On Sep 6, 2007, at 8:25 PM, Paul Bowers wrote:
> Art Montemayor has a nice write-up here:
It is a good explanation, but don't get it confused with Code requirements. The term 'design pressure' has no meaning with respect to the Code, which is concerned only with the Maximum Allowable Working Pressure. That's the biggest difference.
> What is the difference between "Design Pressure" and "Maximum
> Allowable Working Pressure" (MAWP)?
>
> The two terms are not related mathematically; rather, they are
> related in a practical procedure that takes place during the actual
> fabrication of a pressure vessel.
Not exactly--the procedure, which isn't really a procedure but an
ongoing series of trade-offs and compromises, takes place when the
vessel design is being specified, long before a contract is signed or
fabrication begins.
> The "Design" pressure is that pressure that the engineer decides is
> the value of the pressure at which the vessel will normally operate
> (or which it must withstand under operating conditions).
This is a process engineer, not someone who gets significantly
involved with the vessel design details.
> practical factors will result in a vessel that not only meets the
> required design pressure, but often EXCEEDS it.
Absolutely bang on. Don't waste a lot of time 'optimizing' the wall
thickness when plate and pipe comes only in the so-called 'preferred'
thicknesses.
> I consider the MAWP the most important pressure value attached to a
> vessel and one that should be clearly understood and stamped on the
> vessel for all to clearly read.
It will appear on the Code plate which is indeed attached to a
vessel. If there is no Code plate permanently attached turn around
and walk away--the vessel has been deliberately taken out of service,
and likely scrapped. You don't want any part of it, because someone
went to a fair amount of trouble to remove the Code plate and they
did it for a reason. You can bet that reason was not the expectation
of many more years of delightfully satisfactory service.
> The MAWP will change with time (as will the related design value)
> due to wear, corrosion, and vessel fatigue.
No it won't. It's stamped on the Code plate permanently. The vessel
may degrade over time and it may be possible to estimate the effects
of material loss or damage, but the MAWP is one thing only. There are
re-evaluation and repair procedures for doing this according to
National Board rules here in the states and likely in Canada.
> This is why it is so important to religiously keep and maintain
> current and accurate data sheets and calculations on all pressure
> vessels as they are inspected and repaired through the years of
> service.
Absolutely bang on as well.
> When you have a need to set a PSV on a vessel and you don't have
> its MAWP figure, you can employ the "design" pressure value - as
> long as it can be proven that the vessel is in as good a physical
> condition as the day it was fabricated.
The vessel MAWP will be known because the Code plate is permanently
attached. If there's no Code plate don't waste any more time with it.
> And I consider any pressure over 50 psig as HIGH PRESSURE.
Better to consider the applied force, not the pressure. 5 psi acting
over 24 inch floating head exerts a force of just over 2200 lb.
People have been killed opening a quick-acting closure with a
residual pressure of 2 psi behind it. All pressures are dangerous if
the area is right.
> I have never come across the term "Maximum Operating Working
> Pressure" and can only presume it means the same thing as MAWP.
Don't assume--remember 'assume' is 'ass' 'u' 'me.'
> People are forever changing the writing of terms in order to suit
> their own likes and dislikes. MAWP was first described and is still
> employed by ASME in the USA and is the term I have always used to
> define what I have described in the above.
Again--bang on. Insist on the using right names for things.
Misunderstood terminology is the first step on that fateful journey
up Shit Creek.
Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=6smshVFpKb2AoqDvlIidUA-9MUZYfPfrgb31f-f1Z2DcOmLFDW-xuB1VXOpWQphdqh_bTuoGbojmZg">chrisw@skypoint.com</a> | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania1864)
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