Douglas Krantz - Technical Writer - Describing How It Works
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Why did the Dry Sprinkler System Freeze?

A dry sprinkler system has many parts. Failure of any part might cause the system to flood with water.

Greetings Douglas,

I have an investigation in which a flow alarm switch and alarm monitoring system may not have initially functioned properly. I was told a compressor for the dry system failed allowing water to enter the pipes but the flow was not sensed by the system. Later the water froze causing the fittings and sprinkler heads to fail. The water flow from the failure was sensed by the alarm system.

My question is this:

Will static water in the piping be sensed by the pressure switch? In other words, if water filled the system and the switch failed, would the initial pressure change still exist causing the switch to activate while the water was in the line?

Thank you, R E - Professional Engineer

I'm not there to see exactly what happened, but I can at least show a little bit about what goes on.

Analysis of the Dry Sprinkler System

All sprinkler systems are custom built; there is no one correct design, just a general idea of a correct design. Because each building is custom, each sprinkler system is custom.

All of these parts interact with each other, and investigating failures have to take all of these into account to understand the cause.
  • Fire Alarm Monitoring System
    • "Continuous On or Off Signal vs Alarm/Restore Signal"
    • "Alarm"
    • "Supervisory"
    • "Monitoring Company Signals"
  • Dry System Clapper Valve Assembly
    • Clapper Plate (Valve)
    • Water
    • Air Chamber
    • Intermediate Chamber
  • Accelerator
  • "Flow Switch" (Pressure Switch)
  • Alarm Defeat Valve (Maintenance Valve)
  • Low-Air Switch (Pressure Switch)
  • Tamper Switch for the Alarm Defeat Valve

Fire Alarm Monitoring System "Continuous Signal vs Alarm/Restore Signal"

All fire alarm systems work with two different types of signal.

The continuous signal is like a light switch on the wall. The ceiling light is activated by turning on the switch, and stays active the entire time the light switch is turned on (a continuous signal). The ceiling light restores to the inactive state only when the light switch is turned off (a continuous signal).

An alarm/restore signal is like using the telephone. To let someone know that the light switch is turned on, the phone is picked up and dialed at the time the light switch is turned on (a momentary activation signal). Then the phone is hung up. Then to let someone know that the light switch is turned off, the phone is again picked up and dialed at the time the light switch is turned off (a momentary restoration signal).

The "low-air switch" and the "flow switch" (alarm pressure switch) send continuous signals. The fire alarm system receives these continuous signals and "activates" into a supervisory condition or an alarm condition. At this point, the fire alarm system also uses the telephone to send an active signal to the monitoring company.

Dry System Clapper Valve Assembly

Dry System Valve - This is a clapper valve assembly that has 3 chambers. There is a clapper (somewhat flat metal plate hinged on side) that is normally used to separate the 3 chambers.

Below the clapper plate is a wet chamber: a pipe-end. This is the city water pipe that carries water to the dry part of the sprinkler system. When the clapper valve is closed, this pipe-end is blocked by the clapper valve. Commonly, the city water pressure at this point is 90 to 120 pounds per square inch or psi (620 kPa to 827 kPa).

Around this pipe is an intermediate chamber. Because the clapper separates the chambers, the intermediate chamber has ambient air pressure.

Above the clapper plate is the dry sprinkler system. This chamber commonly has 40 psi (75 kPa) of air pressure, which is normally holding the clapper valve closed.

When the clapper valve opens, the clapper plate flips up, and all three chambers are combined to allow city water to flood the sprinkler system. Of course, at this time, all three chambers are at the same pressure - city water pressure.

Accelerator

Some dry systems have an accelerator that will allow the clapper valve to open in a timelier manner.

The dry sprinkler system is a long thin air tank. It holds a lot of air. When heat from a fire melts a fusible link on a sprinkler head, air is let out and the pressure starts dropping. On big dry sprinkler systems, it can take as much as several minutes to let out enough air through a sprinkler head to drop the air pressure in the dry system from 40 psi (75 kPa) to 16 psi (110 kPa), allowing the clapper valve to open. Fire marshals don't like taking that long to start flooding the dry system.

To keep the fire marshals happy, there is often an accelerator attached to the clapper assembly; either this accelerator is part of the valve assembly itself, or added to the assembly. What the accelerator does is to monitor the dry system air pressure, and when the air pressure starts to drop, the accelerator opens the clapper valve right away.

When there is something wrong with the accelerator, the accelerator can be too sensitive to the pressure drops and prematurely open the clapper valve. This can be caused by aging rubber in the accelerator, or poor design for the accelerator.

Waterflow Switches

The dry system waterflow switch is really a pressure switch. The NFPA requires a waterflow switch to be installed, but here, rather than using a paddle type of waterflow switch, a pressure switch is used. This switch is sometimes referred to as a pressure switch or sometimes referred to as a waterflow switch. Physically, it's a pressure switch, functionally, it's a waterflow switch.

It is used to monitor the pressure of the intermediate chamber of the clapper valve assembly. If the pressure of the intermediate chamber is the same as ambient air pressure, the pressure switch is turned off. If the pressure of the intermediate chamber is high, like city water pressure, the pressure switch is turned on.

In reality, whether it's moving or still (static), water has to be under pressure to activate the flow switch. The switch only detects pressure; the normal interpretation of sensing high pressure is that water is flowing.

Alarm Defeat Valve

I call it an alarm defeat valve because that is its function; the defeat valve prevents pressure from the intermediate chamber from reaching the dry system waterflow switch (pressure switch).

There are two reasons that this valve is installed.

One reason is that the fire alarm system cannot be reset as long as there's water flowing in the sprinkler system. In many places, the fire alarm system cannot silenced until the water stops flowing. If the waterflow switch is a paddle type (like the main waterflow switch), when water stops flowing, the fire alarm system can be silenced and reset. If the waterflow switch is the pressure type (like in a dry system), whenever the dry system accidently floods the pipes, the fire alarm system can't be silenced or reset until the dry sprinkler system is drained. Sometimes, with false alarms, this can make for a lot of noise in the building. for a long time.

The other reason for the alarm defeat valve is to allow the sprinkler company to service and test the dry system valve assembly, without evacuating the building.

Tamper Switch for the Alarm Defeat Valve

Yes, common sense says there should be a tamper switch on the alarm defeat valve; yes, I believe that somewhere the NFPA code requires it; no, most installations do not have tamper on the alarm defeat valve. Out of the hundreds of sprinkler systems I have seen, I have only seen one of these valves properly tampered.

With the tamper switch installed, after testing, the fire alarm system will show that the valve need to be opened again; without the tamper switch installed, the memory of the sprinkler system servicer is required to make sure the valve is opened again after servicing. A tamper switch is more reliable than any human memory.

Low Air Switch

This is a pressure switch that monitors the pressure of the dry part of the dry sprinkler system. As long as there is sufficient pressure in the dry sprinkler system, the switch is turned off. When the pressure drops below the level set inside the low air switch, the switch is turned on. It doesn't matter whether it's air pressure or water pressure, the pressure switch turns on or off according to the pressure.

Inside the switch, if the setting for the pressure is too low, the low air switch won't get a chance to turn on. That's because before the pressure has dropped enough for the pressure switch to sense the low pressure, the clapper valve opens and floods the dry system with city water pressure.

If the low air switch is set correctly, but the accelerator accidently trips the clapper valve open before the switch can detect low pressure, the low-air switch won't turn on.

Water Floods the System, but No Alarm

There are three possibilities where the dry sprinkler system can be flooded but not be detected by the fire alarm system.
  1. The "waterflow switch" or pressure switch on the dry system could fail
  2. The alarm defeat valve could be closed (your system might not have the valve tampered)
  3. The fire alarm system might not be programmed correctly


Another question that needs to be asked is "Did the fire alarm system show that there was a low-air condition before the dry system was flooded?"

There are three possibilities where the air pressure in the dry system didn't activate the low air supervisory alarm in the fire alarm system.
    The low-air switch was not adjusted correctly
  1. The air pressure didn't detect any low pressure before the dry system flooded. (This is usually caused by an oversensitive accelerator, if your system has one)
  2. The fire alarm system wasn't programmed correctly


Douglas Krantz
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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