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March 2001: Ice Plugs

What Is An Ice Plug? When, Where, And Why Do They Form?

Ice plugs can be a grave problem since they have the ability to seriously impair sprinkler system piping. Contrary to popular belief, ice plugs can strike at any time of year, not just during the winter months. They are most often found in sprinkler systems entering refrigerated storage.

In most cases, ice plugs form in dry-pipe sprinkler systems. Moisture in the air that is traveling through the system condenses and collects. This condensation can freeze and disable a system. This problem usually occurs in refrigerated storage sprinkler systems This process can happen very quickly and is not often detectable from casual inspection. If a fire occurs, ice plugs can impede or completely block waterflow to the sprinkler heads.

Most ice plugs are actually found in freezer systems. The air supply is cooled down as it travels from the heated area where the refrigerated systems are located, to the extremely cold environment inside the freezer. In addition, ice plugs are more likely to form in systems that are not air tight. The more air in the system, the higher the likelihood of moisture which can cause the ice plugs. In-rack sprinkler systems are especially susceptible to leakage because they are constantly being moved around. Ice plugs also have a greater chance of forming if a system has been flooded with water, such as during a test or a false trip.

Detection and Removal

The most common way to check for ice plugs is to dismantle the piping and visually inspect for ice formation. Obviously this is a very time consuming and tedious task. It is not feasible to dismantle an entire system. Most companies only dismantle sections of the system where they think an ice plug may have formed.

Visual inspection is not a fool-proof method. There has been research done in the use of ultrasound technology. Ultrasound technology can be used to inspect the piping without having to dismantle it. It uses high frequency sound pulses to detect solid ice or liquid. The equipment works on a wide array of pipe sizes, wall thicknesses and materials.

Once an ice plug is located, it has to be removed. There are several ways to do this. The proper procedure is to dismantle the piping and bring it to a warm area to thaw out. Ice plugs can be broken up with a hammer if they are small.

Some companies successfully remove ice plugs without dismantling the piping using steam or hot water to melt the ice. One must ensure all ice is removed and no blockages or clogged branches remain.

f a system is threaded, another possible method is to use a manual pipe cutter to remove the existing pipe that is blocked with ice and replace it with rolled grooved pipe.

Be sure not to use a torch to thaw ice plugs. This poses potential for a loss by introducing an ignition source to system that is out of service.

There are a few things to keep in mind when installing a sprinkler system which will make it easier in the long run to detect and remove ice plugs. Firstly, try to use threaded systems wherever possible. This will facilitate the inspection process since disassembly is easy.

Secondly, try to install your system with inspection portholes. This can make the inspection process much easier and less time consuming.

Thirdly, systems should be small. The smaller the system, the smaller volumes of air that are introduced into the system lessening the likelihood of ice plug. Wherever possible, try to install two smaller systems instead of one big one.

Prevention

There are two ways to prevent ice plugs:

Dehumidify compressed air supplied to the sprinkler system. There are approved pre-engineered systems, which supply dehumidified air to sprinkler systems in refrigerated areas.

Use a regenerative air dryer to dehumidify the compressed air supplied to the sprinkler system. The pressure dew point should be 20° F. lower than the freezer temperature. The compressed air in the freezer sprinkler system will then have a 30% relative humidity. In addition to providing this safety factor, any ice formations that already exist in the pipes will dry out slowly.

Note on Installation

When the air supply is connected above the system deluge valve, a check valve should be installed on the air supply line, at the point of connection to the riser, to prevent water from entering the air lines during system testing or operation. To prevent moisture above the control valves to the sprinkler system from entering the freezer, install an additional check valve (with a bleed hole) just downstream of the trip test cutoff valve. The additional valve may help block moisture from the priming water pool located on top of the refrigerated area system or from water collected above the refrigerated area valve assembly.