Maximum Internal Operating Temperature above Maximum Allowable Working Temperature? 2

[TexasPE]

I am noticing that there are many Amine Contactors that are being dual rated for 1440 psi @ 120F and 1420 psi @ 150F on their nameplates. The UA-1s only reference the lower temperature and higher rating. Yet the amine reaction in the vessel is being designed for a "temperature bulb" in the contactor that exceeds 150F (169F and possibly as high as 186F). Typically I have heard the limits being in the range of 190F to 200F due to 1) reaction reversal or 2) due to corrosion or metallurgy issues. I was always of the belief that the "maximum allowable working temperature" was the coincidental temperature for which the pressure rating was calculated for the vessel Mechanical Engineer and is put on the Nameplate (to ensure it equaled or exceeded the Design Pressure set by the Process Engineer). In the above cases isn't this a "Per Se" Code violation.

I currently have a client that is operating a unit in such a mode and is suggesting that they are considering increasing their operating pressure from a 1000 psig to 1200 psig. The P&IDs show the vessels design P and T to be the 1440 psi and the lower temperature. Their contactor vessel's cylinders and heads are made of normalized carbon steel plate (SA516-Gr70N) but with only a .06" Corrosion Allowance. Is this something that I need to run by a Metallurgist, a licensed Mechanical Engineer or just have a Code Shop that can do a rerate of the vessel recertify it for a higher operating temperature and lower pressure combination. This might result in the client having to lower the relief valve setting as well.

Please no responses that are just guesses. These need to be from knowledgeable engineers familiar with the Boiler and Pressure Vessel Code.

 

[david339933]

Correct....MAWT is coincident temp of MAWP. (UG-98)
An R Stamp shop can perform this Alteration....See NBIC Part 3 - 3.4, this will give you guidance.
Also, contact your local jurisdictional authority.

 

[david339933]

Just re-read your post....For some reason I thought you wanted to increase MAWP....NBIC has no rules for decreasing MAWP....they state to contact jurisdictional authority to "determine if specific procedures should be followed."

 

[davefitz]

The response above is correct insofar as MAWP + MAWT, but there may be 2 other issues at this low temperature. One is that transfer piping metal temperatures above 140F must be insulated in the workspace to meet OSHA rules to prevent burns to workers. The second is that some stainless steels may experience corrosion above 140F in some situations. There may also be limits in the valve and pump seals and packing that does not allow operation above 150 F.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[marty007]

In your post, you mention that the vessel is fabricated from SA-516-70 steel plate. This material maintains full strength up to 500°F, so that would not be the limiting component when it comes to temperature.

I know you didn't ask for guesses, but here it goes... I believe that the reason for your double rating is to match the pressure ratings for the group 1.1 (carbon steel) CL600 flange class in ASME B16.5.

It looks like the equipment is designed on this basis, and if this is the case and they want to run at a higher temperature, they would need to decrease the MAWP to meet the requirements of the flange rating. For the 2017 edition of the ASME B16.5 code for example, the pressure rating of a group 1.1 CL600 flange would be 1360psi @ 200°F.

If they want to increase the MAWT above the rated 150°F, this should be done by a licensed NBIC R-stamp shop. Calculations would need to be re-run, and the shop would stamp the new MAWP/MAWT combination on the R-stamped nameplate. As you mention, the relief valves would also have to be adjusted accordingly.

Cheers,
Marty

 

[JStephen]

It's not clear to me from your post if this is the case, but note that per the definition of temperature in UG-20(a), the temperature inside the vessel could be somewhat higher than the temperature used for design.

 

[TexasPE]

I want to thank everyone for the excellent responses. This is why as a licensed engineer I still like being a member of this site. It is always good to get peer feedback on issues of concern because one engineer definitely doesn't know it all. I do believe the rating issue is due to the flanges on the vessel, but have no paperwork on the rerate to prove it. The vessel has multiple feed trays as low as the 8th tray from the top and the flanges in almost all the four feeds are being exposed to the maximum internal operating temperature in the 170F range. My real concern is that the client was suggesting increasing their operating pressure and I wanted to make sure at a minimum they had a relief valve set at the appropriate Maximum Allowable Working Pressure at a coincidental Maximum Temperature. They need to ensure they have an appropriate rating at that T&P but the immediate concern is are they operating safely.

A code violation does not necessarily guarantee it is not safe, just not legal. That can be resolved with a simple re-rate calculation. It is something that ethically I would need to report to my client they need to do as well as reset a relief valve or ensure they have one in place. If they were operating beyond the limits of the materials, and in the margin then that is unsafe and if reported to the client that it needs to be resolved immediately by reducing the pressure or temperature and it is not, then it would clearly make it the responsibility of a licensed engineer to report it to the appropriate authorities. (That is always a hard call to make and always has negative consequences) Thanks again for the feedback.

 

[r6155]

@ TexasPE.
"A code violation does not necessarily guarantee it is not safe, just not legal" , I disagree
What is the limit of the violation of the code? The code shall not be violated.

Regards
r6155

 

[moltenmetal]

I've always interpreted the MAWT to apply to mean metal temperature through the component which limits the MAWP. The MAWP is determined based on stress values which are related to the properties of the bulk of the material. The inner or outer surface can be hotter than the MAWT as long as the bulk metal temperature is below the MAWT. It is not an absolute maximum exposure temperature.

Brief excursions beyond the MAWT are permitted in both the piping and vessel codes unless I'm mistaken, and there is no explicit requirement in the code for passive layer overtemperature protection. Overtemperature design is left to the engineer's judgment.