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Gasket Fundamentals
- What is Gasket? What is does?
- Forces Acting on Gasketed Joint
- Gasket Assembly Bolts Tightening Guidelines
- Gasket Service Parameters & Terminology
- Flange & Gasket Standard
- Gasket Selection
Gaskets are used to create a static seal between two stationary mating flange surfaces at various operating conditions with Varied Pressure and temperature ratings to prevent leakage from or into the joined objects while under compression.
A gasket fills the microscopic spaces and irregularities of the flange faces, and then it forms a seal that is designed to keep liquids and gases.
The basic function of a gasket is to create and maintain a positive seal between two relatively stationary parts. The gasket must do many functions properly:
- Create an initial seal
- Maintain the seal over a time
- to be easily replaced
Forces Acting on Gasketed Joint
Gaskets are used to create a static seal between two stationary mating flange surfaces at various operating conditions.
The performance of the gasket is depending on many parameters. So must be taken into consideration when selecting a gasket.
The Flange Load: Total force load to compress the gasket to create a initial tight seal than means effective pressure resulting from the bolting load
Internal Pressure : The internal pressure is generated due to internal media will trying to push the gasket and separate the flange so this force we called as ‘Hydrostatic End Force’ This Hydrostatic end force should not be exceed otherwise gasket will get blowout.
Gasket Assembly Bolts Tightening Guidelines
One of the most difficult jobs is to produce the correct assembly pressure on the gasket, low enough to avoid damaging the gasket, but high enough to prevent a leak in the seal. It is vitally important to control accurately the amount of force applied to any particular flange arrangement. Always use a torque wrench or other controlled-tensioning device for tightening. The sequence in which bolts or studs are tightened has a substantial bearing upon the distribution of the assembly pressure on the gasket. Consequently, always torque nuts in a cross-bolt tightening pattern as shown in below picture. Always run the nuts or bolts down by hand. This gives an indication that the threads are satisfactory (if the nuts will not run down by hand, then there is probably some thread defect – check again and, if necessary, replace defective parts).
Gasket Service Parameters & Terminology
Sealability:
Gasket sealability test techniques are used tocharacterize gasket material properties as they relate to sealing performance.
Compressibility:
Compression plays a major role in practical application. This property determines the adaptability of the gasket to the sealing surface. The more a material can be compressed, the greater its ability to adapt to faults in the sealing surface like scratches or grooves. Gasket materials chosen should possess sufficient compressibility to overcome flange imperfections, internal pressure from fluid being sealed, and deflection.
Recovery:
Theoretically speaking, a gasket recovers when the distance between the sealing surfaces in the application is increased by certain peripheral conditions (e.g. internal pressure). In view of this, strong recovery would be a priority.Materials must have sufficient recovery to maintain a seal after initial cycles and relaxation.
Creep Relaxation/Cold Flow:
Creep (sometimes called cold flow) refers to the long term deformation in gaskets from being under a constant load. creep relaxation is the measurement of how much Particular gasket material spreads (thins) out when force is applied. As the gasket loses thickness, the bolts can relax, which leads to a loss of load.
Torque Retention
It is a measurement of how well a bolted joint retains the force applied to each bolt. How much of the initial torque is retained by the fastener after a certain time or exposure? Zero percent torque retention means that the bolts have lost all of their load and are no longer compressing the gasket and thusly, the bolts are falling out and the joint has failed.
Compressibility, recovery, and creep relaxation values have a direct effect on a good torque retention value
Bolt stretch is very important in torque load retention because it will act as the spring to keep the flange interface sealed under dynamic loading, vibration and joint thickness
Chemical Compatibility
Selection of correct gasket material for your application is critical, especially in an environment where you may be sealing against aggressive chemicals (e.g strong acids or alkalis). In this situation you will want to know which chemicals are present; making sure that you are using a gasket with the right chemical resistance or compatibility.
Thermal Stability
Thermal stability is defined as the ability of a gasket do degrade under heat stress.
Blow-Out Resistance
The term blow out refers to the catastrophic failure of a gasketed joint. It occurs when load has been reduced to where the hydrostatic end force approaches that of the bolt load. At that point gasket stress is effectively equivalent to the contained pressure, and the joint is forced apart, causing a rapid release of pressure. The result is an instantaneous and forceful leak of the contained fluid
Cryogenic Service
Cryogenics is the study of material sciences at extremely low temperatures. Cryogenic temperatures are generally regarded to be below -180° Cryogenics has many use cases in manufacturing and is often used in stress tests to evaluate the design of materials and products used in winter environments. Cryogenics often involves the use of liquid nitrogen or liquid helium, both of which are stored compressed at very low temperature.
PTFE Gaskets would be most suitable in Cryogenic services.
Gasket Crush
Over compression of gaskets, especially soft gaskets, will crush the gasket causing premature failure. Many variables are involved when considering the maximum stress or crush strength of a material, including surface finish, gasket width, thickness, material type and temperature and bolt design.We have two options for reducing gasket crush: better torquing procedures, which can be difficult, and better gasket design, which is easier to implement.
PxT Factor
Gasket material is exposed to elevated temperatures the properties of the gasket change, so it is very important to check a PxT (Pressure x Temperature) factor for the material selected and your specific application (pressure, temperature, and media).This PxT factor is the result of multiplying the operating pressure times the operating temperature to arrive at a numerical value. This value is not constant and is different at each temperature and pressure combination.
Gasket Selection
The Selection of gasket and its materials it depends on many parameters while we need to consider below major factors we called as STAMP:
SIZE: if flanges are standard then we need to know Flange standard, flange type, pressure class, flange size.If flanges are Non–Standard then should know flange dimensions, flange type, bolting details.
TEMERATURE: It is very important to know Continuous operating Temperature (COT) at gasket location, maximum spike temperature and minimum temperature, on the basis of these temperature we can select the suitable gasket material composition.
APPLICATION: while selection of gasket needs to know the application that means in which equipment gasket is going to use? Pipeline flanges, Equipment like heat exchanger, Pump casing, valve body bonnet, Manways etc.
MEDIA: It is very important that which media is going to seal by gasket? the gasket material must be compatible with sealing media, Gasket material properties do not be changed during operation due to media under operating pressure and temperature conditions.
PRESSURE: The internal operating and maximum pressure of fluid determine the selection of gasket material. The operating pressure is lower than design pressure.
