O-Ring Between Flat Face Flanges?
O-Ring Between Flat Face Flanges?
(OP)
I have a manufacturer telling my client to use a 7mm o-ring to seal between two 16" 125# cast flanges. There is no o-ring groove. The manufacturers sales rep is saying to just put the O ring between the flange faces, even though there is nothing to locate it. This seems absurd to me. Has anyone ever heard of this being proper piping practice?
The joint in question is between a pump suction nozzle, and a special pump suction elbow, both supplied by the mgf. I am concerned about cracking a flange in the short term, and the o-ring failing longer term as it creeps out. I am tempted to over rule the recommendation, and use a proper gasket (cloth inserted rubber, 1/16th thick). This is at a wastewater plant, so no ASME standards...all the standards are AWWA. Pressure is in the range of 20 psig, digester sludge service.
The joint in question is between a pump suction nozzle, and a special pump suction elbow, both supplied by the mgf. I am concerned about cracking a flange in the short term, and the o-ring failing longer term as it creeps out. I am tempted to over rule the recommendation, and use a proper gasket (cloth inserted rubber, 1/16th thick). This is at a wastewater plant, so no ASME standards...all the standards are AWWA. Pressure is in the range of 20 psig, digester sludge service.





RE: O-Ring Between Flat Face Flanges?
I would use a full-face soft gasket.
RE: O-Ring Between Flat Face Flanges?
I've never heard of it and it is neither common nor good practice to use a o ring in such a situation.
Most flat face cast flanges require a full face gasket, normally made of neoprene (wet suit material) or similar rubber or other resilient material (EPDM) 3-4mm thick.
If you look hard enough, I'm sure AWWA has advise or a requirement for flat face flanges.
I didn't look very hard and found this - see 2nd page under gaskets
http://www.pacinst.org/wp-content/uploads/sites/2/...
Refers you to the manufacturer of the flanges, not the equipment. The o ring idea still sounds very poor advice to me though.....
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
RE: O-Ring Between Flat Face Flanges?
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RE: O-Ring Between Flat Face Flanges?
I think we're all agreed though that unless you put in a groove to hold the O ring, this idea is just nuts.
just for curiosity, how accurate do you need to machine the groove to get your clearance right. I presume you want only a very small gap to squash the o ring just enough and not leave it open to shear forces. what do the flange specs / codes say about O ring grooves?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: O-Ring Between Flat Face Flanges?
While o-rings and o-ring like bulbs have long been used, with appropriate housing or support, and I think generally VERY good (and in many cases superior to at least some flat gasket) success in many sorts of gasketing means used in many sorts of "flanged joints", and going way back I think to at least to the early half of the 1900's, I'm just trying to understand why anyone would want to just throw a floppy o-ring itself in between a couple flat flanges (as I guess all other repsonders are assuming is meant by the OP)?
RE: O-Ring Between Flat Face Flanges?
We machine the grooves per the Parker O-ring handbook, which means typical compression of about .025" on a .12" o-ring section.
As far as cast iron flat faced flanges, I'm not even sure where I could purchase a cast grey iron flange. Maybe I could find a cast ductile or nodular iron flange...but most likely what's in stock at the piping warehouse is plain old steel, and it's what we use.
I'm sure cutting an o-ring groove in a flange violates the piping code somewhere, for some condition. It certainly adds a stress concentration, though it's in compression if the flange is torqued down correctly. For factory test equipment at pressures well below the flange's capacity at room temp., it's just not a concern. We are not putting grooves in the stuff that goes out the door, only on the blind flanges and test connections (factory equipment).
Before we did this, we would routinely have to stop a hydrotest and repair leaks/replace gaskets. My opinion, is that O-rings are much more idiot proof in their ability to seal reliably.
RE: O-Ring Between Flat Face Flanges?
Installing an o-ring without any surface to back it up relative to the direction fluid pressure is applied is poor practice. Since the only thing keeping the o-ring in place is the friction provided by the opposing faces compressing it.
RE: O-Ring Between Flat Face Flanges?
It appears that AWWA 'hub flanges' would have room between the OD of the hub and the ID locus of the bolt ring for an o-ring to be fairly well trapped for suction service, but it would require a very fat o-ring, not 7mm cs. ... or non-standard flange dimensions, which is entirely possible between two parts from the same manufacturer, one of which is even described as 'special'.
We here have not seen photos or drawings of the affected parts, and it smells a little like Slagathor has not personally observed the details, either.
IMHO, it would be a good idea to dig deeper into the joint design before maligning the sales rep. Yeah, many of them are slimy BS artists, but every once in a while you run into one who really really knows his stuff.
Mike Halloran
Pembroke Pines, FL, USA
RE: O-Ring Between Flat Face Flanges?
That would be true if you ignore the pump needing an NPSH.
RE: O-Ring Between Flat Face Flanges?
also pivoting - if you have low grade cast iron yes - it may be squashy, but force is located at a small point creating a high bending moment. full face gaskets spread the reaction load provided by the bolts over the whole face of the gasket. It is one of the problems of FF gaskets as the sealing force is generally low and hence leaks are difficult to eliminate just by tightening the bolts a bit. Ductile iron has got much better over time, but you still get some low grade cast flanges in this type of application.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: O-Ring Between Flat Face Flanges?
It's more a problem when you use a stiff gasket covering less than the full flange face than it is with a more compressible rubber gasket, for obvious reasons. A rubber or composition gasket covering the full face of the flange is the right solution when mating to another flat faced flange. If the mating flange is raised face, you need either a limiter ring of some stiff material to limit the bending when someone gets too aggressive with the bolt torque, or you need to change one of the flanges.
RE: O-Ring Between Flat Face Flanges?