How to fix leaking flange connection of high temperature heat exchanger

[Stefan2211]

Anyone has quick solution:
Problem: We have a large heat exchanger with floating head. The diameter of the tubesheet is around 900 mm with 641 tubes (25.4 x 2 mm). The whole bundle is SS316L (5000kg). The bundle had to be heated in a furance to remove the internal turbulators as there was a severe coking problem. After reinstatement of the bundle (30 m high - vertical)the hydro test failed at 8 bar. There is a leak between tube sheet and floating head. Max. torque of 300 Nm has been already applied. We suspect the heating could have deformed the tube sheet (85mm thick). In the furnace only the tubes were heated - tube sheets were outside the heating zone.

Question: How can the leak be repaired in short time?
Some ideas:
1. Using bolts with higher torque allowance.
2. Using different gaskets to allow a higher compression rate. Right now it uses metal grooved gaskets (2 mm stainless steel with 1 mm graphite layers). Could we use double (sandwich)?

The heat exchanger is operated with 275 Deg C hot oil in the shell and 200 of hydrocarbons in the tube side. A leak would cause major problems.

Thanks for any suggestion.

 

[LittleInch]

Drawing please?

Furmanite?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Stefan2211]

Dear LittleInch,

drawing here:

https://res.cloudinary.com/engineer...8/tips/Leak_floating_headpng_Page1_wvxwve.png

We are using a special solvent which is only chemical resistant to FFKM,PTFE and Graphite. We do not want to use any other seal material than graphite/metall. It is also the internal gasket, which can not be checked for leaks and it is always under thermal expansion when temperatures change.
We are still thinking of thicker graphite lining to have more compression zone than a 1 mm layer. However, I am looking for someone who has experience with such problem.
Thanks anyway.

 

[TGS4]

Have you measured the flatness of the flange surface? Is it within ASME PCC-1 tolerances?

 

[Stefan2211]

We checked and can not find any deformation. However, it looked like the gasket had a manufacturing fault or it was handled badly by our operation. There are two spots with dents in the gasket which means the sealing surface is low at that spot. We will use a new one and try again

 

[mfgenggear]

I am not expert with this line of product.
Did you replace with new gaskets?
It seems to me once the gasket has been installed and compress it will not be reuseable. Wrong or right
Try new gaskets

 

[Stefan2211]

Problem seems to be solved. Deformation could not be detected. Mark in flange has been grinded, welded and polished back to flat surface. Gasket has been replaced. 12 bar hydro test (1.3 x DP) has been finished.
So what was the cause?
1. Mark in flange seal face was known but mech. team did not repair
2. Previous new gasket had a round deformation (bad handling?) and was not suitable.
3. Bolt tightening might have been done uneven
4. Maintenance team is not permanent team but partially contractor = not skilled enough for judgement of gasket condition etc.

Action to be taken:
1. Always use new gaskets and check condition especially on metal groove gaskets with graphite layers.
2. Supervise the reinstatement by experienced mech. staff. Do not believe everything is ok.

 

[mfgenggear]

Stefan
glad it's resolved, and thanks for the reply

 

[Stefan2211]

Yup, lesson learnt. Double checking takes few min - detecting leak, replacing gaskets and still need repair was 3 days....