Pressure reducing valves (PRV) are frequently used in high rise buildings to keep the pressure on sprinkler systems below the allowable maximum, typically 175 psi, when the supply to the systems can be considerably higher. The PRV reduces the pressure at no flow conditions and flow conditions in accordance with manufacturer prescribed performance curves.
Customarily, the curves identify the relationship between outlet pressure and flow through the valve depending on the initial setting of the valve at no flow conditions. The initial setting may be factory preset, or the PRV may be adjustable, allowing for field setting to a location-specific pressure. FM Global Data Sheet 3-11 “Pressure Reducing Valves for Fire Protection Service” (DS 3-11) suggests that fire protection systems be designed to eliminate the need for, or minimize the number of PRV’s. This eliminates reliance on an active mechanical component and the need for maintenance and test costs over time. It also indicates a preference to avoid PRV’s.
NFPA 13-2002 “Installation of Sprinkler Systems” (NFPA 13) requires a test of each PRV at the time it is installed, and that the test results be recorded on the contractors test certificate as a permanent reference record. The means that a flow test at sprinkler system design demand flow is required. NFPA 25-2002 “Inspection, Testing and Maintenance of Water Based Fire Protection Systems” (NFPA 25) requires a full flow test to be completed at five year intervals and the results compared to the initial test results. DS 3-11 recommends this test on an annual basis.
There are two key problems meeting these test requirements often encountered in high rise installations – (1) the ability to flow sufficient water and (2) the capability to measure the actual flow. NFPA 13 requires that all drains and other downstream drain piping be sized to permit sufficient flow (the design demand flow). DS 3-11 is more explicit in that a means to determine the flow rate is required. It also requires drains of proper design to permit the required flow, but in no case should piping be less than 1½ in. diameter. Both documents identify the proper valves, gages, piping and arrangement for systems using PRV’s.
In high rise buildings using PRV’s for sprinklers on individual floors it is not unusual to find a combination Test Connection/Drain that is undersized to permit water flow at the sprinkler system design flow. A site glass instead of a gage or flow meter is often provided, which only establishes that there is flow to drain, without quantifying.
One solution to identifying the flow rate would be the installation of flow meters on each floor level with a PRV. Where a number of floors have PRV’s, the cost of multiple level installations can mount up rapidly. This may be financially unattractive for a test that is conducted annually or less frequently, depending on the facility test protocol. In the long run, it may be more attractive to purchase or rent a portable flow meter and connect the unit to existing fittings when the testing is conducted. Another possibility might be the purchase or rental of ultrasound equipment to measure flow rate. This technology is used in HVAC systems and may be useful on fire systems. The water velocity is measured from outside a closed pipe system and can be converted to flow since the pipe diameter where the measurement is made is known.
Once the flow is established, what is to be done if that flow is well below the sprinkler system design flow? Hindsight suggests that larger diameter drain pipe should have been used, or perhaps that multiple PRV’s in parallel might have been provided so each could be tested at the lower flow available with the existing drain system and then combining the flows. Unfortunately, hindsight can be expensive. There may be economic ways to accomplish this goal using a performance based approach.
Typically, the PRV manufacturer provides curves that prescribe valve performance over a range of flows. There are multiple curves for various valve settings when the PRV is field adjustable. There may be a statistical correlation indicating the PRV is performing properly by establishing a point on the appropriate curve for less flow than the sprinkler system design flow. This would be of value when the drain system is undersized. Once established, the curve could be relied on to identify the pressure on the discharge side of the PRV at the sprinkler system design flow. This can be compared to the sprinkler system hydraulics to assure that the required pressure is available. This extrapolation process should be used only after contacting the manufacturer for assurance and supporting documentation as to the validity of extrapolating the results of one test point on the performance curve to the entire performance range of the particular curve.
Another approach, applicable only to field adjustable PRV’s might be useful. Perform multiple tests at a somewhat constant flow rate below the design flow while changing the PRV setting each time. In this way there is proof that the PRV conforms to test curves, and is therefore working properly. If it is demonstrated that it is working properly, one might imply that it would adhere to a single performance curve for a specific setting over a range of flow rates.
Facilities with a number of PRV’s on several floors may have another alternative available – a statistical approach. It is likely that valve manufacturers have a large data base that supports determining the failure rate for the specific PRV. Knowing the number of PRV’s in a facility, it can be decided what percent of them would need to perform properly out of the total to demonstrate a sufficiently low probability of failure for all of them to be statistically insignificant. That number of PRV’s would be randomly selected and removed for bench scale testing under controlled conditions. If they perform according to the manufacturer’s curves, the correlation to the statistically insignificant failure rate can be established.
A word of caution, please. Keep in mind that the current testing requirements (NFPA and FM) are prescriptive and allow little room for variation. However fire protection engineering is moving more and more toward acceptance of performance based approaches. As with anything new, there will be skepticism and doubt. Therefore, it would be wise to involve all of the interested parties and reviewing authorities from the outset to assure a level of comfort that will lead to acceptance of the test results. Sometimes this can be a lengthy and difficult process.
For those of you who may be responsible for the performance testing of PRV’s, we would like you to keep Risk Logic in mind. We can pre-plan the activities and coordinate with all interested parties, arrange the actual testing, and document the results. Should there be problems, we will work with you to solve them and reach acceptance of successful testing.