blind flange gasket failure cause?

Is it possible (particularly if this condition was not observed after pressure test) that eventual creep/cupping of the plastic? flange under time/service load simply allowed the gasket to in effect "blow-out of this area under pressure (I have heard third-hand of at least a few similar-sounding "issues" over the years with some sorts of flanged joints in plastic pipes)?

I would suspect that the flange was not torqued down evenly which is a common mistake, then when it and the gasket cooled your leak developed.

Dear S Jogia..

Well well .. I assume

1) To start with.. the gasket material , it's not the correct one suited. or the "correct" neoperene, causing the gasket to "swell / grow/ bulge" under pressure, consequently the joint will leak under a pressure / temp cycle. Try using a grade of rubber higher than the one currently used, with less creep

2) There is also a possibility of uneven and / in adequate torque

Are the flanges furnished with RF or FF?

1. Please recheck the mating surfaces of flanges and gasket itself visually.

2. Exchange the gasket with a new one, may be you need a thicker one in case you use a flanges with a FF, and recommend to use a rubber gasket.

3. Be sure that the tighting of bolts is sufficient.

4. Be sure that the bolt tighting is carried out in accordance with the RECOMMENDED BOLT TIGHTENING PROCEDURE: It is important that all bolted joints be tightened uniformly and in a diametrically staggered pattern.

if the mating flanges are smooth, you can get a squeeze-out under pressure. Thus you may need to restrain the gasket if it is elastic enough to get squeezed out.

I have seen some slight concentric reliefs molded into the plastic to reduce squeeze.

Possibly something like this will work? I have also seen the gaskets made with a built in fibrous support cloth-like material embedded in the core for the same purpose. I know head gaskets use restraints like this on engines

Hello there,

ur problem is very tricky. If the flange is flat then it will not last long , if the fixing is done by bolts and nuts the nos. can be increased, for example instead of 4 nos. use six nos. of smaller size. The best way is to have dished end type of flanges then the pressure exerted is reduced considerably. II don't know if u get such shapes in HDPE .

Saradhi

Dear Guest,

In that type of piping systems HDPE, the flanges are standards, therefore the number of bolts and all dimensions are standards in accordance with the flange rating. Therefore, no way to increase the number of bolts, otherwise you have to use another flange with the same NPS but with a higher class (rating).

For example, if you use a flange NPS 6" with rating 150#, and if you need to increase the rating, the flange shall be NPS 6", rating 300# which means that the number of bolts are more and all dimensions are differ than the lower class.

Hello All,

Thank you for all of your feedback. Based on your responses, examining the failed gasket, and after speaking with other experts, I think the most likely cause was incorrect tightening of the flange compounded by thermal expansion/contraction.

The way the gasket was distended in one small corner makes it look like it was pinched out due to the bolts not being tightened in the proper order, and we have weather data showing 20-degree (celsius) temperature swings during the time the gasket was in service.

I'm not sure what type of flange we used (FF vs. RF?), but it is a pretty standard blind flange rated for much higher pressures than our service pressure. Also, the neoprene gasket was rated for water, wasn't exposed to the sun or otherwise damaged prior to installation (it was installed right out of the box), and was well within the proper temperature ranges.

Thanks again for your advice, let me know if you have any other ideas based on the above.

cheers,

S.

Yes SJogia,

"I think the most likely cause was incorrect tightening of the flange"

That is exactly what I was expected at my post #4.

It will be interesting to hear if the contractor agrees or can be persuaded to agree with this assessment, particularly if assembly isntrudctions were not clear or residual bolt torques were not measured etc. after the fact and this dispute involves any significant damages. Is the project still in a contractor's warrantee period (one would think so as you stated the problem apparently occurred only ten days after the acceptance test), and I would be curious also is there a detail available somewhere that describes the flanged plastic fitting, gasket?, and? also plastic? blind flange that encountered this problem?

Yup, once again I've found a thread that had originally escaped me! Better late than never, I suppose:

Whereas there are many factors which can influence whether a bolted joint will hold or fail, proper bolt load certainly ranks amongst those in the top. Note that "bolt load" and not "bolt torque" was written. Although the two are mutually exclusive, many people still seem to think that they're the same thing. Nothing can be further than the truth, relatively speaking. This seems to be the crux of these type of problems.

In this case, even if the "right torque" was applied in a multi-pass pattern with a fully calibrated torque wrench, the result could have been that some of the bolts were too loose, or, perhaps even too tight. Neither condition is conducive to a leak-free joint!

Rather than getting into a long-winded description, here is a link providing further insight: http://www.heviitech.com/Hevii_TorqueDanger.html