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Mechanical Integrity
2

Mechanical Integrity

Mechanical Integrity

(OP)
Hi board,

I am a safety engineer working at a Chemical facility covered by both PSM and RMP.  We have recently found a tank that many consider to have failed it's MI inspection.  Inspectors are API certified inspectors and do their work per API standards.  I am not directly responsible for our MIP, but am responsible for the overall PSM/RMP program.  My questions are:
1) is API 653 the only standard to be reading to learn more about API MI inspections?
2) what section of the ASME code should I be reading?
3) are there any guidelines/rules-of-thumb that I should know about when reviewing the MI inspection reports?
4) isn't there a corrosion allowance allowed in most MIP standards?
4b) if this allowance is exceeded, can the tank remain in service in the same process?
4c) can the tank service be changed to allow for it to be used in a different acceptable service?

thanks for any help!

Bryan
www.SAFTENG.net

RE: Mechanical Integrity

2
1) API 653 is the correct inspection code to follow for atmospheric storage tanks originally constructed to API 650 standards.  Local jurisdictions may have requirements that overide API-653, but in general API 653 is the code to follow.

2) API 510 and NBIC (National Board Inspection Code) cover the inspection of pressure vessels, but they don't relate directly to tanks.

3 & 4)  As a starting point you can use the criteria in API-653 to determine if the tank is fit for service.  If it does not pass this criteria you have a couple of options:
   a)  Make repairs/modifications to get the tank to meet the requirements.
   b)  Use API-579 (Fitness for Service) to perform a more rigirous assesment of the specific corrosion and/or flaws.
   c)  Re-define the operating parameters so that the tank does not exceed safe design limits (i.e. lower the maximum product fill level).

Good Luck!

RE: Mechanical Integrity

Bryan,

Meca has given some good advice. here is some more:

First of all ASME code does not apply to atmospheric (API) tanks other then the portions of the welding procedures that API-650 references. Also, ASME and API-650 are for design and construction of tanks, although, API-650 calculations can be performed on existing tanks with their current shell and roof thicknesses accounted for to "re-rate" the tank.

Tanks may or may not have a corrosion allowance built into the design. The best way to determine if the tank is still fit for service is to calculate the maximum liquid level the tank can withstand (based on API-650 or API-620 and the shell and roof thickness data obtained by your inspectors).

 One important factor is the MAWP (maximum allowable working pressure) of the tank in the vapor space. Most API tanks have a frangible roof/shell joint that will tear at only a couple inches of water column pressure (as low as 0.07 psi). It is very important to calculate this failure point correctly because if you don't and you do not have adequate venting capacity....the tank  WILL fail at that joint and the roof will peel back like a tin can....I have seen it happen.

If the corrosion allowance is exceeded the tank may be able to stay in service depending on the current use (i.e. specific gravity of contents, pressure relief vents, etc.)

Tank could also be used for a different service as long as an analysis, based on the inspector data, is conducted and shows that the integrity of the tank will hold.

I suggest that you consider hiring an expert (consultant) in the field to review the data and possible perform an analysis on the tank.

Hope this helps,
Mike
mgortel@aol.com

RE: Mechanical Integrity

Section 2.3.3.5 of API 653 does allow for minimum required wall thickness calculation per Appendix 4 of ASME Section VIII, Div. 2.  If your nameplate follows API 650, you can obtain most of the information that you will need to do your calculations here.  Use the thicknesses obtained from the UT readings.

Since this is an existing tank and you will not try to increase its design pressure, I don’t think you have to worry about the roof’s frangibility.  I don’t think that any other codes/standards other than ASME recognize the term MAWP.

I agree with Mike that a consultant should be retained to provide you with recommendations for resolving this issue.

K. Phan

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