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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Understanding Gasket Material Hardness

Nov 09, 2015

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.

Rubber Gaskets
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Using Die Cut Gaskets for Electrical Enclosures

Oct 26, 2015

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.

Gaskets & Seals
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Selecting FDA Gasket Material for Food Industry Applications

Oct 12, 2015

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.

Food Grade Gaskets
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How To Bolt Flanges

Sep 29, 2015

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.

FAQs
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Understanding Gasket Compression Curves

Sep 15, 2015

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.

FAQs
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Preparing Flanges for New Gaskets

Aug 18, 2015

Preparation is everything they say, and that’s certainly true for flanged pipe connections. As flanges are brought together and the bolts tightened, the flange gasket compresses and flows into surface irregularities. If those are too severe for the gasket material to fill, the joint will leak. Here’s some advice on flange preparation.

Step 1: Inspection

Examine both flange faces carefully for damage like cracks, dings, burrs and radial scoring. Scoring is the worst problem as this will almost certainly create a leak path. Also check for alignment and verify that the faces are flat and parallel. (It’s possible for flanges to warp if the bolts are tightened in the wrong sequence.) Some softer gaskets will tolerate flanges being slightly out of parallel, but this does depend on the material being used.

Also check bolts, nuts and washers for signs of damage or corrosion. If in doubt as to fitness for purpose, opt to replace.

Step 2: Clean the Mating Faces

It’s common for traces of the old gasket to remain on the flange surfaces. These can be removed with a wire brush or scraper. However, to avoid damaging the flange face, this must be made from a softer material. Brass is usually a good choice. Always brush in a circumferential direction and not radially.

Step 3: Preparation

Inspect the new gasket for damage and ensure that it’s the correct size for the joint. Don’t use any kind of sealant on the gasket or sealing faces unless specifically advised to do so by the gasket manufacturer.

Proper torque tightness is essential to deform the gasket and seal the joint. If there’s excessive friction bolts will seem to be at their torque limit when they’re not, resulting in leaks. This can be avoided by lubricating the threads and under the heads of the bolts. (Ensure the lubricant is compatible with expected service conditions.)

Do it once

Inspection and cleaning may seem time-consuming, but doing a job once is better than having to fix a leak. That’s why thorough preparation of flange surfaces is so important.  Contact Hennig Gasket & Seals for custom manufacturing of flange gaskets to your exact specifications.

Flange Gaskets
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