RE : ASME RF Flange Joint using Spiral Wound Gasket

Thanks Rene and Tony for your insights.

Infact, ASME B16.5 2003 does mention about Gaskets for class 150.

It lists gaskets of group 1 of table C1 of Annex C. This covers spiral wound gaskets also. It however does not mention the use of low stress spiral wound type in the case of class 150. However it does address the use of low stress gasket (para 5.4.2) when 'low strength' (ASME B16.5 Table 1B categorization) bolting is used. A320 Gr B7M falls under 'Intermediate strength'.

ASTM A320 Gr B7M is a NACE MR0175 / ISO 15156 : 2001 (Technical Corrigendum) acceptable material for SOUR service application. The application of ISO 15156 is for upstream processing facilities, where the incidence of H2S related SSC are higher. The flange joint is to be used for an associated gas, offshore facility. Though the NACE compliant material really applies to those bolts which are either directly exposed to the service fluid or those which are confined underground or by flange cover or such arrangement. NACE also cautions that it may necessary to derate the flange joint as a result of the use of SSC resistant bolting.

The bolt loads to be considered under ASME for flange sealing are 2.

1. under operating condition will be,

Wm1 = (3.14*G^2*P)/4 + 2*b*3.14*G*m*P

The first part is the pressure loading and the second is the (m times P) load for seating.

I have had this doubt what is the need to have the 2 in the second part of the expression, when it is not there in the seating load formula below.

3.14*G*b is the effective gasket seating area. Is this for both the sides?

2. Under seating condition will be,

3.14*b*G*y

This is where the class 150 bolt areas multiplied by the allowable stress for bolting (flange joints use a lower allowable) is not able to provide the y (10000 psi) gasket seating stress.

A joint using spiral joint uses the centering ring as a stop when the joint is bolted up. The flange joint bolting should have a strength allowance for this. When the flange face seats on to the centering ring, the load increase in the bolt will be more rapid than it would have increased during the compression of the spiral rings. The bolt strength and the corresponding rigidity of the flange itself should be enough to cater this seating. The following link at Eng-Tips makes some additional reading, <a href="http://www.eng-tips.com/viewthread.cfm?qid=192518&page=8">http://www.eng-tips.com/viewthread.cfm?qid=192518&page=8</a>

ASME B16.5 should have made a note on the use of low stress spiral wound gasket, if they were to be used with ASME class 150 flanges. It is neither addressed in the interpretations included with the 2003 edition that I have.

Sajit

Extract of post from Rene

for 150 lbs flanges you have to use "Low stress" spiral wound gaskets. They have a y-value of 5000 psi. They have also a better performance if you use a Graphite filler.

Even a gasket cut from Graphite sheet gives much better results is this case : y-value of 2900 psi.

Extract of post from Tony

The "y" value is intended for "seating" of the gasket in the bolt-up condition, so that when the proper bolt load exists in the operating condition, the gasket will seal.

Save a tree...please don't print this e-mail unless you really need to. Received on Wed Nov 14 02:35:00 2007