Gasket Leakage Due to Thermal Fluctuation 4

[Metcorr]

We will appreciate views and advice from colleagues on a problem being faced by us at one of our plants. The description is as follows:

In a heat-exchanger, we have a stainless steel jacketted gasket of graphite. The temperature of the heat exchanger swings between 250-300C at a frequency of around twice a month. Normally, it stays around 300C. The service is steam in the shell and cooling water in the tube. The shell is of carbon steel while tubes are of stainless steel 304.

We have been facing frequent leak at the gasket at the interfact of channel head cover and the shell. When we install the gasket, we tight the bolts and face no leakage when the exchanger is taken in service. However, after a 3-4 months, we face leakage. We perform hot-bolting and leak stops. However, it proves to be a temporary as leaks again starts and further tightening does not stop the leak.

Can some one guide us the probable cause of gasket leak and are there any other gasket other than stainless steel which can help resolve the proble. Feedback and advice will be greatly appreciated.

MetCorr

 

[gr2vessels]

I would check first the design calculations, particularly if Appendix 2 was employed. I would look for the allowable stress for the flange material, the rigidity calculations and to see if there is any margin built in the girth flange design. It also could be an excessive overtightening of the bolts with the obvious deformation of the channel flange;- did you check/measure the old gaskets for signs of excessive and uneven flattening?
If you arrive to the conclusion that the flange is not thick enough, then we'll look for options to increase the thickness, within the Code requirements, without altering the original scope and design...
cheers,
gr2vessels

 

[HeviiGuy]

Gasket creep, perhaps?

Although there may have been sufficent load applied to prevent leakage for the first few months, creep may have resulted in bolt load-loss.

Assuming that the designer had put in enough meat into his specified load to compensate for future load loss, it may be moot if the initial tightening procedure resulted in lower preload than desired. Were the studs on this joint simply "torqued". If so, what steps were taken to ensure that the required loads were actually achieved?

Ciao,

HevïGuy

http://www.heviitech.com

 

[unclesyd]

As posted above make sure initial preload of the fasteners is sufficient to seat your particular gasket.

Some questions.

Was this the original gasket by design.

What is the flange and bolt material?

What is the size of the gasket?

No more than you are cycling at your temperatures you shouldn't be having a problem.
We cycle numerous 24" to 30" flanges between 20C and 285C around twelve times a day using a graphite filled flexatallic gasket.

 

[Metcorr]

Thanks for advice by colleagues. As gr2vessels has suggested, we will look for desing calculations. Also, a closer look at the gasket has revealed that in fact, the gasket does have signs of localized damages. Following may please be noted as asked by unclesyd.

1. This gasket is origional gasket by design.

2. The bolts are of B7 and flange is carbon steel overlayed with Stainless Steel.

3. The gasket diamter is around 700 mm.

MetCorr

 

[HeviiGuy]

Metcorr:

Of course, it's very important to review the design. However don't get hung-up on this to the detriment of other major variable: Procedure.

The poor gasket is often the first to be pinned with the blame when something goes awry. Concurrent to your design analysis make sure that you review the actual assembly procedure.

Ciao,

HevïGuy

http://www.heviitech.com

 

[SnTMan]

Metcorr, an option you might consider is change of gasket from jacketed to corrugated metal, graphite covered (Graphonic style). It should prove more resilient than the jacketed. The m & y are also such that your bolting should be sufficient.

If all else is OK, you might also look at adding belleville washers to help retain bolt load, if the thermal cycling is causing the problem. This may require longer studs.

These can be tough to solve, and may require several attempts. Good luck.

Mike

 

[Metcorr]

Thanks SnTMan for your suggestion regarding changing the gasket type. We will look into the corrugated metal type gasket also.

MetCorr

 

[mkkessel]

The poor gasket is typically the last in a series of design calculations made to compensate for any number of sins. Heat exchangers and boilers are maintenance heavy equipment in terms of being opened up for routine inspection. The frequency of removing and reinstalling covers tends to allow for a number of potential sealing issues to occur.

1. Large diameter flanges are more difficult to machine flat than small. Check the mating faces to determine of they remain flat individually and parallel to one another. The repeated dis-assembly and re-assembly of this unit may have resulted in less than stellar mating surfaces.

2. The flange load on the gasket must be evenly applied, either by tightening the bolts in a crossing pattern over several passes until the proper bolt stress is achieved.

3. Metal jacket gaskets are a typical product used in this application, however some PTFE composites have become popular in chemical applications where temperatures are in line (500-550F)

4. Do not use a graphonics or graphite laminate material in this application. The use of graphite is limited by its tendency to facilitate pitting corrosion in some stainless steels, and to promote galvanic corrosion between dissimilar metals (due to its electrical conductivity).

 

[HeviiGuy]

The flange load on the gasket must be evenly applied, either by tightening the bolts in a crossing pattern over several passes until the proper bolt stress is achieved

Being on the same page, I think that mkkessel had meant to add: "...or by simultaneous bolt tensioning (or a high percentage thereof)"

Ciao,

HevïGuy

http://www.heviitech.com

 

[SnTMan]

Metcorr, in light of mkkessel's opinion on the use of graphite, it is possible to face the corrugated metal backing with other materials, check with your supplier.

This style of gasket (with graphite facing) is very common in shell and tube exchangers, however you need to satisfy yourself that you do not create other problems down the road.

Please let us know how this all works out.

Regards,

Mike

 

[mkkessel]

Just to add a point for clarity. Metal jacketed graphite as you use now will probably not create issues with galvanic corrosion however it may be prudent to inspect for it's presence at next inspection. The mention of galvanic corrosion and use of graphite laminates was cautionary primarily because both stainless and carbon steel are present.

Graphonics are also available with a PTFE facing, again keeping in mind limitations on your operating temps. Along the same lines if the gasket is strictly an ID X OD design, the ultimate in sealing technology would be a KAMM-Profile gasket. There is a large initial expense, but the core is reusable and facings relatively inexpensive. Normally considered the ultimate "gasket" problem solver.

 

[SnTMan]

mkkessel, agreed these are top-line gaskets. However the only resiliency is whatever exists in the facings, the core is very rigid. The corrugated metal core may provide some recovery to compensate for the dimensional changes due to thermal cycling. I do not consider this definitive, other styles may be useful as well.

Regards,

Mike