Selecting Gasket Material: Consider Temperature Carefully

All gasket materials have a temperature range they work over. Going outside of this range is a recipe for leakage, but buying one with a wider range than is necessary can be unnecessarily expensive. Why buy a silicone gasket when a nitrile gasket will do the job? The key lies in understanding the expected in-service temperatures.

Effects on the joint

Temperature, and especially temperature cycling, affects sealing in three ways:

  • Expansion/contraction of the joint and fasteners alters clamping loads and gaps.
  • High/low temperatures can result in material cracking of extruding out of the joint.
  • Cycling demands the material recovers to maintain the seal at all times.

The external thermal environment

Gaskets placed outdoors can experience large temperature swings, but ambient temperature is only part of the story. Piping running above a desert floor will absorb solar energy, getting well above 100F. Likewise, a brisk north wind in a Minnesota winter can produce effective temperatures far below zero.

Extreme temperatures are not uncommon indoors either. Foundries and frozen food distribution centers are both examples of where gaskets could see very high or very low temperatures, (although swings between the two are less likely.)

Media temperature

Knowing the mean temperature of the media being transported or sealed isn’t enough. Abnormal operating conditions could lead to unexpected peaks or dips, as can shut-downs and start-ups. Steam cleaning in particular can lead to higher than normal temperatures.

For enclosures it’s important to estimate the worst-case thermal load. Electrical equipment like drives and transformers produce significant heat and while a cabinet might have ventilation, consider the possibility of a blocked filter or failed fan.

Thermal gradients

Temperature differentials across a sealed joint can also challenge gasket materials, especially when that gradient changes. Piping cryogenically-cooled liquids through the desert, or hot gases in the arctic can make joints move and needs materials that recover quickly without taking a compression set.

Consider the worst-case

When selecting gasket material, know what temperatures to anticipate and choose accordingly. For information on gasket materials, contact a product specialist at Hennig Gasket.

Choosing Gasket Material

When changing a gasket most technicians choose a new one made from the same material. If a paper, fiber or cork gasket came out of the joint, then the replacement is usually the same.

That’s not necessarily bad, assuming the gasket hadn’t failed prematurely, but it could also be a missed opportunity. Other gasket materials might hold up better in the application. That would allow more time between inspection and replacement, reducing downtime frequency and saving on maintenance hours.

Gasket materials are specified by multiple criteria, and the importance of each depends on what the application needs. One way of looking at these properties is to divide them into mechanical – their gap-filling ability – and material – how well they handle the media.

Mechanical properties

Whether looking for boiler seals or food grade gaskets, the primary considerations are thickness and hardness. Thickness is easy to understand, (always choose the thinnest that will do the job,) but hardness is less obvious. Gasket material hardness is reported in terms of Durometer, usually on the Shore A scale. (See “Measuring Gasket Material Hardness.”) When comparing two materials of the same thickness, the softer one is usually the better choice.

Other properties to look at are compressibility and creep relaxation. Compressibility measurement is defined by the ASTM F36 standard and describes the load needed to provide a given level of deformation. In general, higher compressibility implies lower loads are needed to secure a joint. Creep relaxation, addressed in ASTM F38, indicates how the gasket thins over time, which reduces bolt loading.

Material properties

Gasket material must be appropriate for the media. For example, nitrile gaskets are preferred for applications involving petroleum, mineral or vegetable oils but don’t perform well with ozones, ketones, esters and aldehydes.

The ability to handle expected temperatures is also important. This is especially critical where the environment causes severe temperature gradients through the joint. (Imagine piping liquid nitrogen in the desert southwest.) Nitrile gaskets may be appropriate for the media but an alternative, like silicone, might handle the temperatures better, (although has poor hydrocarbon resistance.)

The Difference Between Soft, Semi-Metallic and Metallic Gaskets

Gasket selection is driven by the needs of the application. Temperature, environment, media and pressure dictate the gasket required. While there are many different types, to aid selection they are usually separated into three classes:

  • Soft
  • Semi-metallic
  • Metallic

Soft gaskets

These are made from materials that compress easily, such as elastomers like nitrile, (NBR,) EPDM and silicone, as well as graphite, PTFE and fibrous materials. Their corrosion resistance is good but they are limited in the temperatures they can handle. Nitrile gaskets for example only work from -60 to 250°F (-51 to 121°C) and EPDM is only slightly better with a range of -70°F to 350°F (-57°C to 177°C). Silicone gaskets will however go up to 500°F (260°C) and PTFE is effective from cryogenic temperatures up to 450°F (232°C).

Soft gaskets are also limited in their ability to handle high pressures. The best applications are those involving sealing variable gaps as might be found around the doors of an electrical enclosure.

Semi-metallic gaskets

Bridging the gap between metallic gaskets and soft gaskets, the semi-metallics combine features of each. The two main types are spiral-wound and metal-jacketed, although other forms exist. Spiral wound gaskets are made from a ribbon of soft material like PTFE or graphite layered with metal, usually in a ‘V’ form to provide compressibility. Jacketed gaskets consist of a metal cover over a filler material.

Semi-metallic gaskets can handle a wide range of temperatures and pressures up to 6,000 psi, (based on ANSI pressure class 2,500,) so are used in applications ranging from refineries and chemical processing plants to aerospace.

Metallic gaskets

As the name implies, this type of gasket is made from metal. That allows it to resist pressures as high as 10,000 psi but also means it has virtually no compression. Very high bolt loads are needed to create enough deformation for joint sealing.

Metallic gaskets are vulnerable to galvanic corrosion. To minimize problems the gasket metal should be close to the flange material on the electrochemical scale. Alternatively, the material should be chosen to make the gasket the sacrificial element.

Focus on the Cost of Sealing

Gaskets exist to seal joints or interfaces. They’re either keeping something in or keeping something from getting in, and if they do their job no one notices them. That’s probably why some gasket buyers find themselves under pressure to go with the cheapest. Only later do they find that a very expensive mistake.

Gasket failure is expensive

The consequences of a leaking joint range from the trivial to the fatal. At one end of the spectrum, if a pipe flange gasket lets a trace of toxic chemical into the environment the results can be unthinkable, and will probably incur the wrath of the EPA. Fines and clean-up costs could sink the most successful company. Or consider other less serious but still expensive examples. Water penetrating an electrical enclosure gasket could damage equipment inside, causing lengthy unplanned downtime. Failed boiler seals might shut down a heating system, sending employees home. Even when the impact is minor, a lot of time might be spent cleaning up, and a lot of product wasted.

Gasket replacement is expensive

There’s the time and materials to do the job and perhaps other expenses involved in accessing the gasket location, but these pale next to the cost of lost production. A single leaking pipe can bring an entire plant to a halt while a new gasket is installed. Planned replacement is always preferable to reacting to a leak, but either way takes equipment out of service for a period of time.

Lifetime reliability

The price of the gasket is a very small part of the cost of a sealing problem. Logically then, anything that extends the life of the gasket is worth doing.

There are many options for sealing a joint or interface. Gasket materials come with long lists of specifications. Interpreting these and selecting the optimal combination takes in-depth product knowledge and understanding. Gasket experts might find what they need in a catalog, but for most buyers the best option is to ask their supplier. They’ll be happy to explain the characteristics of each gasket material

Open or Closed-Cell Gasket Material

When it comes to gasket material hardness the general advice is that softer is better, providing it seals the joint. Elastomeric gaskets used for sealing enclosures are a good example. When the enclosure door is closed there’s often a large and uneven gap remaining, (especially in the case of light-duty plastic enclosures.) A soft gasket compresses easily where the gap is smaller while filling the larger gaps, providing a seal all the way around the opening.

Interconnected cells

Many softer gasket materials, such as silicone, urethane and neoprene, are available with a cellular structure that makes them very soft. These cells are easily seen in cross-section. What gasket material buyers may not appreciate though is that these cells may be open or closed. This matters because it gives the gasket material different performance characteristics.

In a closed cell material, each cell is completely sealed off from its neighbors. That makes it feel harder because when compressed the air inside has no place to go. In an open material the cells are interconnected, so under compression the air moves through and out of the material, making it feel softer.

Different characteristics

Closed cell materials take on a compression set more readily than do open materials. This is because, under load the air inside permeates slowly through the cell walls. When the load is removed, although the material tries to spring-back it can’t draw air in, leaving the gasket material permanently deformed. In contrast, an open cell material “breathes,” drawing air back in to each cell as the material rebounds.

The weakness of open cell gasket materials is a lack of water-resistance. Just as in a sponge, the interconnected cells let water move through the structure. Although a load may close up the openings and provide some resistance, open cell gasket materials are not recommended for situations where water exposure is possible.

Consider the application

An open cell structure makes for a softer gasket, and one less likely to take a compression set. However, a closed cell material provides better water resistance. Select your gasket material based on the application.



Measuring Gasket Material Hardness

The hardness of elastomeric gasket materials is measured with a durometer. Knowing how this device is used helps in interpreting specifications and selecting gasket material.

Durometer Construction

Durometers come in two forms, analog and digital. Analog durometers look like the traditional stopwatch with a single hand that sweeps around the dial. This dial is mounted on a flat foot, from which protrudes a pin. The pin is spring-loaded, so when the foot is pressed against the gasket material the pin moves up into the body of the durometer. The harder the material, the more the pin moves into the body. Or to put it another way, softer materials let the pin press in deeper.

The dial is marked from zero to 100. These numbers have no units but are related to the spring load and the size and shape of the head of the pin, more properly called the ‘indenter.’

Shore Hardness

Spring strength and indenter geometry are specified in ASTM standard D2240. This fixes every aspect of rubber hardness testing, including the size of the ‘presser foot’, sample preparation, the duration for which the indentor is pressed into the material, and calculation and presentation of results.

Rubber and rubber-like materials can vary enormously in hardness, so ASTM D2240 defines a number of different scales. Each scale has its own indenter form and spring load. Gasket materials are typically measured on the Shore A scale. The ‘A’ indenter is a pin of 1.27mm (0.050”) diameter, tapered at 35 degrees to finish as a truncated cone with a flat area of 0.79mm (0.031”) diameter. At a reading of 100 (no indentation,) the spring force will be 8.05 Newtons.

Determining the Hardness Number

According to ASTM D2240, the test specimen should be at least 6.0mm (0.24”) thick. Hardness is calculated as the mean or median of five measurements taken at least 12.0mm (0.48”) from any edge.

A Comparative Measure

Being dimensionless, the Shore A number tells you little about the properties of an individual material. Its real value is as a standardized test method, allowing comparison of alternative materials for elastomeric gaskets.

Understanding Gasket Material Hardness

The question of how hard a gasket should be comes up quite often. For an answer we need to look at what the gasket actually does.

Gasket function

The job of every gasket is to fill an uneven gap between two surfaces, forming a barrier that stops fluid moving to where it shouldn’t be. Larger gaps and more uneven surfaces need a softer gasket. For example, a gasket between two parallel machined pipe flanges can be hard, resisting loads as the joint faces are tightened together. In contrast, the gasket sealing an electrical enclosure needs to be softer and compress more because the enclosure door will tend to bend as it’s latched.

So a general rule is that a gasket should be as soft as possible in order to fill the gap between two surfaces. At the same time it must be strong enough to resist the lateral forces acting on it.

For elastomeric gasket materials two parameters define hardness: Shore hardness and compression force deflection (CFD.) Here’s what these two terms mean.

Shore hardness

Hardness in this context is a measure of how well a material resists a permanent indentation. The hardness of rubber and elastomeric materials is measured on a durometer and reported as a “Shore A” number. Very soft materials like a rubber band will be around 20, a pencil eraser is between 30 to 40, and car tires measure 60 to 70 Shore A.

Compression force deflection

CFD measures firmness and is defined in ASTM standard D1056 as the force needed to reduce the material in thickness by 25%. According to this standard materials are given a grade correlating to their firmness. Grade 0 material needs less than 2 psi to reduce its thickness by 25%, so is very soft. At the other end of the spectrum a grade 5 material needs at least 17 psi to achieve the same compression. A gasket material that compresses easily accommodates variation in the gap between two surfaces without needing more closing force than can be applied by the clamps or latches.

Using Die Cut Gaskets for Electrical Enclosures

Electrical and electronic equipment should always go in an enclosure designed to protect it from water and dust. However, the enclosure is only as good as the gasket that seals it.

An electrical enclosure gasket needs to accommodate irregularities in the surfaces of both door and enclosure, and continue to seal after repeated opening and closing. It should also resist varying degrees of external pressure, depending the standard applicable.

Outdoors, the threat usually comes from rain, which may be wind-blown but seldom impacts the enclosure with significant force. In industrial situations, especially in the food, medical and pharma sectors, enclosures are often expected to resist high pressure washdowns.

Standards for ‘ingress protection’ are promulgated by several organizations, notably the International Electrotechnical Commission (IEC,) NEMA and UL. The IEC 60529 standard defines the IP levels that many engineers know. (IP65, 66 and so on.) NEMA standards are seen as more demanding in terms of not allowing water penetration but only address design intent. Only UL insists on third party testing to verify compliance.

Three types of gasket are used in enclosures:

  • strip
  • ‘foam-in-place’ (FIP)
  • die-cut

As the name suggests, a strip gasket is cut from sealing material and applied in lengths to the enclosure door. Inevitably that leaves gaps. Dust may be excluded but water can almost certainly penetrate.

For an FIP gasket liquid polyurethane is applied to one of the mating surfaces. As it cures a reaction makes it foam, producing a joint-free gasket. It’s a popular approach but tends to be messy and can be slow.

Die-cut gaskets are stamped from roll or sheet material and are the shape of the enclosure sealing face. The absence of joints means no leak paths. Material selection depends on applications requirements although neoprene or silicone are often good choices. Installation is just a matter of fitting them in place, usually with an adhesive.

Hennig Gasket & Seals can die-cut gaskets as large as 36″ x 62″. For larger gaskets flash (an oscillating knife,) or waterjet cutting are available.  Contact us Today.

Selecting FDA Gasket Material for Food Industry Applications

An FDA investigation of a food contamination incident found a gasket at the root of the problem. Cleaning agents weren’t penetrating small cracks and E. Coli was able to gain a foothold. In their findings, the FDA suggested inspectors should:

  • Look at how frequently a food manufacturer inspects and replaces gaskets
  • Challenge the effectiveness of Clean-in-Place (CIP) procedures
  • Verify that gasket material is FDA approved for use

Responsible food manufacturers conduct rigorous cleaning and inspection regimens. Whenever any doubt exists as to the fitness of a gasket it gets replaced, but that presents those performing the refurbishment with the question of what material to use.

The FDA maintains listings of approved FDA gasket materials. PTFE and many elastomers such as NBR, SBR and EPDM are included, but two points are sometimes overlooked by those shopping for FDA gaskets. First, any markings, such as part numbers or other information used for traceability must also be FDA compliant. Second, any adhesives used to hold the gasket in place must also comply with FDA requirements.

Where should FDA gaskets be used?

Any surface coming into direct contact with food must be manufactured from materials known to be safe. (“Safe” in this context means either materials already listed by the FDA or those it considers “Generally Recognized As Safe” (GRAS).) This includes gaskets used in food preparation equipment such as kettles and mixing vessels, as well as those used in sanitary couplings; the kind of fittings used for moving dairy or brewery products.

How to select appropriate materials?

One approach would be to trawl through the various applicable FDA documents, noting which would work in your application. A less time-consuming approach is to ask your gasket vendor for advice. Describe your application in detail, particularly the temperatures and pressures involved and the cleaning regimens employed, and they’ll know which materials are suitable. You’ll receive FDA gaskets that work in your application and contamination risks will be reduced.

How To Bolt Flanges

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.


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.