Max torque on flange gasket 1

[jmward]

I'm using 1/8" SBR red rubber gasket (various sizes) and I am having a difficult time finding out what the min and max clamping forces on the seal should be. I called the manufacturer and of course they were no help. We had a failed gasket and now our costumer wants a writen spec. So if anyone can help me find a website that would offer clamping forces of the gaskets or better yet the torque ratings for bolts, I'd appreciate it.

 

[rconner]

I see that noone has yet responded to your inquiry on this normally vibrant list. I guess I would therefore be interesting to learn exactly how it has been concluded that the red rubber gasket specifically “failed”, to cause whatever problem occurred in this specific instance (I am aware that flanged joints have been utilized for centuries with all manner of gasketing materials at different times, including I believe leather, lead, various other metals, and all manners of rubber and composites etc. including that of which you speak, and I believe in some designs even with no gaskets!) It is also interesting that the manufacturer of the gasket product involved, presumably making same for the intended service, was apparently “no help”.
That being said, I was told many years ago (by someone I considered much more of a rubber expert than me!) that at least then many “red rubber gaskets” were composed of a very high percentage of some sort of “clay”. While I would not otherwise consider myself any sort of expert in cookie-cut red sheet rubber, I suspect it would follow that those rubbers would quite likely exhibit different physical properties, e.g. some lesser elongation or maybe even significantly greater compression set etc. (than for example some other engineered rubber pipeline seals with lesser or other fillers, e.g. such as those required for push-on and mechanical gasket seals by standards such as ANSI/AWWA C111/A21.11-07 American National Standard for Rubber-Gasket Joints for Ductile-Iron Pressure Pipe and Fittings.)
Flanged gaskets other than “red rubber” are available, and also some with special sealing features from various vendors that may be arguably helpful maybe particularly with variations in bolting diligence/expertise and variable joint loadings (e.g. see gaskets described on page 8-3 at

http://www.acipco.com/adip/products/Sect8.pdf).

While such molded gaskets and even with special sealing features may or may not be more expensive than sheet rubber, depending on factors such as size etc., I suspect in many common at least small piping sizes these molded gaskets are probably at least a little more expensive per each than red rubber basically stamped or otherwise sort of “cookie cut” from larger/bulk sheet.
With abject apologies to William Shakespeare (I think), perhaps “therein lies the rub” (so to speak), I wonder if this may in a sense be a case of one getting what one pays for?

 

[thalon]

We use a lot of gaskets from flexitallic.

http://www.flexitallic.com/

Any time we have an problem they have been a great help.

 

[jmward]

To answer some of your questions.

The failure: The pipe flanges were seperated and the gasket had been torn due to over clamping. It did not leak, but the since it was torn, we believe it eventually would. We torqued the bolts to recommended torque value of the bolt (not considering the gasket). No we are re-assembling and I need to know the max value so this doesn't happem again.

The vender: They are no help because they can not give a min or max clamping value to achieve seal without damaging the gasket. The refer to manufacturer who then refers to material supplier. The material supplier says they do not test for that kind of thing and simply make the material per a standard. The state we should contact the company who stamps the gasskets. So as you can see, this is one big circle.

I can find values for brand name gaskets, but nothing for a plain red rubber gasket. I have found some generic min torque numbers and that is where we are starting at. During our proof pressure testing we are going to increase torque in 10 ft-lb incriments if leaks are found.

So that is my situation better explained. Any suggestions?

 

[rconner]

I am now a little confused by the attempt (that I nevertheless appreciate) to explain what happened. Are you saying an over-clamped gasket somehow caused the flanges to become "separated" (if so I'm not sure I understand the physical mechanism that would cause such, unless the bolts broke etc!), or instead that there was something else unstated that previously occurred (e.g. a leak?) and the flanges were subsequently unbolted/separated for inspection? If it is the latter, can you describe or maytbe even better yet provide a picture of the tears you saw?

 

[jmward]

We unbolted the flange to gain access to a seperate issue on the project and found the gasket had torn. The gasket never leaked and we would have never had found it unless we manually seperated the flanges. Upon finding the torn gasket our customer started having concerns about the torque spec for the gaskets (flanges), rightfully so. This is where I'm at now. I hope this clears the air.

 

[KernOily]

Gasket clamping forces are controlled by the bolt torque (duh) and are set by the stress needed in the gasket material to effect a seal.

If you look in ASME Section VIII, there is a lot of material on gasket clamping, seating stress, flange face type, etc., all as needed to effect a seal.

You did not say what type flanges you have, whether they are spiral-finished, etc.

You probably don't want to hear this but it may be better to switch to a material for which there is a known required seating stress, e.g. Flexitallic or Garlock Blu-Gard. You can't go wrong with Flexitallic - they're cheap, you can buy them everywhere, good for all sorts of fluids, and you can calculate the required seating stress ;-) Hope this helps! Pete

 

[tothepoint]

Sometimes it is difficult to get an answer to a simple question!

Red rubber, all 60-70 durometer elastomers for that matter, should be assembled to a compressive stress range of 500 - 1500 psi. 500 psi as a minimum stress and 1500 as a maximum.
Do not use gasket seating values listed in ASME Appendix 2, as these are design stresses, not assembly stresses (there is a big difference).

Hopefully you are using the red rubber between flat face flanges. If you have raised face flanges you better select another material as you'll max out the smaller gasket contact area long before you develop any appreciable stress on your fasteners. If your flanges are FRP/Plastic, calculate your assembly torque at the low end of the min/max stress range.

 

[rconner]

tothepoint, I'm sure the original inquirer (as well as it appears the gasket manufacturer) were gratified to learn this was such a simple question. I was just curious what is the origin of the minimum and in particular the maximum compressive stress values you mention?

 

[tothepoint]

25 years experience designing, installing and maintaining bolted flanged connections.

 

[corrosionman]

Its a bit " Rule of thumb" but for many years I have understood that rubber gaskets should be compressed to reduce the thickness by 25% and this will give a secure leak proof joint. If you have a full / flat faced flange and the rubber gasket covers the full face then you will find difficulty in overstressing the rubber by normal flange bolt tightening.
The original query was about a torn gasket - - possibly perfectly OK for many years but torn on being dismantled ??
Regards CM

 

[rconner]

tothepoint, Thanks for providing your experience. While I think there are more robust gaskets available, I guess I was nevertheless some surprised by the quite low level of applied stress (that I would think might be applied, maybe even by a not too over-zealous mechanic in the field, with common flanged bolting?) that you indicated these flat red rubber flanged gaskets could withstand (e.g. even if assembled dry with some degree of confinement between say serrated flange faces?) This appears to be a quite confusing field. [I noticed incidentally that the quite extensive document at

http://www.usace.army.mil/publications/eng-manuals/em1110-1-4008/entire.pdf

on page 3-21 appears to give some interesting rules of thumb relative to gasket compression required to seal flat gaskets for internal pressure, and also refers to some sort of “Initial Compression” of “Soft Rubber” of apparently 4,000-6,000 psi.]