installation, a Series 2000 can be expected to meet the following for accuracy of regulation at flow rates between 2 percent and 10 percent of catalog rated capacity.
- Refer to “Planning the Installation,” page 7, and “Installation Detail,” page 8, for general installation information.
- Install PRV station as recommended in HS-601 for a spring and solenoid pilot.
Feedback line to the pilot should be pitched away from the spring pilot (1/4” per foot) to avoid water pockets. Installation of a 1/4” strainer in the feedback line helps reduce dirt contamination in the spring pilot.
The 1/16-inch bleed orifice is standard, and normally should be used on all pressure reduction applications. If a particular application is experiencing repeated opening of the relief valves, do the following: increase the regulator’s closing speed by replacing the standard 1/16-inch bleed orifice with the 3/32-inch orifice. This MAY resolve the problem provided the application falls within the following guidelines:
- Relief valve is opening due to a fast-acting device downstream (pneumatic control valve or solenoid valve), and
- Desired pressure drop (Δ P) across the Series 2000 valve is 50 psig or more.
- Caution! Installation of the 3/32-inch orifice will reduce the sensitivity, capacity and accuracy of control of the Series 2000 valve. This will also increase the lockup pressure and may adversely affect the relief valve.
- Changing orifice orifice size also requires readjustment of pilot settings.
On two-stage reducing stations, it may be necessary to install the 3/32-inch orifice on the upstream valve if pressure surges occur at the intermediate pressure.
Start-Up and Calibration
Complete start-up procedure and pilot adjustment as stated in HS-601 and HS-621 for spring pilots for each valve, as follows:
- Adjust the low pressure PRV for a small amount of flow, about 2 percent of the valve’s capacity.
- Adjust the high pressure PRV while the low pressure PRV is allowing a small capacity of steam through it.
- Adjust the high pressure PRV to the intermediate pressure.
- Adjust the low pressure PRV to reduce the intermediate pressure to the final desired pressure.
For Two-Step Steam Pressure Reducing Valve Selection, see the Steam – 2000 Pressure and Temperature Regulator Selection Tool at http://bellgossett.com/selection-sizing-cad-tools/ or contact your local Bell & Gossett/Hoffman Specialty Representative.