Potter Releasing Systems - Manual (Page 22)

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#3. Wet System With Maintained Excess Pressure
This type of system has a jockey or excess pressure pump that is maintaining a higher pressure on the system
side of the check valve than the supply pressure can be expected to reach. This excess pressure holds the clapper
of the valve down even when the city pressure fluctuates. This eliminates false alarms caused by water surges.
Two different methods utilizing two different waterflow devices can be used to monitor these types of systems.
Excess pressure systems do not require retards on the flow switches.
On both systems the excess pressure pump should be monitored with a supervisory pressure switch. This switch
should be set to trip when the pressure drops 10 psi below normal, this will prevent false alarms due to a drop in
pressure from a pump failure. A typical excess pressure system may maintain a system pressure of 120 psi. The
low pressure supervisory switch would be set to trip at 110 psi. High pressure should be monitored to prevent
damage to sprinkler system and long delay of a waterflow alarm.
A. Straight Riser With Maintained Excess Pressure
A pressure drop type waterflow device is connected to the system side of the check valve. Activation of a
sprinkler head in the system results in a loss of excess pressure, the pressure will drop down to whatever the
supply pressure is. The waterflow switch detects this drop in pressure and sends an alarm signal.
The low pressure supervisory switch is set to trip approximately 10 psi below the normal excess pressure.
The waterflow switch would be set to trip 10 psi below the supervisory switch. The trip point of the waterflow
switch must be higher than the water supply pressure can be expected to reach.
Note: All water control valves should be supervised per NFPA13 2010 8.16.1.1.2.1.