Why So Much Sludge in Dry Sprinkler System?

1/21/2021

I have a project with a typical compressed air dry sprinkler system in an unconditioned attic space (site is at 44 deg latitude). The system is 22 years old.

Site maintenance person reports that they've had issues with this system for at least 15 years. Frequent and repeated leaks, predominately pin holes in the mains. They try to repair before the system gets wet, although not always in time.

They recently replaced a 3 inch main and found that it was about 30% filled with sludge. Sludge was dark brown, almost clay like, sludge.

Any thoughts on how such a large volume of sludge can accumulate in a dry system?

Thanks in advance.

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5 Comments

Dan Wilder

1/21/2021 09:21:08 am

This is gong down the road of corrosion and/or MIC.

Essentially, the air compressor is taking ambient air (and humidity) and pumping it into the system. This air then condenses via temperature gradients across different areas of the system combined with the the initial air introduction (typically warm air due to the action of the air compressor being warm from normal usage).

It sounds like there have been system trips in the past along with (hopefully) inspections per NFPA 25. The combination of moist interior pipe conditions and the introduction(s) and subsequent inability to drain 100% of the water creates these colonies of aerobic/anaerobic bacteria throughout the piping system. Even with the water being introduced quickly, there is no "flushing" type water velocities through most of the system (you may find clean pipe close to the riser, but as the distance increases, I would expect more obstructions) and this is the result - obstructed pipe.

I would recommend a very in depth 5 year inspection, then flush of the entire system, sample drop inspections to verify if there are the same conditions in the smaller drops to start. Verifying correct sloping of the pipe and additional aux drain locations as needed would also be recommended.

Afterwards, integration of N2, Dry Air, or other mitigation and prevention applications to prevent further issues needs to be implemented.

Personally, my company has performed several N2 installations for both wet and dry systems with short term success (up to 5 years worth of mitigation). I say short term because we are still monitoring the sites. The number of leaks and false trips has been greatly reduced (and nearly eliminated the further we get from the install date) and we are now dealing with fixing the already caused damage so it's not just a install and everything is going to be fine fix. Our typical approach is putting out the process, timeline, and costing. There is a large up front investment and not everyone is on board, especially tenants in rentals that will be out of the building in 1-5 years but are tasked with building maintenance & repair.

Potter, ECS, Southtek, General Air, Tyco all have methods available.

Jesse

1/21/2021 09:30:32 am

Its probably all related.Typically, the overall cause is the combination of water, metal, and the oxygen in the air, The overall corrosion may be MIC, galvanic, or electrochemical.

The buildup of corrosion of the inner pipe wall is what's causing the sludge, and also causing the pin hole leaks. A dry systems is never really dry. There will be some residual water in there from the hydro testing, flow testing, trip test, etc. And this water very commonly has micro-organisms in it. These organisms can be acid producing bacteria, sulfer reducing bacteria, or iron related bacteria. They will wreak havoc on internal piping. The presence of these bacterium coupled with the oxygen in the air will keep eating the inner wall of the pipe.

Inevitably, the pipe will need to be replaced. And I'd consider introduction of high purity nitrogen into the system. It won't reverse the corrosion but will inhibit it further.

Franck

1/21/2021 10:14:44 am

You can also provide auxiliary drains to try to remove as much as possible water accumulation in the piping (they normally accumulate at the lower parts).

This will not fully solve the corrosion issue but will enable to gain some time.

Auxiliary drains (also called Low-Point Drains or Drum Drips) are required for each section of trapped pipe in a Dry or Pre-Action Fire Protection System.

The purpose of the auxiliary drain is to collect the moisture that condensates from the pressurized air in the system and minimize loss of air pressure during the maintenance of the systems.

Auxiliary drains located in areas subject to freezing should be accessible (approx. 2.1 m (7 ft) above the floor, to which a hose can be connected to discharge the water in an acceptable manner).

When the quantity of trapped water is less than 20 l (5 gal.), the auxiliary drain should consist of a valve not smaller than 15 mm (1/2 in.) and a plug, or a nipple and cap.

When the quantity of trapped water is more than 20 l (5 gal.), the auxiliary drain should consist of 2 valves of 25 mm (1 in.) and one 50 mm x 300 mm (2 in. x 12 in.) condensate nipple or equivalent and accessibly located.

To avoid water being trapped at the location of the test valve once the system is back into operation after a full trip test (water moisture condensing on the pipe and being chased to the far end of the system), 2 valves can be provided (drum drip leg arrangement). Such arrangement can be found in NFPA 13.

I have seen systems, when performing the full trip from the ITC, where air was "chased" through the "sprinkler opening" and you could hear the water pushing the air... until the orifice was completely blocked by sludge (as well as the last portion of the pipe). No water going out at all !
In case of fire, this means that some of the sprinklers will not operate and this is a critical situation.

Jim

1/21/2021 11:49:08 am

I am in agreeance with comments thus far. I've also run across a few instances where the installing contractor did not pitch/slope the pipe in accordance with NFPA 13 causing trapped water in many areas throughout the system. Overtime these typically will be the first areas to fail.

Phil W

2/4/2021 11:35:54 am

In the UK we are mandated to use hot dipped galvanised pipe (not sure of NFPA requirements) but not everybody is convinced of the absolute effectiveness of reducing pin holes in future; I'd say if is certainly better than an plain internal pipe finish. Not certain of how this might inhibit organic matter developing; N2 is not a normal method in the UK (nor mandated) and so we would always use a compressor with a good quality de-humidifier and be sure to blow out any residual water over a period of weeks (I think you will find this process is described in some of the DPV manufacturers literature. For best results I would say N2 and hot dipped galvanised pipe is about the best you can do, coupled with regular internal inspections at random locations and especially at terminal ranges,

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