7.4 Gaskets
Gaskets are used to provide a fluid‑tight joint. They must be made of a softer material than the metal surfaces. Gaskets are generally incorporated as the seal between flanges. The clamping bolts pull the metal flanges together. By tightening the clamping bolts, the metal surfaces compress the gasket material. The gasket material fills the imperfections of the metal surfaces. This then forms a tight seal. (see Figure 8-13).
Figure 8-13.
There are many types of gasket used on plant equipment. They are made from a variety of materials to meet the demands of process gas or fluid, temperature and pressure. This material can either be metallic or non‑metallic. Other gaskets are cut to size and shape from a large sheet. These are normally only nonmetallic types.
The correct selection of gasket is very important. A lot of things have to be considered when choosing a gasket, such as the operating pressure and temperature, working conditions, and the type of fluid used.
For a gasket to work properly it must replaced every time a piece of plant equipment, is stripped‑down and reassembled. Every mechanical technician must have a good knowledge of gaskets and gasket materials. He must also know how to make‑up a gasket from a sheet of gasket material.
There are three types of general gasket, they are:
- Flat ring gasket. This is the most common gasket.
- Full ring gasket. These are used for flat faced flanges.
- Ring type gasket. These are used for fitting into machined grooves.
Gaskets need to be selected so that the required seating load is compatible with the flange facing, flange rating, strength of the flange and the bolting, and the materials are compatible with the service condition.
For full face gaskets, fairly soft gasket materials are necessary and rubber or synthetic elastomers are generally used. They find application in low pressure (below 100 psi) services.
For raised faces the gasket is positioned inside the bolt circle and use pre-cut rings. A wide variety of materials are available with many claiming to be alternatives to the compressed asbestos fibre grades used as a standard for many years. Composition and compressed fibre gaskets are used for Class 150.
For Class 300 and above, spiral wound gaskets are used. A spiral wound gasket with both inner and outer supporting rings should be used for raised face flanged joints. The outer supporting ring serves to limit the compression on the spiral wound element and the inner ring serves to protect the windings from fluid flow turbulence.
A new gasket should be used when a flange joint is made or remade. Using two or more gaskets to make-up a gap is not to be practised and jointing compounds must not be used. It is important that the faces are properly prepared before making or re-making a joint. see Figure 8-14
Spiral Wound Gasket
Figure 18-14
Ring joint gaskets come in four basic types, oval, octagonal and pressure energised RX and BX. Early applications by ANSI used oval gaskets which fitted in round bottom ring grooves in the flange. Later development used octagonal gaskets in flat bottom grooves. Since both types of gaskets can be used with flat bottom grooves, this is now the standard groove type. Old equipment with the round bottom grooves must be fitted with oval gaskets, since the octagonal gasket will not seal in this groove. These ring joints and groove systems are designated by the letter “R”.
The API pressure energised ring joint gasket to fit in a type “R” groove is known by the designation “RX” and fits the API 6A. Type “R” and “RX” gaskets are interchangeable. A more recent development is the API 6BX flange which is used to handle higher pressures being encountered in the oilfields. The pressure energised ring gasket which mates with this is the BX type. The gaskets are marked to identify the types as R, RX, or BX.
All flexatellic ring joint gaskets are manufactured from fully traceable material and are stress stamped with style, material reference and identification. The ring type joints come in a range of material with their recommended hardness shown by means of identification codes. These range from soft iron, low carbon steel having an electroplated zinc coating to, chrome/moly low carbon steel to various grades of stainless.
Gasket materials have to be soft enough to yield under the bolt load so that they fill the irregularities in the flange faces, jet resilient enough to cope with strains imposed by the piping system. Gasket materials must give the required level of leak tightness and be resistant to the process fluid at its operating temperature and pressure.
7.5 Gasket materials.
The most common types of metallic gaskets are as follows:
- Tin
- Lead
- Bronze
- Copper
- Aluminium
- Stainless steel
- Titanium
- Silver
- Monel
The most common types of non-metallic gaskets are as follows:
- Paper
- Asbestos
- Rubber
- Cork
7.6 Non‑metallic gaskets.
Paper Gaskets.
Paper is used for a wide range of gaskets. It is often used where the surfaces have a very smooth machined finish. Paper sheet can come in a range of thickness’ up to thick card. It is very flexible and can be easily cut to shape using a sharp knife, bell punches and tin-snips. Care must be taken with paper gasket, however, because it can easily get torn.
Rubber Gasket.
Rubber has several properties that make it ideal for gasket material. It is very flexible when it is compressed, and it moulds itself to the metal surfaces. It is also unaffected by many chemicals and gases.
Rubber comes in sheets in a wide range of thickness and density, and is easily cut to shape. It is commonly used for cold water flanged joints.
Cork Gasket.
Compressed cork has many properties that make it ideal for gasket material. It is not affected by most chemicals and gases. It is also flexible and compresses well to form a good seal.` Compressed cork can easily be cut into a range of shapes. Care must be taken with cork gaskets because they can easily get damaged.
Asbestos Gaskets.
Asbestos gaskets are normally impregnated with graphite to resist chemicals and gases. They are usually used for joints that are subjected to heat from water or steam. The graphite also acts as a lubricant and helps stop t he gasket from sticking to the surfaces. It can come in sheet form in a range of thickness’. Asbestos is a common type of gasket material.
7.7 Metallic gasket.
Metallic gaskets are used for high pressure and temperature operations. Some of the most common types are as follows:
Spiral wound. These are used for pressures up to 17200 kPa. The filler between the metallic strips is asbestos. (see Figure 8-15)
Figure 8-15.
- Ring joint. These are used for pressures up to 6900 kPa. They are made from iron, nickel, monel, copper, and stainless steel. (see Figure 8-16).
Figure 8-16.
- These are used for pressures up to 4100 kPa. They are made from aluminium, copper, steel, monel and nickel. (see Figure 8-17).
Figure 8-17.
- These are used for pressures up to 71200 kPa. The filler inside the metal case is asbestos or rubber. (see Figure 8-18).
Figure 8-18
- Flat metal. These are used for pressures up to 10300 kPa. They are made from lead, copper, iron, steel, and monel. (see Figure 8-18).
Figure 8-19.
7.8 Gasket Jointing Compounds.
Gasket jointing compounds are used to provide tighter joints. They also assist the installation of the gasket. The compound acts as an adhesive to hold it in position. They come in a variety of types. Compounds are sometimes called liquid gaskets and can often be used instead of a solid gasket. They come in liquid form and can easily be applied using a brush or from a squeezed tube. (see Figure 8-20).
Figure 8-20.
Compounds are quicker to use because there is no gasket to make‑up. This then reduces the amount of down‑time. The selection of compound is very important in order to produce a good seal. Operating pressures and temperatures are very important. Graphite and oil are very common gasket compounds. Pipe thread lubricants are also used as gasket compounds.
The most important thing to remember when trying to get a good seal, is to always remove all the old gasket and compound. This is done using a cleaning solvent and a scraper. Both faces must have a good clean surface. Make sure there is no damage or burrs on the joining faces.
