Now that we have covered the different types of flanges, let's briefly cover the subject of gaskets and bolts.
We shall only cover what a piper is required to know.
We will first discuss gaskets and then bolts.
A gasket forms the seal necessary between flange connections to prevent leaks;
There are different types of gaskets.
The type of gasket is dependent upon three things:
- Pressure and Temperature of the line;
- Commodity of the line;
- Facing of the flanges they are placed between.
A line at 800° would take a different type of gasket than would a line at150°, even though they both could have raised face flanges.
Also, it naturally follows that a flat face flange would take a different type of gasket than would a ring joint flange.
Who Selects the Gasket Material?
The Piping Material Engineer selects the type of gasket necessary for a particular line and we find this information in the Piping Material Specification.
There are three general types of gaskets.
- Raised face
- Full face
- "Ring Joint Flange" gasket
Raised Face Gaskets
First, let's discuss the raised faced gasket.
It is used on flanges with raised faces.
It's outside diameter is fabricated so as to align itself within the bolts.
The material used may be either metallic or nonmetallic.
The most common metallic gaskets are:
Corrugated, metal, corrugated double jacket asbestos filled, and spiral wound.
Non metallic gaskets would be of materials such as graphite, cardboard or rubber.
Full Face Gaskets
The second type of gasket is the full face gasket.
They are normally used for flat faced flanges and the O.D. extends to the outer edge of the flange face with holes provided for the bolts.
Why would we use a full faced gasket on flat faced flanges?
Flat faced flanges are sometimes made of cast iron, which is brittle.
Unless a full faced gasket is used, we run the risk of cracking the flange as the bolts are tightened.
The common materials used for this type of gasket are:
Graphite, cardboard or rubber.
Both the raised faced gasket and full faced gasket have varying thicknesses, based on pressures and temperatures.
In order to calculate the dimensions to flanges or valves on our isometric, we have to know the thickness, so we can either add or subtract it to arrive at a dimension on the iso.
The most common compressed thicknesses are 1/16" and 1/8".
The material specifications will indicate the gasket compressed thickness.
Metal Ring Joint Flange Gasket
The third type of gasket is the metal ring gasket.
The cross section of the ring is either oval or octagonal.
It fits into machined groves in the flange faces, namely ring joint.
This type of gasket is made of solid metal and is used in high pressure or high temperature service.
In conjunction with gaskets, there are two different types of bolts that are used with flanged connections.
- Stud Bolts
- Machine Bolts
The material of the bolts will not be important in Piping Design for the most part, but there is a maximum temperature at which the machine bolt can be used.
Above that, we usually go to stud bolts.
The temperature depends upon the material and it again is selected by the Material Engineers.
The Piping Designer needs to be aware of the length of the bolt for material purposes and to verify there is adequate clearance for bolt removal. Generally this length is pre-defined, but may need to be adjusted in certain situations
Boss and Cap Screws
There will be particular instances when cap screws will be used instead of bolts; for example, at a turbine or compressor where flange connections may be the "boss type."
Here you would have just a machined surface with tapped holes to accommodate cap screws.
Machine bolts have a square forged head and requires only one hex nut.
Stud bolts do not have heads. They are completely threaded from end to end and require two hex nuts to install.
Cap screws have hex heads on them and are stronger than machine bolts and require only one hex nut.
Stud bolts are the most common type of bolt used in Piping.
They have these three advantages:
- The stud bolt is more easily removed if corroded.
- Confusion with other bolts at the site is avoided.
- Stud bolts in the less frequently used sizes and materials can be readily made from round stock.
In conclusion, the most important thing about gaskets and bolts is the thickness of the gasket (as far as we in Piping Design are concerned).
This information is found in the Piping Material Specification.
For additional information about gaskets and bolts see the "Standards" tab on the pipingdesigners.com website.
About the Author
Anton Dooley is a Piper with over 25 years experience covering process plant engineering, design & training. He is the founder of pipingdesigners.com