Ammonia Storage Tk - Conservation Vents 1

[cannedheat]

We are converting a 20,000 gal horizontal tank to storage of 19% aqueous ammonia. The tank is rated for 15 psig. The question came up as to the pressure setting on the conservation vent (We've decide on the vacuum setting at 2 oz.) The VP at 70F is 4.6 psia. Would we be better off setting the vent at a low pressure or higher pressure? We will be venting to a scrubber.

 

[Montemayor]

Canned:

You are allowed to set the pressure relief at the MAWP (Maximum Allowable Working Pressure) - which is not the "rated" pressure. If you have the original fabrication drawings and calculations for the tank you might have that information - or it might be on the name plate of the tank.

I would set the pressuer relief to vent at the highest, safest pressure - which is the MAWP. You should guide your decisions on the tank by refering to API 2000.

 

[MJCronin]

canned:

I guess I disagee with Mr. Montemayor or perhaps I do not understand some of the terms used. (This is a rare event)

If your "tank" is a certified ASME Section VIII Pressure Vessel (ie has an ASME name plate with registration number), then the design pressure is the MAWP (maximum allowable working pressure) and ASME states that a relief device is required, of a certain capacity, and must be set no higher than the MAWP.

I do not understand the above usage of the term "rated".

In the course that I teach, I find it much clearer to always use the term MAWP (as defined by ASME)

API 2000 will provide guidance on flow rates and relief valve settings based on the specific process conditions. But this is different than the requirement for tank integrity and burst protection, which is what ASME addresses.

I agree with the statement about setting the relief device as high as possible....mindful of other considerations.

-MJC