My experience has been that flanges are usually broken apart after hyrdo testing anyway for the purposes of inspection, etc. In this case, one should most definitely replace the gaskets. No gasket manufacturer will recommend re-use of gaskets. Or another way to state it, just so we're clear, once a gasket has been bolted between flanges and then unbolted, every gasket manufacturer will recommend replacement. I've known of many cases where gaskets have been successfully re-used, but they are such a small part of the overall cost of the vessel, why take a chance? And the compressibility characteristics of the gasket are changed after loading, then unloading, even at ambient temps. as in hyrdo'g. I've had people tell me they think that us gasket manufacturers must conspire as an explanation for our consistency on this policy in order to sell more gaskets. But its not true....I swear! Really!
"If we do
> replace the same how do we ensure proper flange
> tightening?"
Same as original bolt-up. Hopefully some type of method was employed, preferably torquing at a minimum.
The European Sealing Association (ESA) in partnership with the Fluid Sealing Association (FSA) have a pretty good general booklet on gaskets that addresses this point. Its free for download by registering. See page 26.
http://www.europeansealing.com/publications/guidelines.htm<http://www.europeanse\ aling.com/publications/guidelines.htm>
How about the question of flanges that remain buttoned up between hydro and service? I guess that's a bit more complicated. The gasket industry is mute on this point, at least in any published literature I've seen. First off, I think these cases are rare. The customer will almost always want to install equipment inside the vessel or at a minimum do some inspections which will require flange and gasket removal. But I think in rare cases whereby the gasket remains installed between hyrdo and service, the general consensus is that the gasket should be replaced, although the case for doing so may be weaker than as above. Again, it gets down to economics. How much is the cost of a replacement gasket and the time vs. a leak after startup? Most vessels are shipped from a fab shop to the jobsite. The travel involves motion and vibration and who knows what else. At a minimum the gaskets should be re-torqued at jobsite but preferably replaced after hydro. and transfer. Here are some general rules of thumb that can apply to any nonmetallic or semi-metallic (spiral wound) gasket:
-A gasket that is loaded and then unloaded will have more of a tendency to leak
than one that is not.
-The more a gasket is subjected to pressure fluctuations, the more it will tend
to leak.
-A particular gaskets "load required to seal" value is changed after each
loading/unloading cycle.
Hope I've shed some light instead of blinded with brilliance.
-Davis
Replace only gaskets that were removed after the hydrotest.
>
> Hello everyone,
>
> I would like to know if it is mandatory to replace
> all gaskets after pressure test of piping. If we do
> replace the same how do we ensure proper flange
> tightening? I believe one of the major role of
> pressure testing of pipes is to ensure proper bolt
> tightening and leak proof flange joint.
>
> Pankaj Mandal.
>
>
>
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[Non-text portions of this message have been removed] Received on Wed Aug 31 10:13:00 2005
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