This is the third and final installation of a series introducing the new Fire Sprinkler Systems Monitoring Application Guide. Pressure switches are the focus of this article. The previous articles discussed supervisory switches and waterflow detectors.

During a fire condition, sprinkler system reliability is critical. Therefore it is important to continually monitor the proper functioning of the system. One aspect of the monitoring system is pressure switches. These switches are designed to provide early warning of off-normal conditions that occur in a sprinkler system.

To accommodate the four different types of fire sprinkler systems (wet, dry, deluge and pre-action), there are two main types of pressure switches: alarm and supervisory.

Alarm Pressure Switches

Alarm pressure switches detect water discharge from one or more sprinkler heads. While these switches are used in all four types of automatic fire sprinkler systems, they are most commonly installed in dry pipe sprinkler systems. Alarm pressure switches are the only way to detect the flow of water in dry pipe, pre-action and deluge sprinkler systems. These switches do not extend into the open pipe, so there is no risk of damage to the switch from the sudden inrush of water that can result from the operation of these types of sprinkler systems.

Supervisory Pressure Switches

Supervisory pressure switches monitor the status of sprinkler system conditions, primarily the air pressure in dry pipe and pre-action systems. The supervisory alarm helps prevent the accidental operation of the dry pipe valve due to low air pressure from air compressor failure or air leaks in the system. In certain conditions, they also monitor water pressure in wet pipe sprinkler systems.

There are two types of supervisory pressure switches. The first is a low air switch, which monitors the low air pressure within the dry pipe system. It should be set to signal an alarm when the system air pressure drops 10 PSI below normal.

System Sensor’s line of pressure switches includes the EPS10 series for use in wet, dry, deluge and pre-action automatic sprinkler systems to indicate sprinkler discharge. The EPS40 and EPS120 series supervisory pressure switches are intended for dry pipe systems or pressure tanks and water supplies of automatic water control valves. These models offer the choice of initiating low or high/low pressure monitoring alarms, in a variety of pressure ranges.

The second type of supervisory pressure switch, called high/low air, monitors the dry pipe system for both low and high air pressure levels. This is because too much air pressure in the system can also be detrimental to how well the sprinkler system functions. For example, too much air pressure can damage the clapper and rubber gaskets in the dry pipe valve, delaying the operation of the valve due to the time it would take to bleed off the high air pressure within the system.

The Right Location

Installing the pressure switch in the right location is the key to having it work properly. An alarm pressure switch in a dry pipe sprinkler system is installed on the alarm line trim of the dry pipe, deluge and pre-action valves. No delay is required because pressure surges from the water supply are not an issue in these types of systems. In a wet pipe sprinkler system, an alarm pressure switch is typically installed in a one-half-inch tapped outlet on top of the retard chamber.

A low air supervisory pressure switch should be installed to monitor the air on the system side of the dry pipe valve. The supervisory alarm will help prevent the accidental operation of the dry pipe valve due to low air pressure from air compressor failure or air leaks in the system. Both the low air and high/low air supervisory pressure switches are installed in the small trim piping connected to the automatic air compressor or other source.

Finally, low pressure alarm switches should be installed with a ball valve in the line, which is used to exhaust the pressure between the ball valve and the pressure switch. The ball valve allows testing of the pressure switch without any possibility of accidental operation of the dry pipe valve.

Maintenance and Service

After installation is complete, the pressure switch trip points should be tested by slowly introducing pressure from the test source. Test points should be tested several times to ensure the accuracy of the setting. Preventative maintenance and periodic testing should be performed as required by the applicable NFPA standards. It is recommended that testing be done at least bimonthly, or as often as required by the Authority Having Jurisdiction.

Tags: Fire sprinklers, Pressure Switches, Web exclusive

Posted in Commercial, Design and Installation, Sprinkler Systems Monitoring, Winter 2008

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