One of the most cost-effective ways of dealing with severe, corrosive chemicals or process media whose purity must be maintained is to use equipment with process-specific liners.
These liners are available in a variety of polymer-based materials, such as Kynar®, fluorinated ethylene propylene (FEP) and PTFE, as well as exotic metals such as tantalum and titanium.
Glass is commonly used as a liner in the pharmaceutical industry, where it provides a smooth, impervious surface that can be easily cleaned and sterilized (Fig. 1). Although it is not as chemically resistant as PTFE, it provides moderate resistance to many common chemicals.
Due to their impervious nature, glass linings also allow vessels for chemical processing to be made of steel rather than costly exotic alloys.
From a sealing perspective, glass-lined surfaces pose a number of challenges.  If these surfaces are not perfectly flat, the gaskets must conform to them to produce an effective seal.
It should be noted that the ability of a gasket to contain internal system pressure is directly related to the amount of friction between the gasket and the sealing surface. Lacking the serrations commonly found in metallic surfaces, the very smoothness of glass surfaces that allows for easy cleaning and sterilization cannot create the necessary friction.  This low friction factor also reduces the crush resistance of gaskets.
The most commonly used seals for glass-lined equipment are envelope gaskets, which are constructed of slit or milled PTFE envelopes filled with non-asbestos material. In addition some manufacturers incorporate flat or corrugated metal rings between the layers of filler to add tensile or “hoop” strength to their gaskets. These high tensile strength inserts increase the blowout resistance of gaskets installed between smooth glass surfaces.
It is important to remember that the gasket conforms to and seals the imperfections of the assembly, including the unevenness created during application of the glass liner. This is particularly true of glass-lined vessel flanges, where the layering of the glass can result in pronounced waviness of the surface (Fig. 2). 
This can pose a sealing challenge when the imperfections are beyond the capability of the gasket to conform to them.  Envelope gaskets can be “shimmed” by adding traditional fiber gasket fillers in the areas where they encounter irregularities. However, the filler material is only 10 to 20% compressible, so it may be subject to chemical attack if the envelope is damaged during installation or permeated by the service media (Fig. 3). Care also needs to be taken to avoid having the envelope “fold over” during installation, which also can expose the filler material to chemical attack.
Certain highly compressible PTFE products can help reduce or eliminate these potential problems. In some hybrid designs, the traditional core filler in a PTFE envelope is replaced with more compressible PTFE material.  This not only increases the compressibility of the gasket, but keeps it intact even under severe, corrosive conditions if the envelope is damaged, folded over or breached (Fig. 4)
Polymeric Liners
The pharmaceutical industry and other industries also use polymeric liners to improve the long-term performance of their piping systems. Among the more widely used are FEP, PTFE, HDPE and rubber. These materials offer significant cost savings compared with exotic metals and specially polished systems, but they can pose challenges in sealing flanged connections. In many cases, manufacturers claim gaskets are preferred but not required to seal an assembly.  Bear in mind, however, that compressing a polymeric liner is not recommended, as it can damage its face, requiring replacement of the entire section of pipe. A better solution is to use a gasket that is softer than the liner, thereby avoiding deformation of the latter.

Pictured is a typical flange connection for a glass-
lined piping system.

In addition when rubber-lined flanges are assembled without a gasket, they will tend to fuse together over time, resulting in a torn liner when they are disassembled. Using soft, highly compressible PTFE can eliminate this problem, except if the rubber-lined assemblies are being used in high-pressure service.
Another factor to be kept in mind when considering polymer-lined flanges is “creep relaxation,” the tendency of the material to flow perpendicularly to the compressive force and not to be confused with compressibility or conformability.  As noted, gaskets must compress and conform to flange irregularities to create an effective seal. However, once the imperfections are filled and the seal is created, subsequent loss of bolt load results in creep relaxation.  In the case of lined piping, this relaxation is amplified by the liner.

The sealing surface of this glass-lined blind flange
is extremely smooth, but wavy.

To illustrate this dynamic, consider mating two PTFE-lined flanges with a 1/8” liner on each flange, resulting in a ¼” of PTFE in the joint. This in itself will create significant creep relaxation, even without the gasket.
Many users solve this problem through live loading with Belleville spring washers and/or routinely retorquing the flange bolts on a preventative maintenance schedule. However, this can be difficult if the connection is located in a hard-to-access area or, even worse, buried.  In these circumstances, use the best available technology to resist long-term relaxation and provide the desired pressure-resistance and sealability.

Cross-section of a split-envelope gasket that has
been permeated by chemical service media, degrading the filler.

As with all flanged joints, it is of paramount importance to select the right gasket for the application. A simple acronym, STAMPS (size, temperature, application, media and pressure), can serve as a useful guide in this process. 

The curvature of the glass surfaces on flange
connections can create high stress points and
cause traditional envelope gaskets to split.

Size. There are standard sizes for ASME flanges, API valve stems, ANSI pump shafts and bores, etc. Non-standard sizes are best conveyed to the sealing manufacturer in the form of dimensional drawings. Some applications may require field measurements.
Temperature. The first consideration should be the continuous temperature to which the seal will be exposed, including high/low excursions as well as any regular thermal cycling inherent in the process. Note the frictional heat generated by rotating equipment will increase the temperature of the fluid contacting the seal. Temperature data will immediately limit the number of viable seals for an application.
Application. Knowing how the seal is to be used and the function it is expected to perform are keys to making the right selection. This type of information points up the anomalies of an application and the special requirements for optimal seal performance. Defining the parameters of a particular application requires information about where the seal will be installed.
For example, selecting the proper gasket for a flanged piping connection requires knowing the type of flanges involved their material structure and physical condition, the grade of bolts used to secure them, and whether collectively these factors can provide sufficient compressive force to affect a leak-proof seal. This is extremely important, since more than 70 percent of gasket failures are attributed to insufficient load. If the application is a valve, selection of the compression packing will depend on the condition of the stem, whether its motion is reciprocating, helical or continuous, and whether a specific level of leakage must be attained to meet environmental regulations.
Media. Common chemical nomenclature or trade names are used to identify the media that will come into contact with the seal.
Some processes employ secondary media that may not be addressed at this stage of inquiry. For example, a food processing line that is flushed once a day with a sodium hydroxide solution calls for a seal that is compatible both with this corrosive medium and the food being processed.
Pressure. This refers to the internal pressure a seal must contain. Most systems operate at fairly consistent pressure, but as with temperature it is important to know if the seal will be subject to pulses and other variations as a normal part of operation.
Speed. The speed of a rotating shaft or reciprocating rod must be taken into account when selecting oil seals, bearing isolators, mechanical seals or compression packing for dynamic applications. High speeds call for sealing materials that can withstand and effectively dissipate frictional heat.
 When dealing with flanged connections, size and application data are just as important as the service media and the temperature and pressure at which it is operates.

About the author
Matt Tones is director of market intelligence for Garlock Sealing Technologies. He also has served as director of product management for North America, manager of applications engineering, training and customer support, product manager for restructured PTFE gaskets, and as OEM liaison. He began his career in the company’s testing laboratories. He can be reached at 800-448-6688 or