Bolt Torque Values for Spiral Wound Gasketed Joints

[owlscreech]

I have read recently that spiral wound gaskets designed in accordance with ANSI B16.20 are suppose to compress to 0.125-0.135 with a uniform bolt stress of 30,000 psi. This means that torque tables should only depend on the bolt size. Yet many tables are set up by pressure class/fitting size. The SWG manufacturers typically say the torque is the users reponsibility. Suppliers usually fall back on gasket stress.

Does anyone have any ideas on what should be used?

Thanks

 

[Metalguy]

IIRC, Flexitallic recommends something like 30% crush. Some of these gaskets have a solid portion so that the bolting loads are limited wrt the sealing area of the gasket.

The graphite-impregnated gaskets work VERY well, sealing better at lower crush amounts.

 

[arto]

How about ASME PCC-1 "Guidelines for Pressure Boundary Bolted Flange Joint Ass'y?" They give 50ksi prestress as "generally considered suitable" for B7 bolts

 

[Ben4piping]

Very few piping mechanics will agree to use torque wrenches in normal applications. Per Process Industries Practices piping team (PIP), ASME B16.5 flange joints w/ sp-wd gaskets should have bolts torqued until the flange faces just touch all around. Per old API doc (601?), the maximum torque to use with metal (ring joint) gaskets should not exceed half of the specified minimum tensile strength.

 

[tothepoint]

The problem is, most spiral wound gaskets DO NOT meet this compression requirement. If they did, then spiral wound gaskets would be well suited for class 150 flanges- they're not. You should calculate assembly bolt torque based upon developing anywhere from 15 - 20,000 psi stress on the gasket. Remember, for a rigid metal to metal piping joint, you want to compress the gasket all the way down to the guide ring. You want the raised face of the flanges to be biting into the guide ring. This requires compression down to the spec thickness of the guide ring (0.117" - 0.131&quot

 

[TBP]

I've been involved in countless joints involving Class 150 flanges over a lot of years, and the normal allowance by fitters and welders is for 1/8" on spiral wound gaskets after they've been crushed. I have never seen anyone use a torque wrench for Class 150 or 300 flange bolts on steam & compressed air service. I've never seen anyone get out a micrometer measure the gasket crush either.

Unless the fill material has been improperly selected and/or there's an extremely poor fit-up, there are very few field problems with spiral wound gaskets. And from a maintenance standpoint, there's no spending 2 hours with an old wood chisel trying to scrape old gasket material from flange faces that you can only get an inch apart. The spiral wound ones just drop out when the bolts are undone, or they stick to one flange or another, and just pop off with a little push from a screwdriver.

If design engineers want to check things about flanges, go after the idiots that install class 125 flanges instead of the 150s they should use. Check the SAE grade fasteners that are ROUTINELY used in place of B7 studs and 2H nuts. There are FAR bigger issues that need to be addressed. Really.

And if you're really going to specify torque, then get out into the field, and CHECK THAT IT GETS DONE. Good contractors will price this into their quotes, then lose the job to the low-bid short cut artists who WON'T DO IT because they figure that nobody will ever check. And you know what? They're right.

 

[owlscreech]

Nuclear power plants require a controlled method for making up all joints. For most joints this means using a calibrated torque wrench, as well as a specified sequence for tightening the bolts. There are reasons for this ostensibly crazy approach but they would take too long to explain.

As for the contractor problem, I agree that is real...too real. I have seen 10-15% joint leakage after an outage directly linked to contract help.

I agree that SWG joints if made up properly will just keep on tickin'.

Thanks