HEX - De-rating

[cespi82]

Hello everyone,

I work for a company in the Middle East and there are a series of Heat Exchangers in my facilities that were originally design to operate under certain conditions that are not longer valid as the gas wells that feed the system where they are installed in have declined.

The original (and still current) tube side design pressure is 2500 psi considering the down hole pressure at that time. Well has declined and operating pressure is far below the original conditions. PRV of this HEX has been reset to 1970 psi. Every time there is a requirement of Hydrotest, the test pressure used for the tubes is 3750 psi (1.5 x 2500psi)and there are repetitive tube leaks when approaching that test pressure (all leaks reported above 3650 psi). The leaks occur in thinned areas due to general corrosion after over 30 years of operation, so we might now be reaching MAT @ 2500psi design pressure but we might still have a long way to reach the MAT @ 1970 psi.

Under this scenario what I am thinking of is to derate the HEX Tube Side to the PRV Set pressure of 1970 psi and do the Hydrotest at 2955 psi. There's no physical way that the pressure in the system reaches or passes 1970 psi.

I have seen previous threads about de-rating and what I can understand is that there's no such procedure or requirement to derate but at least the name plate must be changed to include the new Design Pressure.

This new Name Plate can be made by ourselves? Is the involvement of a ASME Authorized Repair agency required to do this? Local jurisdiction does not cover this particular case and give no guidance about it.

Thanks a lot for your comments and suggestions.

 

[TGS4]

Did you just write that you have tube leaks at LTAs? I think that this is a more pressing consideration than re-rating the heat exchanger.

Nevertheless, if you are re-rating the equipment, follow the procedure in NBIC (if your jurisdiction provides non guidance), unless your company has another policy/practice. Don't concern yourself about whether a re-rate is increasing or decreasing (your case) the conditions - the same procedure is involved.

 

[Mech2325]

Please clarify "tube leaks". Are these the leaks in "tube to tubesheet joint" or are these the simple tube leaks which can be rectified by plugging the tubes? What is the TEMA type of your exchanger?

 

[cespi82]

Hello TGS4 and Mech2325,

I will extend on the leaks as I think I caused some noise.

The leaks are very seldom leaks in the tubes itself, specifically in the U-Bends (or return bends) and only in the two innermost rows. The leaks are due to Corrosion/Erosion but wall thickness rates are relatively low as they have been in service for over 30 years as I commented before. The leaks are occurring only during the Hydrotest and only when approaching the HT pressure of 3750 psi, which is based on the original, and still current, Design Pressure of 2500 psi. When these leaks have occurred, we have plugged the leaking tubes and put the HEX back on service without no further issues. We conduct the Hydrotest as part of the Turnaround scope every 6 years and historical records show the occurrence of these leaks during the HT only, no leaks during operation.

For the original design parameters, the current wall thickness might be approaching critical levels but we can still have a long way to go under the new proposed Design Pressure of 1970 psi considering the relatively low corrosion rates after over 30 years of operation.

Unfortunately we have no technique or inspection tool available locally to run a volumetric UT test on the tubes to know the exact thickness of the tubes as the return bends are pretty tight so tools cannot pass through. Some companies from US have attempted with no success. We actually have not pushed too much on performing this thickness measurements as our RBI assessment shows these equipment as low risk.

I bring the idea of De-rating to the table based on the fact the operational conditions have changed significantly as I described above (I think I described well) and the tubes are working comfortable at this operating pressure. The problem only comes to the picture when exposing the tubes to a very high HT pressure of 3750 psi.

Mech2325 they are all TEMA type "CGU". Could you please explain how the TEMA Type is affecting/influencing the path forward on this case?

Hope it is a bit clearer now.

Kind regards,

 

[Mech2325]

Cespi82: Your design code is ASME alone or are you using AS1210 standard too (1.5 times MAWP) is the requirement of AS1210. And if you are to only refer to ASME and TEMA, then the test pressure with 1.3 times design pressure comes out to be 3250 Psi which is below the crtical pressure at which the tubes develop leakage.

 

[wsu1975]

Cespi82: I agree with TGS4 on using the NBIC or the jurisdiction requirements for re-rating. Also, It helps greatly if you identify why the equipment has been re-rated to provide a record for future engineers on the job.
As far as operational pressure testing, using the code of record or original design and fabrication hydrostatic testing requirements (1.5 or 1.3 MAWP/Design Pressure), unless required by your insurer, regulator, or owner requirements, does not help much. As an example (yes, I know this is not a nuclear facility), ASME Section XI, Article IWD-5000, the pressure requirements for system leakage testing is normal operating pressure, and hydrostatic pressure testing requirements are 1.1 relief or safety valve setting for T up to 200F and 1.25 for T > 200F. I would recommend finding out what is permissible your case.
If required to use the original hydrostatic test pressures, and you wish to use the current code pressure testing requirements as Mech2325 suggests, I recommend reconciling the current code with the code of record, as design, fabrication and testing requirements may have changed/improved over the years, leading to lower hydro testing pressures.
As far as the NDE of the tubes, if the ID is large enough, you may be able to use eddy current testing to evaluate the tube conditions.
Best regards,
Wsu1975