Design Pressure as MAWP

Not clear what your question is. MAWP must be at least equal to DP, but can be higher, as usually plate etc selected is next standard thickness above that calculated for design. MAWP can be calculated from selected thickness, but this is not mandatory, and if it hasn't been done, must take MAWP = DP.

Hope that helps

Question is

If MAWP has been calculated based on the selected thickness, can MAWP for the vessel still be equal to DP on the Name Plate?

Regards,

Tabby

Yes, but only if (one of) the calcluated plate thicknesses is very close to a std thickness, and that thickness is actually used. There'd then be no need to calculate MAWP, the answer is obviously the DP.

Well I think Designer will obviously tend to mark MAWP=DP for safety reasons wether the actual thickness used is much higher or marginally higher than thickness required for DP.

Well as per ASME BPVC VIII Div 1 definitions, MAWP = DP for all cases where MAWP is not calculated for actual thickness used. I dont find a specific instruction indicating that if MAWP is calculated after selecting the actual thickness for fabrication, then the vessel MAWP marking must be the one that is based on actual thickness used.

Our vendor has done calcuations for MAWP based on actual thickness for all parts (all greater than DP) and still they mark vessel with MAWP = DP. Is that allowed?

Regards,

Tabby

Well as per ASME BPVC VIII Div 1 definitions, MAWP = DP for all cases where MAWP is not calculated for actual thickness used. Agreed, that confirms my post #1.

Our vendor has done calcuations for MAWP based on actual thickness for all parts (all greater than DP) and still they mark vessel with MAWP = DP. Is that allowed? It's probably allowed, I'm not familiar with the latest rules, but it seems a bit odd. Did you instruct the vendor to calculate MAWP? Have you seen the calcs? If no to either, he may not have bothered, so marks MAWP = DP. If he has done the calcs, I assume you could insist on him marking calculated MAWP.

I don't know for sure whether ASME VIII states "...that if MAWP is calculated after selecting the actual thickness for fabrication, then the vessel MAWP marking must be the one that is based on actual thickness used" but it seems pointless not to, if somebody has gone to the trouble of doing the calcs. Note that it says "....if MAWP is calculated....", confirming this is not mandatory.

Cheers.....Codey

The vendor has submitted the design calculation report.

Reviewing the report it is clear that vendor has calculated the MAWP for all parts thus it includes all essential parts. So as per ASME, actual MAWP should be marked.

However, reviewing contracts documents, we did'nt demand specifically for MAWP calculations. However, design and fabrication as per ASME VIII Div 1 is mentioned in contract.

Since design report clearly indicates MAWP has been calculated we can ask vendor on the basis of CODE rules that vessel should be rated for actual MAWP and not DP?

Regards,

Tabby

I don't see why not. I believe both DP and MAWP appear on the vessel nameplate.

Ofcourse both may appear on the Nameplate but the vessel is ASME stamped against marked MAWP. If vessel is stamped for MAWP = DP at this point. Then to operate the vessel near the actual MAWP, it will require "Re-Rating" according to NBIC and so new Tests and blah blah.

BTW, on another forum, it was mentioned that if software used for design calculations has calculated MAWP automatically (obviously thickness selected for fabrication has been provided to software earlier). Then the requirement of UG 99 note 34 is not fulfilled and MAWP may be taken equal to DP.

The calculations require some sort of confirmation (yet to be clarified wat sort of confirmation) for marking MAWP equal to the actual MAWP based on selected thickness. Any ideas? Please do share.

Regards,

Tabby

I don't see what the problem is. You say the vendor has calculated MAWP, so why not instruct him to mark the nameplate with the calculated MAWP? (with or without the DP, which is lower) Then you can operate it up to marked MAWP.

I don't have ASME VIII in front of me and I'm not familiar enough with it to comment on UG 99 note 34 etc.

In principle, MAWP = DP*actual thickness/calculated thickness, but this is likely to vary for different parts of the vessel - shell, dished ends, nozzles etc, and need to use the minimum value. Also thicknesses in the calc are without corrosoin allowance.

Codey