Guidance on gasket thickness for non-metallic flat washers is absent in the ASME B16.21 standard. Selecting the appropriate gasket thickness for a specific application is a recurring problem, difficult to answer succinctly.
Thus, we encourage users, when uncertain about thickness selection, to consult with application engineers.
Today’s article aims to elucidate why this issue is complex, while also imparting relevant application knowledge to prepare users for potential scenarios.
As application engineers for gaskets, we generally advocate for the use of thinner gaskets whenever feasible.
In certain situations, though, a thicker gasket is advisable.
We will clarify this: a 3.2mm thick gasket is necessary and entirely acceptable for some common operational conditions, including the following:
- Thin flanges will become uneven after tightening the bolts, such as 6.4mm thick angle irons or steel plate flanges.
- Large diameter flanges, like those conforming to the AWWA (American Water Works Association) standard, and 3-meter diameter pressure vessels.
- Low pressure, full-faced, large diameter flanges with limited bolt force.
- Older flanges that may have some degree of pitting, warping, or damage.
One reason thick gaskets are used for low pressure, large diameter flanges is that these flanges do not have enough bolts, mainly because the internal pressure is low, so a lot of bolts are not needed in the design.
Limited bolts mean limited gasket compression; thin flanges imply that the flange will deform after the bolts are tightened, compressing the gaps between the bolts to a minimal level or not at all. Thin gaskets do not have enough compression to compensate for these uneven flanges.
This seems to contradict our usual thinking. If you look at our recommended installation stress, we require greater stress (load) with an increase in gasket thickness.
However, in places where the load is very low, such as angle iron flanges, there’s often not enough flange thickness to provide the flatness and sealing required for thin gaskets.
In most cases, these flanges have low internal pressure, so there is no high blowout risk for thicker gaskets.
For instance, let’s consider a 66 flange case: the flange thickness is approximately 6.4mm, with 20 5/8 bolts. For such a large flange, the number and size of the bolts are insufficient.
The customer inquired about a 1.6mm oil-water self-expanding gasket for pressureless oil, but a better choice would be a 3.2mm thick one, for two reasons:
- Such a large bolt spacing results in very small compressive loads between two bolts. Thin gaskets can’t adapt well to warped flanges.
- With no internal pressure, using a thicker gasket has no disadvantages or drawbacks, as the gasket will not blow out.
However, for flanges designed for higher pressure, the situation differs greatly. These flanges are much thicker, which typically allows them to maintain flatness, achieving a flatness of 0.1mm when the bolts are tightened.
In such cases, the thinner-the-better approach is suitable.
There are numerous advantages to using thin gaskets:
(1) Greater blowout resistance due to a smaller area exposed to internal pressure.
(2) Reduced leakage rate, also due to the smaller area in contact with the internal pressure.
(3) Better torque retention in fasteners because of the lower creep relaxation properties of thinner gaskets.
(4) Lower cost due to less material used.
While the thinner the gasket the better “as far as possible”, this principle is the most challenging to define; using thin gaskets isn’t always possible.
Thicker gaskets are more suitable for severely damaged or warped flanges. The ability of a gasket to fill uneven flanges is based on the compression amount under a given load. This compression rate is expressed as a percentage of the original thickness of the gasket.
Thicker gaskets, with a larger original thickness, also have a larger actual compression amount. For a 1.6mm gasket, a 10% compression rate means a compression amount of 0.16mm, while a 3.2mm gasket compressed to 10% has a compression amount of 0.32mm.
This additional gasket compression means that thick gaskets can better fill deep scratches or pits than thin gaskets.
However, the advantages of using a thick gasket can be misleading. Regardless, when thicker gaskets are used to seal more flawed flanges, they may lead to more problems in the future.
Thicker gaskets result in higher creep relaxation, meaning that throughout the lifespan of the flange connection, users may need to retighten the bolts to maintain sufficient gasket compression.
Thicker gaskets can also lead to a higher blowout force, exacerbated by an increased area in contact with the internal pressure, which generates a larger total force trying to push the gasket out of the flange (blowout force).
(The unit of internal pressure is MPa, and a thicker gasket appears “taller” in the direction facing the internal pressure, which means a larger surface area. The larger force results from the internal pressure MPa multiplied by the larger area.)
Lastly, since all gasket materials are somewhat permeable, the medium can penetrate the gasket body. Thicker gaskets create larger permeation channels, resulting in a higher leakage rate.
Note that the reverse can also occur. If a gasket is too thin to compensate for the flaws in the flange, the medium will leak instead of seeping through the gasket body, and the leakage rate may be higher than with a thick gasket.
Flanges that require thicker gaskets can lead to problems that gasket manufacturers cannot control.
The best solution is to use or design flanges that can provide a higher compression load, maintain a good flange surface condition, and use gaskets with thicknesses of 1.6mm or even 0.8mm.
When designing to use non-asbestos sheet gaskets, users should consider using the higher 3.2mm thickness “M&Y” values in their design calculations, but install a 1.6mm thick gasket. These recommendations will eliminate some of the most common causes of flange connection failures.
In special cases, washers with very specific thicknesses are required. There are numerous washer connections that necessitate a particular washer thickness. For these types of connections, it’s crucial to remember that the final compressed thickness of the washer must be taken into account. This could include the following scenarios:
Split pumps: The final thickness is critical as it affects the clearance between both sides of the pump. These pumps often use 0.4mm non-asbestos compression washers.
Customers sometimes require sheet metal with small thickness tolerance and minimal thickness variation. It’s important to remember that washers with large compression are generally not applicable here, as the final thickness is different.
Long-distance pipeline systems designed for specific washer thickness. For instance, a standard spiral wound washer, when compressed, measures approximately 3.2mm in thickness. There could be a spacing issue in long-distance pipelines if thinner washers are used, and there are numerous flanges on a single pipeline, creating a large gap at the last flange.
Washers used in slots: When using tongue and groove surfaces or concave flat flanges, the washer must fill the entire space before the metal of the flange contacts other metal. The compressed thickness of the loaded washer must be calculated and must exceed the gap created after flange contact.
For example, if the slot is 3.2mm deep, the tongue is 0.6mm high, the compressed thickness of the washer must exceed 2.6mm, otherwise, the flanges will contact before the washer is fully compressed.
The type of washer material and the allowable compression load can also affect the thickness of the washer sealing a specific flange connection. Under the ASTM F36 standard test, washers with a higher compression ratio do not require the same thickness as those with a lower compression ratio, as a more easily compressed washer does not need to be as thick to accommodate flange defects.
We are always asked to seal flawed flanges. This can usually be achieved by carefully considering all the variables of the application conditions when choosing the type and thickness of the washer material.
However, sometimes the defects of the flange or bolt are not entirely compensable by the washer. Similarly, the proper installation of the flange connection system is also crucial.