Answered by Wayne Hable, chief engineer, email@example.com
A: We offer two types. A Darley discharge pressure relief valve is intended to only allow as much as a 30-psig pressure rise when going from the rated flow capacity to a dead headed condition (zero flow rate) if the input speed is kept at a constant speed. When doing this, the discharge valves cannot be closed faster than three seconds and no longer than 10 seconds. A Darley discharge pressure relief valve works with discharge pressure as low as 90 psig and as high as 250 psig.
A Darley discharge pressure relief valve is set to open at a predetermined discharge pressure. It is not a pressure governor—if you want to apply more speed, which will create more discharge pressure, you can drive the discharge pressure higher than desired even when the discharge pressure relief valve is trying to relieve the discharge pressure.
A Darley discharge pressure relief valve is intended to protect operators who are using the same fire pump but are unaware of each other’s operating condition, i.e. if two or more discharges are being used and one of the discharges is suddenly closed off, the operators on the still open discharge lines should only see as much as a 30-psig pressure rise.
We also offer a Darley thermal relief valve which is intended to dump water from the pump when water internal to the pump has been found to attain a predetermined temperature. Normally this predetermined temperature is 140˚F. What we have witnessed is pumps that are dead headed and have significant power being supplied to them heat up the water that is contained within the pump. Essentially the pump is a water heater in this dead headed condition. Without knowing the pump can act as a hot water heater to dead headed water, the first glug of water discharged can be quite warm when the operator finally opens up a discharge line. A thermal relief valve ensures that the first glug of water will be no warmer than that pre-determined temperature.
Pumps can have both a Darley discharge pressure relief valve and a thermal relief valve installed upon them. Or they can have one or the other. Or they can have none at all—it simply depends on how you want it configured.
For more information, visit www.darley.com.