Nitrogen in dry pipe systems 1

[FFP1]

I have a facility where two of the dry pipe systems feed freezers (28F and -10F). Both systems feed cooler areas (40F) with a portion of the system also in the fairly large freezer areas. This facility has a history of ice plugs.......no surprise to anyone in the industry. I am fully aware of the requirements for refrigerated area systems and I plan to suggest this approach to my client; however, I am considering the possibility of using nitrogen in place of the current compressed air supplies.

I plan to suggest Reliable nitrogen regulators and continue performing the annual trip testing activities and the ice plug investigations where the sprinkelr piping initially enters the freezer area. I have essentially no experience with nitrogen filled sprinkler systems. I cannot find any real good information in NFPA or via the internet. Is this a good approach? Are there any special procedures associated with the conversion from compressed air to nitrogen? How about the annual trip tests; are there any significant differences? Is nitrogen cost prohibitive in the long haul (compared to the refrigerated area systems)? I am hoping for some guidance from someone who has experience with nitrogen in dry pipe systems.

Thanks

 

[cdafd]

Looks like your major problem is cost.
how many square feet??

I beleive have seen one system use nitrogen with no problem.

Look at 7.2.6.8, 7.8.2.4, 7.8.2.7 nfpa 13 2002

They use it in fire extinguishers if that counts.

 

[JoeWill123]

Define "Ice plug" please....

Joe G. Willett
Jacksonville, AR

 

[LCREP]

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.
March 2001:

http://www.risklogic.com/mar2001.html

 

[fireguy519345]

I have also seen an isolation valve between the cold storage and the dry pipe valve. The isolation valve is used to insure no water gets into the cold storage area during testing.

A pressure washer using hot water and small diameter hoses w/sewer jetting nozzles can be used to melt the ice blocks.
fireguy

 

[FFP1]

I agree with most of what LCREP has stated in his post (VERY good post by the way); however, the post by Fireguy519345 causes some concern......the last thing we want to do is introduce moisture into the system. I realize isolation valves are important when conducting functional trip testing, but I disagree with using hot water to met the ice plugs because the moisture left in the system will evenutually collect at the coldest point and condense forming ice. Once an ice plug is identified, the system needs to be removed, dried and reinstalled or the freezer area needs to be brought above freezing and the systems should be fully and completely drained to remove all of the moisture.

I was looking for information specifically about nitrogen in dry pipe systems. I am seriously considering conversion from compressed air to nitrogen and I was hoping for some solid pro or con advice. Based on my research and the fact we are dealing with an existing system, the nitrogen approach seems easier and more cost effective than a refrigerated area sprinkler system.

 

[LCREP]

FFP1,

The down side is the building owner has to maintain the nitrogen supply. Not a problem, if you have a good client. I would make sure the tank has a low pressure sensor and I would have double the supply required so 1 tank could be filled and the other in service. Other then the expense I do not see any down side, only good things, like preventing the ice plug!

 

[FFP1]

Thanks LCREP!

Your post confirms what I was already thinking, but as I stated in my original post I do not have much experience with nitrogen supplies. I wanted to make sure there were not serious cons with this approach.

I sincerely appreciate your time and knowledge.

 

[fireguy519345]

I should have been more specific about the hot water introduced into the piping system. NFPA 13 tells us that the piping needs to be level or slightly pitched to drain. The choice to use hot water to thaw ice, would depend on the pitch of the piping. Grooved fittings would allow the pipe to be removed, thawed and reinstalled.

I have seen one dry pipe systems with an air dryer. The air dryer removes the moisture before the air is introduced into the system. The dryer did not produce enough dry air for the initial charge, but did produce enough air for maintenance. The initial charge was from Nitrogen tanks. Nitrogen tanks were also used as a back-up should the dryer fail. A low pressure switch was used to moniter the air pressure, send an a trouble alarm and to open a solenoid to the the nitrogen tanks.

Fireguy