FAQs

Leaking volatile organic compounds, (VOCs) can cause breathing difficulties for people living or working near the source. Some VOCs, known as volatile hazardous air pollutants (VHAPs,) may even cause cancer or birth defects.

According to the EPA, valves are the largest source of leaks. They account for some 60% of the 70,000 tons of VOCs that escape each year. Next up are flanged connections, responsible for around 30% of “fugitive” emissions. Valves generally leak around the stem or gland. For flanged connections gaskets play an important role leak prevention.

Create a Formal Program

No business wants to be associated with problems like these. That’s why a leak detection and repair (LDAR) program is so important. LDAR is required under 25 different Federal standards, but even if a business doesn’t come under one or more of these, a program still provides benefits.

Leaks are lost product, and that’s wasted money. Leaks tend to get worse over time too, so early detection can prevent a larger problem in the future. Relying on informal, ad hoc inspections is no way to monitor equipment condition. Instead, use a LDAR program to formalize the process.

LDAR Basics

The EPA publication, “Leak Detection and Repair: A Best Practices Guide” explains how to run an effective program. The five key steps of this are:

• Identify components that need testing for leaks – valves and anything that uses a gasket to seal fluids.
• Define “leak” in terms of a ppm level. (The EPA suggests using a tighter level than required by applicable standards.)
• Implement a monitoring program – “Method 21” is a formal process documented in the EPA booklet referenced above.
• Repair leaks. The EPA recognizes that not all leaks can be fixed immediately but expects prompt action. Either keep gaskets on-hand or find a reliable supplier who can make-to-order and has short lead times.
• Keep records

Choose Quality

Replacing gaskets can be expensive, but neglecting them could cost more. Buy quality gaskets from an established supplier and install them with care. That way they’ll last longer and you’ll have fewer problems.  Hennig Gasket & Seals has been in the business of manufacturing custom cut gaskets and seals for over 100 years.  Contact us today.

Selecting the right material for a gasket is never easy. Temperature and pressure must be considered, and so too must the nature of the fluid being sealed. Some combinations of fluid and gasket material are just incompatible. Choose wrongly and the gasket will fail prematurely. When purchasing gasket material, mention the intended purpose. That way a material specialist can tell you if there’s a better alternative.  View our rubber sheet gasket material compatibility chart for chemicals.

Learning More About Material Compatibility

Some of the most frequently used gasket materials are neoprene, nitrile rubber, EPDM, silicone and Viton®. (Technically, this last one is a DuPont tradename for fluoroelastomer or FKM as it’s known in the ASTM standards.) Each has strengths and weaknesses in terms of chemical compatibility. “Rubber Gaskets & Seals” on our website provides a summary of what chemicals various gasket materials do and do not work with.

The other way to approach material selection is from the perspective of the chemical. Unfortunately that means identifying every possible chemical, which makes for a very long list. For some commonly used fluids the lists below highlight good and bad combinations. But don’t rely solely on these – always ask advice!

Good Material Compatibility Pairings for Chemicals

• Acetone (a form of ketone): EPDM
• Aldehydes (found in many food ingredients): SBR
• Ammonia: Good choices are neoprene and EDPM gasket material
• Animal fats: Nitrile, Viton®
• Automatic transmission fluid: Nitrile rubber (NBR) and Viton®
• Brake fluid: EPDM and styrene-butadiene rubber (SBR)
• Ethylene glycol (antifreeze): nitrile rubber, Viton®, EPDM and neoprene work well
• Fuels and oils: Nitrile rubber (NBR), Viton®
• Ozone: EPDM, silicone, Viton®

Bad Gasket Material Pairings for Chemicals

• Acetone (a form of ketone): Avoid contact with nitrile rubber, neoprene, silicone and Viton®
• Aldehydes (found in many food ingredients): Nitrile
• Ammonia: Poor with SBR and Viton®
• Animal fats: SBR
• Automatic transmission fluid: Avoid EPDM, SBR and Silicone
• Brake fluid: Viton®
• Fuels and oils: EPDM, SBR, Silicone
• Ozone: Nitrile and SBR

Seek Advice for Material Compatibility with Chemicals

These lists give only a superficial overview of a complex subject. The only sure way of selecting the correct gasket material for any given chemical is to seek guidance from a material specialist. At Hennig Gasket, if we don’t know we’ll find out.  Contact us Today.

Gaskets seal gaps of varying size by compressing under load. Best practice is usually to keep that load as low as possible, which is why softer gasket materials are preferred. For products like silicone or PTFE gaskets durometer numbers give a good indication of material hardness, (following the ASTM D2240 standard,) but they don’t show how that material will perform in a joint. That means turning to the ASTM F36 test data.

Compressibility and Recovery

When selecting gasket material it’s important to understand its compression and recovery behavior. This is because joints tend to move, whether due to varying temperatures, (media and environmental,) or loads. A material that compresses easily but has no recovery may not do a good job of sealing a joint that experiences a lot of cycling.

ASTM F36 provides a standardized method of testing and measuring compressibility and recovery. The test has two parts. First, the material is put under a load of 5,000 psi for 60 seconds and the reduction in thickness measured. Then the load is taken off and the material given another 60 seconds to spring back before the thickness is measured again. Both compressibility and recovery are expressed as percentages.

Caveats

The conditions F36 testing is done under don’t necessarily reflect the actual usage conditions as temperatures, pressures and loads will almost certainly be different. Neither do they take time into account, which in reality is a significant factor when dealing with viscoelastic materials, (where properties change over time.) What the numbers do provide is a basis for comparing between different gasket materials.

Typical F36 Numbers

Compressibility and recovery values vary greatly between different materials. For example, expanded PTFE has a compressibility of around 68% but recovery of just 12%, while the same numbers for a neoprene gasket could be 7 to 17% compressibility and 50% recovery. This would suggest the neoprene material would perform better in an application where flange faces are in good condition but gasket loads cycle. Of course, other factors such as temperatures, media and pressures must also be considered.

According to the Fluid Sealing Association (FSA,) incorrect tightness is the leading reason gasketed joints fail. This can be prevented by following good bolting practice.

Torque

After installing a new gasket or seal it’s essential to tighten the fasteners with a torque wrench that’s been recently calibrated. Without this it’s impossible to know if the joint has been tightened to the required level.

Friction between the nut, washers, flange faces and thread increases the torque measured at the wrench, possibly resulting in insufficient clamping force being applied to the gasket. Avoid this by applying a thin, uniform coating of high quality lubricant to the underside of bolt heads, nuts and washers and the thread itself. Take care to keep it off the gasket.

Tightening sequence

The gasket must be compressed uniformly to avoid material displacement. It’s also important to avoid deforming the flange faces. There are two aspects to consider: the bolt pattern and the tightening sequence.

Bolt pattern

To bring the joint together, fasteners should be tightened in opposite pairs. Start at 12 o’clock and then move to 6 o’clock. Then halve the angle between them, moving to the 3 and 9 o’clock pair. Halve the angle again, going to the pair closest to 1:30 and 7:30. Keep repeating until every bolt has been tightened.

Tightening sequence

1. Following the pattern described above, insert the bolts and run up the nuts by hand.
2. Set the torque wrench to 30% of full torque and, using the pattern, tighten each fastener.
3. Repeat with the torque wrench at 60%.
4. Repeat again with the torque wrench at 100%.
5. Make a final pass, this time in a circumferential direction, ensuring each fastener is at the required torque.

Do the job once

Replacing gaskets and seals can be expensive, so whenever joints are made in pipes and ducting it’s important to ensure they don’t leak. One factor in achieving a good joint is to follow good bolting practice. Control the torque applied, the bolting pattern and the tightening sequence to avoid leaks.

Selecting gasket material requires knowledge of how it’s going to perform in the joint. There are a number of material properties that designers or engineers use to guide their choice for the fabrication of a custom gasket. One of those is compressibility. Essentially a measure of material stiffness, compressibility is defined as the percentage reduction in thickness that occurs under the application of a given load. It’s often presented graphically with thickness reduction along the x-axis and load in pounds per square inch on the Y.

All non-metallic gasket materials compress or densify under load. It’s how they adapt to the mating faces, filling hollows and compensating for poor parallelism. (Metal gaskets are usually designed with compressive features for the same reason.) In general, a softer gasket material is going to deform more easily, so resulting in a leak-tight joint at the lowest possible clamping force.

Complicating the selection process, softer materials often have a tendency to flow or extrude. Bolt loads push material out through the bolt-to-hole clearance and from around the flanges. Internal loads can also lead to the material extruding out, ultimately creating a leak path.

Another issue is relaxation. The compression curve shows the initial load to create a given deflection. However, as with most materials, gasket materials undergo both elastic and plastic deformation. Elastic deformation is temporary: remove the load and the material springs back. But plastic deformation is permanent: the material takes on a ‘set.’ So when the joint is first made the compressive force is high, but over time, (minutes rather than days,) it reduces. This stress relaxation is another important material property for the designer to consider.

Plastic deformation has implications for gasket life too. When a joint is undone some of that initial compressibility has been lost, which is one reason why gaskets shouldn’t be reused.

Gasket compression curves indicate the stiffness of a material. They should be used as an aid to selecting the softest material for an application, having given regards to the other properties needed. If in doubt, it’s always best to consult a specialist!  Contact Hennig Gasket & Seals today for fast quotes and accurately cut parts.