MAWP question 4

[USP4]

Hello dear members,
Could you help me with following questions:
- Does MAWP is usually governed by shell or head but not flange? Will it not be flange governing the MAWP? Correct me if I am wrong,the reason for MAWP governed by flange is that if in future the plant personnel decide to change pressure then they will be adjusting according to MAWP based on flange, and this will protect the head or shell of vessel?

 

[saplanti]

MAWP is for every pressure retaining component of pressure vessels. Vessel limit goes on the outer surface of the flange which is welded to the nozzle. So, you have to maintain MAWP for this flange as well.

 

[Some Curious Guy]

MAWP is governed by the weakest component of the vessel. It can be the flange , shell , head , nozzle etc.

Most client specification say that MAWP should not be governed by nozzle standard flange. One reason I know is that a nozzle standard flange MAWP is often governed by leakage criteria. So if the MAWP is exceeded the flange will leak. While if MAWP is exceed for a shell or head it might just deform plastically but should not leak. If a vessel handles hazardous fluid leakage is the last thing you want. There are many ifs and buts here but that's a simple wisdom I know.

Some client also do not want the nozzle reinforcement to govern the MAWP apart from the nozzle flange. Nozzle opening being a discontinuity is a localized high stressed joint. Also for a pipe and pad construction it difficult to monitor the shell thickness lost due to corrosion at the area inside the pad.

There might be several engineering, economic, operation and maintenance related reason to not allow flange or nozzle to govern the MAWP. Each Owner / Client has their own preference.

Some client also do not want the bottom head govern the MAWP. If the bottom head governs the MAWP then there will be limit to the liqiud height of the vessel. In todays time I see a lot of client want to reutilize an existing vessel for certain optimized process instead of buying a new vessel.

So to summarize
The BPVC code does not mandate that a particular component govern or not govern the vessel MAWP. Owners dictate those terms based their past experience and for engineering , economic , operation or maintenance related reason.

 

[USP4]

Thank you @Some Curious Guy for detail explanation. An auxiliary question to get better understanding of my primary question, as follows: Lets say we have 23 kg/cm2 as design pressure and MAWP is 28 kg/cm2 in hot and corroded condition (head governing). If someone in plant wants to change pressure will he be exceeding 23 kg/cm2? If not then what purpose MAWP will serve to client, of course he will be happy as weakest component is at comfortable pressure with reference to design pressure? AND if client is trying to achieve pressure which will be tending towards 28 kg/cm2 in future, will it not destroy the critical parts of our vessel that is head or shell, if we select as head or shell as governing MAWP?

 

[mk3223]

In the example above, the design pressure 23 kg/cm2 is based the process design condition, and the MAWP 28kg/cm2 can be based on the conditions of the mechanical design and/or the shop fabrication.
The actual operation of the vessel can be at the design pressure of 23 kg/cm2 or higher according to the process requirement, as long as it won't be over 28kg/cm2 which is limited by the vessel MAWP per ASME Code.

 

[Some Curious Guy]

What is the practical layman meaning of MAWP?
It is the maximum pressure a vessel can handle. You can say that its the 'strength' of the vessel in very non technical sense.
Have you heard a famous proverb that chain is as strong as the weakest link. Similarly a Vessel is a combination of different component and is as strong as its weakest component.

When you say that the vessel MAWP is 28 units, then you mean that its weakest component strength is 28 units. The plant personnel should not operate the vessel for a pressure more than 28 units.

Refer UG-98 and Appendix 3-2 of BPVC Sec VIII Div 1 for a more technical and legal definition of MAWP. You might be confused if you are reading it for the first time. You might even start cursing the people who wrote the code. However over time as you gain experience in the field you will appreciate the wisdom of people in writing everything in a very legal ( ifs and buts type ) language.

All the best and welcome to the industry.

 

[Christine74]

It's common for EPC firms to specify that the MAWP must not be limited by nozzle reinforcement, must not be limited by "minor components" such as nozzles, or that the MAWP must be limited by the cylinders or the heads. I don't know if this is done for safety concerns or done for possible future rerates, but in practice this requirement doesn't really accomplish anything.

When a vessel manufacturer is told that minor components cannot limit the MAWP, many of them will just order the heads exactly to the required thickness. So if the design pressure is 1000 psi and the minimum required thickness of the head is 0.91738 inches, they order the heads as "0.91738 inches minimum" and list the MAWP as exactly 1000 psi limited by the head, even though 1.000 inch thick plate was used to form the head.


-Christine

 

[LittleInch]

USP4,

The use of the terms design pressure and MAWP are different in pressure vessels than in pipe or plant design. It took me a while to grasp it, but in essence, as mk3223 says, the design pressure is e=what the process or the rest of the plant is designed to. For a pressure vessel it is the start point of the design and the flange and other components are based on that design pressure, though the flange ratings are usually higher.

Then when the design of the vessel is complete, the design then goes back and looks at the actual materials used, their actual thickness and the geometry and calculates a MAWP. This can be equal to or often higher than the design pressure. Now what limits the MAWP can vary - often it is the head or shell thickness or geometry, but sometimes it can be the flange rating or some other attached component.

But in you example above the vessel MAWP is 28 kg/cm2. That means what it says, i.e. the MAXIMUM WORKING Pressure is 28 kg/cm2. So the vessel doesn't care what pressure the rest of the connecting system is designed or operated at so long as it doesn't exceed 28 kg/cm2.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r6155]

If we calculate the MAWP then the ideal is to do the hydraulic test according to UG-99 (c).

Regards

 

[KevinNZ]

I think MAWP is poor term to use. Maybe it should be called "pressure to use for hydro test calculation"

You can not just up the maximum pressure for a vessel from design pressure to something else without reconsidering the all the as designed external equipment, loads and design actions. MAWP should not be allowable until these factors are considered. For example increasing the pressure may also increase temperature and expansion at the nozzles or down bolts.

 

[fakhfakh]

You have to calculate/check the MAWP for each component of a PV including bolts. The lowest will be governing the MAWP of the item.

 

[r6155]

@ KevinNZ
See UG-99 (c) again.
This is for new and uncorroded conditions at the shop. No external loads are considered for this special test.

See definition in 3-2 calculated test pressure: the requirements for determining
the test pressure based on calculations are outlined in
UG-99(c) for the hydrostatic test and in UG-100(b) for
the pneumatic test. The basis for calculated test pressure
in either of these paragraphs is the highest permissible internal
pressure as determined by the design equations,
for each element of the vessel using nominal thicknesses
with corrosion allowances included and using the allowable
stress values given in Section II, Part D, Subpart 1
for the temperature of the test.

Regards

 

[USP4]

Thank you all for kind information, really appreciate. Further to question related to MAWP, if plant guys decide to go with MAWP pressure of vessel, which will be higher than Design pressure, does the vessel needs to be recalculated for stresses generating from combined loading such as wind etc? What will happen to stress concertation related to MAWP pressure? How about behavior of weld joints related to MAWP pressure (hot and corroded)?