Flange Gaskets

Elastomeric gaskets, like those made from nitrile rubber and neoprene, work because they are flexible. Compressed in a flange, the material fills the gap between joint faces, regardless of how each side moves. When a joint gets cold the gap often grows wider, and the gasket material is expected to recover enough to maintain a good seal.

When elastomers become very cold they lose this ability. When choosing gasket material for an application that could see low temperatures it’s important to check it has sufficient low temperature flexibility. This is indicated by their TR10 number.

Low-temperature Behavior

Nitrile rubber and neoprene gaskets are flexible because they are made from entangled chains of molecules that move over one another. When temperatures drop, contraction limits the movement of these molecules and the material gets stiffer. Eventually, there’s no room for any movement and the material becomes brittle.

The “freezing” temperature of an elastomer is termed it’s Glass Transition Temperature (Tg). Strictly speaking, freezing happens over a range, and the brittle point is several degrees lower than the Tg. What matters for gaskets though is when the material loses any ability to recover.

The Temperature of Retraction

ASTM D1392 advises that “..retraction rate is believed to correlate with low-temperature flexibility of … rubbers.” Retraction is measured by first stretching and then freezing an elastomer. It’s then gradually warmed up, and as the molecular chains regain some freedom to move it starts to shrink back.

This recovery is measured against temperature and indicates when the elastomer transitions into rubber-like behavior. Experience shows this happens when it recovers by around 10% of it’s elongated length. The point at which this happens is the temperature of 10% retraction or TR10.

Gasket Material for Low-temperature Applications

If an elastomeric gasket will get cold, two things are needed: the lowest anticipated temperature, and the TR10 value for the candidate materials. If TR10 is not available use Tg as a substitute. For reliable in-service performance, ensure these values are lower than the worst conditions the gasket will experience.

Heating, sterilization, humidification and power generation are a few of the ways steam is used in industry and commerce. Steam systems combine heat with pressure so any leak in pipework is potentially dangerous. This is especially true when using steam that’s superheated, and therefore not visible to the naked eye. To reduce the risk of leaks, material for steam system gaskets should always be selected with regard to the specific requirements of the equipment being worked on.

Steam Gasket Requirements

Steam is water vapor at or above 212°F (100°C). In steam systems, it’s under pressure and usually at higher temperatures. The temperatures and pressures used vary by system and these need determining before selecting a steam gasket.

Superheated steam is steam at temperatures above the saturated steam curve for that particular pressure. Temperatures of 500°F (260°C) or more are possible, creating special requirements for gasket materials. Here we’ll address materials suitable for gaskets in conventional saturated steam systems.

Steam Gasket Material

Many common gasket materials don’t handle steam well. Neoprene and silicone are two examples. Nitrile rubber or NBR is another material seldom recommended, mainly because depending on formulation its upper temperature limit is around 300°F (150°C)

One good steam gasket material choice is Ethylene Propylene Rubber (EPDM.) EPDM gaskets will handle steam at temperatures up to 395°F (200°C) and also provide good resistance to dilute acids and alkalies, alcohol, ketones and automotive brake fluids.

Styrene-butadiene rubber (SBR) gaskets are another good choice. While lacking the upper temperature limit of EPDM – 340°F (170°C) is about their limit – they do hold up well to steam. Particularly in cloth-inserted grades SBR gaskets resist both pressure and high compression loads.

Select for the Specific Application

Steam systems can be difficult to gasket because temperatures and pressures vary between systems. When seeking material for steam system gaskets always determine the properties of the system to be sealed. Don’t use a gasket purchased for one system in another that may have different characteristics. As always, material specialists at Hennig are available to answer any questions.  Contact us today.

Like everything manufactured, gaskets have tolerances on their key features. These are dictated by a combination of material characteristics and manufacturing process. It’s important to understand these tolerances if your new gasket is going to fit.

Key Features

Unless a gasket goes in a channel the outer dimensions aren’t usually critical. What does matter though are the bolt hole positions, bolt hole diameter, and the inner shape, (because the gasket should not intrude into the flow.)

Gasket Material Thickness

Tolerances are dependant on the type of material and industry standards.  RMA commercial gauge sheet rubber tolerance chart shows typical tolerances. 

Manufacturing Process Tolerances

Here at Hennig, gasket material is cut in four ways: die cutting, oscillating knife (flash cutting,) water jet and by hand. Die cutting, used for quantity orders, is probably the least accurate yet also the most repeatable. The oscillating knife and water jet machines provide excellent accuracy, and as they cut one gasket at time repeatability is about the same. Hand cutting is the least accurate.

Here’s some more detail on each process:

• Die cutting. Accuracy, defined as conformance to design, is set by the precision to which the steel rule is set into the mounting block. A laser-cut block can usually hold +/- 0.015” (+/-0.381mm) although this tolerance increases with the size of the tool. Larger tools are less precise. Softer, thicker material deforms more during cutting and a convex edge profile may result. Die-cut gaskets are very consistent, with the first piece identical to the last, (until the blade starts to wear.)
• Water jet cutting. Accuracy is set by the precision of the gantry and table motion. In general, this is around +/-0.007”.
• Oscillating knife. Machine repeatability is claimed as 0.010mm but in practice +/-0.003” (+/-0.076mm) is more typical. Softer and thicker materials can show greater variation.

Consider Tolerances When Ordering

Critical gasket dimensions influence the choice of cutting methods. In such cases, it’s important to let us know about these at the quotation stage.

One of the most important properties in a gasket material is compressibility, and this leads many gasket buyers to think they need rubber. In many cases though there is an alternative: fiber gasket material. “Fiber gaskets” is a broad heading as there are many different types. Here we’ll explain what “fiber” means, how it differs from rubber and other rubber-like elastomeric materials, and when you might want to use it.

What is a Fiber Gasket?

Fiber gasket material is made through a process similar to papermaking. Strands of fiber are spread out and impregnated with a resin material. This dries to form sheets that are easily cut to shape.

Many different type fibers are used to produce gaskets with differing strength, compressibility and temperature ratings. These range from vegetable fiber and cellulose to more exotic materials like aramid, (a strong, heat resistant synthetic fiber.)

When additional compressibility is needed cork or a rubber binder (often NBR) is added. Alternatively, cellulose fiber material can be vulcanized to make a paper-like material that’s both hard and lightweight. When this has electrical insulating properties it’s known as “fish paper.”

Possibly the oldest type of gasket still in use is the vegetable fiber or “Detroiter” gasket. This is made from vegetable fibers impregnated with a glue-glycerine compound. It remains a popular choice in some applications.

Fiber Gasket Properties and Applications

Fiber material makes gaskets with high tensile strength, (so they’ll resist internal pressure,) and an upper temperature limit of 250 – 350°F. They have excellent resistance to chemicals, particularly oils, so are used in many industrial situations, especially chemical and petroleum product manufacturing.

The Contrast with Rubber

While true rubber is a natural product, the rubber used in gaskets is almost always a synthetic version. Synthetic rubbers function over a wider temperature range and are less vulnerable to damage by UV light. Commonly used materials are nitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), neoprene and EPDM. Available in a range of thicknesses and grades of hardness, these have generally good compressibility but are vulnerable to the effects of petroleum oils.

Hennig Gasket & Seals manufactures custom gasket from a large variety of fiber gasket material.  Contact us today for a fast quote.

When sealing raised or flat face flanges there are two choices of gasket shape: full-face gasket and ring-type. Each has advantages, so before ordering you should know which will suit your application best. You should also understand the measurements your gasket supplier needs before cutting material.

Flange Basics

ASME standards describe several designs, but the most common are the raised face and the flat face. The difference between them is that the raised face flange has a raised region surrounding the pipe bore. The bolt holes are outside of this. The flat face flange has no such step.

The Ring-Type Gasket

This is positioned inside of the flange bolts and around the pipe bore. In a raised face design it sits on that surface. This design:

• Requires less material and less cutting.
• Can be installed without completely dissembling the joint, (making it a “drop in” gasket.)
• Is harder to clamp in position.

When specifying a ring type gasket only three measurements are needed: ID (which corresponds to the pipe bore,) OD (which is the same is the OD of the raised face,) and gasket thickness.

The Full-Face Gasket

Like the ring-type gasket, this seals on raised flange faces, but has an OD the same as the flange. That means it needs holes for the securing bolts to pass through, and these help locate it on the flange, making alignment easier. Extending out to the flange OD has the added benefit of filling the gap between bolting surfaces, which stops dirt getting in. However, the joint must be completely dissembled for installation.

Specifying a full-face gasket requires these measurements:

• ID (same as the pipe bore.)
• OD (same as the flange OD.)
• Bolt circle diameter (the diameter on which all the bolt hole centers are located.)
• Number of bolt holes (and spacing if they’re not be regular – which would be very unusual.)
• Gasket thickness.

Finding The Bolt Circle Diameter

While ID and OD can be measured with calipers or even a tape, this dimension is harder to determine.

1. Pick two holes diametrically opposite. One we’ll call left hole, the other will be right hole.
2. Measure from the outer edge of left hole, (the side nearest the flange OD) to the inner edge of right hole, (the side nearest the bore.) That dimension is the bolt circle diameter.
3. As a check, measure a second pair of bolt holes and make sure the distance is the same. Remember the rule: outer edge to inner edge!

Installation

Full-face and ring gaskets will do an equally good job of sealing the joint. The difference really boils down to installation preferences and priorities.

HVAC system seals and gaskets maintain efficiency by preventing the loss of heated and cooled air. Whether installing new ductwork, modifying an existing system, or just replacing worn out gaskets, it’s important to choose appropriate material. Many HVAC specialists consider neoprene gaskets the default choice, but it’s possible better performance could be achieved with EPDM or silicone gaskets.

HVAC Gasket Applications

Gaskets have three main roles in HVAC systems:

• Sealing opening panels, flaps, and doors
• Reducing transmission of motor or fan vibration
• Allowing for thermal expansion and contraction

Sealing

Almost every ducting system includes access doors and panels, along with dampers that close off airflow through “legs” of the system. To minimize closing forces, these need a soft material with good compressibility. Combined with appropriate thickness, such gaskets will also take up the dimensional variation and uneven edges inevitable in most systems.

Reducing Vibration Transmission

Fans and motors can cause a vibration in flat ducting that’s audible as a low hum. To avoid complaints from building tenants, incorporate gaskets at appropriate interfaces. The cellular structure absorbs the vibration and prevents it spreading throughout a system.

Expansion and Contraction

Metal ducting experiences significant dimensional changes in response to switching between warmed and cooled air. A gasket with good recovery takes up these changes while still maintaining a leak-tight seal.

Environmental Factors

Outdoor applications challenge HVAC gasket material as UV light degrades some materials, and moisture penetration must be avoided. Low temperatures and ozone might also be a concern in some applications.

HVAC Gasket Materials

Neoprene gaskets and those made from thermoplastic elastomers (TPE’s) generally perform when soft and resistant to compression set. EPDM gaskets work well outdoors as they stand up to sunlight and other weathering effects. Where air or gas temperatures are high silicone gaskets can be a good choice.

Closed cell materials may be preferable because air and moisture cannot pass through, although these are firmer, requiring higher closing forces.

Installation is simplified by using a pressure sensitive adhesive (PSA). This can be laminated on to the gasket material or can be applied in tape form.

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

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.

A gasketed joint is rarely static. Changes in temperature can cause mating flanges to move apart or closer together, creating a variable gap that the gasket has to fill. That’s why understanding the influence of temperature helps when selecting the gasket material for flange gaskets.

Flanged joint dynamics

In service a gasket is compressed between two flanges. Sufficient load must then be applied to hold the joint closed, regardless of how conditions change.

Fluid moving through the pipe creates hydrostatic end thrust that opens up the joint. Internal pressure also creates side loading on the gasket, trying to extrude it out between the flanges. And changes in temperature result in expansion and contraction of both the piping and the fastening bolts.

Temperature influences

Temperature changes have two sources: the temperature of the fluid being transported, and the environment through which the pipe runs. In a continuous process media temperature may vary very little, but a pipe exposed to hot desert sun could experience a range of 80 deg F or more over a twelve hour period.

The influence of media temperature changes, (perhaps at start-up or shut-down,) will depend on the details of the pipework installation. However, most likely higher temps will act to close the gap between mating flanges.

Higher temperatures will make the flange bolts grow, so reducing the clamping force. Tightening to recommended torque levels creates some elongation that compensates for expansion, which is why proper jointing procedures should always be followed.

Of lesser importance, gasket materials and piping usually have different coefficients of thermal expansion. This may cause differential movement between flange and gasket which could, in marginal situations, open up a leak path.

Material selection impact

The ideal gasket possesses both good compressibility and good recovery or resilience, enabling it to maintain a seal as the gap between flanges changes and the compressive load varies. Natural rubber is one of the most effective materials, but is not always suitable.

The prudent approach is to discuss the application with the gasket vendor, being sure to make them aware of the various temperatures to which the joint will be exposed.  Contact Hennig Gasket & Seals today to discuss your flange gasket application.