Insulating Bolted Connections

[scrobe]

Can anyone point me in the right direction to obtain technical information to show why one should NOT insulate over flanges, especially those in hot service?

Our Energy Team wants to insulate to reduce heat loss; others want to leave them exposed since potential bolt relaxation will cause leaks, and as such, will create HSE incidents, and add cost for removal and replacement of the insulation every time there's a leak.

Thanks for your help.

 

[mroz]

I can't point you to a bona-fide publication, but I strongly support NOT insulating flanges. Especially in a corrosive service. Exactly for the reason you mention, LEAKS. If the service is exceptionally hot, a stress analysis should be done, and at this time a flange report could be generated to show potention trouble at flanged joints. I know that insulation manufacturers make removable flanged joint covers which is another possibility.

Mroz

 

[Dan76]

I Can't point to any publication either but insulating over pipe flanges is definitely a BAD idea with permanent insulation any water that gets into the insulation will creep and cause corrosion, especially in carbon steel. I agree with mroz, removable lagging or insulation pads will insulate when needed but can be removed for inspection, maintainence or whatever. If Bolt relaxation is a worry just inspect and retorque as necessary, once the system is setup the lack of any differentail temperatures may even cause less problems and efficency goes up!

Of course you could always ask the pipefitters, the old guys might not always have the Education but they've got a ton of experience and "Horse Sense"

 

[kstaylor]

If the flanged joint was not designed originally for insulation I have found it is usually difficult to add insulation and NOT have the flange leak. The leakage will usually not manifest itself until the system goes through a temperature cycle where the fluid (and thus the flange) is cooling down and the bolt temperature lags behind the flange temperature. Bolts stay hot and long, the flanges cool and shrink, gasket load is reduced and a leak is very likely.

The use of spring washers (i.e. Belleville washers) can sometimes eliminate the leak potential by maintaining a minimum bolt load above the critical load required for a seal. Spring washers are available for pipe flanges from both Key Bellevilles and Solon. Other spring washer manufacturers may have them also.

The correct size and number of washers have to be determined by differential thermal expansion calculations of the flanged joint, including the gasket, and the bolts. Watch out that the bolt load needed to seal the flange with spring washers may exceed the strength of the flange at the new higher temperature and cause rotation (i.e. cupping) of the ring of the flange, especially in the lower pressure classes. This will almost guarantee leakage, even with the washers installed.

As I said earlier, if the joint is designed in the beginning to be insulated it is possible to have leak free joints. Retrofits are more difficult.

 

[scrobe]

You're right kstaylor. Another thing that we have found in our petrochemical facilities is a contract EPC may have designed the piping systems with a certain class flange that cannot meet the design temperature criteria that results from insulating the flange; therefore a next higher flange class should have been specified. Something to think about when developing a piping standard!