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Welding, Explosions and Ammonia

01/29/2010 6:00 AM

The ammonia can cause explosions during weld repairs (New 1" nozzle) of the tank that contains it ?
I understand that it is not inflammable

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#1

Re: Welding, Explosions and Ammonia

01/29/2010 10:45 PM

This is a complicated topic.

Ammonia does have a flammable range between about 16 and 25 percent in air. This concentration is not very readily achieved, but it can happen with a large leak into a small space, for instance. Then, if an ignition source is present, such as welding equipment or arcing electrical contacts, a combustion-type explosion could occur.

In the case of ammonia, I understand that this is called a "deflagration," in which the flame front moves slower then the speed of sound. I suppose this would make it sound like a big whump or whoosh, but not a sharp crack or blast. [Not sure of this.]

If a pressure vessel ruptures, say by impact rather than fire, the resulting explosion is termed a "BLEVE" (boiling liquid expanding vapor explosion).

In refrigeration systems, some amount of oil typically circulates with the ammonia therein. If a leak occurs, the oil can be blown into a mist, which may be ignitable even if the ammonia concentration itself is less than the lower flammable limit.

These varying scenarios can lead to confusion about what type of event has happened.

Ammonia can also be flared in a controlled way by passing the vapor through a fuel-supplied flame. The heat dissociates the nitrogen from the hydrogen. The hydrogen adds to the flame, and I think the nitrogen just goes to atmosphere, but I'm not sure about this or about secondary reactions. This technique can be used for neutralizing the discharge from a relief valve system, for instance.

If cutting into or welding on a pipe that has contained ammonia, it is possible to ignite an oil mist/ammonia mixture from the residual vapor in the line. This tends to be exciting or hair-singeing, but usually quickly over. However, it may be enough to ignite any nearby flammables.

As may be seen, there are many variables here.

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#2
In reply to #1

Re: Welding, Explosions and Ammonia

01/30/2010 1:23 AM

Where do you want to weld, in gas region or liquid region? Has the tank contains only gas or liquid too?

'New 1" nozzle' - welding, fine, how about hole to have this nozzle.

It is too dangerous affair, should not attempt with out taking necessary precautions.

How about 'Hot Tapping', a process in which you weld and then drill in controlled envelope. Wish you could locate a suitable third party contractor near-by to do this.

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#3

Re: Welding, Explosions and Ammonia

01/30/2010 5:22 PM

Not sure of this, but I believe if ammonia soaks into wood or other organic material at the site, bacterial decay of the wood can produce nitrates which are oxidizers. If you weld/burn near these, can ignite the organic material impregnated with nitrites.

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#4

Re: Welding, Explosions and Ammonia

01/30/2010 5:33 PM

Don't assume that it's only the ammonia you must be careful about. A cloud of ignitable dust injured welders many years ago where I worked. There was no flammable material near the welding or on the ground. Nevertheless, the dust (in this case starch) which was blown through by wind from a nearby starch processing area flashed enough to cause serious burns, even though there was no explosion. Many serious explosions are caused when the initial flash kicks up ignitable dust like sawdust, starch, flour, sugar, etc, and causes a small secondary explosion and cloud which ignites and kicks up more dust etc, etc getting bigger and bigger. That's the sort of thing that caused a sugar-based explosion in a manufacturing plant near here a few years ago.

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#5

Re: Welding, Explosions and Ammonia

01/30/2010 10:47 PM

I used to work in a plant where open flame and ammonia vapors were present.We had an automatic shutdown that triggered at 25% ammonia vapor, to prevent explosions.

Household ammonia is only 5%. 25% is fatal. Pure anhydrous NH3 causes caustic burns as well as frost bite and irritation.It seeks out moisture, such as eyes,armpits,etc.and anything it contacts begins to cause immediate pain.It begins as a menthol feeling, then progesses to burning sensation.I would never weld on or around it.

Consult a chemical engineer first.

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#7
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Re: Welding, Explosions and Ammonia

01/31/2010 12:33 AM

Then thrill--or shudder--to this "sea story"!

I have heard of a two-stage ammonia refrigeration intercooler (vessel) in which the anti-siphon hole in the inlet dip pipe had been omitted. This back-flowed liquid ammonia onto the heads of the low-stage compressor.

To fix this would normally require evacuating all the liquid ammonia out of the intercooler, a time consuming process. Moreover, in the wilds of the Alaska fish industry, where are you gonna get an R-stamp shop to do this--when you have 100,000 lb of salmon to freeze by tomorrow?

Well... by reducing the high-stage suction pressure from its normal 25 psig to zero, and watching really closely, they stabilized the intercooler at atmospheric. Then they cut a hole in the head, reached in with a drill to produce the anti-siphon hole in the internal pipe, beveled the cutout piece, and finally welded it back in. Wearing gas masks, of course.

The way this worked was that inside the vessel the ammonia concentration was always almost 100%, way above the flammable limit (and almost no oxygen there, anyway).

I suppose this might be called a "cold tap" operation, considering that the ammonia was at -28°F. As the saying goes, "kids, do not try this at home." But sometimes ya do what ya hafta do, whether kosher or not.

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#6

Re: Welding, Explosions and Ammonia

01/30/2010 11:07 PM

Your statement (not inflammable = flammable) is a little confusing... did you mean it was not flammable? Combustion of ammonia will occur if the mixture in air (oxygen) is correct (approx. 20%). In fact it will run a combustion engine...producing about 10000 BTU/pound. 4NH3 +3O2 -> 2N2 + 6H2O producing a pail yellow flame (electromagnetic wave/spectrum).

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#8
In reply to #6

Re: Welding, Explosions and Ammonia

01/31/2010 12:40 AM

Thanks for the clarification. It appears that the reaction you describe could be exactly that of the flaring process mentioned earlier. Not too great as an energy provider, though; 10,000 Btu/lb is rather modest.

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#10
In reply to #8

Re: Welding, Explosions and Ammonia

02/01/2010 2:13 PM

I believe the combustion of NH3 will create approximately half the energy that diesel fuel would produced (given similar conditions)...so it is modest.

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#9
In reply to #6

Re: Welding, Explosions and Ammonia

02/01/2010 9:53 AM

I would expect some nitric or nitrous oxides to be produced, also.

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#11
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Re: Welding, Explosions and Ammonia

02/01/2010 2:41 PM

You are essentially correct, but nitric oxide (NO) has a very short half life (0-2 seconds) and would typically (given these conditions) be considered an intermediate reaction. (Your internet name indicates you might be interested in cardiovascular disease...checkout nitric oxide and Dr. Ignarro). NO's are acid anhydrides so in the presence of liquid water will convert to nitric acid.

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