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.

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.

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.

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.