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Starflex Sealing India Pvt. Ltd
About Starflex Sealing(formerly Flexitallic) India Pvt. Ltd
An improperly designed, casually installed, or poorly manufactured gasket can lead to plant shutdowns, financial losses, plant capacity reductions, environmental damages and safety hazards that are sometimes fatal. Therefore there is an increased awareness from customers to enhance sealing performance. Additionally, there is also a need to understand the advantages and limitations of newer products such as asbestos substitutes and other innovative designs.
A gasket is designed to be placed between two static faces of a flange in order to affect a seal. The selection of gasket is defined by taking into consideration factors such as pressure, temperature, and type of media, etc. The gasket should withstand these parameters in order to maintain a seal.
Gasket function can be divided into two stages
  1. Gasket compression during Assembly
    The gasket needs to be properly seated during assembly. In order to achieve this, the bolt stress should be sufficiently higher than the internal pressure, hydrostatic end force, and minimum required bolt stress for gasket seating.

  2. Gasket resiliency and recovery during a running plant
    In a running plant, the gasket needs to maintain resiliency and recovery properties in order to maintain the same initial bolt stress and fight against hydrostatic end force and flange movements due to pressure-temperature fluctuations.
Many factors can influence gasket function. External factors such as bolts and flanges should also be considered in conjunction with gaskets as they influence sealing performance.
  1. Application parameters need to be first considered before selecting a gasket. These include

    1. Temperature in the system
    2. Pressure in the system
    3. Corrosive nature of media
    4. Viscosity of media

  2. The following parameters should be considered for gasket selection

    1. Compression characteristics
    2. Recovery characteristics
    3. Chemical resistance
    4. Stress Retention
    5. Shouldn't corrode/ damage flanges
    6. Blow out resistant
    7. Creep resistant
    8. Easy to handle

  3. The above need to be complimented by proper plant maintenance practices. These include the following

    1. Flange Condition

      1. Flange Surface Finish: Different gaskets require different flange finishes. The flange finish should be concentric or phonographic and correspond to the gasket being installed. (Refer to datasheets for surface finish recommendation)

      2. Flange Surface Damage: Flange surface damage can lead to a leakage path, and should be avoided.

      3. Flange Flatness: An out of flatness flange or a non parallel flange would lead to uneven compression on the gasket, and thus effect performance.

    2. Bolting

      1. Bolt stress during assembly: In order to maintain a seal, it is extremely important that proper bolt stress is maintained during assembly. Proper bolting equipments should be considered to maintain these stresses. (Refer to gasket selection for bolt stress information)

      2. Bolting Procedure: In order to maintain uniform bolt stress, it is recommended to follow the above sequence. Bolting should also be done in four stages with 33% torque during the first stage, 66% torque during the second stage, 100% torque during the third stange, and a 100% retorque during the fourth stage.

      3. Bolt Lubrication: Good lubrication is necessary to maintain uniform torque. In the absence of proper lubrication, torque will be lost to friction.

Section 8 of the ASME Boiler and Pressure vessel code covers flange design. It also includes m (gasket factor) and y (minimum gasket seating stress) values for design purposes. Two conditions need to be considered separately for gasket design. They are a) Minimum required bolt load for operating conditions, and b) Minimum required bolt load for gasket seating. The flange and bolt design should be sufficiently stronger than these two conditions. This ASME code has certain limitations as it does not consider certain information such as leakage levels. Therefore, committees such as PVRC are developing new standards and constants that would consider design based on maximum permissible leakage.