3 replies to this topic

[AK47]

Hi-

I'm new to the forum and have a twist on a question that seems to be repeated quite often here.  I have an API 650 tank that is rated as "atmospheric" and it is NOT an appendix F tank.  So, per API 650 and 2000, the MAWP should be estimated as the weight of the roof / cross sectional area of the tank (or ~1.5 inWC every time)!  But these tanks also have a noted " air test pressure" ranging anywhere from 0.5 to 3 psig. 

 

I understand all of the previous discussions with sizing relief vents for tanks that don't have a real MAWP.  But, can I use the "test pressure" as the MAWP for vent sizing purposes?  The problem is these tanks have been in operation for a number of years with a Nitrogen blanket of 3 inWC and current pressure vents set anywhere from 28-45 inWC.  So, I don't think the MAWP is really 1.5 inWC!

 

Thanks for your input.

[proinwv]

Several thoughts.

 

In most standards the test pressure is 1.5 x or more of the MAWP.

 

How do you determine your MAWV?

 

If you intend to work around the code, have you discussed this with your insurer?

 

We can not tell you what you can use for a MAWP or MAWV for such a tank.

 

Since these tanks have been in service and are vented, what is your purpose of asking this question?

[fallah]

 

 

In most standards the test pressure is 1.5 x or more of the MAWP.

 

 

Paul,

 

The OP did mention about "air test pressure", not hydrotest pressure, and in most standard the air test pressure for pressure vessels is almost 1.1*MAWP. On the other hand, as far as i know the air test pressure isn't applicable for atmospheric storage tanks under API 650. Am i right? 

[proinwv]

Hi Fallah,

 

That may be so, but I was speaking more in generalities. Sometimes I can be too brief. My point was to impress that most standards require a factor of safety. In this case I do not believe that 10% would be adequate. My point being that the tank is not designed for the pressures he mentions and that we should not be expected to tell him or anyone that his statement "But, can I use the "test pressure" as the MAWP..." is acceptable. Even if this method were to be acceptable, it would certainly require more investigation than a simple pressure test. Beyond that vacuum need also be considered.

 

I must admit that I do not have a copy of API 650 handy, but my memory tells me that these tanks were configured for atmospheric applications and only tested to resist the head of liquid.

 

Possibly I am overly cautious, but I have refused to take venting jobs where the tanks were not qualified by code and design to withstand the necessary pressures.