PCC-1 Flanges Too Tight? 1

[PTMX]

We have recently adopted PCC-1 torque figures for bolting B16.5 flanges.
Feedback from the technicians is that the gaskets are now being found with the spiral windings crushed into the pipe bore and the outer rings are tending to cup.
We use Flexitallic spiral wound gaskets with 'flexicarb' filler.
Has anyone else had this problem?

 

[PAN]

PTMX,

Do you use inner ring with your spiral wound gasket?

 

[PTMX]

We only use inner rings for the 1500# flanges, the ones in question are 150# and 300#

 

[GasketBob]

The phenomenon you're seeing is called radial buckling and it happens with all spiral wound gaskets because the sealing surface thickness is .175" thick and the centering or guage ring is .125" thick. When you compress the gasket the windings have to go someplace and they protrude to the ID. Inner rings eliminate the radial buckling; however, then you create a required gasket stress to seat the gasket which is higher than can be achieved by either the flange thickness or the strength of the bolts.

 

[unclesyd]

GasketBob,

Inner rings eliminate the radial buckling; however, then you create a required gasket stress to seat the gasket which is higher than can be achieved by either the flange thickness or the strength of the bolts.

This statement isn't correct as the spirals create the sealing surface of the gasket. One ring or two the required seating stress is the same. These ring are only for location of the spirals.

 

[PTMX]

Maybe they have always bulged inwards and we are now more aware since the training to introduce PCC-1. Alternately could it be the change to graphite filler makes radial movement more likey with the lower coeff of friction?

 

[GasketBob]

All standard spiral wound gaskets, without inner rings buckle! PTFE spiral wounds actually buckle the most because of the extremely low coeff of friction and the uncompressible nature of PTFE. ASME now dictates the use of inner rings in all PTFE spiral wound gaskets as well as many other gaskets above Class 900 (See ASME B16.20 code, 3.2.5)

Graphite filler is fairly compressible; however, once it reaches its full compressive state, it causes the spiral wound to buckle inwards.

There are spiral wound gaskets on the market that eliminate inward radial buckling by allowing this buckling phenomenon to occur to the OD of the gasket and not into the pipe stream.

 

[tothepoint]

ASME PCC-1 torque values are reference values only as they represent 50% bolt yield for high strength B7 bolting. Flange/gasket/bolt stress analysis is required to identify the optimum assembly bolt load for different flanges. This work is currently being performed by the PVRC Bolted Flange Connection Committee, and will be published this fall.

GasketBob's comment regarding the stiffness of inner ring spiral wound gaskets is 100% correct. Inner ring spiral wound gaskets require significantly higher bolt loads to compress as the spiral windings are no longer free to deform towards the ID- thus they are also less prone to buckling.

Anti-buckling spiral wound gaskets are indeed a good solution for eliminating buckling WITHOUT the added cost and bolt load penalty of inner rings. Two companies manufacture anti-buckling spiral wound gaskets- Garlock and Teadit.

 

[GasketBob]

"tothepoint" commented on two companies manufacturing "anti-buckling" gaskets. I did a little research and have found out that Garlock calls their's the "EDGE Gasket"; I haven't been able to learn anything about the Teadit gasket, but will keep researching.