Re: Design and Working Pressure

On Apr 10, 2005, at 10:59 PM, D Kannan wrote:

> The operative word here is "completed". The vessel is
> completed when a fabricator, according to the code
> laid down by ASME, has designed it.

That isn't correct. A completed vessel is one which has undergone all the required examinations and passed the vessel hydrotest and been Code stamped as required.

> The vessel's fabricator, not the Process Engineer, determines MAWP.
Actually the vessel designer, who is to be the user or the user's designee [per U-2(a)] determines this value by performing all the required calculations to size the vessel and its various parts. The vessel's MAWP may also be determined by a burst test if the vessel design details aren't covered by the Code rules.

> Not quite because the certified vessel drawings, which are delivered
> way before the vessel is, contains this information
Not necessarily, probably not in many cases, particularly for larger vessels where construction may begin before the entire vessel is completed. In cases where vessel details are changed during fabrication, as-built drawings may also be required. In the US Div 1 vessel drawing also do not require certification beyond the usual manufacturing approvals. No written specification is required by the code, although only a very trusting user would want to handle the vessel requirements verbally.

> The design pressure is the value obtained after adding a
> margin to the most severe pressure expected during
> normal operation at a coincident temperature.
What normally happens is that a vessel's normal service pressure is decided on to suit the particular process or service. The vessel is then designed for a higher pressure to accomodate such off design conditions as pressure surges during service, which will set the maximum allowable working pressure, which in turn determines the setting of the vessel's pressure relief device. You want enough margin between the maximum expected service pressure, including excursions, and the relief setting so the you're not venting product all the time. Pollution and environmental concerns play a big role here, since you don't want hazardous materials being vented every time you get a temperature excursion. In some cases you may want to leave a margin for plant upgrade. You also have to account for corrosion since the design calculations are always based on the corroded thickness.

A common although untrue belief is that a vessel may be operated at 110% of the maximum allowable operating pressure. The misunderstanding comes from the allowance between the cracking pressure and the full open pressure of a relief device. The relief device must crack at the maximum allowable working pressure, but the full open pressure can be no more than 10% in excess of the cracking pressure. Moreover the relief device must be sized to prevent the pressure from exceeding 110% of the maximum allowable working pressure.

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania