Flange Gaskets

What is a Constant Seating Stress Gasket?

If flange faces were perfectly smooth no one would need gaskets. Once the faces were bolted together there’d be no leak paths and the joint would seal perfectly. Fortunately for those selling gaskets and gasket materials, perfection is impossible, at a reasonable price anyway. As a result, it’s important to insert some compressible material, or as we like to say, “a gasket”, between the flange faces. This seals surface imperfections and resists internal pressure, ensuring the joint stays leak-free.

Uneven Loading

When flanges are bolted together the resulting load on the gasket material is uneven. The outer edges of each flange bend inwards towards the pipe centerline, putting more load on the outer edges of the gasket. As a result, the material compresses more at the outside diameter than at the inside.

The load on a gasket is referred to as the gasket sealing stress. Higher load equates to higher gasket stress. Uneven gasket stress is a bad thing, primarily because more creep relaxation is experienced where load is higher. Especially when coupled with high internal pressures, temperature cycling, and vibration, this leads to reduced service life and higher maintenance costs.

Achieving Uniform Sealing Stress

Gaskets are available which even-out the sealing stress. These are sold as “constant sealing stress gaskets”. They work by placing an incompressible metal annulus, (usually steel,) between the flanges. This creates a minimum gap either side of the annulus, which is then filled with compressible gasket material.

The gasket material, typically PTFE, expanded graphite or vermiculite, is layered onto a metal backing thinner than the main annulus. As the flanges are brought together the gasket material compresses, but only until the flanges close up on the annulus. This prevents uneven sealing stress and results in longer joint life.

Sealing Problem Joints

When replacing a failed gasket examine it carefully for signs of uneven compression. If it looks like the outer edges suffered excessive compression, consider replacing it with a constant sealing stress gasket. The material specialists at Hennig can help you understand your options.

Understanding Stress Relaxation and Torque Loss

Compression is an important part of getting a gasket to work. Closing the joint up tight holds the gasket in place and helps it resist internal pressure. Joints often work loose over time though, and that leads to leaks and even blow-out. Here’s how to reduce the chances of this happening.

Understand the Joint

After putting the gasket between the flanges the bolts are fastened. As torque increases the bolts stretch, creating a load that pulls the flanges together. That compresses the gasket material and pushes it into irregularities on the surfaces.

Over time some of the stretch put into those bolts becomes permanent. Take them out and measure them 24 hours later and you’ll find they’ve lengthened slightly. In addition, the gasket material deforms in a process known as creep, becoming thinner while the outside diameter increases and the bore shrinks.

High temperatures, as might be caused by environmental conditions or the media being sealed against, accentuate these effects. Bolts expand, reducing bolt torque, and creep increases. Temperature cycling can accelerate this loss of torque.

Vibration is another problem. A pulsing pump or water hammer can quickly loosen the joint and lead to leaks.

Bolting Procedure

Counter relaxation by following correct bolt tightening procedure, as detailed in “How to Bolt Flanges”. Conical spring washers can help maintain load as bolts lengthen. In addition, some people suggest torquing-up the bolts, then releasing and retorquing. They argue that this “conditions” the gasket material.

Material Selection

Some gasket materials resist creep better than others. Silicone and nitrile rubber are particularly good, as are compressed non-asbestos materials that incorporate a nitrile binder. Conversely, PTFE is a high-creep material.

Also consider gasket thickness. Creep is proportional to thickness, so using a thinner gasket results in less loss of bolt torque.

Use the right material

A gasket that’s not installed properly will almost certainly leak. Taking steps to counter stress relaxation and torque loss, as detailed here, will help extend joint life. To learn more about the part played by gasket materials, call or email the specialists at Hennig Gasket.

There’s a Standard for That!

One of the biggest applications for gaskets is sealing joints between pipes and devices like pumps and valves. Welding isn’t an option as it may be necessary to take the unit out of service at some point. Instead, each side of the joint has a flange and they’re bolted together with a gasket in between.

Whenever a flanged joint is opened up it’s important to install a new gasket. Obviously, this has to be the right size and to minimize downtime you need the gasket at hand before taking the joint apart. So how do you determine what size is needed? Well, the answer is, use the ASME standards.

Know the Flange Standards

Two main standards define pipe flanges: ASME B16.5 and B16.47. B16.5 covers flanges used on pipes from 1/2” NPS to 24” NPS. B16.47 addresses pipes from 26” to 60” NPS.

NPS (Nominal Pipe Size) refers to bore diameter. That makes it difficult to determine what size you’re dealing with. If you put calipers on the OD you also need to know the wall thickness. Alternatively, measure the outside diameter of the flange and refer to the appropriate standard.

Two other standards used to define pipes and flanges are MSS SP-44 and API 605. Fortunately, both are also part of ASME B16.47. Flanges complying with MSS SP-44 are defined as Series A while those meeting API 605 are Series B. The Series A flanges are intended for higher clamping loads, so are thicker and have a larger bolt circle diameter.

Standards for Flange Gaskets

Sensibly, ASME has two gasket standards that are closely related to those for flanges. These are B16.21: Nonmetallic Flat Gaskets for Pipe Flanges, and B16.20: Metallic Gaskets for Pipe Flanges.

Ask The Experts

If you don’t have a copy of B16.5, B16.47, B16.20 or B16.21 to hand, don’t guess at what gasket you need. You can look it up online, but it’s easier to call us with the details. One of our gasket material specialists will discuss your application with you and help figure out what size and material you need.

Don’t Overspecify Your Gasket Requirements

When choosing a gasket always consider the application’s TEMP – that’s the temperature, environment, media and pressure. This will lead you to the best material for the job, providing you can predict what those values will be. In reality, all but the media can vary. In response, engineers sometimes select gasket material to handle the worst possible combination of conditions. This is not a good idea, and here’s why.

Compromising Performance, at a Price

Consider an application where a simple nitrile rubber gasket will handle the normal operating conditions. Then throw in the remote possibility of exceptionally low ambient temperatures or hotter-than-normal media. This could lead you to look at silicone or PTFE gasket materials.

These will handle extremes better than nitrile rubber, but both are considerably more expensive. And there’s another point to consider: will they work as well as the nitrile over the normal working range? If nitrile best satisfies the typical needs of the application, that’s probably the material to go with.

Consider Risks and Probabilities

There are of course exceptions. If the likelihood of failure is related to the chance of extreme deviations from normal conditions, how much deviation do you design for? Four-sigma? Six? More? It depends on how much risk you’re willing to accept, and that is driven by the cost of failure.

If you’re sealing steam in an accessible location the consequences of a gasket failure are probably not too severe. But if the application is sealing-in sulfuric acid in a high volume processing plant, the costs of both downtime and failure could be very high.

Make an Engineering Decision

Here’s the bottom line: gasket failure always has a cost. You can probably reduce the risk of failure and extend the period between gasket replacement by specifying more sophisticated gasket material – Viton/FKM rather than neoprene for example. But, this increases the upfront costs. So estimate risks and costs – a Failure Mode Effect Analysis (FMEA) might help – and make an informed decision about the right gasket material for your application.

Why Steel Rule Die-Cut Gaskets are Disappearing

No one likes waiting for a gasket, especially when it’s for an urgent repair. No one wants to keep spare gaskets in inventory either. That’s why more gaskets are being water-jet cut. As tooling-free processes, they offer a faster turnaround than die cutting. Here’s more detail.

Steel Rule Die Lead Time and Storage

Each die is made to cut a unique non-metallic gasket. The sharpened blade is pressed into a slot cut in a backing board. Then ejection rubber is fitted around the blade to push out the cut gasket shape.

These take days or weeks to make so gasket manufacturers keep previously used tools in storage. At Hennig, we have thousands taking up space. If a customer wants a die cut gasket we check whether we have the tool needed. If we don’t, water-jet cutting is faster.

Tool Maintenance and Repair

Steel rule edges wear. That affects quality and accuracy, so they need sharpening and/or changing. Likewise, ejection rubber needs regular replacement.

In addition to faster order turnaround, these processes offer:

  • Better edge quality – no compression means straight and square edges.
  • Higher accuracy – water-jet machines can maintain tolerances as tight as +/- 0.0005” while die cutting struggles to beat +/- 0.010”. That’s because the machines are more precise and there’s no deflection or compression of the gasket material.
  • No tooling charges.

Die Cutting for Higher Volumes

Water-jet machines cut quickly, but not as fast as a tool in a press. (And rotary die cutting is even faster.)

Against that, the die cutting press takes time to set up. The breakeven point depends on gasket size, but water-jet and laser usually win for small quantity orders. Die cutting gets cheaper per piece for large quantities but don’t overlook tooling charges.

Speed Wins

With water-jet machines, gaskets can be cut to order. That enables a rapid turnaround that avoids holding spare gaskets in inventory. No wonder die cut gaskets are going away.

Custom Die Cut Gaskets

Practically every non-metallic gasket is cut from a sheet or roll of gasket material. Cutting methods include oscillating knife, laser, waterjet and steel punch dies although the most common process is steel rule die cutting. It’s cost-effective for medium quantity orders but needs special tools. Knowing a little about the process will help gasket buyers understand when this might be the one to use.

The “cookie cutter” process

A steel rule die is a steel strip bent to the profile of the part being cut. This is embedded into a plywood or phenolic board that holds it in shape. The exposed edge of the steel knife is sharpened so it can cut through the material. The board is then mounted into a press. Bringing the tool down into the gasket material cuts out the gasket.

To use the material efficiently gasket makers typically “nest” steel rule dies. This might mean cutting a small gasket from the center of a much larger one or combining a number of different gasket shapes in one tool.

For higher volume production a gasket maker might use a rotary die machine. Here the steel rule die is wrapped around a cylinder. Strip material is fed underneath and the die cuts out the gasket shapes as it rotates.

“Kiss cutting”

When cutting all the way through normal practice is to have a relatively soft sacrificial material underneath. This helps extend the life of the cutting edge. However, sometimes it’s better not to cut all the way through. This is “kiss cutting,” because the knife just “kisses” the backing material. It keeps the gaskets together in a single sheet while making them easy to separate when needed. This simplifies storage and is particularly useful when the gasket material has a pressure sensitive adhesive backing.

Dies for your gaskets

We’ve been cutting gaskets at Hennig for many years and have a lot of dies in inventory. If you want die cut gaskets it’s possible we already have the tools needed. Call or email to find out.

Yes, Cork is Still Used as a Gasket Material

Cork, the bark of the cork oak tree, has been used as a sealing material for centuries. The Romans “corked” wine bottles with it – something it still does today – and before modern elastomers were developed it was widely used as a gasket material. Cork gaskets are less common today, but they still have a role.

Properties of Natural Cork

Cork has a closed-cell structure that gives it excellent resilience. A layer can be compressed to around half its thickness and still recover when the load is removed. It’s also lightweight, flexible, and resists attack by water, many oils and even ozone. At around 275°F (135C) its upper temperature limit is lower than some elastomers, but that’s not its biggest weakness. Those are a vulnerability to mold, fungi and acid attack, and as an entirely natural material, its properties are somewhat unpredictable.

Composite Cork Gasket Material

Cork- rubber composites address these deficiencies. Typically these are 70% cork with a synthetic elastomer binder making up the balance. The elastomer imparts some of its own characteristics to the composite and is usually chosen to improve chemical compatibility and sealing performance. Processing also takes out much of the natural variability.

Composite cork gasket material is available that incorporates a number of different elastomers. EPDM, Neoprene, Nitrile and silicone are just a few. The material is produced as a sheet up to ¼” (6mm) thick and is easily die-cut. It readily takes a pressure sensitive adhesive coating, making it easy to apply in many gasketing and sealing applications.

The “Green” Dimension

Cork is cut from the cork oak tree once every nine years. The tree doesn’t die but instead grows a new layer of protective bark. This makes cork a renewable material, something that may be important in some applications and for some users.

There’s Still a Place for Cork Gaskets

Cork is waterproof and has great compressibility. Modern elastomers may offer better chemical resistance and a wider temperature range, but for sealing water and oil, don’t overlook this oldest of gasket materials.

Gasket Material and the PxT Factor

PxT stands for pressure times temperature. It’s a factor used more and more to characterize the performance of gasket material. The advantage is that it addresses how elastomers like silicone, EPDM and neoprene lose strength at higher temperatures.

An Inverse Relationship

Specifications for elastomers call out temperature limits. What’s rarely appreciated is that these values may not be good in all applications. Unlike metals where hardness changes very little as temperature rises, elastomers soften. For example, neoprene sheet material may have a quoted upper limit of 250°F but at this temperature it’s strength will be far lower than at say 68°F.

In many sealing applications pressure isn’t a concern. If the neoprene gasket only seals against warm air, that reduction in strength may not affect performance. However, if it’s sealing a pipe flange in a steam system reduced strength could be a problem. Even though the temperature might not exceed the upper limit, sustained pressure could extrude the material out through the flange.

Pressure AND Temperature

PxT shows how a gasket material performs under varying pressures and temperatures. Here are two examples.

Specs for a neoprene gasket material show the upper-temperature limit as 250°F and the maximum pressure as 250 PSI. However, the maximum PxT factor for the material is only 20,000 (°F x PSI). If used in an application with a continuous operating temperature of 250°F peak pressure is just 80 PSI. (20,000/250 = 80)

A sheet of EDPM gasket material has a maximum temperature of 300°F, a pressure limit of 250 PSI and a PxT factor of 30,000. In this case, the material manufacturer is saying that if sealing against media with a pressure of 200 PSI, the temperature should not exceed 150°F. (30,000/200 = 150)

If In Doubt …

Every gasket material has an upper-temperature limit. It may fail if taken beyond that, but it could also fail at a lower temperature if the pressure is high. The best approach is always to consult a material specialist like those at Hennig Gasket.

Sponge vs. Foam Gasket Materials

The terms sponge and foam are used interchangeably but they’re not the same. Buy foam for a gasket application and you may find the joint leaks. Use sponge material for cushioning and you may not get the protection you expected. Sponge and foam both have cellular structures, but there are important differences between the two.

Different Processes

Producing foam material is similar to baking bread. A chemical reaction in the liquid mixture creates carbon dioxide gas that leaves a very open structure.

Sponge materials also use a chemical reaction to create gas bubbles, but these remain self-contained cells, each one isolated from its neighbors. Some manufacturers can control the size and distribution of these pores to produce very consistent material with highly predictable behavior.

Open and Closed

The foam production process leaves an open structure more like a mesh than a solid. The stiffness or rigidity of this structure depends on the polymer used – typically that’s PVC, polyethylene or polyurethane.

In contrast, in a sponge the pores influence material behavior. Unlike a foam material, when sponge is compressed the gas in each pore has nowhere to go. That results in strong compression set resistance and good compression recovery.

Sealing Properties

Foam and sponge materials both have the advantage of low density but behave very differently when asked to act as a barrier. The open cellular structure of foam material lets fluid pass through readily, even when compressed. Sponge however blocks the movement of gases and liquids. That makes sponge material a good choice in HVAC sealing applications.

Cutting sponge will open up the edge pores. This allows a limited amount of liquid retention but the body of the material still acts as a barrier.

Materials for Sponge Gaskets

Practically all elastomers can be manufactured with a closed cellular structure. Consequently, neoprene, EPDM, nitrile and silicone gasket material are all available as sponges. As with their solid variants, these can be purchased with varying levels of temperature resistance and strength. Talk to a material specialist at Hennig Gasket if you need more advice.

Adding a PSA to Your Gasket Material

Not all joints hold the gasket in place. Electrical cabinet doors, food mixers and HVAC access panels are situations that often lack mechanical gasket retention features. In such cases the gasket must be adhesively bonded to the frame, cover or lid. Adhesives can be messy and awkward to use, and that’s why it’s worth asking about gasket material with a coating of pressure sensitive adhesive (PSA.)

The advantages of “peel and stick”

PSA is bonded to the gasket material before shapes are cut out. On the reverse side, (away from the gasket material) there’s a release film which is peeled off before the gasket is installed.

The advantages of PSA-coated gaskets are:

  • Easier to install.
  • Stay in place when the joint is opened.
  • Facilitates “kiss-cutting”. This is when the die cutter goes all the way through the gasket material but stops short of the PSA release film. Kiss-cutting holds the individual gaskets together in a sheet or roll, making them easier to handle.

Material compatibility

Most gasket materials will take a PSA coating. For example, both neoprene and silicone gasket material can be ordered with PSA. The major exception is PTFE gasket material as this is naturally non-stick.

PSA cautions

A PSA used on gasket material becomes part of the joint and experiences the same temperatures, pressures and chemicals as the gasket itself. Accordingly, the PSA must be matched to the application. Silicone gaskets are a good example. As these are used for both high and low temperatures the PSA must be chosen appropriately. In some cases the available PSA’s will limit the service conditions.

PSA selection is of particular importance when installing food grade or FDA gaskets. For these applications the adhesive must also be food grade.

Replacing a PSA gasket can be time-consuming. This is because the old adhesive must be completely removed from the joint faces before the new gasket goes on.

Make life easier

Depending on the gasket material and the nature of the application, a PSA can simplify gasket installation. If “peel and stick” gaskets would simplify your next sealing job, talk to Hennig.