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20
To:
Subject:
Deformed ring grooves after
make-up and
break-out
Question:
We have a job in process using
7-1/16” 5M flanges and RX-46 ring gaskets. We have made-up connections,
applying 722 ft-lbs torque (using Easyrun 100 as a lubricant). Upon
dismantling of the connections we have observed metal deformation around
the whole of the outer sealing surface of the ring groove. We have not
seen this before. Can we accept this condition, and should we consider
the condition as normal?
Answer:
The phenomenon you have observed
does frequently occur. We do not know the
make-up tension you produced
in the bolts using the combination of lubricant and torque you mention,
as we do not know the lubricant’s characteristics. The
tension in the
bolts ultimately determines everything about a bolted connection.
Despite its common use, bolt torque serves as a poor predictor of bolt
tension.
With this said, the answer to your
question should come from a practical test. Assuming the deformation
appears slight, we at WOODCO USA believe a repeat make-up and test of a
typical portion of your assembly will demonstrate that the condition
does not get worse to a degree that might cause leakage. The number of
make-up and test repetitions should approximate the number you might
expect in the field during the reasonable expected useful life of the
equipment.
If your testing produces
disappointing results, you may want to consider the following
information.
API
weld neck flanges made of
45K
yield strength material may have a hardness as low as 140
HBW. API integral
flanges made of
60K yield strength
material may have a minimum 174 HBW hardness,
for “sour service” these flanges would have a specified maximum hardness
of 237 HBW. Standard service equipment has no specified maximum
hardness.
RX ring gasket design produces a
specific “self sealing” characteristic. Without explaining all the
details ( see
http://www.woodcousa.com/field_appr.htm
), this factor of
design contributes to your problem. The RX gasket profile intends the
gasket to seal only on its’ O.D. The wide surface on its’
I.D. bears
over more surface of the ring groove in the flange, and as the flanges
come together and compress the ring gasket, this wider surface
engagement on the I.D. causes the great majority of the “coining” force
to bear on the narrower O.D. of the gasket (and the O.D. of the groove).
API specifies hardness maximums
for ring gaskets specifically to reduce the problem you have
experienced. Ring gaskets without an API Monogram may not have this
hardness control. Even with controlled hardness ring gaskets, repeated
make-up will slightly deform the ring groove of flanges that fall within
the hardness limits required for “sour service” applications.
If your situation involves using
weld neck flanges made from 45K yield strength material, you can expect
this problem to continue. Substitution of higher strength material for
weld neck flanges will improve the situation in regard to deformation of
the ring groove, but will increase problems in welding.
At WOODCO USA, we have found this
problem limited to appearance. Repeated make-up and test has not
produced connection leakage, even when ring groove distortion on 6B test
rack flanges that seal with R or RX ring gaskets has reached a point of
widening the ring groove beyond allowable tolerance.
Click here for
Q & A 21 Ring Gasket Evaluation
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