Diaphragm Seals (Remote Seals): Working Principle, Types & Applications

Brian Craig

June 08, 2020

Diaphragm seals are used to isolate pressure instruments from process media that could damage, contaminate, clog, or otherwise interfere with pressure measurement. They are commonly used with pressure transmitters, differential pressure transmitters, pressure gauges, and other pressure-measurement devices.

Also known as remote seals or chemical seals in many industrial applications, diaphragm seals create a barrier between the process medium and the instrument's sensing element. Process pressure acts on the diaphragm and is transferred through a fill fluid to the connected measuring instrument.

This arrangement is particularly useful for corrosive, viscous, abrasive, high-temperature, crystallizing, sanitary, or solids-containing processes where a conventional direct pressure connection may not be suitable.

Selecting the right diaphragm seal requires more than choosing a compatible material. Process pressure, temperature, fluid characteristics, diaphragm material, fill fluid, process connection, capillary length, mounting configuration, and required measurement performance all need to be considered.

What Is a Diaphragm Seal?

A diaphragm seal is a pressure-measurement device that separates the process medium from the sensing element of a pressure instrument. A flexible diaphragm forms the barrier between the process and the instrument while allowing process pressure to be transmitted to the measuring element.

The diaphragm seal is typically filled with an appropriate transmission fluid. When process pressure acts on the diaphragm, the diaphragm deflects and transfers that pressure through the fill fluid to the pressure instrument.

This configuration allows pressure measurement without allowing the process fluid to enter the sensing element or connecting impulse passage.

Diaphragm seals are also commonly called chemical seals. When the diaphragm seal is physically separated from the pressure transmitter and connected through a capillary, the arrangement is generally referred to as a remote seal system.

Diaphragm seals are particularly useful when direct exposure to the process could cause corrosion, plugging, contamination, excessive temperature exposure, or damage to the pressure instrument.

Types of Diaphragm Seals

Diaphragm seals are available in different configurations to suit process connections, fluid characteristics, temperature conditions, sanitary requirements, and installation constraints. The most appropriate design depends on the process rather than simply the instrument model.

Flanged Diaphragm Seals

Flanged diaphragm seals are used where the process connection requires a flange. They are commonly applied to vessels, pipelines, tanks, and process systems where a robust flanged connection is preferred.

Threaded Diaphragm Seals

Threaded diaphragm seals provide a compact connection for applications using threaded process connections. Their pressure and temperature ratings must be matched to the specific application and manufacturer specifications.

Flush Diaphragm Seals

A flush diaphragm minimizes the amount of the seal projecting into the process connection. This configuration can be useful for viscous fluids, slurries, suspended solids, and processes where material buildup around the pressure connection could affect measurement.

Extended Diaphragm Seals

Extended diaphragm designs position the sensing diaphragm farther into the process connection. They can be useful for applications involving vessel walls, lining arrangements, or specialized process connections.

Sanitary Diaphragm Seals

Sanitary diaphragm seals are designed for applications where hygiene, cleanability, and specialized process connections are important. They are commonly considered in food, beverage, pharmaceutical, and biotechnology processes.

Remote Diaphragm Seals

Remote diaphragm seals are separated from the pressure transmitter and connected through a capillary filled with transmission fluid. They can be useful when the transmitter needs to be positioned away from a high-temperature, inaccessible, or otherwise difficult process connection.

Diaphragm Seal Materials and Wetted Parts

The diaphragm material must be compatible with the process medium because it is directly exposed to the process.

Common material options can include stainless steel and specialized alloys such as Hastelloy, tantalum, titanium, and other application-specific materials. Polymer-based materials may also be available for particular chemical or sanitary applications.

Material selection should consider:

  • Chemical composition of the process medium
  • Concentration
  • Process temperature
  • Pressure
  • Corrosiveness
  • Abrasiveness
  • Solids content
  • Cleaning chemicals
  • Hygienic requirements

There is no single diaphragm material that is suitable for every process.

For example, a material that performs well in one chemical service may be unsuitable for another chemical at a different concentration or temperature. The diaphragm, process connection, gasket, and other wetted components should therefore be evaluated as a complete process-contacting system.

How Does a Diaphragm Seal Work?

The working principle of a diaphragm seal is based on hydraulic pressure transmission.

The process medium applies pressure to the exposed side of the flexible diaphragm. The diaphragm responds to this pressure and transfers the pressure through an incompressible fill fluid located behind the diaphragm.

In a direct-mounted arrangement, the fill fluid transfers pressure directly to the connected pressure instrument. In a remote seal system, the fill fluid also occupies the capillary connecting the diaphragm seal to the pressure transmitter.

The basic pressure path is:

Process medium → diaphragm → fill fluid → capillary, where applicable → sensing element

The process medium therefore remains isolated from the pressure transmitter's sensing element.

This arrangement is particularly valuable when the process fluid is corrosive, viscous, abrasive, hazardous, crystallizing, or contains suspended solids.

A diaphragm seal should therefore not be viewed simply as a protective membrane. It becomes part of the complete pressure-measurement system, and its design can influence measurement response and accuracy.

Fill Fluid and Capillary: Why They Matter

In a diaphragm seal system, the fill fluid provides the hydraulic connection between the process diaphragm and the pressure instrument.

The selected fill fluid must be suitable for the operating temperature and measurement requirements. Changes in temperature can cause the fill fluid to expand or contract, potentially affecting the measurement.

In remote seal systems, the capillary carries the fill fluid between the diaphragm seal and the transmitter. Capillary length and temperature exposure can therefore influence measurement performance.

Important considerations include:

  • Capillary length
  • Ambient temperature
  • Process temperature
  • Fill-fluid characteristics
  • Installation location
  • Temperature differences between capillaries
  • Mechanical protection
  • Response requirements

For this reason, capillary routing should be planned during instrument installation rather than treated as an afterthought.

Remote Diaphragm Seal vs. Direct-Mount Diaphragm Seal

A direct-mounted diaphragm seal is installed directly at the pressure instrument, while a remote diaphragm seal is physically separated from the instrument and connected through a capillary.

Consideration

Direct-Mount Seal

Remote Seal

Instrument Location

At process connection

Away from process connection

Capillary

Generally not required

Required

High-Temperature Separation

Limited

Greater physical separation possible

Installation

More compact

Requires capillary routing

Temperature Effects

Must still be considered

Capillary temperature effects require attention

Typical Use

Difficult media with accessible process connection

Difficult media plus challenging temperature or location conditions


Remote mounting can be particularly useful when the transmitter needs to be positioned away from a high-temperature or difficult-to-access process point.

Why Are Diaphragm Seals Used in Pressure Measurement?

Diaphragm seals are primarily used when direct contact between the process medium and pressure instrument could create reliability, maintenance, safety, or measurement problems.

They provide a physical barrier between the process and sensing system while maintaining pressure transmission.

Corrosive Media

Aggressive process fluids can attack conventional wetted components. A diaphragm seal allows the process-contacting material to be selected according to chemical compatibility.

Viscous Fluids

Highly viscous fluids can restrict small pressure passages and make conventional impulse connections difficult to maintain.

Slurries and Suspended Solids

Solid particles can accumulate in pressure connections and interfere with measurement. A suitable diaphragm configuration can reduce exposure of the pressure instrument to these materials.

High-Temperature Processes

Remote diaphragm seals can allow the transmitter to be positioned away from the process connection, reducing direct exposure to process temperature.

Crystallizing or Polymerizing Media

Some process fluids can solidify or form deposits when exposed to pressure connections. A suitable diaphragm seal can provide a more appropriate process interface.

Sanitary Applications

Specialized diaphragm seals can provide process connections and surfaces suitable for applications where cleanability and hygiene are important.

Factors That Affect Diaphragm Seal Measurement Accuracy

A diaphragm seal is part of the measurement system, so its configuration can influence the overall measurement performance.

Process Temperature

Temperature changes can affect the diaphragm and fill-fluid system and may contribute to measurement shifts.

Ambient Temperature

Remote capillaries exposed to changing ambient temperatures can introduce additional thermal effects.

Capillary Length

Longer capillaries can increase the influence of temperature effects and affect the dynamic response of the measurement system.

Fill Fluid

The fill fluid should be selected for the expected temperature range and application requirements.

Mounting Elevation

Elevation differences between the diaphragm seal and transmitter can introduce hydrostatic effects, particularly in differential-pressure applications.

Diaphragm Characteristics

Diaphragm design, size, flexibility, and material can influence pressure transmission.

Vacuum Conditions

Applications involving vacuum or vacuum cycling require a diaphragm-seal configuration specifically suitable for those conditions.

Installation

Incorrect capillary routing, excessive mechanical stress, or unsuitable temperature exposure can affect measurement performance.

This is why the accuracy specification of the pressure transmitter alone should not be used to evaluate the accuracy of the complete remote seal system.

Diaphragm Seal Applications

Diaphragm seals are used across industries where the process conditions make direct pressure measurement difficult or undesirable.

Chemical Processing

Chemical processes often involve corrosive or reactive fluids. Diaphragm seals allow suitable wetted materials to be selected for the process while isolating the pressure instrument.

Oil and Gas

Remote diaphragm seals can be used for pressure and differential-pressure measurement where process temperature, fluid characteristics, or installation conditions make conventional connections unsuitable.

Refining and Petrochemical Processing

High temperatures, aggressive media, and difficult process conditions can require specialized diaphragm seal configurations.

Food and Beverage

Sanitary diaphragm seals can be used where cleanability, hygiene, and specialized process connections are required.

Pharmaceutical Processing

Pharmaceutical applications may require hygienic process connections and materials compatible with cleaning and sterilization procedures.

Pulp and Paper

Slurries, fibers, and viscous process materials can create plugging and maintenance problems in conventional pressure connections.

Wastewater and Sludge

Suspended solids and deposits can interfere with pressure measurement, making suitable diaphragm configurations valuable.

High-Temperature Processes

Remote mounting can separate the pressure transmitter from high process temperatures while maintaining pressure transmission through the fill-fluid system.

How to Select the Right Diaphragm Seal

Diaphragm seal selection should begin with the process conditions and then be matched to the pressure instrument.

Consider the following factors.

Process Medium

Identify whether the medium is corrosive, viscous, abrasive, hazardous, sanitary, crystallizing, polymerizing, or contains suspended solids.

Pressure Range

Consider normal operating pressure, maximum operating pressure, pressure fluctuations, and any potential pressure excursions.

Process Temperature

Specify both normal and maximum/minimum temperatures. Temperature affects material selection, fill fluid, and remote-seal performance.

Diaphragm Material

Select wetted materials according to the actual chemical and physical characteristics of the process.

Process Connection

Choose the appropriate threaded, flanged, flush, extended, or sanitary connection based on the process equipment.

Fill Fluid

The fill fluid must be suitable for the operating temperature and measurement requirements.

Capillary Length

For remote systems, specify only the required capillary length and consider its temperature environment and routing.

Direct or Remote Mounting

Determine whether the transmitter can be mounted directly at the process connection or needs to be physically separated.

Instrument Compatibility

Confirm that the diaphragm seal is compatible with the pressure transmitter, differential pressure transmitter, gauge, or other connected instrument.

Application Requirements

Consider hygiene, cleaning, vacuum service, hazardous media, corrosion resistance, and other application-specific requirements before final selection.

Diaphragm Seal Installation Considerations

Proper installation is essential because the diaphragm seal and pressure instrument function as a complete measurement system.

For remote systems, capillary routing should avoid unnecessary bends, mechanical damage, excessive vibration, and unfavorable temperature exposure.

The seal and transmitter should be installed according to the manufacturer's specifications.

For differential-pressure applications, the elevation difference between the two seals and the transmitter should also be considered because the fill fluid can create hydrostatic effects.

Installation should also prevent unnecessary mechanical stress on the process connection, diaphragm seal, and capillary.

A correctly selected seal can still produce poor measurement performance if it is improperly installed.

Common Diaphragm Seal Problems and Troubleshooting

Slow Response

Remote diaphragm seal systems can respond more slowly than direct pressure connections because pressure is transmitted through the diaphragm and fill-fluid system.

Temperature-Related Zero Shift

Changes in ambient or process temperature can affect the fill fluid and create measurement shifts.

Diaphragm Damage

Mechanical damage, corrosion, excessive pressure, or unsuitable process conditions can affect diaphragm integrity.

Capillary Damage

Crushing, sharp bends, excessive stress, or physical damage can compromise the remote connection.

Process Buildup

Deposits on the process side of the diaphragm can affect pressure transmission.

Incorrect Material Selection

Chemical incompatibility can lead to corrosion or premature failure.

Incorrect Mounting

Improper elevation or installation configuration can create additional measurement errors, particularly in differential-pressure applications.

Troubleshooting should therefore examine the seal, fill fluid, capillary, installation, and transmitter together.

Diaphragm Seal vs. Impulse Line

Factor

Impulse Line

Diaphragm Seal

Pressure transmission

Direct through tubing

Through diaphragm and fill fluid

Process contact

Process reaches pressure line

Process isolated from instrument

Best for

Clean, compatible fluids

Corrosive, viscous, high-temperature, or solids-containing media

Plugging risk

Higher

Lower with suitable seal design

Temperature

Can expose instrument to process heat

Remote seal can separate transmitter from heat

Maintenance

Impulse lines may require cleaning

Seal and capillary require proper installation

Cost

Generally lower

Generally higher

Bottom line: Use an impulse line for clean, compatible processes; choose a diaphragm seal when the process could cause corrosion, plugging, crystallization, viscosity, or excessive temperature exposure.

When Should You Use a Remote Seal?

A remote seal should be considered when the pressure transmitter needs to be isolated from the process or physically separated from the process connection.

Typical reasons include:

  • High process temperature
  • Corrosive process media
  • Highly viscous fluids
  • Slurries
  • Suspended solids
  • Crystallizing fluids
  • Polymerizing fluids
  • Difficult-to-access process connections
  • Sanitary requirements
  • Hazardous or toxic media

A remote seal is not automatically the best solution for every pressure application. It should be selected when its isolation, mounting flexibility, or process compatibility provides a clear engineering advantage.

Remote Diaphragm Seals for Industrial Applications

The Transmitter Shop supplies new, surplus, and reconditioned industrial instrumentation, including remote seals and pressure transmitters from established manufacturers. Selection should be based on the process medium, pressure, temperature, seal configuration, material compatibility, and transmitter requirements.

Conclusion

Diaphragm seals provide an effective way to isolate pressure instruments from difficult process conditions while maintaining pressure measurement.

They are particularly valuable for corrosive, viscous, abrasive, high-temperature, sanitary, crystallizing, and solids-containing applications where conventional pressure connections may create reliability or maintenance problems.

The right diaphragm seal depends on more than the process connection. Process medium, pressure, temperature, diaphragm material, fill fluid, capillary length, mounting arrangement, and instrument compatibility all influence the final measurement system.

For remote installations, the diaphragm seal, fill fluid, capillary, and pressure transmitter should be treated as one integrated measurement system. Proper selection and installation are therefore essential for reliable long-term performance.

Frequently Asked Questions (FAQ's)

  • What is the difference between a diaphragm seal and a remote seal?

A diaphragm seal isolates the pressure instrument from the process. A remote seal specifically refers to a configuration where the diaphragm seal is separated from the instrument, typically by a capillary.

  • Why are diaphragm seals used with pressure transmitters?

They are used when the process medium, temperature, viscosity, solids content, or chemical properties make direct connection unsuitable for the pressure transmitter.

  • Does a diaphragm seal affect pressure measurement accuracy?

Yes. Fill-fluid characteristics, temperature, capillary length, mounting elevation, diaphragm design, and installation can influence the performance of the complete measurement system.

  • What material should be used for a diaphragm seal?

The wetted material should be selected according to the process medium, concentration, temperature, pressure, and chemical compatibility requirements.

  • Why is a capillary used with a remote seal?

The capillary transfers pressure hydraulically through the fill fluid while allowing the transmitter to be positioned away from the process connection.

  • Can diaphragm seals be used for differential-pressure measurement?

Yes. Diaphragm seal systems are commonly used with differential-pressure instruments, but installation elevation, fill-fluid effects, temperature, and capillary configuration must be considered.

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