The ASME B16.21 standard does not provide guidance on gasket thickness for non-metallic flat gaskets, and selecting the proper gasket thickness for a given application is a question that often arises and is difficult to answer simply. Therefore, we encourage users to talk to their application engineer if they cannot determine how to choose a gasket thickness, and today's article is intended to explain why this is such a difficult question and share some application knowledge to help users understand some of the possible scenarios.
How do I determine the thickness of a gasket?
As gasket application engineers, we recommend using thinner gaskets whenever possible.
However, there are some situations where thicker shims are required and recommended, which we will first explain. There are some common situations where a 3.2mm thickness shim is necessary and perfectly acceptable, as follows:
- Thin flanges will become uneven when the bolts are tightened, such as angle iron or steel plate flanges of 6.4mm thickness;
- Large diameter flanges, such as AWWA (American Water Works Association) standard water pipe flanges, 3-meter diameter pressure vessels;
- Many low-pressure full-flat large-diameter flanges with limited bolt force;
- Older flanges that may have some degree of pitting, bending, or breakage.
One of the reasons thicker gaskets are used for low-pressure large-diameter flanges is that these flanges need more bolts, mainly because the internal pressure is not high, so the design does not require many bolts. Limited bolts mean limited compression of the gasket, and thin flanges mean that the flange will deform when the bolts are tightened, and the gap between the bolts will compress very little or not at all. A thin gasket needs more compression to compensate for these insufficiently flat flanges.
This is contrary to our normal thinking. If you look at our recommended installation stresses, we require more stress (load) as the gasket thickness increases. But where the load is very low, such as in angle iron flanges, it is rarely possible to provide the adequate flange thickness needed to keep the flange flat and seal the thin gasket. In most cases, these flanges have low internal pressures, so there is no high risk of a blow-out for thicker gaskets.
For example, there is an example of a 80" flange: a flange thickness of about 6.4mm with 20 5/8" bolts. The number and size of bolts must be increased for such a large flange. For unpressurized oil, the customer asked for a 1.6mm oil-water self-expanding gasket; a better choice would have been a 3.2mm thickness for two reasons:
- First, such a large bolt spacing would result in a very small compression load between the bolts. A thin gasket will not fit well into a bent flange.
- Second, without internal pressure, there is no disadvantage or drawback to using a thicker gasket because the gasket will not be blown out.
However, the situation is quite different for flanges designed for higher pressures. These flanges are much thicker, which means that when the bolts are tightened, they can usually remain flat to 0.1 mm, in which case the recommendation "the thinner, the better" is appropriate. The advantages of thin gaskets are many:
1). Higher resistance to blow-out due to smaller surface area in contact with the internal pressure.
2). Also, the leakage rate is lower due to the smaller surface area in contact with the internal pressure.
3). Better torque retention of the fastener due to thinner shims with lower creep relaxation characteristics.
4). The gasket itself is also less costly.
Thicker gaskets will be better suited for severely damaged or warped flanges.
This is because the ability of a gasket to fill an uneven flange is based on the amount of gasket compression at a given load, and the compression at a given load is expressed as a percentage of the gasket's original thickness. Thicker gaskets have a larger original thickness and compress more. For a 1.6mm shim, 10% compression means 0.16mm of compression, while a 3.2mm shim compressed to 10% would have 0.32mm of compression. This extra shim compression means thicker shims can fill deep scratches or pits better than thinner shims.
The advantages of using thicker gaskets may be misleading and, in any case, may lead to more problems in the future when thicker gaskets are used to seal more defective flanges. Thicker gaskets result in higher creep relaxation, which means that the user may need to re-tighten the bolts to maintain sufficient compression load on the gasket throughout the life of the flange connection. Thicker gaskets will result in greater blow-out forces. This condition is exacerbated by the increased surface area of the part of the gasket that is in contact with the internal pressure, which will create a greater total force trying to push the gasket out of the flange, i.e., the blow-out force. (The unit of internal pressure is Mpa, and a thicker gasket is "taller" in the direction of the internal pressure, which means more surface area. Multiplying the internal pressure in Mpa by the larger area gives a larger force.)
Finally, since all gasket materials are somewhat permeable, the medium can penetrate the gasket body. Thicker gaskets create larger permeation channels and, therefore, higher leakage rates. It is important to note here that the reverse of this occurs as well. If the gasket is too thin to compensate for the flaws in the flange, the media will leak instead of through the gasket body, and the leak rate may be higher than with thicker gaskets.
Flanges requiring thicker gaskets can create problems beyond the manufacturer's control. The best solution is to use or design flanges that can provide higher compression loads, keep the flange surface in good condition and use 1.6mm or even 0.8mm thick gaskets. Users should consider using the higher "M & Y" value of 3.2mm thickness in their design calculations when designing for non-asbestos sheet gaskets but installing 1.6mm thickness gaskets. These recommendations will eliminate some of the most common factors for flange joint failure.
- Split Pumps: The final thickness is critical because it affects the clearance between the two sides of the pump. These pumps typically use a 0.4mm non-asbestos compression gasket. Customers sometimes require sheets with very small thickness tolerances and small thickness variations. It is important to remember that large compression gaskets are usually not applicable here because the final thickness is different.
- For special gasket thickness settings for long transmission piping systems. For example, when compressed, a standard spiral wound gaskets measures about 3.2mm thick. There may be spacing problems in long transmission pipelines where a thinner gasket is used and many flanges on a line; there will be a large gap between the last flange.
- Gaskets placed for grooves: When using tongue and groove face construction or concave flat flanges, the gasket must fill the entire space and be compressed before the flanged metal touches the metal. The thickness of the gasket compressed after loading must be calculated and greater than the gap created by the flange connection. For example, if the groove is 3.2mm deep and the tenon is 0.6mm high, the thickness of the gasket after compression must be greater than 2.6mm, or the flange will make contact before the gasket is fully compressed. In one example, the user requested to replace the original design modified PTFE gasket with an expanded PTFE gasket; the expanded PTFE gasket was not able to fully compress, thus causing a leak, as this expanded PTFE gasket needed to be compressed to nearly 1/3 of the original thickness to be acceptable.
The type of gasket material and allowable compression load also affects the gasket thickness for sealing a specific flange connection. Under ASTM F36 standard testing, a gasket with a higher compression rate would not require the same gasket degree thickness as a gasket with a lower compression rate. A more easily compressed gasket would be less thick to accommodate the flange's defects.
We are always asked to seal defective flanges. This can be accomplished by carefully considering all application variables when selecting the type and thickness of gasket material. However, sometimes a flange or bolt defect is not completely compensated by a gasket. Likewise, proper installation of the flange connection system is critical.