PRESSURE SETTINGS FOR FIRE PUMP CONTROLLERS
Selecting the pressure points for pump cut-in and cut-out is dependdent on the features of each individual pump assembly or system. Factors influencing the selection are maximum and minimum pump suction supply pressure at churn (static pressure), churn (pump shut-off) pressure, total discharge pressure at churn (static plus pump), desired sequence of operations (multiplepump or multiple-source systems), and operating range and degree of sensitivity of pump controller pressure-operated switches.
A good rule of thumb is to keep the range from cut-in to cut-out as short as possible to avoid water hammer. The pressure-operated switch in the controller should be sensitive enough to permit this range to be as short as 10 to 13 psi.
The cut-out point should be determined first. In the case of a booster pump making suction from a variable pressure source, the cut-out point must be lower than the minimum anticipated churn. The minimum anticipated churn is equal to the pump net churn pressure determined by the pump acceptance test plus the lowest static pressure at the suction side of the pump. If the cut-out point selected is higher than churn pressure, a pump with an automatic-stop controller will continue to operate until the suction pressure increases sufficiently or the controller is manually disengaged.
If the pump assembly includes a jockey pump, the jockey pump controller should be adjusted to cut out at approximately 3 psi less than fire pump cut-out. The jockey pump cutin should be approximately 10 lbs. less than jockey pump cut-out. The fire pump cut-in should be set for approximately 5 psi less than jockey pump cut-in.
The discharge relief valve setting should be 3 to 10 psi higher than fire pump cut-out but never higher than the rating of the fittings in the system.
Following are examples of typical pump assemblies or systems:
- Booster pump and jockey pump in bypass taking suction from a connection to the city water main.
- Fire pump and jockey pump taking suction from reservoir.
- Underground fire protection loop with two city connections, booster pump and jockey pump in bypass at one city connection, and two fire pumps with suction reservoir.
- Multiple pumps in series.
