9 replies to this topic

[chemks2012]

Hi all,

I am working for a project wherein I need to review the existing relief devices on the vessel.

The vessel is installed with relief valve (RV) and a bursting disc (BD) independently.

The design pressure (DP) if the vessel is 2.3barg. RV is set pressure is 2barg, BD set pressure is 2.8barg.

Vessel has been tested at the test pressure (note: this is not MAWP) of 3.45barg.

My query is:

We don't have data on how test pressure was applied but it seems like it was a hydraulic test pressure (1.5 times DP) at normal temperature.

I understand that MAWP should be equal to greater than DP. But vessel don't have MAWP data and I read definition of MAWP in API and as its name suggest MAWP should be measured (or calculated?) at maximum working temperature of process by excluding corrision allowance of the vessel thickness?

Or in other words, as we don't have MAWP data and the BD set pressure is higher than DP of the vessel, i am not sure why the BD set pressure is higher than DP of vessel. Cluent is ready to carry out some testing/calculation by a third party to establish MAWP but I am not sure how to establish MAWP?

Of course, there is a simple solution of setting BD at DP but I am sure MAWP data of this will be helpful while the test pressure (hydraulic test pressure) has nothing to do with MAWP.

Any help please?
Thanks
KS

[fallah]

 KS,

 

As you mentioned the test pressure (hydraulic pressure test) you calculated based on DP has nothing to do with MAWP. You can evaluate the MAWP of the vessel by making a contract with an authorizing company, and set the RV on evaluated MAWP and BD on 105%*MAWP (as per code allowance). But as long as you wouldn't do that, you have to rely just on vessel's design pressure. Then, the set pressure of the RV is equal to 2.3 barg and of the BD equal to 105%*DP, i.e., 2.415 barg (not 2.8 barg).

 

Edited by fallah, 15 May 2013 - 03:35 AM.

[chemks2012]

Hi Fallah,

Thanks a ton for your reply but I want to know the difference between test pressure and MAWP.

Am I right in saying that there is no direct comparison between these two pressures but MAWP is measured at the maximum expected process pressure by ignoring vessel corrosion allowance?

API does not say explicitly at what temperature MAWP is to be measured at.

Thanks
KS

[fallah]

KS,

 

Hydrostatic pressure test, as per code, is equal to 1.3 times the MAWP to be marked on the vessel multiplied by the lowest ratio (for the material of which the vessel is manufactured) of the stress value for the test temperature on the vessel to the stress value for the design temperature. Therefore, standard hydrostatic test pressure and MAWP are related to each other by mentioned relation.

 

MAWP is to be determined at the designated coincident temperature specified for that pressure.

Edited by fallah, 15 May 2013 - 03:34 AM.

[chemks2012]

Thanks Fallah,

 

Also, the vessel is 50years old now and I am not sure if design pressure is still valid.

Could we have third party check to establish if design pressure is still valid?

 

Thanks

[Art Montemayor]

chemks2012:

 

I have written various posts in this Forum in the past specifically identifying the MAWP, how it is defined, calculated, and why it exists.  Use the SEARCH engine in this Forum and you will find information answering your questions.

 

Fallah is correct in his recommendations.  The Design Pressure (as long as you have the documented calculations to show this value) is appropriate to be used as the set pressure in a PSV because it is a "conservative" value.  It is almost always lower than the MAWP.

 

The Maximum Allowable Working Pressure  (MAWP) is a product of the vessel's fabricator.  When designing a pressure vessel, it is rare (if not unlikely) that the process design pressure generated by process engineers and specified on the vessel's Data Sheet will be exactly the same as the actual safe pressure that can be applied to the vessel once it is fabricated.  This is so because the fabricator takes the specifications noted on the Data Sheet and uses the available steel plates and heads to fabricate the vessel specified.  The available steel plates and heads are fixed in dimensions because of industry standards and economics and have nothing to do with the desires or specifications of a process engineer.  Therefore, what actually occurs in real life is that the fabricator's choice of a steel plate thickness will be in excess of the design pressure - because of economics and availability and the necessity to warrant that the product will meet the design pressure requirements.   Because of these practical facts, the finished vessel results with an ability to safely sustain a pressure in excess of the specified design pressure.  This "excessive" pressure is calculated in accordance with the materials, thickness, and design temperature and is labeled the MAWP - which the vessel can safely contain.   With time, the vessel ages, corrodes, and errodes in accordance to its use and consequently the MAWP and (possibly) the design pressure must be re-rated for a lower value identification and for resetting the PSV (if need be).  This is usually done every 5-10 years, with a subsequent hydrostatic test to verify the safe operating condition.

 

Does this respond to your query(ies)?

[chemks2012]

Hi Art,

 

Thanks for your reply and I now recalled one of your posts regarding MAWP & design pressure. Agree what you said.

 

But my query is:  As the vessel is now 55years old and it may be possible that the vessel may have passed its design life [no data available on design life but I am assuming it may be about 25years?]. As this is the case, to identify if the MAWP and/or design pressure is still valid, could we have third party check? If yes, will it be done by some sort of non destructive test [NDT]?

 

Regards,

Edited by chemks2012, 15 May 2013 - 08:19 AM.

[Art Montemayor]

  

Chemks2012:

 

The normal depreciation life of a process vessel – as with most industrial investments – is approx. 15 years.  That means your vessel is well over its depreciable life (over a factor of 3!).  I replied to this point by inferring:  “This (inspection & testing) is usually done every 5-10 years, with a subsequent hydrostatic test to verify the safe operating condition”.  Although you may opt to not inspect and test on this time period, the decision is left to your best, prudent engineering judgment in consideration of the operating conditions your vessels work under and local regulations and laws.

 

From what I can surmise of your operation, I would have a program that religiously inspected and tested all my pressure vessels and storage tanks every 7 to 10 years if I am depreciating them over a life of 15 years.  There is little logic in waiting until the equipment is fully depreciated if your primary intent is to safeguard your workers, neighbors, and the environment.

 

A licensed, professional agency or mechanical engineer would be the entity to inspect and rate the condition and usable service life in the vessels.  This should be totally documented with a hydrostatic test as well.  In addition to this safe best practice, all the PSVs associated with your equipment should also undergo similar NDE (non-destructive evaluation) or NDT (non-destructive testing).

 

I hope this response specifically addresses your concerns and query.

 

[chemks2012]

Hi Art,

 

Thanks for your reply and detailed explanation.

 

Regards,KS

[chemks2012]

How to select the 'best answer' from each person?

 

I think Art and Fallah both have got 'best answer' but unfortunately I can't select two!

 

Thanks.