In our case...There is a single chamber, in top of the section, design pressure is 7.8 kg/cm2 (g) and at bottom section, design pressure is 8.2 kg/cm2 (g)... and the governing MAWP is 8.1 kg/cm2 (g) which is governed by shell of the top section... but as per definition of MAWP, MAWP must be equal to or grater than the design pressure.... in our case MAWP is higher than top section, but lower than btm section ... is it failure for top section?? or i need to increase the thickness of the top shell to meet the MAWP requirement..... Kindly give suggestion...
You mean, i have to consider the MAWP at least equal to the design pressure of Lower section ( pressure 8.2 kg/cm2 (g))...
but neither PV Elite nor Compress gives deficiency / failure report for this type of multipal pressure in the same chamber..
If I understand correctly, you have a single pressure chamber (eg: a pressure vessel that does not have any internal heads or pressure-separating internal devices). The specified design pressure varies for the shell components in this chamber (this might just as well have varying temperatures).
Due to process operations it is not uncommon for the operating temperatures to vary across a chamber, it’s probably less common for operating pressures to vary (how would the pressure distribution be contained?...well, there are packed beds, etc). But I have seen this in design specifications for some vessels.
How does ASME Code consider this case? (Other Codes may be similar on this issue.)
Section VIII Division 1, Appendix 3 defines the “design pressure” as the pressure used in the design of a vessel component for purpose of determining the minimum permissible thickness of the different zones of the vessel. Static head, if any, shall be included. Thus per this paragraph the design pressures may vary for the components in the pressure chamber.
The same Appendix defines the “maximum allowable working pressure” (MAWP) as the maximum gage pressure at the top of the completed vessel in its normal operating orientation, at the design temperature. Thus you need to determine the MAWP of each of the vessel components, adjust each for the static head, then the smallest such value is the MAWP for the completed pressure vessel.
The pressure vessel must have the name plate stamped with the design information as per UG-116 through UG-119. The ASME nameplate provides for only a single pressure value at a given temperature. You can have multiple different design temperatures with different corresponding pressure values, but for any given temperature there is only a single pressure value used for that chamber. Thus if you have a chamber where the operating or design pressures vary, you are limited to stamping the chamber to the lowest MAWP for that temperature.
You may be able to define the vessel as having two chambers, each with its own stamp and nameplate. This could be so even if there is no physical barrier to the different pressures (however, the process must be considered very carefully so that the lower pressure region is not subjected to the higher pressure during some catastrophic event). Different MAWP/temperature pairs would apply to each chamber.
If you expect to be able to design a component for the lower design pressure in one zone, but have its MAWP be acceptable for higher design pressure zone, then you must make certain that the thickness (or other properties) of the component is able to sustain that higher design pressure…essentially you must “design” the components in the lower design/operating pressure zone for the pressure of the higher design/operating pressure zone.
But the software will not complain if you tell it that the design pressure is “X” and the corresponding MAWP of the component is less than the design pressure in another region of the chamber. This is not what you told the software you wanted to do, so it is not a failure.
I should disclaim that I work for the publisher of COMPRESS, so I am privy to some other information. The input design pressure for the components varied throughout the pressure vessel. In fact, COMPRESS warns of this condition as it may be a data input error (but sometimes is not).
Static head is handled separately and is not the issue here. Static head is present in likely the majority of vessels and how to address this is well known.
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Dear Mr. Tom Barsh,
I am not agree & confuse to your response. As it is a multi pressure chamber (Top Sec-8.2kg/cm2 & Bttm Sec-8.9 Kg/cm2) & there no pressure boundaries between the two sections it is considered as a single Chamber. As a result the calculated MAWP shall be the lowest/governing component of that chamber. In our case if you see the calculated MAWP (Hot & corroded), i.e. 8.5 kg/cm2 is the lowest/governing which is Shell-1. The calculated MAWP is greater the design pressure for the component. Further more we calculate MAWP for every single component, So what is the purpose of that ? Top section will never create such or same pressure as the bottom section.
Also please refer respnse from Codeware regarding the same as below.
Mr. Paikray,
In your current design, COMPRESS is treating the entire vessel as a single chamber. As a result COMPRESS is setting the "Chamber MAWP" to the lowest (governing) component's MAWP in the chamber which is Cylinder 1. I did not see anything wrong with the calculations for the cylinder, the MAWP > Design P for this component. To analyze the chambers separately you will need to include an "internal head" as shown in the attached sample file. Note how the pressure summary is split where I have inserted the internal head. If you add an internal head to your COMPRESS design this will allow each "chamber" to be analyzed separately.
Regards
Devin Bligh
Mechanical Engineer
Codeware, Inc.
5224 Station Way
Sarasota, FL 34233
United States Work: 941.927.2670 x 20119
Fax: 941.927.2459
Email: devin.bligh@codeware.com
I assumed that what ever you want to say is right, than in the actual operating condition i can not go beyond my MAWP for entire equipment!!! ... in this case it is less than the design pressure of btm section...!!! in that case there is no use of extra pressure for the btm section and the thk of the bottom section is not justifying for that pressure ....
But for Mechanical design ... we need to consider severe condition of the equipment which is 8.9 kg/cm2 (g) (MAWP of the entire equipment should be equal to or grater than 8.9 kg/cm2 (g))...
For clear your view in real condition your top section pressure is always less than the bottom. Bottom section pressure is always higher due to my liquid level.
Further more i think you have certain pressure drop in your column.i.e. Overall pressure drop of column mentioned in our MDS.
Further you may consult with any of your A.I to clear your view.
I just want to ask you one question..... if your MAWP is limited by 8.5 ... can you go beyond that pressure for equipment in any condition ???
one more question to you.... suppose as u said ... lets assume that in any condition ... pressure at the bottom is 8.9 ... r you sure that at the top of the section .. my pressure will never be more than my MAWP??
can you calculate the pressure at the top for me for above mentioned condition (dont considered pressure drop )??...
Regarding software ... if u have seen in above post .. compress had make their stands clear ...i dont know what pv elite says (Although i hv done design in both and ask the question in pv elite also - both software doesnt give any warning for the MAWP)..
From the posts that have been going back and forth, I sense there is additional communication being done through email... but I'll give my two cents anyway.
MKshah: If you want the MAWP to be equal to or greater than 8.9 kg/cm2 (g), then why don't you just set the design pressure in Compress to this value? Then, the software will work to find the minimum thicknesses required to satisfy this requirement, and a slightly higher MAWP will result.
Once you have your design, you could then lower the design pressure of any individual component, and the MAWP will be unchanged (as long as you don't touch thicknesses).
HP-IP-LP Turbine casings have a comparable problem (different pressures through the assembly without any dividers or valves between the different casings, but the solution there is different. Within turbines, the assembly is not rated as a PV. The rules are "bypassed".
But here, the blades and vanes that bleed off the pressures between stages are not present: the system design is very different.
If i want to, i wouldn't do as we are in very much advance stages of engineering (We just got the comment from client)... all material had been procured and fabrication is already started .... if i vl design as u said at 8.9 kg/ cm2 (g) component having MAWP of 8.5 kg/cm2 (g) vl never pass at design pressure of 8.9 kg/cm2 (g) ...
The ultimate solution would be come out if you consult with your client. Further more the least way out is if you change your joint efficieny. I think probably you are considering the joint efficiency as 0.85. So you can change that to 1 , As a result you will definetly meet your MAWP requirment.That will not much hinder your project costing.
MKshah, was it clearly stated by your client when you quoted the work that the MAWP should be 8.9kg/cm2 (g) or higher, or did they just give you those two different design pressures?
If it was stated from day 1 that the MAWP should be 8.9kg/cm2 (g) or higher, then I think your company would have to pay for any changes required to meet that requirement.
However, if this requirement was never stated and they are adding it through a drawing review process, I think you would have grounds to charge your client extra to meet this 'new' requirement.
Just my thoughts. Either way, it sounds very frustrating. Good luck.
Ya its not from day 1, they just commented during the review of the drawings and design calculation....
Thanks all for your explanation. In my case governing is internal pressure for the top section.
Any how to avoid this condition what i did is , i just considered the Joint efficiency as 1 instead of 0.85 for the top section. As a result, i can meet the mawp requirement with lieu to max design pressure of 8.9kg/cm2(g). Now the calculated MAWP for the section is 8.93kg/cm2(g). That could be a pricing implication but not that much what it would be for a 30mtr length & 6mtr dia shell course.
Thank you all for sharing your valuable opinions. Keep sharing its really help full.
