Re: Stress Temperature and pressure

On Jul 24, 2007, at 1:08 AM, Ashish Mishra wrote:

> If your Process people says the line is going to
> experience 190 DEg C ....then how can you do the
> analysis in operating condition only ?
>
> Think....What happen if you design a sysytem which is
> not capable to withstand the design parameters.
Sounds to me like there's some confusion over what's meant by design pressure. When you use the term 'design pressure,' clarify exactly what you're talking about--the process design, the Code allowable pressure, the relief valve set pressure, or some other pressure not requiring Code compliant 'design.'

There are all kinds of 'design' pressures. The lowest of these is the system pressure in the system under normal service, which is usually set by the process designers because that's what they need. Then you add what's expected for 'normal' excursions: pressure changes during start-up or shut-down or under and over-shoots by process controllers. Then you have pressures and temperatures caused by abnormal excursions--unintended pressures and temperatures that you need to withstand without any safety compromise or damage.

All of these can be considered 'design' pressures and temperatures, but when you come to Code compliance, you have to consider any or all of them independently. You may end up having to insure Code compliance for an enveloping case--the highest temperature and the highest pressure, even though they may not be coincidental. So you establish what Section VIII calls the maximum allowable working pressure (MAWP) which is the 'design' pressure you use for Code compliance.

The relief valve needs to crack at pressures no less than the MAWP and be fully open without allowing a pressure excursion greater than 10% above the MAWP while doing so. There's another 'design' pressure to consider.

Sometimes you'll also have a 'design' pressure which the system may need to withstand in order to prevent release of hazardous materials or contain a fire. Here you may expect damage but the system may need to be more or less intact or able to shut down. This 'design' pressure normally exceeds the MAWP.

Finally don't confuse the shop test pressure, either hydrostatic or pneumatic test, as a 'design' condition. The MAWP is such that no damage will result to a properly constructed system when the shop test pressure is applied--no special design provisions are needed to handle the shop test, so it isn't a 'design' condition requiring Code compliance. In fact the shop test is really intended as leak checks, and were originally set to verify that riveted vessels were sound. The shop test levels are too low to provide any indication of structural strength. Any damage encountered during the shop test is the result of a gross design or fab error. That said, you may end up with a situation for a large vessel where you have to consider the weight of the water used in hydrotesting, which requires 'design' for test conditions. You may also have to 'design' for the possibility of burst during a pneumatic test.

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

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