API 650- Tank Overturning Stability 2

[BitaSh]

Please advise if my interpretation as below of the criteria#1 of 5.11.2.1 of API-650 is appropriate.

This criteria appears to be applicable only when the tank is empty of its contents. A tank empty condition could exist before commissioning when the internal pressure would be zero. In our case, we have an operating condition (when the tank liquid level is anywhere between the high and the low levels) and a shutdown condition ( when the tank content is at the low liquid level). These conditions (operating and shutdown) could not be considered while using the above criteria, If we are to use the above criteria for operating or shutdown condition, on the right hand side of the equation, a resisting moment from the tank content as well as tank base plate needs to be applied. The criteria does not have this provision and hence is not considered applicable in our case.


Thanks for your time.

Sincerely,
BitaSh

 

[IFRs]

If you are going to design per API 650, you have to satisfy all overturning criteria despite your contention that one or more are not applicable. 5.11.2 says "Unanchored tanks shall satisfy all of the following uplift criteria". Surely the tank will be empty when first built, when being emptied for inspections or repairs or for emergency or non-emergency situations you can not foresee now. On the other hand, if this is the controlling case and it is very expensive to anchor the tank you can take it to your risk management department and see what they say. The other criteria include the tank contents so I would not add it to the right side of this equation. My opinion only...looking forward to seeing other posts.

 

[fegenbush]

You are, of course, free to design the tank however you wish. API 650 is a standard, and does not carry the weight of law in most if not all jurisdictions. That being said, the committee has spent much time on the subject of overturning and anchorage of late, and the approach given in the standard is considered to be both conservative and economical.

As IFRs mentions, there are numerous situations which cannot be foreseen where designing, fabricating, and erecting the tank to the letter of the standard would be beneficial: Example, the facility where the tank is erected is later sold to a company that is safety conscious. While there is only one situation where not doing so is beneficial: It's a little more difficult and costs more right now.

If we don't have time (or money) to do the job correctly, when are we going to find the resources to correct the defects?

 

[BitaSh]

Thanks for your responses.
IFRs,
I agree with you. My point is if the first criteria is for empty condition, can I put atmospheric pressure for Mpi,as for our case, there is no internal pressure in empty condition other than atmosphere ?
Based on the API safety factor table, factor of 1.5 is for empty and 2.00 for half full condition. So the first criteria is for empty and the second is for operating (half full) condition.

 

[JStephen]

I think there are two different issues here.
First, does a tank that does not meet the uplift criteria when empty comply with API-650? And the answer is "no".
Secondly, can you safely build and operate a tank that doesn't comply with that criteria? The answer is, "Quite likely". The problems come up when you start putting name plates and writing certifications that the tank meets the standard when in fact it doesn't.

 

[IFRs]

Regarding the internal pressure, the code requires that use the tank's design internal pressure. Section 5.2.1.c defines this as having no minimum but a maximum of 18 kPa. Use the design pressure that will be inscribed on your tank's name plate.