13 replies to this topic

[naihusna]

I always confuse with design pressure (DP) and maximum allowable working pressure (MAWP). Do they carry same meaning? What about maximum operating working pressure (MOWP)? Can anybody clarify me on this? TQ.

[Art Montemayor]

naihusna:

It is not unusual for engineers to get confused with the terms "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.

Every Chemical Engineer - sooner or later - will have to deal with a pressure vessel fabrication or operation and it is sound and good advice that he/she should familiarize themselves with engineering terms employed and the logic of their application. In dealing with or specifying a pressure vessel, an engineer must resort to filling in or using a Vessel Specification Sheet - much like the one I have included in the attached Excel Workbook that gives you a host of Specification Sheets that you should be familiar with and employ in the course of your work.

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 value must include any normal excess pressure that can occur during the vessel's operation. This is a discretionary value that depends on the background and experience of the design engineer. Sometimes the design value can be 10% over the pressure calculated (as in a simulation) or as much as 25% more. Good engineering judgment is employed in arriving at this design figure.

Once the Specification Sheet is received by the vessel fabricator, mechanical fabrication design takes place in which alloys, fabrication techniques, available materials, and other factors are taken into consideration to generate a fabrication drawing. Although the design pressure given is employed to generate the required vessel physical characteristics, some practical factors - such as available materials, fabrication efficiency factors, and alloys employed - will result in a vessel that not only meets the required design pressure, but often EXCEEDS it. This is a fortunate and conservative procedure because it ensures that the vessel will meet pressure safety expectations. The Maximum Allowable Working Pressure (MAWP) is a result of back-calculating the ultimate resulting fabricated vessel and is the prime factor in setting the pressure at which the corresponding vessel Safety Relief Devices will be activated. 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. The MAWP will change with time (as will the related design value) due to wear, corrosion, and vessel fatigue. 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.

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. Note that I'm going to lengths to define the physical condition of the vessel. We often neglect to mention that we are ASSUMING that the physical condition of the vessel doesn't change from the day it was fabricated. This can be a dangerous assumption that doesn't necessarily apply. A vessel can undergo corrosion and wear as well as other chemical attacks through its use and lifetime. Physical and meticulous inspections and reports are essential to ensure that the vessel can be safely applied to a process --- especially to a high-pressure application. And I consider any pressure over 50 psig as HIGH PRESSURE. When a vessel explodes, it isn't the pressure that kills you; it's the amount of shrapnel and steel pieces that are blown about that do the damage. And even 50 psig can cause a considerable amount of serious damage if allowed to trigger a vessel failure.

I have never come across the term "Maximum Operating Working Pressure" and can only presume it means the same thing as MAWP. 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.

I hope this helps you understand the terms you have been confused with.

Attached Files

•  Art__s_Process_Spec_Sheets.zip   191.47KB   7728 downloads

[djack77494]

naihusna:

Art has provided a very useful and complete picture of the meaning and significance of these two terms. I want to take a moment to stand on a sopbox and say to all that you should REQUIRE vessel manufacturers to indicate the MAWP in their documentation. They should always do so, but I have seen a very disturbing trend wherein the design pressure is represented as being the MAWP. They are not the same and should not be represented as the same. If only one MAWP is listed, I like to see what is called the "hot and corroded" MAWP. This means merely the MAWP when the vessel is at design temperature and after the corrosion allowance you specified has been taken into account. I have seen alternative numbers represented as "the MAWP", though I'm not sure of the legality of doing this.
Doug

[pleckner]

Adding:

The reason the MAWP, as stamped on the vessel nameplate, is more commonly the specified design pressure is cost and nothing more. By selecting the next standard plate thickness after calculating the minimum required to meet the design pressure at the coincidental design temperature, they are assured of meeting the desgin requirements and may stamp the vessel as such. But to get the true MAWP requires the manufacturer to do some more calculations (back calcualte the value as Art stated) and then offer that guarantee. If you, the owner want the true MAWP, then ask for it on your specification and expect to pay some more.

[naihusna]

A lot of thanks to Art and others for that lengthy explanations. Also, thanks for the process spec sheet. I'll try to utilise it in my workplace.

[jerald04]

Thank you Art and everyone here for the clear explanations on the difference between these two terms. I was having difficulties trying to digest the definitions provided in ASME codes for a young engineer like me, and this forum made the explanations in so much easier to swallow.

Appreciate it.

[jerald04]

I apologise if my following post is going to go a little off topic, but it is somewhat related to the MAWP.

I came across a pressure vessel recently, with a design pressure of 43kgf/cm2G and a MAWP of 43.95kgf/cm2G. Obviously, the penumatic and hydrostatic test pressure for the vessel would be at 47.3k and 55.9k respectively.

May I find out if the test pressure values is supposed/ intended to be always higher than its MAWP? If the test pressures have to be higher than MAWP, wouldnt it exceed the max pressures that the vessel is designed for?

[djack77494]

jerald,
You raise some interesting points and seem to suffer from some confusion about this topic. Concerning pneumatic and hydrostatic testing, it's "either/or". You either hydrostatically test the vessel OR (if absolutely necessary) you pneumatically test the vessel. A discussion on these two types of testing was recently active in this forum, and I suggest you review what is said there.

You correctly note that the test pressure, whether hydrostatic or pneumatic, is greater than the MAWP. If you think about it a bit, you'll note that the vessel is not designed so that it will explode just above the MAWP (duh). In fact, you want to test it at a somewhat elevated pressure using an innocuous fluid - e.g. water - so that if its integrity is found lacking you have not released some toxic or flammable fluid. You don't want to do that "test" with a hot nasty operating fluid filling the vessel, so you test with water or air. Codes may vary some, but I'm accustomed to the hydrostatic test pressure being 1.3 x the MAWP. (It was formerly 1.5 x the MAWP.) There are correction factors for the lower temperature conditions during testing when the design temperature is elevated. I'm not sure how corrosion allowances are handled. Test pressures are different (lower) if you test pneumatically. In any case, the vessel is actually designed to be able to fully withstand the test pressure, providing a bit of a "buffer". Never try to infringe into this buffer; it is both dangerous and illegal.

I hope this provides somewhat of an explanation of how these terms interrelate.
Doug

[JoeWong]

Doug,
Good explanation...

I'm not sure how corrosion allowances are handled.

I guess short term exposure will not be considered.

I believe normally there are requirement to testing medium e.g. water with Chloride content less than 250 mg/l (in some area and some project) to be used...some client may have even more stringent requirement (demin water for reactor) on this aspect.


JoeWong

[fallah]

.

I came across a pressure vessel recently, with a design pressure of 43kgf/cm2G and a MAWP of 43.95kgf/cm2G. Obviously, the penumatic and hydrostatic test pressure for the vessel would be at 47.3k and 55.9k respectively.

I think, because of conservative recommended values of allowable stress for material in standards(for example in B31.3 , max. recommended allow. stress is two third or 2/3 of min. yield strength) the short therm exposure to higher pressure(1.3 times that still lower than yield point) results in any problem.

Fallah

[jerald04]

Hi Doug,

Thank you for your detailed explanation. Now I can confirmed that things are in order from the observation I made for the pressure vessel MAWP and its test pressures.

I'm not sure if its part of the ASME code to do both types of pressure testings for pressure vessels. Its a requirement for my company though.

[pleckner]

It is NOT a requirement of ASME to pressure test using both methods. I would go back and double check if this is indeed your company standard because it makes no sense to me at all.

[djack77494]

I have to weigh in on this topic again. Phil is absolutely correct to note that it makes no sense to "double test". The hydrostatic test is a good vessel integrity test; the pneumatic test is a lousy test.

Besides the great potential for disaster, it is very difficult to detect a small leak in a pneumatic test. Think about it. If you have a small leak, a small amount of air escapes. What happens? A very small drop in pressure that is very difficult to detect. Switch to a hydrostatic test of a vessel with a small leak. If you've done a good job of filling the vessel with water, a small leak is readily detectable as a LARGE drop in pressure. That's what you want - amplification of problem detection.
Doug

I registered to this forum just so I could reply to this thread that I ran across in search for something else. Not long ago, I worked as a mechanical engineer in a pressure vessel company. We dealt in very high pressure stuff (30 ksi?... that's midrange). I'm no P.E. (yet) and I was the most junior engineer on the staff, but I can address this dual testing thingy that some folks are soapboxing about.

We ran both a pneumatic and a hydro test. The hydro test was always done per ASME code and is required by that code. Water is fairly safe for the test per comments earlier in the thread... mainly because it has few toxins (so spills and ruptures don't require NBC gear), is relatively inert to most construction materials (chlorides might be a sticky point to some because spots of precipitates are thought by some to be sites for stress corrosion cracking later on) and is relatively incompressible (especially when compared to ANY gas). You can think of pressure as storing energy in a fluid. A gas is like a springboard... a springboard (or diving board) deflects quite a bit and will return it when released, flinging the diver high. A liquid like water will be less compressible... more like a diving platform. The diver must use shear muscle to launch with no springs to help out (there is still spring... but almost immeasurable in comparision to the springboard).

Puttting the asides aside (sic)... the hydrotest is supposed to be a test of the structural integrity of the vessel. If it should fail, water will spray and spill, but will not explode. The stored energy content is too small. A gas will explode, so it needs to be heavily barricaded and is all sorts of trouble and expense... so it is avoided if at all possible.

After a vessel passed the hydro test, we usually did a pneumatic test. Most of the time with air sometimes with nitrogen. Nitrogen is fairly inert so leaks don't cause secondary problems (just ventilate enough so no one gets suffocated). The purpose of the pneumatic/gas test was to test for leaks. Granted to some of the posters, a leak of an "incompressible" fluid will show on a pressure guage faster. On the other hand, gases are generally smaller and more slippery and will sneak out around seals and migrate through elastomers where a liquid will not. Some process fluids will accumulate in a pit until they reach some critical density and then something tragic will happen, so we test for smaller leaks than would be caught on a pressure guage. We gas test at 1 or 1.1 of MAWP to see if the little buggers are getting out. Usually, a bubble test is enough. In some cases, we actually used a sniffer to detect really small leaks. The costs go up as you add more test but none of them are useless. You have to balance these costs against the criticality of the system.

I hope that this helps straighten things out.
Paul