Recommended proceedure for rerateing air coolers 1

[dcasto]

I don't have all the details so I'm just checking on a proceedure needed to accomplish the following. I have an air cooled exchaner rated at 1050 psig AT 650F. I want it in a new service rated at 1150 psig at 250 F. Can I redo the calculations for the box and nozzel with the stress allowed by the materials at the lower temperature and the do a hydrotest and reuse the cooler. Also, would it require an R-stamp or if in a noncode state a new tag by the owner stateing the new rating?

Then to add to the complexity, I have cooler rated for 1050 at 250F and I would like it to operated at 670 at 550F, do the same thing?

 

[jte]

dcasto-

Rerating the header boxes of an air cooled heat exchanger is really no different than any other vessel. As you mentioned, rerun the calc's using the appropriate allowable stress. What I've found to be a stumbling point in the past is in running the nozzle reinforcement calc's for header boxes with swaged nozzles - detailed geometry information was not available and the original fabricator was long gone...

If you are choosing to follow typical NBIC rules for ASME vessels, the rerate will be an alteration and upon approval by the AI and a hydrotest if necessary (sometimes the original hydrotest is sufficient for the new conditions) the exchanger will get a R stamp nameplate with the new conditions.

If you are choosing to not follow NBIC/ASME rules (as you seem to endorse in thread1203-189944)... then you're free to do whatever you want as long as you stay out of trouble!

jt

 

[dcasto]

Thanks jte. In my other post I was pointing out that you have a choice, risk based. A 50 psi vessel used at 5 psi is so low, why make an issue, or as I pointed out whats the cost to comply. I just hate when people try to intimidate others saying something is a law and that implys you could be thrown in prison or something.

Does API have a procedure similar to ASME too? I gave you two scenerios, rating the pressure down as the temperature goes up and rating the pressure up as the temperature rateing goes down. On the first, how much does the temperature affect the nozzel reinforcement?

BTW I'll be doing R stamps as these coolers are in high risk service.

 

[code1]

Previously, I had some vessels that needed to have nozzles added. They were previously U-Stamped and subsequently having an R-stamp after the modification.

It should be necessary as well for the Client to inspect and measure the local thicknesses as well to ensure that the original corrosion allowances were not exceded. Else the theoretical CA which you enter to re-rate may not be valid.

 

[jte]

dcasto-

It makes no difference whether you're going up in pressure and down in temp or up in temp and down in pressure. The procedure will be the same. One thing to keep in mind is that the allowable compressive stress ("B" factor) tends to begin to drop at a lower temperature than the allowable tensile stress. Probably not an issue with your ACHE's. Another alternative to NBIC would be API 510

http://www.api.org/Publications/new/api-publication-510.cfm

but I have virtually no experience with that standard - seems that most AI's that I've dealt with prefer (strongly) using the NBIC.

As for the other thread, with an existing installation I'd see if it makes sense to remove the U stamp nameplate and put a non-code nameplate on with a MAWP listed as 14 psi or with a new vessel I'd buy it with a 14 psi MAWP and specify no stamp. I agree with you though that in some cases folks can get over enthusiastic about sending people to jail... though there are times that can be a valid approach...

code1-

Good point about needing to get a good inspection prior to the rerate. Just to be clear, though, if I had a 48" nominal ID vessel with a 1/8" CA, I would calculate it with a 48.25" ID wether I was doing calc's for the original construction or for a rerate - as long as that 1/8" CA has not been entirely used up. If I still have 0.025" remaining CA and need to squeeze out a 2 year run with a max corrosion rate of 0.010" per year at some new rerated condition before I can get a new vessel delivered then it still makes sense to go through with the rerate. I've seen very few rerates where the original CA is increased, perhaps due to increased wind/seismic or other support (generally compressive stress)conditions governing.

jt

 

[dcasto]

API 510 then. I've had a separator re-certified under 510, I didn't realize it could do air cooler boxes/nozzels. I like to know the answer before I send the vessel off for an R stamp, normally I'll do as code1 suggest before wasting time at the shop only to find there was a wall thickness problem. Most of my stuff is clean dry gas so I end up with a lot of vessels having 0.001" to .125" CA.