Flange Gaskets

When choosing rubber gasket material it’s important to consider chemical compatibility. What’s sometimes overlooked though is that compatibility relates to more than just the fluid being sealed. Cleaning agents and solvents that will come into contact with the sealing material must also be considered.

One widely used cleaner/solvent is acetone. While compatible with some types of rubber, it reacts negatively with others. Here’s what to consider.

Acetone Exposure

Although perhaps best known as a nail polish remover, acetone has many other uses. It’s a key ingredient in lacquers used in the automotive and furniture industries, it’s used in some textile manufacturing, and it’s an industrial cleaner, particularly in the printed circuit board sector.

Chemists classify acetone as a ketone. It’s generally harmless to humans, although is very flammable. However, it will damage some types of rubber.

Rubber Takes Many Forms

In its original sense, “rubber” refers to an elastic material made from the sap of the rubber tree. Today though it’s often used as a general term for almost any elastomeric material.

The closest materials to natural rubber are SBR or red rubber, and NBR, also known as nitrile and Buna-N. (These were originally developed as synthetic forms of natural rubber.) Then there are other elastomers like neoprene, EPDM, silicon and FKM/Viton® that behave in similar, often superior, ways to natural rubber. All these are sometimes lumped under the heading of “rubber”. This causes a problem because they react differently to chemicals like acetone.

Positive and Negative Compatibility

The good news is that acetone won’t react with or degrade EPDM. Unfortunately it reacts negatively with many other rubber-like gasket materials.

With natural rubber, SBR and neoprene, the degradation is minimal: brief exposure is unlikely to cause any problems. However, in NBR and FKM/Viton it causes swelling which can quickly lead to failure of the seal. For this reason these types of rubber are best kept away from acetone.

If in Doubt, Ask an Expert

Material compatibility is an important part of gasket material selection. If you have any concerns, contact us and speak with a specialist.

Used on its own or blended with rubber, cork makes an excellent gasket material. It’s flexible over a wide temperature range and very compressible. It resists oil, it’s fire-resistant and it doesn’t creep. Why then, do some gasket and gasket material buyers express concern when presented with it?

The answer is, in some quarters there’s a perception that cork absorbs water.  Does cork absorb water is a frequently asked question.  If this were true, wine bottles would leak when laid horizontally, but it’s not entirely incorrect either. For an explanation, let’s delve into the properties of this natural material.

The Structure of Cork

Cork grows on trees, and like all biological materials, is composed of cells. As a paper from North Carolina State University (NCSU) noted in 2015, the cells in cork contain air, so it can be considered a closed cell foam. (“The rationale behind cork properties: A review of structure and chemistry” Pereira, H. (2015).)

This similarity to a sponge material is probably the origin of the belief that it absorbs water, but there’s a bit more to it. The NCSU paper notes that, “The porosity coefficients of cork range from below 2% to over 15%”. Or in other words, it will absorb a little water. This occurs at the cells that have been cut open on the surface and also through defects in the cell structures.

Countering this though, the cell walls are made mostly of a substance called suberin. Naturally hydrophobic, this repels water, which works against absorption.

So, as a conclusion, yes, cork can absorb water, but the amount is very small. It’s not a sponge.

Relevance to Gasket Materials

Natural cork, often described as composition cork, can be used for gaskets. More often, it’s blended with a synthetic rubber like neoprene, nitrile or silicone. This increases strength and flexibility while enhancing oil resistance.

Thanks to high compressibility, cork-rubber blend gasket material works well on uneven surfaces and in low pressure applications. It’s available in thicknesses from 1/32” to ¼”, can be die, water jet or laser cut to shape, and no, it won’t absorb water – to any significant degree.

Like gaskets, a mechanical seal stops fluids leaking into places they are not supposed to be. The difference is that gaskets are for static applications like pipe flanges while mechanical seals prevent leaks from around rotating shafts. Hennig Gasket & Seals is in the business of cutting gaskets to shape and supplying gasket material, but we do offer a mechanical seal repair service. Here’s a primer on mechanical seals for those who’d like to know more.

The Shaft-Sealing Challenge

Pumps, compressors, and similar rotating machinery are built around a rotating shaft that passes into or through a housing. Bearings hold the shaft in place but don’t stop the fluid from escaping. Historically, this was done with gland packing material. This looks like rope and is packed around the shaft to prevent leaks.

Gland packing has several limitations:

• Wears out, requiring adjustment and eventually, replacement
• Can wear the shaft
• Creates friction for the motor to overcome

Modern gland materials incorporate PTFE, graphite or other friction-reducing compounds, but the risk of leaks remains. As awareness of the environmental and health and safety consequences has grown, many machinery manufacturers have replaced packing with mechanical seals.

Mechanical Seal Basics

In principle, mechanical seals are straightforward. They comprise one element fixed to the shaft and a second element on the housing or body. These are arranged with surfaces that are perpendicular to the shaft axis and are brought together so the rotating shaft element rubs against the fixed housing element.

This perpendicular configuration accommodates runout between shaft and housing. A small gap – less than 0.0001″ – between the two contacting surfaces stops particles getting into the fluid but can allow a degree of leakage. The alternative is to put combinations of low friction materials like graphite and ceramic or carbide in contact. This drives leakage rates almost to zero. For additional sealing, many modern mechanical seals spring-load one element to push it against the other.

Contact Hennig

Hennig Gasket & Seals are experts in all types of gasket and sealing applications. Contact us for gasket materials or mechanical seal rebuilds.

PTFE is an excellent material for a wide range of sealing applications, including PTFE washers. Being chemically inert, it resists attack by almost all acids and alkalis. It’s soft, so it deforms readily to fill uneven gaps between surfaces, and it remains flexible and useable over a wide temperature range, (in comparison to other gasket materials). It’s a good thermal and electrical insulator, but it has one other useful attribute: low friction.

Low-Torque Tightening

Low friction means other surfaces experience almost no resistance as they slide over PTFE, (also known by its Chemours tradename, Teflon.) That’s advantageous when closing up a joint where one surface slides against the seal or gasket as it means all the applied force goes into the joint.

A product that exploits this behavior is the PTFE washer. Consider this a special type of gasket that benefits from low friction. Slippery surfaces help ensure the fastener fitted over the washer seals without moving or tearing the washer surface.

Applications for PTFE Washers

• Plumbing — use PTFE washers under bolts and when fastening fittings to delicate or easily marked surfaces
• Electronics — insulating properties and high-temperature resistance make PTFE washers a good choice for distributing screw loads in electrical assemblies
• Mechanical assemblies — PTFE washers let bolt heads tighten down without putting and stresses into the surface underneath
• Food industry – PTFE is available in food grades and withstands aggressive cleaning regimens

PTFE Washers:  Shapes, Sizes, and Forms

PTFE comes in sheet form and both filled and pure PTFE grades. It’s readily die-cut, water jet cut, and flash cut.

While there’s an assumption that washers are circular, they don’t have to be. Hexagonal shapes will utilize the PTFE sheet better while squares and rectangles may be more useful in some applications. Tabs can help resist rotation during fastening, although the slipperiness of PTFE means this is rarely an issue.

We are happy to provide a quote for PTFE washers custom fabricated to your specs.

EPDM is a soft, silicone-like material suitable for a wide range of gasket applications. It’s often molded into seals and ‘O’ rings but it’s also produced in sheet form of various sizes and thicknesses. This is ideal for laser, waterjet and die-cutting custom gaskets. Like all gasket materials though, EPDM does have some limitations. Here’s some guidance on when and when not to use EPDM Rubber Gaskets.

EPDM Gasket Material Basics

EPDM is an acronym for Ethylene Propylene Diene Monomer. The short version of the chemistry explained in, “Understanding EPDM Gasket Material,” is that this forms long interlocked molecular chains. This gives it an elastic behavior somewhere between neoprene and silicone.

Strengths of EPDM Rubber Gaskets

EPDM has characteristics that make it good for a wide range of gasket applications. Chief among these are:

• Withstands attack by a wide range of chemicals including acids, alkalis and ketones
• Good resistance to steam and caustic cleaners
• Good resistance to UV light
• Remains flexible at temperatures as low as -40°F (-40C)
• Upper temperature limit around 225°F (107°C)
• Good ozone resistance
• Good dielectric properties mean it’s a good choice for electrical sealing
• Good abrasion resistance
• Excellent elastic recovery.
• Won’t impart a taint to liquids or solids it comes into contact with

Limitations of EPDM Rubber Gaskets

EPDM should not be used with oils and fuels as these cause swelling. The upper limit to strength is around 1100 PSI.

Good Applications for EPDM Rubber Gaskets

EPDM works where the application needs a gasket material more robust than neoprene and less expensive than silicone. The three main classes of application are:

• Food and drink processing equipment (mixer seals is a common application) as it’s available in food grades and withstands aggressive cleaning regimens.
• Sealing outdoors, where it withstands low temperatures and exposure to UV.
• Electrical cabinets and enclosures, where ozone may be generated.

More Information

If you have a gasket application that needs a more robust material than neoprene, EPDM might be the answer. Contact the specialists at Hennig Gasket to learn more.

Natural rubber is an imperfect material for gaskets. While soft and compressible at room temperature it hardens in the cold and has poor weather resistance. Styrene Butadiene Rubber (SBR) is a manmade alternative that comes in mainly in extruded and sheet form. For gaskets, SBR sheet is inexpensive and performs well in a range of applications. Here’s a closer look.

Properties of SBR Sheet

SBR is an abrasion-resistant elastomer manufactured in hardnesses ranging from 50 to 80 Shore A. (50 is firmer than a pencil eraser while 80 feels like a leather belt.) It’s also less variable than natural rubber.

SBR has good impact strength and tensile strength from 400 to 1,000 psi. Consider fabric-reinforced grades for applications needing higher strength.

For gasket applications, SBR sheet has high resilience and low compression set. It resists acids, brake fluid, and moisture but is attacked by oils and hydrocarbons. Useful temperature range is -40°F to 180°F (-40°C to 82°C) though limits vary by grade.

SBR Sheet Availability

SBR is available in sheet and roll form up to 36” wide. Thicknesses are from 1/64” to 1/4” with wider and thicker sheet available for special order. SBR sheet comes mainly in black, red and gray. For other colors please ask.

Applications for SBR Sheet

SBR is a good choice for applications needing flexibility and abrasion resistance. Examples include:

1. Flaps (access doors and panels, seals around doors.)
2. Scrapers and wipers (especially those used in low-temperature environments.)
3. Lining rubber (Placed over workbenches and in drawers, SBR sheet absorbs impacts and prevents damage to tools and workpieces.)
4. Conveyor Belting
5. Skirtboards (Rubber strips at the base of walls in high-traffic and washdown environments.)

High resilience and low compression set make it a good two choice for two other types of application:

1. Brake system seals
2. Gaskets

The Manmade Alternative

Natural rubber would make an excellent gasket material if it didn’t stiffen in low temperatures and had better weather resistance. SBR sheet has similar useful properties – abrasion resistance, resilience, and strength – without the weaknesses. If you’d like to learn more, specialists at Hennig Gasket can help.

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.