Water-Flow-Actuated Detectors

Sprinkler water-flow alarm-initiating devices are switches, just as fire alarm initiating devices are. Normally open switches that close upon alarm are frequently used in end-of-line resistor circuits, though some normally closed switches are used in normally closed loop circuits. However, the alarm-initiating devices for sprinkler water-flow mount differently and sense different conditions from fire-alarm-initiating devices.

Sprinkler water-flow detectors are generally pressure actuated or vane actuated. Pressure switches are used on both wet- and dry-pipe sprinkler systems. Vane switches are widely used on wet-pipe sprinkler systems.

PRESSURE TYPE OF WATER-FLOW DETECTORS.\Numerous styles of water-flow pressure switches of the pressure-increase type are found in wet- and dry-pipe systems. (Figure 7-15 shows one style.) The usual arrangement for switch actuation includes a sealed accordionlike bellows that is assembled to a spring and linkage. The spring compression or tension controls the pressure setting of the switch and may be adjustable and/or factory set to the desired pressure. As water or air pressure in the bellows increases, it expands, providing motion against a spring. The linkage converts the motion of the

Figure 7-15.\Pressure-increase type of water-flow detector.

bellows into the desired motion to actuate the electrical switch. If the pressure switch is used on a wet-pipe system, it is usually mounted at the top of a retarding chamber, which reduces the speed of pressure buildup at the switch.

There are also water-flow pressure-increase detectors that incorporate a pneumatic retarding mechanism within the detector housing. The retard time is adjustable to a maximum of 90 seconds with usual settings in the 20- to 70-second range. The retarded switch would be connected to the alarm port of a wet sprinkler system alarm check valve. The usual pressure settings for these switches are in the range of 8 to 15 psi.

Pressure-drop detectors can be used in wetpipe sprinkler systems equipped with a check valve that holds excessive pressure on the system side of the check valve. These detectors are most frequently used where a water surge or hammer causes false alarms with other types of water-flow detectors.

The construction of pressure-drop detectors is similar to that for pressure-increase detectors. The switch for a pressure-drop detector is arranged to actuate on a drop in pressure, and there is no retarding mechanism or chamber. A typical switch of this type would be adjusted for some normal operating pressure in the 50- to 130-psi range. The alarm pressure would be adjustable to 10 to 20 psi below the normal pressure.

VANE TYPE OF WATER-FLOW DETEC-TOR.\A vane type of water-flow detector,

Figure 7-16.\Vane type of water-flow detector.

used only in wet-pipe sprinkler systems, is shown in figure 7-16. The vane (a flexible, almost flat, disk) is made of corrosion-resistant material. The detector is assembled to the pipe by drilling a hole in the wall of the sprinkler pipe. The vane is rolled up to form a tube and inserted into the pipe through the hole. Once inside the pipe, the vane springs open, almost covering the inside cross section of the pipe. The whole detector assembly is clamped to the pipe with one or two U-bolts. Gaskets and other sealing devices prevent leakage of water out of the riser pipe and into the detector housing. Operation of a sprinkler causes water to flow in the system, moving the vane. A mechanical linkage connects the vane to an adjustable retarding device in the detector.

The retarding device, which is usually a pneumatic dashpot, actuates the alarm switch or switches and/or signal transmitter if the vane is still deflected at the end of the adjustable delay period. The retarding device prevents spurious alarms by delaying the mechanical actuation of the alarm switch(es) and/or transmitter to allow the vane and retarding mechanism to return to their normal positions after momentary water surges. The retarding-device setting is usually in the range of 30 to 45 seconds, though the maximum setting may be as high as 90 seconds.

TESTING WATER-FLOW-ACTUATED DETECTORS.\Water-flow-actuated detectors should be inspected monthly for physical damage and for paint on information plates and labels. Replace or repair damaged devices immediately. Clean or replace painted plates and labels. Correct other deficiencies promptly.

Test wet-pipe-sprinkler-system-water-flow devices by causing a flow of water equal to that from one sprinkler by opening the inspector's test valve fully. This valve is usually near the end of the sprinkler system on the opposite side of the building from the system riser. For sectional water-flow detectors, the inspector's test valve is usually on the opposite side of the section of the building from the riser. The inspector's test valve is left open to allow full flow until an alarm is indicated at the local control unit or, if the control unit is connected to the base alarm system, until a clear alarm is received at the alarm headquarters. One person with radio or telephone communications at the test valve and one person at each alarm-receiving location are usually needed for testing.

The delay between the start of full flow and receipt of the alarm signal should be between 15 to 90 seconds for retarded signals. Detectors that sense a pressure drop should respond in less than 15 seconds. If the alarm has not been received after water has been flowing for 3 minutes, stop the test and determine the cause of the problem.

Dry-pipe sprinkler systems have an alarm test valve at the sprinkler riser in the trim piping that allows water from the supply side of the dry-pipe valve to exert supply pressure on a water-flow detector of the pressure-increase type. The alarm test valve is frequently a small lever valve but may be a globe valve. It should be permanently tagged Alarm Test Valve to expedite future testing.

The regular trip test of a dry-pipe sprinkler system to check the operating condition of the sprinkler system can also be used to test the waterflow detector and alarm system if the tests are coordinated. However, it is not practical to trip-test the dry-pipe valve for every alarm system test. Do not open the inspector's valve at the end of a dry-pipe sprinkler system for an alarm system test unless a trip test is desired.

The purpose of these initiating devices is to detect a fire condition and provide that information to the control unit. The control unit energizes the indicating circuit to warn building personnel for evacuation and to inform fire personnel of a fire.