Elastomer Seals for Instrumentation: In-Line Process

Gallagher recently released our High Performance Elastomer Seals for the Instrumentation Industry White Paper.  This was written by Russ Schnell, an Elastomer Consultant contracted by Gallagher Fluid Seals, and a former Senior Application Engineer with the Kalrez® perfluoroelastomer parts business at DuPont.  This white paper is now available for download on our Resources page.

Below is the first section of the white paper, diving into applications where the measurement is made at the process and the results then transmitted to a control system.  This section will review the four types of in-line measurement devices, all involving slightly different elastomer sealing applications.

In-Line Process Applications

Flowmeters

Flowmeters are used to measure the flow of liquid. In this section we will only consider the measurement of liquid flow in a closed piping system. Several examples of flow measurement devices include: flowmeters, Venturi tubes and orifice plates.

Note that these devices are “in-line” and require isolating the process line to remove and repair, or replace the measurement device. Shutting down a process to remove a device is time consuming, involves loss of production, and may require specific procedures to protect the operators and environment when a line is opened. All of these devices require seals to prevent leakage of the process to the environment and the elastomer seals should last the life of the flowmeter. For aggressive chemicals or high temperature applications, FKM or FFKM seals are an excellent choice. These products offer a long service life and resist deterioration in harsh environments.

When selecting an elastomer seal, first consider the process temperature, including a margin of safety for temperature excursions. Product data sheets usually list the upper service temperature for elastomers in dry air. In a chemical environment involving aggressive fluids, the upper service temperature for the elastomer will typically be lower than that listed on a data sheet. As an example, fluoroelastomers are rated for use up to 200°C, but exercise caution when using these seals over 175°C. For temperatures over 175°C, perfluoroelastomers may offer the best performance, especially in harsh chemicals. FFKMs can be used in services up to 325°C, but time in service may be limited at this top temperature. However, different formulations have subtle differences in performance, so be certain to understand the elastomer product in use, be it an FKM or FFKM. For low temperature applications, chemicals are less reactive and fluoroelastomers can provide excellent performance and sealing down to -30°C. For lower temperature applications, specialty formulations of FKM or FFKM products may be required.

Second, consider the chemical environment. Ideally, the elastomer should be tested in the actual fluid environment to determine its performance. Unfortunately, outside test data is rarely available on the exact conditions to which the seal will be exposed. Further, consider whether the seal is in a dedicated process line, or exposed to a variety of different fluids. Next, are there trace contaminants in the process that might attack the elastomer seal, even though the elastomer is resistant to the bulk process fluid? Finally, verify the elastomer is resistant to any cleaning fluids that may be used in the process.

Third, consider the pressure requirements for the elastomer seal. High pressure processes (>1000 psi) may require a higher durometer elastomer seal (e.g. 85-90 durometer Shore A), or anti-extrusion rings to prevent elastomer seal extrusion and subsequent failure. Also, will the elastomer be exposed to any rapid decreases in pressure? Specialty elastomer formulations are available that provide superior resistance to rapid gas decompression. For vacuum applications, specialty elastomer formulations or specific seal designs may provide the best performance. All these factors should be considered when selecting a high performance elastomer seal.

Pressure Gauges

Some types of pressure gauges use a diaphragm to separate the process fluid from the gauge’s internal mechanism. These gauges are often used with particularly aggressive or high viscosity fluids that could affect the proper functioning of the gauge internals. Elastomers are one choice of diaphragm material which readily flex with pressure changes in the line. The mechanical properties of fluoroelastomers typically make them a better choice for diaphragms than perfluoroelastomers. However, these applications usually involve only a small diaphragm deflection which is a benefit if an FFKM is required. Diaphragm design features can also aid in the ability of the part to flex without damage. Be sure to select an elastomer that meets the chemical resistance and temperature requirements of the application. Finally, discuss the application with your seal supplier regarding elastomer formulations with improved flex life and design options for these applications.

Probes

In-line or tank probes are used to measure process attributes such as pH, conductivity and various ion concentrations. These probes are inserted into a fitting in the process line or tank so the probe is in direct contact with the process fluid. These probes typically use o-ring seals which must withstand the application environment. Care must also be taken with the o-ring seal design for the probe. If the probe is screwed into a fitting, overtightening could result in damage to the o-ring due to over compression. The o-ring groove must have sufficient space to allow for part expansion in hot environments in order to avoid extrusion of the part. Finally if the seal is a circumferential seal around the probe, the o-ring compression and durometer (hardness) must be considered. Too high a compression or too high a hardness of the o-ring can make insertion of the probe into the tube well difficult and could result in damage to the probe. As in the previous examples, the elastomer seal should offer long term performance and chemical resistance to the range of chemicals used in the application.

Sensors

Sensors, used to relay information, are another instrumentation application that involves seals. For example, cars use sensors to monitor exhaust gases. Computers use this information to “adjust” the engine in order to maximize performance and minimize air pollution. Elastomers used in oxygen (lambda) sensors need to perform in hot aggressive gas environments. Seals need to perform for extended periods of time and in all types of environmental conditions. During steady state operation, these seals may be exposed to high temperatures (> 225°C). However, when a car engine is first started, the seals need to work in a relatively cool or possibly cold condition. Temperature cycling can be difficult on seals because of thermal expansion and contraction of the seal. Finally, car engines are designed to run for many years and failure of an oxygen sensor seal results in poor engine performance as well as personal frustration. It is important to understand the exact seal requirements for these applications in order to deliver the performance needed for satisfactory vehicle operation.