6 Tips for Preventing Gasket Blowout

Gaskets in flanged joints fail in two ways. Either they allow a very slow leak or they blow out. A blowout can be quite spectacular, but also dangerous and expensive. As it’s something to be avoided, here’s an explanation of what causes it and some ways of making sure it doesn’t happen.

Recognizing a Blowout

A blowout occurs when the gasket material fails catastrophically. This can happen when the gasket is resisting significant internal load. A steam system might be one example but it could be any application where the media being sealed against is under pressure. When this pressure exceeds the strength of the gasket the material is likely to fail fast, forming a hole that releases the media.

In a steam system, this results in a dangerously hot jet. In other situations, it could be acid or a flammable fluid that escapes.

Blowout Prevention

One factor is the tensile strength of the gasket material, but a flange system is more complex than that. The gasket is held in place by the clamping force. This creates friction between the flange surfaces and the gasket, and that resists movement.

Analysis of the forces in a flanged joint shows that clamping force is a bigger factor in blowout prevention than material tensile strength. (The full analysis is available on the Fluid Sealing website.) Thus anything that reduces clamping force can cause a blowout. Here are six tips to prevent that happening.

  1. Follow best practices when bolting flanged joints together. Use the recommended tightening pattern.
  2. Don’t overtighten. This makes the flanges rotate, compressing the gasket at its outer edge while there’s little or no contact on the inside. It can even crush the material, guaranteeing a failure.
  3. Minimize vibration through the joint.
  4. Choose gasket materials with low creep. (Silicone and nitrile rubber are particularly good.)
  5. Consider the effect of temperature. High temperatures will lengthen bolts, (which reduces clamping load,) and let gasket material creep. Conical spring washers may help.
  6. Use the thinnest gasket possible that handles unevenness in the flange faces.

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.

Sealing Helium: Best Gasket Materials to Use

Helium is used for cooling in electronics manufacturing. It’s used extensively for leak testing and it can be the inert gas in MIG and TIG welding. That means a lot of equipment needs gaskets to keep helium contained. Here’s some advice on what to use.

Helium Properties

Only hydrogen has a lower atomic number than helium. That means, even compared to other atoms, helium atoms are small and light. Unlike hydrogen though, helium, (symbol: He) is very unreactive. It won’t burn or oxidize and doesn’t form compounds, all of which make it safe and easy to handle.

This “friendly” nature and small atomic size make helium the preferred gas in leak testing. If a pressure or vacuum chamber has even the smallest crack or pore helium will find a way through. In fact, helium is so good at finding holes in materials that it’s quite difficult to contain: at the atomic scale many materials have pores that helium can pass through.

Low Permeability Materials Needed

A material that lets helium pass is considered permeable. (The same material may be impermeable to larger atoms.) Permeability is measured in terms of the volume of gas that can pass through a given area in a set time.

Most polymers have a helium permeability coefficient. This indicates how well the material blocks the passage of helium. These values are useful when choosing appropriate gasket material.

Gasket Materials for Helium

Helium is so unreactive it can be used with any gasket made from an elastomeric polymer. The issue to watch for is permeation. (A gasket that lets the gas escape isn’t working very well!)

The polymer with the lowest helium permeation coefficient is nitrile rubber, (a.k.a. NBR or Buna N.) EPDM has only slightly higher permeation, closely followed by FKM/Viton. The material to avoid is silicone as helium can pass through it quite quickly.

When selecting a gasket for helium it’s also important to consider temperature and pressure along with compression set resistance. In most room temperature applications nitrile rubber/NBR/Buna N works well, but if in doubt, consult a material expert 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.

Silicone Gasket Material – The Ends and Outs

What a difference an “e” makes! Silicon is the material of electronics. It’s hard and brittle and makes lousy gaskets. Silicone on the other hand is soft and elastic, which makes it a good choice in many gasket applications. Here’s what makes this polysiloxane material so useful.

Basic Chemistry

Silicon and silicone are closely related. Silicon is a naturally-occurring element while silicone is a polymer that combines silicon atoms with those of oxygen and the H3C hydrocarbon compound. The result is a soft, plastic-like material that springs back after being compressed.

More Silicone Properties

In addition to compressibility, other useful features of silicone include:

  • Poor adhesion, so it doesn’t mark surfaces
  • Low toxicity, making it useful for food and medical applications
  • Resists degradation by ultraviolet light (sunlight)
  • Low electrical conductivity
  • Repels water
  • Ozone resistant
  • Retains its flexibility over a temperature range of -94 to +392°F

These properties make silicone gaskets a good choice in a range of food, medical and electrical applications. It won’t taint foods and it handles a wider temperature range than many other gasket materials.

Silicone Weaknesses

In some regards, silicone performs less well as a gasket material than the alternatives. Some others have higher strength and better compression set recovery for instance. It’s also attacked by hydrocarbons like most oils and fuels, and resistance to acids and alkalines is poor. In short, unless you need the special properties of silicone there may be better alternatives.

Silicone Forms

Silicone is available in both solid sheet and as a foamed or cellular material. Silicone foam may be either closed or open cell.

Silicone comes in many colors, (which may not be food grade, so check before ordering.) There are also many variants tailored for specific application needs. Some will go to lower temperatures than that given above, while others have been engineered for higher strength or even electrical conductivity.

If you’re considering using silicone gaskets we respectfully suggest speaking with one of our material specialists. There are many instances where silicone is an excellent choice, but sometimes other materials may perform better.

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.

Where to Buy Gasket Material

Gasketing is complicated. Every application has a unique combination of temperature, environment, media, and pressure to consider and there’s almost never a ‘one-material-suits-all’ solution. As a result, even a small HVAC, food processing or chemical application might need gasket materials ranging from nitrile rubber to EPDM, silicone, fiber and even PTFE.

Many companies will sell you some of these. The danger is that when they don’t carry a full range they’ll propose a material that’s not ideal for your application. Here’s some advice for deciding where to buy your gasket material.

Consider Material Range

Stocking nitrile rubber and neoprene foam does not constitute a full range. You should look for a vendor that carries all these types of gasket material:

  • Fiber
  • Felt
  • Cork and cork/rubber
  • Fiberglass
  • Compressed Non-Asbestos
  • Various types of rubber
  • Closed and open-cell foam
  • Graphite
  • PTFE
  • Specialized brand name materials like Viton and Santoprene

Look for Rapid Turnaround Capabilities …

It’s rare to have the luxury of buying gasket material just to put into storage. Usually, there’s an immediate need, and you can’t wait for delivery. Keeping a full range of gasket material in stock ties up money and space though, so some businesses don’t want to do it. Always look for a material supplier who maintains a comprehensive inventory.

… And Gasket Cutting Services

Material is one thing, but you still have to cut gaskets from it. A stockist with waterjet cutting capabilities can produce installation-ready gaskets in just a few hours, sometimes faster. That saves you time and results in a more accurate gasket with better edge quality.

Location is another factor to consider. Today’s rapid delivery services mean material can ship almost anywhere in the country in 24 to 48 hours, but there are still advantages to finding a local vendor. Closer is faster, and likely cheaper too.

Range and Speed

When searching for gasket material, similar isn’t good enough. It’s essential to buy the right material for each application and get it without a long wait. That’s why you should look for a supplier with a broad inventory and the ability to deliver quickly.

Cutting with Water: How the Waterjet Machine Works

Waterjet cutting produces small or large gaskets in just minutes. There’s no tooling, it’s fast and edge quality is extremely good. Waterjet eliminates the wait for tooling that goes with steel rule die cutting and it avoids the heat and fumes of laser cutting. That’s why we prefer it for many different gasket materials. Here’s how it works and why we’re such fans.

High Intensity

A jet of water from a garden hose can sting the skin. In a waterjet cutting machine that jet is far thinner and traveling at supersonic speeds. The water is powered by an intensifier pump that produces 60,000 psi or more. This forces the water through a 0.006” diameter hole in a piece of sapphire.

With such high pressure and velocity, the jet of water slices through material like a knife. For cutting harder materials, adding an abrasive powder to the water increases cutting speed.

Machine Operation

The cutting head is mounted on a gantry. This spans the machine bed, which is where the material being cut is placed. Here at Hennig Gasket we can handle sheets up to 8′ x 6′. The gantry provides one axis of motion and the sheet moves under the gantry for the second. By interpolating motion between the two it’s possible to cut incredibly intricate shapes.

Waterjet machines are programmed directly from CAD files and there’s no special tooling needed. The programming package helps maximize material utilization by nesting gaskets efficiently. This, plus a very small kerf (the cut width,) helps minimize waste.

Types of Gasket Material

Cutting speed and maximum material thickness depend on hardness. Speeds of up to 500”/minute are possible, but this drops as thickness increases. Sheet EPDM, neoprene, cork and silicone gasket material can be cut quickly and accurately, and maximum thickness can be up to 6”.

Fast Turnaround

Using a waterjet to cut gaskets from sheet material is fast and cost-effective. Perhaps the biggest benefit though is the speed with which we can satisfy an order. Curious how fast that is? Put us to the test.

Gaskets for Vacuum Chambers

Vacuum chambers are used in many industries. Their largest application area in the physical vapor and directed vapor deposition, (PVD and DVD,) process industries. Here they are used for applying both decorative finishes and hard protective coatings. Vacuum eliminates contaminants that would cause oxidation or reduce purity. The cutting tool, semiconductor and nuclear industries are all big users. They’re also used in scientific research even for growing engineered diamonds.

A Difficult Sealing Environment

The gaskets used for sealing these chambers are critical pieces of the equipment. They fit around access ports where they have to withstand high clamping forces as well as extremely low vacuum. That means they need strength and good compression set resistance. Another requirement is a wide temperature range and there’s also a fourth, more specialized challenge.

Outgassing

Emptying a vacuum chamber of air, (pumping it down,) takes time because molecules cling to the interior surfaces. These surfaces must be given time to give up these molecules in a process called “outgassing”.

Materials give up their attached air molecules at different rates, which makes outgassing behavior an important consideration when selecting gasket material for vacuum chambers. Slower outgassing means longer pump-down times, which in turn reduces chamber throughput.

Suitable Materials for Vacuum Chamber Gaskets

The most popular choice is Viton®. Technically a polymer from the fluorocarbon family, (Viton® is the DuPont trade name,) this has a wide temperature range, (-20 to +400°F) and good compression set resistance. Most importantly though, it provides shorter outgassing times than the alternatives.

These alternative materials are silicon, Butyl, Buna-N and EPDM. Silicon gasket material outgasses more slowly than Viton® but has a wider temperature range and good ozone resistance. In ultra-low vacuum applications, meaning pressures below 7.5×10-10 Torr, elastomeric gaskets are replaced by copper.

Finding the Right Material for Vacuum

Every vacuum chamber has access ports, and access ports need gaskets. An important consideration for the gasket material is outgassing behavior as this affects pump-down time. If outgassing is a concern in your gasket application, the specialists at Hennig Gasket will be happy to offer advice.