Corrosion in Sprinkler System

[nodnerb]

Hi,
I am on a ship at the moment where it has just been discovered that the Fire Fighting Sprinkler System has serious corrosion problems.

To better understand it I will give a description of how it is supposed to work and then ask my questions.

The System was custom built by the Shipyard as the vessel was constructed. It contains 3 x 500 Lt Pressure Vessels that are first charged with 5 bar ( 75 PSI) Air and then Fresh Drinking Water is added via a dedicated pump that feeds off the domestic ring main to a press of 10 Bar ( 150 PSI).

The whole lot is retained by an Electro Hydraulic Valve that can either be opened by remote control from two places onboard or it is opened when there is a Fire Alarm in any of the Sprinkler protected areas onboard. (The section between Valve and the Sprinklers themselves is usually kept under 3 Bar (45 PSI) Air Pressure monitored by Press Switch integrated into the Main Alarm System.) The Branch Lines are then charged if there is actually a fire the Bulb(s) will burst in the area and water will spray out when the pressure drops to 4.5 Bar (67 PSI) in the Pressure Vessels a 50,000 Lt/Hr Pump starts to fill the Branch Lines with Sea Water.

If this pump fails another Valve can be opened by remote control and any one of the Fire Pumps onboard can be brought into service.

There is a non return valve to prevent any back pressurising of the Tanks by either the Sprinkler Pump or the Fire Pumps.

An accidental discharge yesterday of the Tanks whilst part of one of the branch lines was open for maintenance caused a stinky black mud and grit type mix to hit the deckhead and leave a deposit of greasy black chunks some 20mm square and 5mm think ( More than big enough to block a sprinkler if it were being used for real). After the mess was cleaned up a closer inspection revealed that all 3 tanks are suffering inside from what looks like the worst case of Acne ever seen. I had to remove some pipes and peer in with a flashlight so that is about the best way I can describe it.

Today we have filled and discharged the tanks 4 times each time there is less and less dirty crap coming out.

As far as I can see the Water was not drained out of the tanks for 5 yrs at least. Once when the boat was brand new the system was charged with Fresh Water by a bloke welding setting off the Fire Alarm. To the best of my knowledge the Pump has only been run a few time through a dedicated overboard discharge line in the system for this very purpose.


Here are my questions:

1, What caused the Tanks and anywhere the Fresh Water was sitting to fester so badly ?

2,If they are cleaned out/ replaced is there a corrosion inhibitor that could be added to the water and would not have any nasty reaction if it were sprayed onto a fire?

3, Any suggestions on how to remove the scale from the system without removing the tanks and tumbling( Access is only through fittings holes)?

Ok That's about all for the mo

Thanks in advance

Cheers

Nodnerb

 

[Metalguy]

Sounds like you have a MIC (microbiologically influenced/induced corrosion) problem. The bacteria are probably SRBs (sulfate reducing bacteria). They live in anerobic (low/no oxygen) conditions and cause extremely corrosive conditions under the masses they create. Carbon steel and many stainlesses cannot withstand the environment they create.

Best thing to do is somehow remove all the crud inside the piping/tanks, because SRBs can survive under nearly any kind of deposit-even the deposits created by other, aerobic bacteria. IF you can heat the piping/tanks to ~212 F it will probably kill them. In the future you must always refill with water either heated or chemically treated to kill them. It will also help if you can raise the pH to ~10.5 or so with NaOH or something.

 

[aspearin1]

What affect would a 10.5 pH shower from a sprinkler head have on a worker caught near a fire? Is that pH high enough to add chemical burn to the hazard? I would think so. As far as everything else goes, I'll have to agree with Metalguy. If it is bacteria, it could have been feeding on any grease or rubber components for the past 5 years. If it was bacteria, you'd know for sure from the smell of the water. Did the seals disintegrate as well? I'd suggest NOT letting the water sit for 5 years anymore. Make it part of the ship's regular maintenance to replace the water and purge the system each time. If kept to frequent enough change-outs, city water would most certainly suffice. There are plenty of anti-microbial additives already in there to make it last. Microbial growth isn't necessarily bad, either. Most, if not all sprinkler systems have plenty of growth too. It's stagnant water. Things will grow. It's not unique. (Just don't drink it) However, your system has been neglected. This is probably the source of the problem.

aspearin1