We are often asked about the differences between elastomeric (or soft) and hard gaskets, and which style is better suited on hinged and bolted manways.
From a technical standpoint GRI feels the “hard” style gaskets are better suited for several reasons. This is important because hinged and bolt manways are the number one cause of non-accident releases (NARs) in the industry.
What Are Elastomeric Gaskets?
Elastomeric gaskets are made of homogeneous rubber materials which include Nitrile, Buna-N, SBR, Viton®, and EPDM.
The advantages of these materials are that they are very compressible and seal quickly under low flange loads, but on the high end the compressive load range for these materials is to approximately 1500 psi, above which these materials can crush resulting in leaks.
Hard Gaskets to Consider
Hard gaskets come in various material styles and compression ranges can vary by the type of material.
For hard gaskets each material type has its advantages and disadvantages. For hinged and bolted manways, we recommend two materials types:
- Filled PTFE, such as Durlon® 9000
- Compressed Fiber, such as Durlon® 8500
Reducing NARs with Hard Gaskets
The Non-Accident Release Reduction Program of the AAR details various causes for NARs on an annual basis. As already stated, hinged and bolted manways are the number one cause of tank car NARs. The AAR lists four main cause codes for manways, these include:
- Bolts loose
- Gasket deteriorated
- Gasket missing
- Gasket misaligned.
We believe two of the above codes, loose bolts and deteriorated gaskets, are mainly a result of the use of elastomeric gaskets.
The code for deteriorated gaskets is easy to identify as being related to the wrong elastomer being selected for a commodity. In contrast, a filled PTFE is chemically resistant. We also believe loose bolts may be related to soft, elastomeric gaskets as well.
With elastomer gaskets the bolts cannot be overly tightened or the gasket will be crushed. As a result of the low torque used with elastomer gaskets the bolts are not stretched to become spring-like, and they have no stored energy to react to pressure or temperature fluctuations. This leads to a greater tendency to relax and loosen with the vibration that is common on tank cars.
Hard gaskets on the other hand can be torqued to a much higher compressive load. A higher bolt load will stretch the fasteners so they become spring-like with stored energy and fewer tendencies to relax and loosen.
Learn more about best practices for torque values and bolt tightening by downloading a free copy of our Gasket Installation Worksheet.