Fire protection pumps: Updates to NFPA 20
Pressure relief valve
Issues with pressure relief valves continue to surface, with one committee member reporting a significant number of events where a pressure relief valve that was piped back to the fire pump suction would apparently be completely open. This results in a significant impairment to the fire protection system that may go unnoticed until the water is needed for an actual fire. This issue can occur any time the discharge from a pressure relief valve cannot be observed. When a pressure relief valve is piped back to a suction tank, a means to observe the discharge should be provided (see the sight glass in Figure 3, item 2).
A fire pump adds energy to the water passing through it. When no water is discharging, the energy will be in the form of heat added to the water. To prevent water overheating at churn, a circulation relief valve located between the fire pump and fire pump discharge check valve is required to operate at churn pressures, but should not operate when the fire pump is flowing water (see circulation relief valve in Figure 4). When water is flowing through a pressure relief valve or a test loop piped back to suction, the pressure will be lower than churn pressure and the circulation relief valve designed to operate at churn will not operate. However, when circulating water though a closed loop, a fire pump adds even more energy to the water passing through it than under churn conditions. An additional circulation relief valve is required when a relief valve is piped back to the pump suction. This additional circulation relief valve should be set to operate below the 150% pump discharge pressure.
Without proper operation of the circulation relief valves, the temperature of the water will rise. In addition, if the pressure relief valve is wide open, it may cause a diesel engine (if used) to exceed its rated horsepower. When the higher temperature water is sent to the heat exchanger for cooling, the engine may overheat. In one case I am aware of, a diesel engine was destroyed and this scenario appears to be the cause.
An argument was presented that if the pressure relief valve discharged to the atmosphere, the extreme flow of water would not be ignored. Currently pressure relief valves are permitted and required only to prevent overpressurization for two reasons:
- Diesel overspeed conditions
- Failure of the variable speed mode on variable speed pumps.
Under normal operating conditions the pressure relief valve should not operate even under churn conditions. However, older fire pump installations may use the pressure relief valve to “trim” the fire pump discharge and the pressure relief valve may discharge a large volume of water well beyond churn conditions. The committee felt that the pump installations in the reported incidences did not conform to current requirements and returning pressure relief valve pipe to the pump suction was appropriate for the limited use of pressure relief valves permitted by NFPA 20.
While piping a pressure relief valve or test loop back to suction is still permitted, a requirement was added for heat exchanger cooled diesel engines to monitor the temperature of the cooling water and shut 4down the engine while operating in the test mode (only) whenever the water temperature at the engine inlet of the heat exchanger exceeds 104 F.
As noted earlier, a separate circulation relief valve is required by NPFA 20 whenever the pressure relief valve is piped back to suction. The additional circulation relief valve is critical to prevent the fire pump from overheating. Tier 3 environmental requirements that limit the intake air temperature cannot be maintained without limiting the cooling water temperature. A circulation relief valve on a pressure relief valve piped back to suction is shown in Figure 3, item 9.