The proper selection of the gasket should be taken into consideration at the time of installation for the prevention of a subsequent impact on the operator or on the system.
Selection of the board
With the diversity of materials available on the market, it is a challenge to find the product that meets the specifications required for a given application. The most effective product selection begins with a conditions assessment. Four basic conditions must be considered:
- Operating conditions
- Nature of the medium and chemical compatibility
- Flange design
- Legal or environmental issues
The operating conditions include parameters such as pressure and temperature, as well as their fluctuations. If the system is subjected to pressure spikes or temperature cycling, this can adversely affect the performance of the otherwise adequately performing gasket material. Vibration should also be considered.
The chemical compatibility of the gasket material with the medium material must also be taken into account. What is the nature of the medium? Is it a gas, a liquid, or a combination of both? In addition, chemical properties such as pH and concentration must be taken into account. The gasket material must not contaminate or be chemically attacked by the product to be sealed. It is also necessary to consider what changes in temperature affect the dew point, since they can affect the aggressiveness of the medium. In critical applications, laboratory tests may be necessary to simulate operating conditions and to ensure chemical compatibility.
The flange design plays a very important role in the selection of the gasket material. Flanges made of plastic, FRP, glass or other brittle materials will require materials that seal at low pressure loads. Material selection for these flanges differs from high pressure metal flanges in that the bolt torque must be high to ensure sealing. The geometry of the flange is also important. Male-female flanges can be applied higher pressure compared to flat flanges. Sealing surface finish and condition, such as the presence of pitting or corrosion, warping or rotation, can also determine material selection. The number of bolts, the bolt material and, consequently, the tightening torque, will also influence the selection.
Legal and environmental issues should also be carefully considered. There may be regulations that prevent the use of a certain material in an application. For example, when the material is required to be flame retardant, or when it comes to food or pharmaceutical products and material that complies with FDA standards is required.
Many joint failures result from failure to consider these four basic steps. With an adequate analysis of the application, a great advance can be made in the selection of the gasket material, which will perform its function safely.
Once the necessary attention has been given to the choice of material, proper installation is necessary to ensure the success of the joint in the flanged joint.
One of the most common causes of leaking gaskets is bad installation. The following procedures are suitable for installing a gasket on a flanged joint:
- Inspect the gasket. It is important that the correct gasket has been chosen. Verify that the material and design are as specified and carry out a visual inspection of the joint to locate any defects or visible damage.
- Inspect the surfaces on which the gasket sits. Look for tool marks, cracks, scratches, or corrosion pitting. Radial machining marks on surfaces are virtually impossible to seal, regardless of the type of gasket used. Therefore, special attention should be paid to this aspect.
- Use only new bolts, washers and nuts. Make sure they are of appropriate quality for the application.
- Lubricate all threaded joints and bolt faces. Proper lubrication will provide a low coefficient of friction for increased bolt tightening. When an anti-seize compound is used as a lubricant, it will facilitate subsequent disassembly.
- Install boltsloosely. On RF and FF flange installations, drive bolts loosely into bottom half of flange. Insert the gasket between the flanges to allow the bolts to center the gasket on the mount. Install the rest of the bolts and screws and tighten them slightly. In a grooved or pocketed installation, center the gasket in the middle of the groove or pocket. If the joint is vertical it may be necessary to use a minimal amount of grease, joint cement or other fluid compatible adhesive to hold the joint in position until the joints are tightened. Then install the bolts and screws tightening lightly.
- Identify the proper sequence of bolts and number them accordingly. Each bolt should be numbered so that the tightening sequence can be easily followed. A failure in the tightening sequence can result in off-center flanges. Thus, despite subsequent tightening, the flanges cannot be placed parallel again. This can greatly contribute to a leaky joint.
- Tightening of bolts. The bolt should be tightened in a proper sequence, in a minimum of 4 stages.
- Tighten the bolts to a maximum of 30% of the final required torque value following the recommended sequence.
- Repeat step 8, increasing the tightening to approximately 60% of the final tightening required.
- Repeat step 8, increasing the tightening value to the final value.
- Retighten all bolts. All bolts should be retightened in a rotational pattern to the final tightening value until no further rotation of the nuts can be achieved. This may require several steps, as tightening one bolt typically causes relaxation of adjacent ones. Continue squeezing until balance is reached.
- Some flanged joints need to be retightened right after start-up, to compensate for loosening of the bolts and joints. This practice has been reported to be successful in heat exchangers, with certain types of gaskets and flanges, when the bolts are retightened during initial heating, prior to loss of lubricant.