Gasket is a soft sealing material closed between two flanges in order to make a leak free sealing application.To achieve a successful seal, the gasket must be resilient enough to conform to any irregularities in the mating surfaces. The gasket must also be sufficiently tough (rugged) enough to resist extrusion, creep and blowout under the operating conditions and unexpected pressure/ temperature excursions.
Often when there are leaks at a flange joint, it is assumed to be a “gasket failure”. The FSA and the ESA both have Gasket Divisions whose membership consists of many of the gasket material manufacturers in the world; report that a high percentage of reported “gasket failures” are actually Bolted Flange Joint Assemblies (BFJA) that have been poorly designed, installed and/or assembled.
Uses of gaskets are:
If there is a leak for flange joint, it results in loss of product and energy and will be disastrous. No plant operator wants to leak of toxic or hazardous material that can harm humans and the environment. The gasket can help in achieving reliable sealing to prevent the leak from the flange joints. It also has to overcome the misalignment of the flanges in order to prevent leakage between gasket and the flange surface. The gasket should be Gas-Liquid tight so that it does not leak or cause emission.
Parameters that depends the selection of gaskets are:
Temperature– Material of gasket should withstand the design temperature range of the fluid it handles.
Pressure– Gasket material must withstand the entire design pressure range of the fluid it handles.
Corrosion resistance– Gasket material should not corrode when it comes in contact with the fluid it handles or by the environmental exposure.
Types of fluid – Gasket material should be capable of dealing with different types of fluids if installed in line that handles more than one type of fluids.
Robustness– The gasket must be capable of withstanding all movement that may occur due to a change in temperature and pressure.
Availability & cost – Gasket should be easily available and cheaper
Gasket Materials commonly used:
Non-Metallic Gasket is manufactured from flexible materials. Such as
Compressed Non-Asbestos fiber
And Ceramic fiber
For metallic gaskets is given in ASME B16.20 following materials are used
Low carbon steel
4-6% Chrome, ½ Mo
Stainless steel Type 304,316,347,410
Or metallic gasket can be manufactured from Service specific material as suggested by the designer.
Gasket dimensions are covered in the following standards:
BS 3381 – Metallic Spiral Wound Gaskets
ASME B 16.20 -Metallic Gaskets for pipe flanges
ASME B 16.21-Non-metallic Gaskets for pipe flanges.
Selection of Gasket
Factors that depends the proper selection of gaskets are compatibility of the gasket material with the fluid, ability to withstand the pressure-temperature of the system and the service life of the gasket. It is important to understand the requirements of particular applications before making gasket selection. Gaskets must maintain a seal for an acceptable period against all the operational forces involved.
Following are the properties which any gasket must possess to achieve its functionality:
1. Impermeability– The gasket should not be porous to the fluid being sealed.
2. Compressibility– The gasket should compress into the imperfections on the flange sealing faces to create the initial seal.
3. Stress relaxation(creep resistance)– The gasket should not show significant flow (creep) when subjected to load and temperature. Such flow will allow the bolts to relax, reduce gasket surface stress, and cause leakage.
4. Resilience– Although normally stable, flanges do in fact move slightly relative to one another under the influence of cycling temperature and pressure. The gasket should be capable of compensating for such movements.
5. Chemical resistance– The gasket should withstand chemical attack from the process medium being handled. Likewise, the gasket material itself must not contaminate the process medium.
6. Temperature resistance– The gasket should be able to withstand the effects of the maximum and minimum temperatures within the process and the external atmospheric temperatures.
7. Anti-stick– The gasket has to be easily removable after use.
8. Anti-corrosion– The gasket must not cause corrosion of the flange faces.
Points to clear doubts on gasket requirements:
· You will need to specify the type of gas, working pressure and temperature. Flange dimension is also important when recommending a gasket material.
· It’s very important to know the chemical concentration, pressure and temperature. When checking chemical compatibility you can check the manufacturer’s chemical compatibility chart or contact them directly.
· Contact the gasket manufacturer for certification, or an accredited test facility for compliance with various standards such as API 6FB
· The gasket material has to be soft in order to compress this in the irregularities of flanges.
· Gasket should be Gas-Liquid tight so that it does not leak or cause emission.
· The gaskets should not creep under influence from stress and temperature. This could result in lower bolt stresses and possible leakage.
· The resilience of gasket should take static and dynamic effects due to stress, temperature and pressure.
· The gasket has to withstand internal pressure without being blown out.
· The gasket should be capable to achieve sealing at elevated temperatures.
· The metal of the eyelet protects the inner diameter of the gasket from mechanical and chemical attack of the flowing medium. This makes the gasket seal better and longer. In addition, the metal beading prevents the medium from penetrating and diffusing into and/or through the gasket.
· Taylor Forge calculations aim at verifying the mechanical integrity of the bolted assembly. Assumptions are done in the calculations that can lead to insufficient gaskets stress.
Causes for leakage in Gasket:
Gasket damage or relaxation due to flange rotation
Gasket damage due to differential thermal expansion
Incorrect assembly bolt load
Load loss due to thermal fluctuation
Gasket load loss due to pressure and or piping loads
Excessive gasket relaxation
Excessive gasket load
Methods to prevent leakage
Proper selection of the correct assembly bolt stress
Correct location, constraint and width of the sealing
Consideration of bending loads, misalignment.
Quantify effects of gasket creep/relaxation
Quantify effects of temperature and pressure
Maximum permissible assembly load
Correct gasket selection
Root cause analysis
Unless the bolted joint is properly designed for all the operating conditions in a process, including system upsets, sealing reliability cannot be ensured. Historically, design codes for bolted joints have been successfully based on experimental methodology, while tightness based approaches were introduced more recently. Both have proven to provide the required system reliability. The more practical and prevalent problem encountered in practice is ensuring the integrity of bolted joints with standard flanges, such as ASME or EN. Standards for the material, dimensions and identification of gaskets employed in these flanges enable proper selection. The proper selection and condition of each component, including the gasket, their correct assembly and installation are fundamentally important to preventing leakage and joint failure. The proper training and qualification of personnel safely employing the appropriate tools, also is essential to joint integrity.
**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.