Gasket Safety: The Role of a Gasket in Relieving Pressure

Gasket Class Or Pressure Rating System

The most common standard prescribing the geometry of flanges is ASME (previously ANSI). Within the ASME pressure rating system there are seven pressure classes:

• 150
• 300
• 400
• 600
• 900
• 1500
• 2500

A Class 300 flange will handle more pressure than a Class 150 flange, simply because it has been made with more metal and so can withstand more pressure; and so on up through the classes. The pressure class or rating for flanges is given in lbs. For example: 150lb, 150 lbs, 150#, or class 150 – are all equivalent.

The flange class or pressure rating system extends to gaskets designed for those flanges. So, for example, a class 150 gasket is designed to seal under a load of up to 150 lbs of pressure in a class 150 flange.

The overall pressure rating of the gasket ultimately depends on the material used for the gasket and the operating temperature.

Pressure and Temperature Variations

As pressure increases the temperature that the flange will maintain falls. Conversely, as the pressure decreases a higher temperature can be maintained. The selection of suitable gasket materials must be considered together with the flange design bolting and materials of construction.

Pipe and Flange Construction and Gasket Pressure

Gaskets are typically fixed by bolts under load around the flange face. The gasket either encompasses the bolts (called a ‘full faced’ gasket) or sits inside the bolts (known as an IBC or ‘ring type’ gasket).

To maintain seal integrity pressure must remain on the gasket surface to prevent leakage. Under operating conditions this pressure is relieved by internal pressure which acts to separate the flanges. The gasket itself is also subject to a side loading, where the internal fluid pressure can cause the gasket to extrude through the flange clearance space. To continue to maintain a seal the pressure from compression on the gasket must be greater than the internal pressure by some multiple, depending on the gasket type and level of tightness required.

How Do Gaskets Behave Under Pressure?

Gaskets need to perform in many different conditions, which is why there are a huge number of gasket materials and configurations to choose from. The main factors that need to be considered when specifying a gasket are temperature, chemical resistance, and pressure.

Even in the same environment, gaskets can be subject to different operating conditions. Below are some of the conditions affecting the pressure to which a gasket will be subject; and how the gasket is likely to act under load.

Stress Relaxation of Gasket Material

The performance of the gasket is directly related to the stress retention of the gasket material. As a material decays or becomes brittle or soft, the stress relaxation of the material is compromised and so is its ability to withstand pressure. Generally, rubber based materials have a shelf-life of seven years. In critical application it is important to ensure that rubber based material is used within its shelf life. Where required we can supply material or parts with batch and cure dates, so that customers can be sure to only install gaskets that will not fail due to perishing material.

Gasket Material Thickness & Pressure

As a general rule, the thinnest possible gasket material for the application should be chosen. The reason for this is that thinner materials present a surface area (the smallest ID, or inner diameter) for pressure to act upon, and so are less likely to fail. Having said this, the choice of material thickness also needs to take into consideration the amount of compression required to take up any flange distortion or misalignment – and this is especially true when using fibre based gasket materials.

Flange Quality & Pressure

The quality of the finish of the metalwork on a flange is critical to the correct sealing of a joint using a gasket. The surface finish should not be too rough, otherwise a leak-path can form under the gasket. Standard pipe flanges often have a groove across the sealing face, which the gasket deforms into under pressure; and this also helps to limit the displacement of the gasket across the flange face. Any flange damage should be fixed before re-inserting a gasket. The mating flanges should be made from the same material and machined identically to allow pressure to be evenly distributed across the bolt and flange surfaces.

Tensile Strength

The strength of gasket material as an isolated piece is not critical to its sealing performance. For example: graphite is soft, pliable, and cracks and breaks easily. However, when compressed between flanges it forms an excellent seal that can be subjected to high temperatures and steam without failing. As with fibre gasket material, the thinner the graphite gasket the better the resistance to overall pressure.

Load Seal-ability

All gaskets leak to varying degrees (even if the leak is so small that it can only be detected with a mass spectrometer). If all gaskets leak, this raises the question: why use gaskets at all? Why not just machine and weld all surfaces? The answer is that huge lengths of pipework require servicing. Gaskets perform well at preventing leakages at the joints in lengths of pipework, whilst allowing the joints to be uncoupled; and the gaskets replaced as and when required.
If testing of a leakage is required, such as in the manufacture of aeroplane wings, parts are often pressurised with helium and the leak-rate is tested with a helium detector (mass spectrometer). Such leaks may be considered undetectable in every-day practical applications – but it is important to measure them in critical sealing applications to test the quality of the gaskets and bolt loading of the joint. We can supply certified samples of gaskets in different materials for testing.

Minimum Gasket Pressure, Installation, and ROTT testing.

A minimum amount of compression is needed to seal a gasket on the flange surfaces. Tightening the bolts on the flange adds additional compression which blocks any permeability through the gasket. This permeability varies between different materials, but as a general rule leak rates decrease as compressive load increases.

The state of the contents of the pipe, such as molecular size (liquid, gas) will affect the stress needed to create a seal. The stresses required to seal gases are higher than the minimum stresses necessary for the gasket to conform to the flange surfaces.

Metal Gaskets require a greater stress to compress and seal than flexible gaskets. When using flexible non-metallic gaskets, the ability of the joint to hold internal pressure depends on friction. The minimum compressive stress will need to be high enough to maintain the friction needed to keep the gasket from blowing out from the internal pressure.

The test that determines the constant sealing pressure is the ROTT (Room Temperature Tightness) test. Increasing temperature creates gasket relaxation, and subsequent relaxation in the bold load (sometimes bolt-load losses can be as high as 50% of the initial gasket stress). For this reason, depending on the gasket type, it is advisable to re-torque after the first heat cycle.

Ultimately if a flexible gasket is under too much pressure it will extrude out around the flange, and eventually exit right out of the flange space both internally and externally. In this situation, if it is an old gasket, servicing and replacement is sufficient. If this is a continuing problem a more rigid material that can cope with: greater stress relaxation, more diverse operating temperatures, and produce no swelling when in contact with chemicals, will need to be used. If you require support then please contact us for technical advice.

Low Pressure Gaskets (Vacuum Environment)

Sealing a vacuum presents unique challenges. Generally softer materials are more effective at sealing in a vacuum: for instance, consider using natural rubbers and butyls. Polyurethane is another soft polymer with a great ‘rubbery’ consistency, that deforms and seals effectively when creating a vacuum. Our technical department can support the correct choice of material for your particular requirements within a low-pressure environment.

High Pressure Gaskets

Chart showing the Upper Pressure, Common Gasket materials can be expected to perform to: