API Guidelines for Fire Protection 2

[GHartmann]

I have a pressurized liquid ammonia storage tank (12' ID x 124' long = 108,000 gallons) that requires pressure protection for:

Heat input from steam coil
Blocked flow

The design pressure of the vessel is 275 psig.

What I am wondering is since ammonia is not easily flammable (although the vapors could be explosive) is the case for fire really relavant? I know it would be on the safe side, but the capacity required is around 85,000 lb/h which must go to a flare system.

The 85,000 sets the sizing for the flare.

Its not like I have a tank full of flammable liquid like kerosene, etc.

 

[TD2K]

The content in the vessel isn't material.

If your pressure vessel can be overpressured, you need to protect it against that scenario (you do not want that vessel to fail from overpressure).

275 psig equals a bulk temperature of 120F for ammonia so if your steam coil can heat it over that temperature, it's a valid scenario in my opinion. You'll also have heat losses from the shell to offset the heat input from the coil but you'll have to consider the worse case for heat losses, maximum ambient temperature and still air, have those been considered in determining the 85,000 lb/hr relief flow?

Is the steam coil required to ensure sufficient ammonia vapor is available for the process?

Is fire exposure not a viable case?

 

[Latexman]

Does OSHA 1910.111 apply to this vessel? If so, it specifies the minimum relief rate. If other credible scenarios exceed this, the greater rate prevails.

Is the vessel in a dike? Anything else in the same dike? Is it flammable or combustible? Three yes answers and fire exposure is credible.

Is the vessel close to a fire area? It may see indirect fire exposure.

Ammonia is weird. The U.S. DOT says it is non-flammable, but it has a LFL and UFL! You may want to check with your insurance carrier.

Good luck,
Latexman

 

[don1980]

Check OSHA 1910.111 (Ammonia storage and handling) to see if contains a fire relief requirement. I don't think it does, but I didn't check the reg. Ammonia is one of the few chemicals I've seen which has conflicting information with respect to flammability. Ammonia/air (O2) mixtures can explode, but most references say it won't start a fire unless it's mixed with hydrocarbons. So, by itself I don't think a pool of liquid ammonia is pool fire threat.

So, unless it's mandated in OSHA 1910.11, the fire decision is one that the owner is empowered to make.

 

[GHartmann]

Thanks folks.

Clarifications:

Yes we are protecting the tank against overpressure (blocked flow, heating coil left on)

Excluding fire, the worst case is with the heating coil left on which is only 15000 - 16000 lb/h

The tanks are not in a diked area (most of the liquid will evporate, making an extreme safety hazard for ammonia release). But the ammonia vapor itself is not readily ignitable.

OSHA 1910.111 does seem to prevail and the sizing is essentially based on the API wetted area calculation for fire.

And no OSHA 1910.111 doesn't say anything about fire. But it's implicit in the method they use to arrive at the required. relief capacity

Thanks for pointing me in the right direction.

 

[GHartmann]

One more thing. The vessel is in an ammonia plant and OSHA 1910.111 doesn't apply to ammonia production plants (funny rule ehhh???). But we were suggested to follow the rule any way.

 

[Latexman]

GHartmann,

No, not so funny. The *main* intent of OSHA 1910.111 was to make it difficult for illegal drug dealers/makers to steal ammonia from farmers, co-ops, low tech distribution facilities, etc. As such, it also addresses security, pin-hole orifices under pressure guages and transmitters, detectors, alarm systems, etc. OSHA figured an ammonia producer would have good security anyway, thus the exemption. The ammonia producer lobbyists saw to that. I installed an ammonia tank in the largest chemical plant in North America in 2007/8. We had security out the wazoo because we made several versions of methyl ethyl death as intermediates. Unfortunately, we did not produce ammonia, so I had to tick off every line item in 1910.111. Oh well, it made a good checklist!

Good luck,
Latexman

 

[rutherford703]

Based on API PR 505 on page 12 Note 2 " Experience has shown that ammonia is hard to ignite, and a gas release will dissipate in the open air, so any explosive gas atmosphere will be of negligible extent."