Storage Tank Design Codes 3

[ANK72]

Hello all,
I have two questions:
1) There are two existing bulk fuel storage tanks that require nozzle load checking. The tanks were originally designed to API 650 but it appears that no calcs were ever done for the max allowable nozzle loads imposed by the piping. The tanks experience a considerable amount of subsidence and rebound. As a result the nozzles are subjected to cyclic loads. We have done AutoPIPE stress analysis and have the load values.
Appendix P of API650 describes how to plot a nomogram to determine the allowable loads. However the calcs are for tanks with diameters 120ft and greater. My tanks are only 18m(59ft) and 25m(82ft) in diameter.
Could you please suggest what design code to use. Will Bijlaard or WRC bulletins address this d/D ratio?

2)We also need to back check the design of an existing low pressure (20psig) storage tank. It is a vertical cylindrical tank with domed ends and four legs attached to the sides. Tank diameter 3.2m(10.5ft), height 8m(26.2ft)including legs.
What would be the most appropriate design code for this unit? The owner requires that the check be done against a known standard (ASME, BS, API, etc).

Your expert advice will be greatly appreciated.
Many Thanks

 

[pmover]

this posting might get appropriate responses in API forum, under Engr Codes and Stds.

good luck!
-pmover

 

[jte]

ANK72-

1) The October 2003 Addenda to API-650 will be valuable to you. It includes an addition to Appendix P, titled Appendix P.3, "Alternate Procedure for the Evaluation of External Loads on Tank Shell Openings." This appendix is based on work presented by Lengsfeld et. al. from 1995 thru 2002 at the ASME Pressure Vessels and Piping Conferences. Your tank diameters will be covered by this appendix.

2) At 20 psig, your "tank" is within the scope of ASME B&PV Section VIII.

jt

 

[ANK72]

jte, pmover,
Many thanks for your input. We've just been told that the second tank (question 2 above) will be fitted with a pressure relief vent and a vacuum breaker set at 5kPa and 0.5kPa respectively. This is outside the scope of ASME VIII Div.1 (External or internal pressure not exceeding 15psi).
Can you please recommend a design code that would meet the new operating conditions?
Many thanks

 

[jte]

ANK72-

If you can drop the vacuum to about 430 Pa (1/16 psi) then you can go with API 620. I'm not sure about the allowable vacuum for API 650; check Appendix F for your internal pressure.

Seems to me that the type of tank you're working with, vertical with "domed" ends (is that / could it be 2:1 SE or F&D heads), you could take the "designed and built to ASME VIII-1 but not stamped" approach. Even if you want it stamped, there's no reason you couldn't build (and stamp) it to VIII-1. There are a few other threads on this site which discuss in and out of scope for VIII-1.

jt

 

[MJCronin]

ANK72,

Who is requiring the checking of nozzle loading on an existing storage tank ? is this your boss or perhaps the tank owner ?

Most typically, API-650 tanks are designed, built and delivered with the standard nozzle configuration of the code of record. Only when the purchaser specifies additional loadings to the fabricator are there any more detailed nozzle evaluation performed.

More info......


MJC

 

[ANK72]

MJC,
Thanks for your input. The tanks have been in service for over ten years now and the reason why we are checking the nozzle attachments is that the owner is now making some dramatic alterations to the pipework. As I mentioned above the tanks experience a lot of vertical movement due to rather unstable subsoil conditions (the tank farm was built on a swamp). We have established that the new piping configuration will exert much higher stresses on the nozzles and we need to compare them against the allowables. I'm getting a copy of Appendix P.3 to API 650 as has been suggested by jte above.
ANK

 

[unclesyd]

Keep all unnecessary loads off any tank nozzle. Tanks don't make good supports or anchors.

Our plant standards require that any line attached to a 650 or 620 tank normally impose no more load than dead weight of the line to the nearest support or anchor. The support or anchor point on the line is placed within a minimum distance determined by a flexibility analysis. Be most careful with the thermal loads, expansions, and vibrations.


In evaluating nozzle loads on existing tanks it is absolutely essential that you confirm the details of the nozzle as built, don't depend on print details. You will find a lot tanks were built with typical details, meaning as the erection foeman saw fit.
If any loads on tank nozzles were absolutely unavoidable, like space constraints, usually the existing nozzle was removed and replace with a heaver nozzle and integral reinforcing pad, min of 1/2".

 

[LSThill]

Nozzle Load for API 650/API 620 Design: FE-PIPE Home Page at the bottom is the "Videos available for download !! Click Here to See Video Clips" Lower Tank Nozzle