Large diameter gasket types ?

[jonesey]

We have the requirement for a 42" diameter overhead line from a 50m high vacuum tower. Our required piping materials specification only goes up to 36" and needs to be expanded to 42". We use graphite reinforced stainless steel flat ring gaskets normally in this material spec. Someone has raised the question of wouldn't a graphite/stainless steel spiral wound gasket, with inner and outer ring, be better in this application. AS i have no experience in lines of this size, can anyone advise ?

 

[unclesyd]

A 42" spiral wound gasket is available. The problems is whether you have a flange and bolting that will seat the gasket. This has to be calculated. Generally you should be able to seat a spiral wound gasket if you are using a metal gasket at present.

Can you give a little more information on the existing flange and gasket like is this an envelope type or something else. You will get a more informative answer with more information.

Is the flange a standard type or calculated type?
What standard?

Are you having trouble with existing gasket on 36" flange?

 

[jonesey]

There is no existing flange/gasket. It is a new line from a new vacuum tower. The flange will be a standard MSS-SP-44 (ASME b16.47 series A)42" 150#. Known to me the largest line we have here on the efinery is a 36" flare line. This uses a graphite stainless steel reinforced flat ring type of gasket, and known to me does not have problems with sealing.

 

[unclesyd]

Before I would suggest using a spiral wound gasket on a 42" MSS 150 Cl flange a thorough mechanical evaluation needs to be made of this flange. This includes the flange and associated piping.

It was in our standards that a spiral wound gasket was not to be used in conjunction with a Class 150 flange of any description.

 

[codeeng]

Don't see any problem using a spiral wound gasket on either Series A or B flanges.

 

[pigmeat]

ASME B16.47 permits the use of spiral wound gaskets for Class 150# flanged joints. I see two issues however -
1) There is a recognised problem with 150# joints and spiral wound gaskets, in that the required bolt stresses needed to fully compress the gasket can be excessive. Some gasket manufacturers have developed a specific low-stress range of spiral wound gaskets to overcone this potential problem. Consult with your proposed gasket manufacturer prior to purchase.
2) 150# gaskets of this sort can be mechanically unstable during assembly and sometimes have tendency for the windings to fall away from the inner/outer rings before the flanges can be brought together. Careful handling is essential. Once the windings have separated, throw the gasket away and use a new one (expensive at 42").
You proposal of using a stainless steel reinforced graphite ring gasket looks fine to me and is used extensively by my company.

 

[gasketguru]

It is worth noting that a cut from sheet gasket at this size would need to be made in several sections, dove-tailed together due to limitations in commonly available sheet sizes. If you are working at height on a vertical flange face, then handling could be a problem. One option is to pre-bond two cut gaskets to a solid metal ring to improve handling.

A spiral wound would at least be in one piece so easier to handle, but as already mentioned it can “spring” apart if handled roughly. On this flange I would not worry about the available bolting unless you are using B8 bolts which have a relatively low yield. Generally it is the ½”-24” class 150 range that are a little under-bolted, especially 3” and 8” N.B. – the 42” series A has enough bolting to achieve a decent level of initial and operating stress on the standard ASME B 16.20 size gasket.

If you are worried about the possibility of springing the windings during handling, then a kammprofile type made to the same dimensions as the spiral might be a better option.

 

[unclesyd]

We use thousands of spiral wound gaskets on every size flange from Class 300 to 2500 routinely, but never on Class 125.
It had been my experience that decent or just adequate bolting is not viable for real world operations. We have tested every type and style of spiral wound gasket both in the laboratory and field and have never been able to get the Class 125 flanges with spiral wound gaskets to perform satisfactorily even with a lot of tender loving care. I agree that everything calculates outs but operations never seem to operate by the rules agreed on.
Another problem is if you have a proliferation of types and styles of gaskets you are asking for trouble with today’s maintenance and management schemes.

A note on the installation of spiral wound gaskets. Never allow any type tape to be used on the gasket especially in the radial direction. We had a very expensive incident occur when a mechanic put 3 pieces of green fabric tape on 24" gasket to hold it togather and in place while they moved a blind flange into place. After a few hours operation at 650 F at 70 psig a small leak was noticed and within a couple of hours a second leak on the same flange was noted. By the time I arrived there were three leaks about 60 degrees apart. We had to shut the equipment down and the lost of this piece of equipment resulted in the loss of additional equipment with a maintenance bill of over $200,000. Examination of the gasket revealed 3, barley discernible, radial depressions across the spirals exactly 2" wide, the tape width.

 

[codeeng]

Unclesyd, I guess you are referring to Class 150 not Class 125? From my experience in the Refinery Industry we use spiral wound gaskets extensively and I'm not ware of any real problems with Class 150. I've seen a few issues where they; used the wrong surface finish (too smooth), used low strength bolts (should be intermediate or higher strength), over-tightened the bolts (resulting in flange distortion) or improper bolt-up sequence.

 

[jte]

Interesting that the entire focus has been on spiral wound... We are trending away from spiral wound towards Grooved Metal Graphite Coated (GMGC aka kammprofile) and Corrugated Metal Graphite Coated (CMGC aka Graphonic) gaskets for mostly exchangers but also large diameter piping. They are softer gaskets which are more tolerant of radial deformation (e.g. hot tubesheet vs cold shell side flange). For this diameter, I'd probably recommend a kammpro due to its rigidity (vs CMGC) which makes it easier to handle. More expensive, but rebuildable and much more reliable sealing. What we've paid for in more expensive gaskets we've made up in fewer flange leaks which need to be clamped or wire wrapped etc.

jt

 

[unclesyd]

codeeng,
Please excuse my error as it was Class 150 instead of Class 125.
Not getting off of my soapbox I will have to admit that properly designed, selected, and installed spiral wound gaskets will work very well if used in a some what steady state piping system. It is probably just that we have nothing that doesn't cycle, shake, wiggle, or run very long.
One additional point to be made for using heavier flanges is that the metal spirals alone can seal if someone uses a gasket with the wrong filler. It has happened to us several times where a graphite filled gasket was substituted for a ceramic filled gasket in NOX service at 1200 F.

We do also use the all metal gasket in a couple of styles, with and with out graphite. One problem we have had with the graphonic style on SS at high temperature is that we sometimes get a pronounced embossing of the flange face. This has resulted in having to make a skin cut on one set of flanges to clear it up.

A lot of people may be aware of this but I'll mention it anyway. A long long time ago we were having trouble with centering (2) 72" dia 1/2" wide metal gaskets on high temperature heat exchanger flanges. Since we used set-in studs on the inside flange I welded 4 thin (0.020) tabs on the gasket edge that would allow it to properly hang on the set-in studs. The gaskets are now purchased with the tabs.