Tank shell evaluation for change of service

[sajk14]

Hello all,

I have an existing tank originally constructed in 1980 and the client now wishes to store a product with a higher SG value in the tank. All other design parameters such as pressure, temperature, corrosion allowance etc. will remain the same.

The original design SG value is assumed to be 1.0 and is to be increased to 1.2. The existing material grade for the shell is also known.

With regards to checking the existing shell for the higher SG value, is it ok to use the 'tmin' formula and max allowable stress values 'S' per clause 4.3.3.1.a in API 653 (5th Edition). Or should the tank shell be assessed using the formulas and allowable stress values in the latest API 650 standard for this change of service?

Also once the relevant 'tmin' value has been determined is it reasonable to check it against the average thickness reading for each shell course (from the Ultrasonic examination report). Or should the calculated 'tmin' be checked against the minimum thickness reading found on each shell course to determine if the existing plate thickness is acceptable? Similarly should we use average or minimum thickness readings when checking the tank for stiffening ring requirements.

I have already advised the client that he may need to reduce the fill height in the tank and that the existing foundations should be checked for the higher loadings from the heavier product. Shell stability and anchorage requirements would also need to be checked based on the latest thickness readings.

Look forward to your thoughts and opinions.

 

[JStephen]

I was assuming that was covered somewhere, but not seeing it.
I would use the as-built thicknesses and current API-650 stresses to calculate a new maximum operating level with the new specific gravity. Also check manway cover plates, flush manway base plates, etc.
Then use the current corroded thicknesses and API-653 evaluation to further reduce that height if necessary to account for current condition of the tank.
My thinking is that the higher stresses allowed for continued operation in API-653 wouldn't normally be applied where the entire course was uniformly that thickness.

 

[sajk14]

Thanks JStephen,

Looking at API 653 Section 4.2.4 it clearly states that for a change of service relating to 'internal pressure' and 'operation at lower temperature than original design' the current applicable standard shall be used. It also states that for 'operation at elevated temperature' and for 'external pressure' then Annex M and Annex V shall be used ( I assume Annex M and Annex V are to be used from the latest code? ).

Section 4.3 for Tank Shell Evaluation also mentions change of service and refers you to formulas in section 4.3.3 to calculate minimum thickness for 'continued operation'. Note that you could interpret the wording 'continued operation' to imply there are no changes from the original design/service parameters of the tank.

Unfortunately a change in Product Density is not discussed in the above sections. However as this would also be classed as a 'change from original design' i think a check against the latest applicable code should be carried out ( particularly if the S.G of the product is going above 1.0 ), with a follow up evaluation under API 653 using the existing thicknesses as you have already suggested in your comments above.

Finally when evaluating the tank shell under API 653 with corroded thickness, for each shell course would you use the average of the thickness readings from the UT inspection or the lowest/minimum thickness reading?

Thanks again for your feedback