Coating of Flange Faces 1

[xema77]

I have an air receiver (DP = 1034 kpa) for which the bottom head and 1/3 of shell requires internal epoxy coating. Per the client specification, the nozzle neck, flange and first third of the flange face also require coating. The flange face coating detail indicates that a reduced coating thickness of 75 micro meters shall be used to coat the first 1/3 of the flange face. The concern arose regarding gasket sealing.
1) Is it common to coat flange faces if the vessel is internally coated?
2) Will the gasket seal properly?

I would appreciate hearing anyones experience.

Thanks.

 

[eadwine]

From what I'm reading here this is a carbon steel vessel, and that the owner of the vessel wants the bottom head & shell, 1/3 rd up, coated with epoxy. This for corrosion resistance when the water condenses out of the gas. Normal requirements are that the nozzles are also coated wthin this zone. As for the flange facings, I take it these are std. raised face flg'S. On any R.F. flange the raised portion of the the flange facing has machined concentric rings to a 125 rms surface finish. If the epoxy gets into these concentric sealing rings they will not seal correctly. But, this raised face portion does not cover the full face of the flange. There are three sectors to a flange facing, the bottom portion that has no concentric rings and does not provide sealing. Then there is the raised face portion that does the sealing. Finally there is the outer portion that does no sealing, but has the bolt holes circle that holds the flange faces together and provides the sealing forces on the gasket at the raised face portion of the flange face.

Now, if you provide epoxy coating on the nozzle neck and bottom portion of the flange and not cross over into the raised face portion of the flange, you will have satisfied the owners requirement and not compromised the sealing capabilities of the flange itself.

Make sense?

 

[roca]

In my experience the flange gasket seating machined face is never coated.
The only time I have seen this area covered (other than by a gasket) is when the vessel is rubber or plastic lined. This type of lining normally covers the fulll flange face - including the gasket seating area.

 

[rmw]

The gasket seating is a function of the clamping force spread over the flange face surface area. Not knowing the thickness of the coating, still, there will be a step across the coating and that will change the reaction of the flange face surface area to the gasket sealing area.

So, if you coat, coat the whole flange face. If you don't... then it is a moot point.

Frankly, I wouldn't coat. The clamping force can't be too kind to the coating possibly damaging it and causing it to fail. But I would coat the metal surface right up to the edge of the flange face where the gasket would be.

rmw

 

[moltenmetal]

Your pressures are modest and the service is air. Sealing will not be a problem. Sealing and in fact mechanical integrity of the vessel WILL be a problem if you lose your flange face and ultimately nozzle neck too due to corrosion.

Choose another gasket type- nobody says that you have to use a spiral-wound gasket for 10 bar air. Use a gasket material with some elastomer content, so it'll be more compliant to the inevitable irregularities in the coated surface.

This is the client's call since they're the ones who are asking for the 1st third of the flange face to be covered. But I'd coat the entire flange face with the same thickness as the rest of the vessel was coated with. If you have to blast before applying the epoxy, unless you do a very good job of masking, you're going to end up damaging the rest of the flange face anyway.

 

[rneill]

I'm not a coating expert but my prior employer's coatings folks used to call for full coating over the flange face with the first coat (primer) and then coating up to the flange face with the second or subsequent coats. They also recommended using a flat sheet gasket if suitable or a kammprofile gasket for higher pressures if I recall to minimize damage to the coating on the flange face.

 

[EngAddict]

You shouldn't need spiral wound gaskets for air applications but if you did use one with an inner ring then you could coat part of the face before the spiral w/o it affecting the seal.

I think with a flat gasket that the 75 microns would create a problem if only one third of the face was covered. If they whole face was covered then it would be ok provided the gasket material was suitable sealing on a painted surface.

However generally I would always avoid coating a raised face, only times I would do it is for rubber lining but then that would be a flat faced flange instead. Paint the neck and leave the flange face unpainted and allow for a suitable corrosion allowance in your design.

 

[gr2vessels]

eadwine,
It makes sense, right on the spot.
Cheers,
gr2vessels