One Pressure Vessel and Many Design Pressures

You need to review Bernoulli's Principle to understand what you are asking.

MAWP will be based on the strength of the weakest point in the vessel.

The vessel has to be able to withstand, safely, the extremes. Which means that if there is any risk of the multiple sections of the vessel becoming one, perhaps by valve being left open inadvertently, then the greatest extremes will apply.

Hydrotest will be a multiple of the maximum working pressure.

If the vessel is not being built to any recognised design code, then open a dialogue with the pressure vessel's insurance company at the earliest opportunity.

THANK YOU very much for the good answer what i want...

Each component in the vessel must be designed in accordance with the pressure imposed on that component. The clear example is when we proceed such a design for an elevated tall column, where the design pressure at the top = MAWP, and the design pressure at the bottom = MAWP + Pressure due to static head of liquid. And in many cases, you can find that the shell thickness at the lower part of column is higher than the shell thickness of upper part, and the same for lower and upper heads. Also the rating of flanges of upper portions can be found 150# (for example), where the rating of flanges at lower portions of the column must not less than 300#.

Hello GALALA sir, one last question to you...

as you see that New edition 2007 VIII-2 uses greater of 1.43 time MAWP or 1.25 x MAWP x S_T/S.....for hydrotest pressure.

Now why they have changed DESIGN PRESSURE from ED. 2004 hydrotest pressure and replace it with MAWP in new code test pressure....???

why MAWP is more significant than DESIGN PRESSURE.....????

Please sir help me....

I'd like you to know that there is only one MAWP for the whole of vessel to be designed under its accordance, and there are a lot of Design Pressures related to the location of vessel component under consideration. And the design pressure of any pressure component under consideration is equal to the MAWP plus the static pressure affect on that pressure component.

And per ASME VIII-2, 2007, there is a lower limit for the hydrostatic test to be the greater of [1.43 MAWP] or [1.25 MAWP (S_{@ ambient temp./}S_{@ design temp.)}], just a limit per code to be taken into consideration, and there must be a stress analysis to this figure to avoid any increasing of stresses (due to hydrostatic test) higher than the permitted figure.

during hydrotest which design pressure is taken in to account ?

The ASME Code defines the hydrostatic pressure as P_h = MAWP (S_{@ ambient temp.} / S_{@ design temp.)}, where the pressure must be measured at the higher point of vessel. Therefore, the gauge pressure used for hydrostatic must be located at the top of vessel. Locating the gauge pressure at any other locations will cause false indication to hydrostatic pressure.

Therefore, during hydrostatic test, the pressure measured at the bottom of a vessel will be higher than the pressure measured at the top of the vessel and the difference shall be equal to the component of pressure due to static head of fluid. Therefore, during design phase of a pressure part the designer must carry out a stress analysis for the hydrostatic condition.

one more question GALALA sir...look at following and tell me what i understood is right?

if not then correct me.....i want some questions to be cleared by your expert comments....please sir look at this....(AS I M STUDENT I am keep on asking such a stupid questions)

I have one vessel (3 section) with bottom head with pressure = 9.50 MPa including 0.50 MPa static Head

central portion of the vessel(shell) is with pressure = 9.25 MPa including 0.25 MPa static head and

Top head is with 9 MPa pressure...then what will be the MAWP? it is 9 MPa?

what pressure is taken in to account for test pressure according to ASME VIII-2 edition 2007? and what pressure should be if vessel designed as per VIII-2 Ed. 2004?

From that data, I can say that the MAWP is 9 MPa since it can be easily calculated by:

• @ bottom head : MAWP = Design Pressure 9.5 - Static Pressure 0.5 = 9 MPa

• @ middle height: MAWP = Design Pressure 9.25 - Static Pressure 0.25 = 9 MPa

• or can be taken directly as the MAWP at the top of vessel (top head where there in no static head): 9 MPa.

So you can calculate the hydrostatic pressure value by substituting the MAWP as 9 MPa.