Fire Alarm, Empty Pressurized Line

[jmellc]

A couple of us were discussing a sprinkler line recently that had sprung a leak. Part of the discussion was about it passing through a breezeway from one building to another, little or no climate control.

I've not done much fire alarm and even less in recent times. But I recall something from class about a method where lines unprotected from weather are left empty but pressurized with air. If a sprinkler activates, it spouts air for a few seconds but a valve opens on supply side and sends water. That way, no frozen lines. What is this method called? I'm sure it had a name or a letter/number code.

 

[stretch12]

A dry system I believe

 

[don_resqcapt19]

Yes that is a dry system and a lot more troublesome than a wet system, especially from a corrosion stand point. You have to fill it with water to test, and then drain it setting up conditions for excessive rust. A lot of the newer systems are using dry nitrogen and not air as the pressure medium to combat the rust.
For a short section in an unheated area, I would be looking at heat tracing long before I would look at a dry system. However, the heat tracing must be listed for freeze protection of fire sprinkler systems.

 

[hillbilly1]

Still see a lot of them freeze, air pressure keeps the check valve closed, but a sustained power outage, or compressor failure can allow water back in if there is a small leak In the system. Big Orange installed nitrogen systems on their’s. The compressor is 240 volt only, so they had to boost the existing 208 circuit up to 240.

 

[don_resqcapt19]

Still see a lot of them freeze, air pressure keeps the check valve closed, but a sustained power outage, or compressor failure can allow water back in if there is a small leak In the system. Big Orange installed nitrogen systems on their’s. The compressor is 240 volt only, so they had to boost the existing 208 circuit up to 240.

Typically, when the dry system operates you get a water flow alarm....when the temperature is below freezing, they should drain the system and go on fire watch until the system can be restored to normal operation.

 

[hillbilly1]

Typically, when the dry system operates you get a water flow alarm....when the temperature is below freezing, they should drain the system and go on fire watch until the system can be restored to normal operation.

Yes, but with a slow leak, water seeps in once the air pressure is too low to keep it out, the flow switch does not activate the alarm, only after the pipe thaws, water starts to flow enough to trigger the flow switch, the check valve is not a monitored point.

 

[Joethemechanic]

We had one in a big old building. For whatever reason some of the pipes were low and collected water. Damn thing would freeze and break. Seemed like it was always 2AM. The damn thing must have leaked quite a bit of air because the compressor seemed to run a lot.

 

[don_resqcapt19]

We had one in a big old building. For whatever reason some of the pipes were low and collected water. Damn thing would freeze and break. Seemed like it was always 2AM. The damn thing must have leaked quite a bit of air because the compressor seemed to run a lot.

It is important to design and install a dry pipe system so there are no low points. Often you will see the taps for the head come off the top of the main pipe and than drop down, that way unless the head is actually open no water will get to the head as it is air locked.

 

[don_resqcapt19]

Yes, but with a slow leak, water seeps in once the air pressure is too low to keep it out, the flow switch does not activate the alarm, only after the pipe thaws, water starts to flow enough to trigger the flow switch, the check valve is not a monitored point.

The only way water gets into a dry pipe system that has been properly designed is when the flapper in the dry valve flips. That happens any time the pressure of the air is too low. When the dry valve opens, the fire alarm system should activate. Often the dry pipe system uses a pressure switch and not a flow switch like a wet system. Because of the difference in area between the wet and dry side of the valve, the pressure after the valve opens will be much higher than the pressure with just the air there. I guess if you the system had used a flow switch, there may not be enough flow to activate the fire alarm.

 

[winnie]

I don't know these systems, but I'd guess that with just the right amount of air leak the condensation from compressing air could leave enough water in the wrong place to freeze.

Probably why it is preferable to pressurize with Nitrogen.

Jon

 

[don_resqcapt19]

I don't know these systems, but I'd guess that with just the right amount of air leak the condensation from compressing air could leave enough water in the wrong place to freeze.

Probably why it is preferable to pressurize with Nitrogen.

Jon

That is probably possible, but given the size of the piping, I would be very surprised if you could have enough condensate to cause an issue if the system has been properly designed and installed.

The nitrogen is really used as a corrosion reduction measure, but it will also eliminate condensation.

The compressors often have an air accumulation tank with auto drain valves as most of the condensation will take place in the accumulation tank.

 

[gadfly56]

The only way water gets into a dry pipe system that has been properly designed is when the flapper in the dry valve flips. That happens any time the pressure of the air is too low. When the dry valve opens, the fire alarm system should activate. Often the dry pipe system uses a pressure switch and not a flow switch like a wet system. Because of the difference in area between the wet and dry side of the valve, the pressure after the valve opens will be much higher than the pressure with just the air there. I guess if you the system had used a flow switch, there may not be enough flow to activate the fire alarm.

Yes, these systems are go/no go. Once the system air is low enough, as you pointed out, the flapper pops and locks in place. You will almost immediately get a flow alarm. Long before this, however, you will get a low air pressure supervisory alarm, which everyone simply ignores.