Hydrotest Using Map in Lieu of MAWP

Even allowing for secondary language issues, you are not yet explaining this well. What are "PTS" and "MAP"? I know MAWP, but others may need an explanation of that, also.

What are the actual conditions under which this vessel is expected to operate?

As a pressure vessel manufacturer, I've seen the request to use MAP (new & cold maximum allowable pressure) rather than MAWP (hot & corroded maximum allowable pressure) for calculation of the hydrotest pressure. This is OK. Per the ASME Code VIII-1, 1.3*MAWP is the minimum. It does not give a maximum. 1.3*MAP is still safe.

The issue with the client requesting you to use 900# flanges in place of 600# flanges sounds like a customer relations problem rather than a technical one. I would not do it unless the client paid for the difference. You may also remind the client that the attached piping will also have to have its flanges uprated to match.

Have you and client allowed for 600# flange being rated for 600 psig at some elevated temperature? At ambient (and hydrotest) temperature < 100°F, 600# flange is good for 1480 psig. http://www.engineeringtoolbox.com/steel-flanges-pressure-rating-d_49.html.

I believe flanges are tested at 1.5 x working pressure = 2220 psig which could easily be higher than the vessel hydrotest pressure. That's my take on it but you need to confirm details.

hi,

in general some companies used to calculate the Hydro-test pressure by the "MAP" after the moment of the fabrication complete, since the vessel at this case is not corroded. but even the designer will use the MAP or MAWP in the hydro-test pressure shall not pass 90% of the yield strength of the governed pressure vessel elemnt which will be in your case the shell part.

And i thinik the client request regarding change the flange rating, is not related to the pressure issue in the vessel, it is related to the flange type, since most of the technical practices of the oil and gas companies (shell, BP, and APATCHE) stated that the #600 rating flanges are RF (rasied face) and the #900 rated flange are RTJ (ring to joint), so this change will related to give more safty tightness to the flange connections and hadn't any relation with the stress on the flange since it si not the governed part of the vessel.