Keep pressure under control with the right diaphragm seals

July 9, 2008
Contributing Editor Sheila Kennedy details smart ways to keep pressure at bay.

Diaphragm seals, also known as chemical seals, isolate pressure-measuring instruments from the process media. The system pressure is transmitted to fill fluid in the upper housing of the diaphragm seal, and from there to the pressure-measuring instrument itself.

Choosing the right diaphragm seal depends on a number of factors. You must consider process characteristics such as temperature extremes or whether the media is corrosive, viscous, toxic, abrasive or contains suspended solids. The fill fluid must be chemically compatible with the process fluid in the unlikely event of a diaphragm rupture. You need to consider the transmitter sensor range, mounting position, regulatory requirements and other criteria. Here are some examples of the ways process applications influence diaphragm seal specifications, and how software tools simplify the selection process.

Sanitary applications: The food and beverage, pharmaceutical and biotechnology industries are subject to stringent government and industry sanitary standards. Sanitary diaphragm seals eliminate dead space, cracks, crevices and cavities, thereby preventing bacterial growth and contamination in or on the process connection.

Wika’s proprietary series of Inline seals, mounted directly into process lines, make them well suited for sanitary applications. Used for gauge or differential pressure measurements in flow applications, Inline seals provide a continuous process flow that removes dead space and avoids turbulence, corners and obstructions. The self-draining design simplifies cleaning. “The sanitary Inline diaphragm seal is installed between two flanges, becoming an integral part of the pipeline. It replaces the conventional T-piece,” says Raimund Weissner, Wika technical director of diaphragm seals.

Clamped sanitary diaphragm seals that compress the gasket can impinge on the active pressure-sensing area of the diaphragm. PI Components virtually eliminates the “clamp effect” with its improved-finish sanitary seals and patented attachment technology.

Biodiesel and ethanol applications: Biofuel processing is subject to the same harsh environmental conditions, mechanical vibration and process pulsation found in oil and gas processing. Breaks in the traditional threaded gauge-to-seal connection can compromise readings or lead to system failure.

In Wika’s all-welded system (AWS), the process gauge and diaphragm seal are integrated into a single assembly, and each connection and leak path is welded. “[The] system is leak-free, tamper-resistant and durable — a true dual-containment system,” Weissner says. “We are seeing a lot of growth in the acceptance of the all-welded approach because it reduces shutdowns, improves safety and controls emissions.” The addition of a port to the assembly allows a clean fluid to flush residue out of the pipe, preventing instrument damage and clogging.

Pulp and paper: The manufacture of chemical pulp and treatment of recovered and recycled paper involves viscous, aggressive and hardening media. PI Components’ flush diaphragm seal for pulp and paper will not allow debris to plug the impulse lines, so the measurement remains constant. The company developed a super-duplex diaphragm material to handle black liquor applications.

Wika’s pulp and paper units meet the industry’s seal specification and process-connection requirements. An optional Wikaramic coating can be applied for further protection against abrasive media to avoid scouring and premature diaphragm wear.

Nuclear power applications: In this highly regulated industry, safety, reliability, stringent design and manufacturing quality control are foremost concerns. The Model 1159 remote diaphragm seal by Rosemount Nuclear Instruments is specifically intended for use in nuclear power stations. The threaded remote diaphragm seal has been tested to IEEE’s seismic, radiation and environmental qualifications. The seal, combined with Rosemount’s nuclear-qualified pressure transmitters, supports both boiling water and pressurized water reactor applications.

Seal selection software: With so many factors to consider and process reliability at stake, some seal manufacturers offer software to simplify the seal selection. For example, Rosemount’s Instrument Toolkit uses exclusionary logic to eliminate transmitter and remote seal models that don’t meet your application requirements. The software factors in conditions that can affect performance to calculate the total installed performance of the transmitter and seal assembly. It also produces reports, installation drawings and specification sheets.

BAsecal is Badotherm’s chemical-seal calculation program, from which users can select the seal and transmitter combination for the needs of the application. The Windows-based software package uses exclusionary logic to align Badotherm chemical seals with well-known industrial transmitters in the market. The software then calculates the total performance of the seal/transmitter combination based on the models selected and application data.

E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at [email protected].

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