Does Your Application Need a Graphite Seal or Gasket?

Mar 14, 2016

High temperatures challenge many gasket materials. Nitrile gaskets will go up to about 95oC, silicone gaskets to 200oC and PTFE to 260oC, but what if you need to go higher? One option is to go with a metal gasket. A better one is to ask about graphite. Graphite seals and gaskets retain their properties at temperatures as high as 450oC, and have some other very useful sealing characteristics.

Properties of graphite

Graphite is a form of carbon where the atoms are arranged in layers or sheets. That lets them slide over one another easily, which translates to a slippery feel when rubbed between finger and thumb. (It’s also what lets a “lead” pencil write – graphite actually rubs off onto the paper.)

This slipperiness or low coefficient of friction is useful in gasketing or sealing. As mating surfaces are brought together graphite allows some slip. That accommodates any rotation or sideways movement as the clamping load goes on without damaging the sealing material. Used as a dynamic or shaft seal the low friction properties of graphite minimize energy losses and heat build-up while maintaining an effective barrier.

Graphite is also a very soft material, (which might seem odd when you consider it’s a cousin of diamond,) but that lets it flow into surface irregularities, which is what provides the sealing function.

Like PTFE, graphite is quite inert. It resists attack from most corrosive chemicals, even at high temperatures, and likewise doesn’t contaminate them.

Graphite seals and gaskets

It’s possible to buy graphite for use as a gasket or seal. It’s also used as a coating for some metal gaskets, (Kammprofile gaskets are an example,) where it provides excellent sealing performance along with temperature and chemical resistance. If you’re looking for gasket material for a high temperature application, ask Hennig Gasket if graphite seals might be right for you.

Graphite Gaskets
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Failure Mechanism at High Temperatures

Mar 07, 2016

Previous blog posts emphasized the importance of assessing the peak temperatures anticipated in a gasketed joint. Left unsaid has been why this matters. It’s probably obvious that polyurethane, nitrile and silicone gaskets all have a temperature at which they melt. More important, all will likely fail under prolonged exposure to temperatures near their melting point, due to a phenomenon called “creep.”.

Viscoelastic materials

Most gasket materials are viscoelastic. The “viscous” part means they have a propensity to flow slowly, like a thick gel and “elastic” refers to their ability to stretch and return to their original dimensions. However, elasticity has its limits. If the material is stretched too far it can’t return to its original size or shape, resulting in permanent “plastic deformation.”

Place a viscoelastic gasket material like polyurethane under load and it becomes thinner while simultaneously spreading outwards. This is “creep.” Releasing the load lets the material recover, but only to the extent that it has not been deformed plastically.

Creep relaxation

In a gasketed joint the material is compressed, either by the stretch of the flange bolts as they are tightened or by other retaining clamps. When first placed under load it starts to creep, but as the gasket thins the load lessens until the creep stops. This is termed creep relaxation. With good design this happens before the gasket reaches a point where the joint starts to leak.

Higher temperatures

Creep is related to temperature. When a polymer like polyurethane or styrene butadiene rubber gets warmer the molecular chains slide more readily. As a result it takes less force to produce a given movement. As the temperature approaches the melting point of the material, the force needed to produce a given movement falls quickly. To take one example, this means that at temperatures over 200 F (93 C) a nitrile gasket starts shows considerable creep.

Consider the material properties

Always select gasket material with the knowledge of the maximum temperatures expected. The more safety margin can be incorporated the less creep will be experienced, leading to a longer lasting gasket.

Gasket Material
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Why Thinner Gasket Material Usually Works Better

Feb 15, 2016

Gasket materials come in many thicknesses. To give one example, at Hennig Gasket neoprene gasket material is available from 3/32” all the way up to 2” thickness. Customers will sometimes ask what thickness they should buy, but a gasket material supplier really can’t help with that. It depends completely on the application. However, it’s generally agreed that a gasket should be as thin as possible, providing it still seals. There are four reasons. A thinner gasket:

1. Has greater blow-out resistance. Being thinner, the gasket present less area to the internal pressure, so is less prone to deformation and failure.

2. Has a lower leak rate. All gaskets will allow some quantity of fluid to pass through. This is just a natural function of their structure and the make-up of the fluid being constrained. (Anyone who’s ever tried piping helium knows how its small molecules let it escape from almost anywhere!) So the less gasket material that’s exposed to the fluid, the less will leak.

3. Retains fastener torque better. This stems from the creep relaxation characteristics of the gasket material. When there’s less thickness there’s less creep, (think of it happening on a percentage basis,) so more torque is retained.

4. Is less expensive. Material cost relates more to volume or weight than area, and thicker gaskets need more material. Secondly, thickness also influences cutting method and thicker materials could be more expensive to cut to shape. Neoprene gasket material 3/32” thick die cuts readily, but a thickness of 2” may call for a waterjet.

Note though that points 1 and 2 really only apply to situations where the gasket resists pressure, such as in pipelines. In no-pressure situations such as a gasket sealing around an electrical enclosure, the benefit is primarily Point 4 – cost.

All About the Gap

How thick a gasket should be depends entirely on the application. Remember that it’s purpose is to take up an uneven gap between two surfaces. The key is having enough thickness that the gasket compresses and fills the voids, but no more.

FAQs, Gasket Material, Neoprene Gaskets
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HVAC Sealing Material Primer

Feb 13, 2016

HVAC system seals and gaskets maintain efficiency by preventing the loss of heated and cooled air. Whether installing new ductwork, modifying an existing system, or just replacing worn out gaskets, it’s important to choose appropriate material. Many HVAC specialists consider neoprene gaskets the default choice, but it’s possible better performance could be achieved with EPDM or silicone gaskets.

HVAC Gasket Applications

Gaskets have three main roles in HVAC systems:

  • Sealing opening panels, flaps, and doors
  • Reducing transmission of motor or fan vibration
  • Allowing for thermal expansion and contraction

Sealing

Almost every ducting system includes access doors and panels, along with dampers that close off airflow through “legs” of the system. To minimize closing forces, these need a soft material with good compressibility. Combined with appropriate thickness, such gaskets will also take up the dimensional variation and uneven edges inevitable in most systems.

Reducing Vibration Transmission

Fans and motors can cause a vibration in flat ducting that’s audible as a low hum. To avoid complaints from building tenants, incorporate gaskets at appropriate interfaces. The cellular structure absorbs the vibration and prevents it spreading throughout a system.

Expansion and Contraction

Metal ducting experiences significant dimensional changes in response to switching between warmed and cooled air. A gasket with good recovery takes up these changes while still maintaining a leak-tight seal.

Environmental Factors

Outdoor applications challenge HVAC gasket material as UV light degrades some materials, and moisture penetration must be avoided. Low temperatures and ozone might also be a concern in some applications.

HVAC Gasket Materials

Neoprene gaskets and those made from thermoplastic elastomers (TPE’s) generally perform when soft and resistant to compression set. EPDM gaskets work well outdoors as they stand up to sunlight and other weathering effects. Where air or gas temperatures are high silicone gaskets can be a good choice.

Closed cell materials may be preferable because air and moisture cannot pass through, although these are firmer, requiring higher closing forces.

Installation is simplified by using a pressure sensitive adhesive (PSA). This can be laminated on to the gasket material or can be applied in tape form.

Flange Gaskets
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Selecting Gasket Material: Consider Temperature Carefully

Feb 01, 2016

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.

Gasket Material
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Choosing Gasket Material

Jan 18, 2016

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.)

FAQs, Nitrile Gaskets
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The Difference Between Soft, Semi-Metallic and Metallic Gaskets

Jan 04, 2016

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.

FAQs, Nitrile Gaskets, Spiral Wound Gaskets
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Focus on the Cost of Sealing

Dec 21, 2015

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

Flange Gaskets
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Open or Closed-Cell Gasket Material

Dec 07, 2015

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.

 

 

FAQs, Rubber Gaskets
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Measuring Gasket Material Hardness

Nov 23, 2015

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.

Rubber Gaskets
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