Raised face gasket dimension

[ccc123]

All,

My concern is about the width (OD-ID) of raised face gaskets that will be installed for ASME B16.5 flanges, for each Class of the B16.5 flanges the width of gasket (OD)for the same flange diameter is different (Starting from 6" flange diameter),I think that this width of gasket shall cover the raised face section of the flange, In my project we have flanges with different classes 150,300,600 for the same diameter but they have the same raised face section width, So why the gaskets width is different if the rased face setion has the same width? is it related only to pressure rate ?

 

[gdti]

The outside diameter of the gasket should be sized to locate inside the flange bolts to align the inside diameter with the bore. The bolt circle diameter and bolts size vary with pressure class and hence the gasket OD will differ.

 

[ccc123]

Hey gdti,

what you are talking about is for flat face gaskets not raised face,the bolts should not go through the raised face gasket, I mean there are no holes in the raised face gasket, it is flat with no holes.

 

[TBP]

No, gdti has described a ring gasket. If a gasket is cut or manufactured such that it "just fits" the raised face on flanges, it'll slop around inside the bolt hole pattern, until the bolts are tightened. While this CAN be dealt with, it's a real pain. It's far faster/easier to have the OD of the gasket ride just inside the bolting, as it will pretty much self-centre. On horizontal piping, put a couple of bolts in the bottom of a 2-holed flange (just get the nuts started - don't make them even finger-tight), drop in the ring gasket, then install the rest of the bolts.

 

[ccc123]

Thanks a lot guys,

What I understand from you is that this extra width of the gasket comparing to riased face width is to facilitate the installation for horizontal piping application and exactly to ensure the centering of the installed gasket, also it's clear now that the difference of width for gaskets having different classes is related to the difference in bolt hole and bolt dimensions. Is that correct ?

 

[TBP]

I think you've got it. And it's not necessarily just for horizontal piping - it's easier no matter what the orientation. If you're using, for example, spiral wound gaskets, the metal outer ring centres the actual gasket surface on the raised faces of the two flanges. If you're using gaskets cut from sheet material (we keep some for emergencies & odd dimensions/shapes) and somebody mistakenly cuts the ring to "just" match the raised face width, it's a real PITA to get this ring to sit where you need it. It's normally easier just to cut another one with the proper OD.