Introduction to Pump Packing Materials

Packing is a simple but cost-effective way to minimize leakage from around pump shafts. Over time though, it needs periodic adjustment to take up wear. Eventually, when the pump packing material no longer seals effectively, it must be replaced.

This blog provides an overview of packing and discusses the options for pump packing materials.

Sealing Around the Shaft

A fundamental challenge in pump design is to prevent the fluid being pumped from escaping along the impeller shaft. Mechanical seals are an option, but they add complexity and cost. As a result, most pumps rely on compression packing.

Packing involves surrounding the shaft with a braided, rope-like material. This is pushed into a region of the pump housing called the stuffing box. A cylindrical packing gland is passed over the shaft and used to compress, (or stuff), the packing material around the shaft. The gland, sometimes called the follower, is bolted into place to maintain compression on the packing. As the packing wears, these fasteners are tightened to maintain the seal.

Friction, Heat and Leakage

As the packing material presses against the shaft, rotation generates heat from friction. To prevent overheating, the packing gland is tightened to a level that allows a very small leak. Thus the fluid being pumped the both lubricates and cools the packing-shaft interface.

This creates two challenges: the packing material must withstand sliding contact with the shaft, and it must also have appropriate chemical compatibility.

Options for Pump Packing Materials

The principal material choices are fabric reinforced rubber and graphite. Homogeneous rubber with no reinforcement works in some applications while expanded PTFE with graphite fiber is used in others.

At Hennig Gasket & Seals we carry a wide range of Garlock pump packing materials. Their Chevron® line has solutions for most packing needs, while for abrasive liquids Garlock SLUDGE-PAK® is often the answer.

Packing material selection is driven by shaft speed in surface feet per minute, and fluid type and temperature. For advice on what will suit your application, please have this information to hand when you contact us.

What is Compressibility of Gasket Material

Compressibility goes to heart of what gaskets do. Here we’ll explain what gasket compressibility is and why it’s such an important property of gasket materials.

Compress to Seal

Gaskets are used to seal mating services, particularly between flanges. They take up imperfections in the two surfaces that would otherwise let the fluid being sealed leak out, and accommodate any changes in the gap as might be caused by temperature changes or vibration.

Gasket material works by deforming against the harder flange to take up even microscopic deviations in the surface. Compressibility is a measure of how readily the material does this.

Recovery From Compression

Gasket compressibility goes hand-in-hand with recovery. This is the degree to which the material springs back to its original thickness after being compressed.

Recovery matters because every joint is somewhat dynamic. Whether it’s opened and closed repeatedly, like an access door or panel, or just subject to varying temperatures, loads and vibrations, the effective gap being sealed will keep changing size. To maintain a seal the gasket material must expand and compress with these changes.

Closely related, compression set is when a material doesn’t spring back to its original thickness as the clamping load is removed.

Compressibility Measurement and Specification

The ASTM F36 test provides a standardized way of measuring gasket compressibility. The data needs some interpretation because the test conditions almost certainly don’t replicate your application, but it lets you compare materials.

Compressibility is stated as a percentage of the original thickness. Recovery is given as a percentage of the compression.

Material Selection Considerations

In general, materials that compress readily under light loads lack strength. Open cell foam is a prime example. You wouldn’t use this to seal a pressurized gas or liquid as it would almost certainly fail.

EPDM, neoprene and cork are examples of materials with good compressibility. Higher strength materials tend to have lower compressibility.

Gasket material manufacturers publish ASTM F36 test data, but for advice on a specific application, consult a gasket specialist, like those at Hennig Gasket & Seals.

Synthetic Cork for Gaskets: Advantages & Properties

Cork is one of the oldest gasket materials, but it still has a role in sealing applications. It’s a flexible, closed-cell material that resists water and oil, and has excellent compressibility. However, as a natural material – cork is the bark of the cork oak tree – it can vary in quality. It’s also susceptible to various types of mold.

Synthetic cork is intended to have the same wide set of useful properties but without these limitations.

Composition Cork Vs. Cork Rubber

The natural cork used for gaskets is almost always composition cork. This is a conglomeration of cork particles held together with a binder. It’s produced in sheets up to ¼” thick and is usually die cut to produce gaskets.

The synthetic cork used for gaskets, also known as rubberized cork or cork rubber, improves upon composition cork by adding an elastomeric material. Rubberized cork is usually composed of 70% natural cork particles and 30% neoprene or NBR.

In addition to its role as a gasket material, natural cork has long been used as a stopper for wine bottles. In recent years supply shortages and problems resulting from natural variability have prompted development of synthetic cork. Made primarily from polyethylene, this works for sealing bottles, but isn’t going to make good gaskets.

Properties of Rubberized Cork

Neoprene resists oils and greases. Cork made with NBR resists these plus fuels and many solvents. The upper temperature limit for both is 250 °F.

Good applications are those that need a very compressible material. Flanges with uneven surfaces and those made from material that deforms under load are often best sealed with rubberized or synthetic cork gasket material.

Hennig, For All Your Gasket Material Needs

When replacing a gasket it’s usually best to stick with the material that came out. If that looks like a composition or synthetic cork, we can help. We carry rubberized gasket material in sheet and roll form, up to 48” wide and ¼” thick. Synthetic cork is readily die cut to the shape needed. Contact us for a quote.

Measure the Hardness of Rubber Gasket Material

When specifying gasket material, along with factors like strength, working temperature range and chemical resistance, it’s important to consider hardness. Hardness determines how well a material fits against uneven surfaces, with softer materials performing better.

The hardness of rubbers and other elastomeric materials is specified primarily in terms of durometer or Shore number. Here’s an introduction to this rubber hardness scale.

Measurement by Indentation

Hardness is generally measured by pressing a point into a sample of material. Measuring the size, depth, or both, of the resulting indentation indicates the hardness.

For materials softer than metals, hardness is measured with durometer. This uses a calibrated spring to push a conical foot into the material. The foot penetrates further into softer material with depth inversely proportional to hardness.

A procedure for durometer testing is given in ASTM D2250. This standard covers factors like test duration, material temperature, material thickness, and minimum distance of the indenter from an edge.

Rubber Hardness Scales

The readout from a durometer is a dimensionless number on a scale from 0 to 100. A 0 shows the indenter went through the material while a reading of 100 means it left no mark at all.

As the hardness of rubbers and other elastomers covers a wide range – think latex gloves to golf balls – hardness values are reported using one of three scales. Developed by Albert Shore in the 1920s, these scales are identified as 00, A and D.

Originally there were other scales, but these are the only ones used today, and only Shore A and D are relevant for gasket material. The scales overlap so for example, the hardness of a particular rubber could be either 75 Shore A or 50 Shore D.

Specifying Rubber Gasket Material Hardness

Softer rubbers and elastomeric materials are preferred for sealing applications, with Shore A the rubber hardness scale used most often. Common gasket materials like NBR, EPDM and silicone are produced in a wide range of hardnesses: most are available from 30 to 80 Shore A. If you need assistance with hardness specification, Hennig Gasket and Seals can help.

Is PTFE Safe?

Some PTFE material can be used for gaskets and seals in the food and medical equipment industries, and some cannot. Here’s why you might want to use PTFE, and the difference between material that is and is not FDA approved.

PTFE as a Sealing Material

Polytetrafluoroethylene, a.k.a. PTFE, also sold under the tradename Teflon®, has properties that make it an excellent choice for many sealing applications.

  • Extremely inert chemically – fluorine and nitric acid are its only vulnerabilities
  • Soft, so it conforms to uneven sealing surfaces
  • Remains plastic down to -400°F and is still usable at 500°F (-240 to 260°C)
  • Slippery surface assists fastener tightening

PTFE is sold in both sheet form and as expanded PTFE. Expanded, it’s softer and more compliant than the sheet version.

In high bolt load applications PTFE suffers from creep. This is reduced by adding fillers. Glass, carbon and graphite are the most commonly used. They increase strength without significantly impairing the other properties.

Safe and UnSafe PTFE

Pure or virgin PTFE is completely inert so can safely be used in applications where it will come into contact with food, pharmaceuticals or healthcare products. In fact it’s so safe the FDA classes it as “Generally Recognized As Safe” (GRAS). (The relevant regulation is 21CFR177.1550.) This means it’s suitable for gaskets, seals and washers on mixers, storage vessels, kettles and similar equipment.

Filled PTFE does not meet the GRAS criteria and is therefore not FDA approved for food and medical applications. Despite their higher strength, these grades of PTFE must not be used in places where they could be ingested or otherwise taken into the human body.

Expert Advice on Gasket Materials

When choosing a gasket material for food processing or medical equipment applications, a PTFE gasket can be a good choice. However, only the virgin grades are considered safe, and these are prone to creep under high bolt loads. Filled PTFE offers more strength but doesn’t come in FDA-approved grades.

Gasket material selection can be complicated. For expert advice on materials, chemical compatibility and FDA-approved materials, contact us and speak to a product specialist.

When Should You Ask for Butyl Rubber?

Synthetic rubber materials like SBR were developed to increase the supply and compensate for the limitations of natural rubber. For most of these man-made elastomers, while they excel in some regards, they fall short in others. Poor weather and ozone resistance are two of the biggest weaknesses.

Butyl rubber is a synthetic material that plugs this gap. Suitable for a range of sealing applications, it also has great damping and permeability characteristics. Here’s a closer look.

Butyl Rubber Basics

As polymerized at the factory, butyl rubber is white or colorless. It’s usually blended with carbon black to make it black, although other colors are possible.

Butyl rubber is much more dense than other synthetic rubbers, so it’s heavier, but this means it’s a very good absorber of vibration. The same chemical structure that creates high density also renders it impermeable to liquids and gases. (Other synthetic rubber materials like SBR and EPDM have a degree of gas permeability.)

Butyl rubber is on the softer side, (It’s available with Shore A ratings of 40 to 90 but is usually around 65), and has a working temperature range of -40 to 285°F. It’s also resistant to water, dilute acids, and animal and vegetable oils.

Limitations of Butyl Rubber

Butyl rubber will tend to take a compression set, limiting its use as a gasket in applications where the joint will be opened up periodically. Its abrasion resistance is only moderate and it is attacked by hydrocarbon fuels and oils.

Good Applications for Butyl Rubber

The biggest market for butyl rubber is automobile tires, where it’s used as a liner material. In terms of seals and gaskets, applications include:

  • Tank liners
  • Pond liners
  • Cushioning
  • Shock absorbers
  • Flange and full-face gaskets, especially for outdoor applications

Butyl Rubber Sheet Cut to Shape

Ask for butyl rubber when you need impermeability, weather-resistance or cushioning. Hennig Gasket & Seals has butyl rubber sheet form and a range of thicknesses. We can die, flash or waterjet cut it to the exact size and shape you need. Contact us to discuss your needs and get a quote.

Will Acetone Damage Rubber Seals?

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.

What is Food Grade Silicone?

Silicone is an excellent material for gasket applications. It resists most chemicals, stays flexible over a wide temperature range, has good elongation and doesn’t take much of a compression set. If used where it could come into contact with foodstuffs or beverage products though, it must be food-grade.

This isn’t just a recommendation. If you’re making or handling food the Food and Drug Administration (FDA) mandates the use of materials from their approved list. Here’s what to know about food-grade silicone.

Silicone Basics

Silicone, (notice the letter ‘e’ that distinguishes it from silicon,) is produced by heating silica, (also known as silicon dioxide or SiO2,) with carbon. This produces polymer chains that can be processed into liquids and elastomeric solids and gels.

Solid silicone gasket material is sold in sheet form in thicknesses from 1/32” up ¼”. Important points for gasket applications are:

  • Temperature range: -67⁰F to 450⁰F (-55⁰C to 230⁰C)
  • Durometer: 30 to 80 Shore A
  • Resists UV light and ozone
  • Resists most chemicals, except for chlorine, methane and acetates

Why and When to Ask For “Food-Grade”

FDA regulation 21 CFR 177.2600 is a list of materials considered suitable for use with food. Silicone is on the list, but unless you specify food-grade silicone you’ll almost certainly get material that isn’t suitable for use around food products.

Silicone that isn’t food-grade contains additives, mostly colorings, that could contaminate product or make it taste off in some way. Food-grade silicone is white, and for this reason is sometimes called “white silicone”.

Food-grade silicone gaskets should be used in food processing and handling equipment. Storage vessels, kettles, mixers and even freezer doors are all good applications. Be sure to check what cleaning chemicals will be used, as those containing chlorine could damage the silicone.

Hennig Gasket for Food-Grade Material

People involved with food understand the importance of avoiding contamination. No one wants to make someone sick, which is why kitchens and food processors should always use food grade-materials. Stainless steel is ubiquitous for hardware, but when it comes to gaskets, food-grade silicone is often the way to go.

Does Cork Absorb Water?

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 chemistryPereira, 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.

Is Graphite Metallic or Nonmetallic?

Graphite is something of a wonder material for gasket applications. Not much will attack it and it seals over a very wide temperature range. Graphite gasket material is sold mostly in sheet form and has a dark gray, silver-ish color that looks a lot like lead. This is one reason people sometimes assume it’s a metal. The other reason is that graphite conducts electricity, which is something associated with metals.

In this blog we’ll set the record straight.

What You Should Know About Graphite

Graphite is a form of carbon. Carbon atoms can arrange themselves in several ways, which determines its properties. In graphite the atoms are arranged in layers. These can slide over one another, which is what makes graphite slippery, but at the same time, the layers are surprisingly strong.

Carbon is found on the top right side of the periodic table, between boron and nitrogen. This tells us it’s not a metal, because those elements are grouped in the center of the table.

Useful Properties for Gaskets

Graphite is soft and conforms to match uneven surfaces, a key requirement for a gasket material. It also retains its properties over a temperature range from -400°F to 950°F in air (and higher in non-oxidizing atmospheres.)

Another requirement for gaskets is chemical compatibility, and here graphite excels. It resists almost everything except for a few highly oxidizing chemicals.

Graphite Reinforcement

One weakness of graphite is a tendency to creep under load. This is remedied by mounting it to a stainless steel carrier via little hooks or tangs stamped into the metal. Alternatively, it’s often bonded to stainless foil for increased creep resistance.

Not for Food and Beverage

Unfortunately, graphite is unsuitable for applications where it would come into contact with food or drink. This is because carbon particles can detach and enter into the media. PTFE is usually an acceptable alternative.

Graphite for Gasket Applications

Graphite, particularly when reinforced with stainless steel, makes an excellent gasket material for many applications. Hennig Gasket carries a wide range of types, widths and thicknesses. Contact us to learn more.