2 replies to this topic

[stu]

Dear sir,
I am fresher in design, I am confused to the following terms.Test pressure, Design pressure, MAWP.
Normally we design one vessel according to design pressure but we test it with test pressure of 1.5 times the design pressure.How is it possible , our design pressure is smaller than the testing pressure so the value exceeds the designed one, if it is so the vessel may break during test pressure.
Please explain with example to get clear picture.
Thanks
STU

[Art Montemayor]

Stu:

I don’t know what a “fresher” is. Do you mean you are a first-year, chemical engineering student? If so, you are trying to understand some terms that are a couple of years in front of you – particularly in plant design and strength of materials courses that you will take later.

But if you are interested, then consider doing a search in our Forums for the terms you are interested in. These have been discussed and defined many times in the past. For example, go to:

“Design Vacuum - Design vacuum for pressure vessels” posted by benoyjohn on Apr 1 2004, 05:20 AM.

Note the reference to the Eugene F. Megyesy book titled “Pressure Vessel Handbook”. If you are interested in learning about pressure vessels, then obtain a copy of this book and read it thoroughly. Even if you don’t understand certain features in it now, you will always be referring to it in the future should you continue to become an engineer someday.

Briefly, you hydrostatically test a pressure vessel to a level above its design pressure – but below its maximum allowable tensile stress (before it fails). This makes good sense. It would be bad sense to design a vessel to operate normally at a pressure (its design pressure) barely below the point where it is going to fail. Don’t you agree?

[djack77494]

Stu,
Art offers you the best advise possible - obtain a good reference book on the topic of interest and proceed to master the areas of interest. I'll try to enable a bit less ambition on your part, understanding that you'll get to the better approach as time and inclination permit.

As Art explains, the design pressure is well below the pressure at which the vessel will fail. This practice allows for years of safe operation during which corrosion, erosion, stressing/cycling, and other generally detremental processes occur. The concept is that the vessel may safely be operated over any range UP TO the design pressure over its lifetime with confidence (but see below). I like to at least mentally add "(hot and corroded)" following the design pressure value, since that is what you will normally mean when you refer to the design pressure.

You confirm that the vessel is adequately constructed by doing a (normally) hydrostatic test on the newly built vessel. Thus you want some margin, knowing that the vessel is at the best condition of its operating life during the test, and that you are using a benign fluid at nearly ideal temperatures. This is also appropriate considering that you would like the test to take a minimum of time while still detecting any likely problems. The appropriate code will stipulate the TEST PRESSURE required, which was frequently 150% of design but is now more commonly 130% of design. (There are alternative factors that depend on how the test is performed.) Really, the test pressure should be factored higher in many cases to account for, for example, elevated operating and design temperatures.

In fact, the situation is a bit more complicated. The above assumes that the vessel is suitable for service. You should confirm this through a mechanical integrity program that should include metal thickness measurements, etc.

HTH,
Doug