Pressure Maintenance/ Jockey pump sizing & Jockey pump pressure relief valve application.

1. “JOCKEY PUMP- OVERSIZED & UNDERSIZED”
A Poor Design Practice:
Conservative engineering practices often result in
the specification, purchase, and installation of
pumps that exceed process requirements.
Unfortunately, NFPA20 doesn’t clearly specify
the required flow for pump but its says:
4.26.2 Pressure maintenance pumps shall not be
required to be listed. Pressure maintenance pumps
shall be approved.
4.26.2.1* The pressure maintenance pump shall be
sized to replenish the fire protection system
pressure due to allowable leakage and normal drops
in pressure.
4.26.3 Pressure maintenance pumps shall have rated
capacities not less than any normal leakage rate.
So, most of the designer & manufacturer ends up
with Oversized & Undersized jockey pump.
UNDERSIZED PUMPS –
An Undersized pump will be slow in full filling
the system compare to the leakage. And in-
turn cause main pump to start early unnecessarily.
OVERSIZED PUMPS –
An Oversized pump will immediately fulfill the
system as it come to duty and shut down.
The pump will work for short time and pressure
rise rapidly which leads to water hammering in
lines.
These large jockey pumps with improper pressure
setting will cause the more sprinkler head to fuse
before the main pump come to run.
AN OVERSIZED PUMP WILL STOP THE
MAIN PUMP TO START, UNTIL A BIG
FIRE BROKE OUT
Even Now-a-days, you can see the requirement of
1 Electric + 1 Diesel + 2 Jockey, especially for
middle east projects. However these jockey will be
set with different cut-in & cut-out pressure using
dual Jockey pump controllers.
But IT WILL AGAIN DELAY OUR MAIN PUMP
TO START. In-turns more sprinkler head need to
fuse to drop the system pressure till the Main pump
cut-in pressure meet. Which is a poor design
practise.
SO, QUESTION IS WHAT SHALL BE THE
PROPER SIZE OF JOCKEY PUMP?
One guideline that has been successfully used to size
pressure maintenance pumps is to select a pump that
will make up the allowable leakage rate in 10
minutes or 1 gpm (3.8 L/min), whichever is larger.
Based on NFPA 24 (1995) Standard for the
installation of private fire service mains and their
appurtenances, maximum leakage per 100 joints is
0.5 gallon per hour. The standard requires the water
and the pressure to be replenished within ten
minutes. We also need to take into account :
• Net positive suction pressure available
• Effect of rise to shutoff the system to prevent
over pressure
Since 1 GPM jockey will work on intermittent duty,
so we suggest to keep jockey pump as:
“8 GPM - 20 GPM as max duty flow, till 1500
GPM main pump flow"
“RELIEF VALVE FOR JOCKEY”
Another common question arises with Jockey
pump Is Relief Valve required for jockey pump?
YES, If Jockey pump has a total discharge pressure
with the pump operating at shutoff exceeding the
working pressure rating of the fire protection
equipment, a relief valve sized to prevent over-
pressuring of the system shall be installed on the
pump discharge to prevent damage to the fire
protection system.
xxx
3/01/18