In fire protection service, pressure reducing valves (PRV's) can be found on standpipes and automatic sprinkler systems in high-rise buildings, in pump houses, on public and private gravity water systems and other situations where conditions warrant their use.
The most common application of PRV's is in combined standpipe/sprinkler system risers in high-rise buildings. In these combined systems, the PRV's are used to maintain maximum pressures at the hose connections within the limits given in NFPA 14, Standard for the Installation of Standpipes and Hose Systems, and the maximum static pressure in sprinkler system piping within the limits given in NFPA 13, Standard for the Installation of Sprinkler Systems.
Sizing and Selection of Pressure Reducing Valves
There are generally two types of PRV's: direct-acting and pilot-operated.
Direct-Acting PRV's – Sizing of direct-acting PRV's is based on flow charts available from the manufacturer of the valve. It is typical for manufacturers of direct-acting valves to provide flow charts showing a plot of the outlet pressure versus the inlet pressure for a given flow rate through the valve. In these graphs, each curve represents the performance of the valve at a given pressure setting
Other manufacturers provide flow charts that show a plot of the outlet pressure versus the flow rate through the valve for a given inlet pressure range or condition. This type of chart gives better indication of the valve performance than the previously described "inlet pressure vs. outlet pressure" charts.
Pilot-Operated PRV's – Similar to direct-acting valves, pilot-operated valves should be sized and set to handle both the minimum and maximum water demands of the system, and to prevent excessive outlet static pressures. The valve should be sized to operate within 20 to 80% of its maximum flow rate capacity to avoid instability, system oscillation and damage to the seat and trim caused by localized high velocities, and to ensure adequate differential across the valve so that the valve can provide quick controlled response. Therefore, in a system where there is a wide flow range, parallel installation of two or more valves may be required. In a system where a large pressure drop is needed, installation of two or more valves in series may also be necessary.
Correct sizing of a PRV is an important step in system design. Under static condition, a PRV needs to be able to prevent outlet static pressures from exceeding the rated pressure of system components, or the maximum system pressures allowed by NFPA 13 and NFPA 14. Under flow conditions, the valve needs to be able to use the available inlet supply pressures to deliver the flow rates and outlet pressures required by the fire protection system being supplied through the valve.
In general, good installation practice for PRV's should include a bypass to allow for emergency use of the system or maintenance without shutting off the water supply, a relief valve downstream of the PRV to relieve excess pressure should the PRV malfunction, and pressure gauges both upstream and downstream of the valve for setting and checking the PRV.
Locate all pressure reducing valves in dry, accessible areas, arranged for convenient maintenance and testing.
Adequately sized drain lines (3 in. diameter minimum for combined systems) should be provided to allow for periodic PRV flow testing.
To reduce the possibility of foreign material becoming lodged in the valves, pressure reducing valves should be installed in a horizontal orientation with the stem up. The installation site should be accessible. Provide sufficient clearance to allow proper operation and maintenance of the valves; this is particularly important with rising stem valves.
Some direct-acting pressure reducing valves are tagged with a "set pressure" and must be installed at the factory-designated floor; this tag should not be removed nor should the valve be installed in any other location without factory agreement.
Any special tools required to adjust the setting of field-adjustable direct-acting pressure reducing valves should be maintained available on-site.
PRV Maintenance and Testing
As recommended in NFPA 25, Standard for the Inspection, Testing and Maintenance of Water-Based Fire Protection Systems, all valves shall be inspected quarterly. The inspection shall verify that the valves are in the following condition:
a. In the open position
b. Not leaking
c. Maintaining downstream pressures in accordance with the design criteria
d. In good condition, with handles installed and unbroken
NFPA 25 also indicates a full flow test shall be conducted on each valve at 5-year intervals and shall be compared to previous test results. Record the static and residual inlet and outlet pressures of the valve. Results should be compared with the manufacturer's performance curves to determine if the valve is operating in a satisfactory manner. Investigate any changes in test results and perform adjustments as needed. If adjustments are necessary, they shall be made in accordance with the manufacturer's instructions.
A partial flow test adequate to move the valve from its seat shall be conducted annually.
FM Global recommends weekly visual inspections, monthly partial flow tests (through Inspector's Test Connections) and annual full flow tests be conducted (using the main drain noted above).