Understanding Diaphragm (Chemical) Seals - A Comprehensive Technical Guide to Protecting Pressure Instruments

Prepared by Merit Instruments

Executive Summary

Pressure gauges and pressure transmitters are designed to accurately measure pressure under a wide variety of operating conditions. However, many industrial processes expose instruments to media that can damage sensing elements, clog pressure ports, cause corrosion, freeze, crystallize, or operate at temperatures far beyond the safe operating limits of the instrument.

A diaphragm seal—also commonly called a chemical seal—isolates the pressure instrument from the process while allowing accurate pressure measurement through a sealed hydraulic system filled with an engineered fill fluid.

When properly selected, diaphragm seals can significantly extend instrument life, improve measurement reliability, reduce maintenance costs, and improve operator safety.

This paper explains diaphragm seals in detail, including:

• How diaphragm seals work

• Why they are used

• Common styles and designs

• Materials of construction

• Fill fluids

• Accuracy considerations

• Installation methods

• Industry applications

• Selection guidelines

• Common mistakes to avoid

1. What Is a Diaphragm (Chemical) Seal?

A diaphragm seal is a pressure-isolating device installed between a pressure instrument and the process.

Instead of allowing process media to enter the Bourdon tube, pressure transmitter, or pressure switch, process pressure acts against a thin flexible diaphragm.

Behind this diaphragm is an incompressible fill fluid that hydraulically transfers pressure directly to the measuring instrument.

The instrument never contacts the process media.

Instead, it only "feels" pressure through the sealed hydraulic system.

Basic Construction

A diaphragm seal consists of:

• Process connection

• Flexible diaphragm

• Upper housing

• Lower housing

• Hydraulic fill fluid

• Instrument connection

The diaphragm is usually laser welded to ensure a permanent leak-free seal.

2. How Does a Diaphragm Seal Work?

When process pressure increases:

1. Pressure pushes against the diaphragm.

2. The diaphragm flexes slightly.

3. Fill fluid transfers the pressure.

4. Pressure reaches the gauge or transmitter.

5. Instrument displays pressure normally.

The process media never enters the instrument.

No contamination occurs.

No clogging occurs.

No corrosion occurs inside the instrument.

3. Why Use a Diaphragm Seal?

Diaphragm seals solve many process measurement problems.

Corrosive Chemicals

Examples include:

• Sulfuric acid

• Hydrochloric acid

• Sodium hydroxide

• Nitric acid

• Chlorine solutions

Without isolation, these chemicals can rapidly destroy instrument internals.

Viscous Fluids

Examples include:

• Asphalt

• Paint

• Resin

• Syrup

• Heavy oils

These products can clog small pressure ports.

Flush diaphragm designs eliminate dead spaces where material can accumulate.

Crystallizing Products

Products including:

• Sugar solutions

• Salt solutions

• Fertilizers

may crystallize inside pressure ports.

A diaphragm seal prevents buildup inside the instrument.

Sanitary Processes

Food and pharmaceutical applications require hygienic process connections.

Typical industries include:

• Dairy

• Beverage

• Pharmaceutical

• Biotechnology

• Food processing

Flush diaphragms eliminate crevices where bacteria could collect.

High Temperatures

Excessive temperature can damage:

• Bourdon tubes

• Pressure transmitters

• Electronic sensors

Diaphragm seals combined with capillary tubing can dramatically reduce instrument temperature.

Freezing Applications

Outdoor installations often experience freezing conditions.

Fill fluids can be selected for low-temperature operation while keeping process media out of the instrument.

Hazardous Media

Some process fluids are:

• Toxic

• Radioactive

• Flammable

• Explosive

The seal minimizes exposure should the instrument require maintenance.

4. Common Types of Diaphragm Seals

Threaded Diaphragm Seals

The most common industrial design.

Advantages include:

• Economical

• Compact

• Easy installation

• Suitable for general process service

  
  

Typical applications:

• Water treatment

• Hydraulic systems

• Chemical plants

• Industrial manufacturing

  
  

Flush Diaphragm Seals

The diaphragm is flush with the process connection.

Ideal for:

• Slurries

• Pulp

• Food

• Wastewater

• High-viscosity media

There are no cavities where material can accumulate.

In-Line Diaphragm Seals

Installed directly into process piping.

Excellent for:

• Continuous flow

• Sanitary systems

• Pharmaceutical production

  
  

Flanged Diaphragm Seals

Designed for larger process connections.

Used in:

• Refineries

• Chemical plants

• Power generation

• Large process vessels

Available in ANSI and DIN standards.

Extended Diaphragm Seals

Designed for tanks and vessels with insulation or thick vessel walls.

The diaphragm extends into the vessel for direct media contact.

Sanitary Diaphragm Seals

Manufactured using hygienic process connections such as:

• Tri-Clamp

• DIN 11851

• SMS

• RJT

Often polished to pharmaceutical standards.

5. Materials of Construction

Material selection is critical.

Common diaphragm materials include:

Stainless Steel 316L

Suitable for:

• Water

• Steam

• General industrial service

Excellent corrosion resistance.

Hastelloy®

Excellent for:

• Strong acids

• Chlorides

• Aggressive chemicals

  
  

Monel®

Suitable for:

• Saltwater

• Marine service

• Hydrofluoric acid applications

  
  

Tantalum

Outstanding corrosion resistance.

Often used with:

• Concentrated acids

• Extremely aggressive chemicals

  
  

Titanium

Ideal for:

• Seawater

• Chlorine

• Offshore service

  
  

PTFE Coated Diaphragms

Provide additional chemical resistance.

Common in chemical processing.

6. Fill Fluids

The fill fluid is critical to system performance.

Common options include:

Silicone Oil

The most common choice.

Advantages:

• Stable

• Wide temperature range

• Excellent pressure transmission

  
  

Food Grade Oils

Used where incidental food contact may occur.

High Temperature Fluids

Special synthetic oils allow elevated operating temperatures.

Low Temperature Fluids

Maintain accuracy in sub-zero conditions.

Oxygen Service Fluids

Special fluids compatible with oxygen systems.

7. Capillary Systems

Sometimes the instrument cannot be mounted directly on the seal.

A capillary tube filled with hydraulic fluid connects them.

Benefits include:

• Reduced temperature exposure

• Easier instrument access

• Lower vibration

• Improved readability

Longer capillaries may slightly affect response time and accuracy.

8. Accuracy Considerations

Several factors influence performance.

These include:

• Temperature

• Fill fluid expansion

• Capillary length

• Diaphragm size

• Instrument range

• Orientation

Properly matched assemblies minimize measurement error.

9. Selecting the Correct Seal

Consider the following questions:

What is the process media?

Is it:

• Corrosive?

• Abrasive?

• Viscous?

• Hygienic?

  
  

What is the operating temperature?

Extreme temperatures influence:

• Fill fluid

• Diaphragm material

• Instrument selection

  
  

What pressure range is required?

Very low pressures often require larger diaphragm diameters.

Is sanitation required?

Food and pharmaceutical systems require sanitary connections.

Is the media likely to clog?

Flush diaphragms are generally preferred.

Is maintenance accessibility important?

Capillary-mounted instruments can simplify servicing.

10. Typical Applications

Diaphragm seals are commonly used in:

Chemical Processing

Protect instruments from corrosive chemicals.

Oil & Gas

Protect against hydrocarbons, sour gas, and high temperatures.

Water & Wastewater

Suitable for sludge and corrosive treatment chemicals.

Food Processing

Meet hygienic requirements while preventing contamination.

Pharmaceutical

Allow sterile pressure measurement.

Pulp & Paper

Prevent clogging from pulp slurries.

Mining

Handle abrasive slurries and process chemicals.

Power Generation

Protect instruments from high-temperature steam and chemicals.

11. Common Installation Mistakes

Avoid these common errors:

• Incorrect fill fluid selection

• Wrong diaphragm material

• Excessively long capillary tubing

• Instrument mounted in direct sunlight without temperature consideration

• Using threaded seals with plugging media

• Using seals with incompatible process connections

• Improper venting during installation

• Mechanical damage during handling

Proper installation significantly improves long-term reliability.

12. Advantages of Diaphragm Seals

• Protects pressure instruments

• Extends instrument service life

• Improves operator safety

• Prevents clogging

• Prevents corrosion

• Allows measurement of difficult media

• Reduces maintenance costs

• Supports sanitary processing

• Improves measurement reliability

• Enables remote instrument mounting

  
  

13. Limitations

Diaphragm seals are not a universal solution.

Considerations include:

• Higher initial cost

• Slightly slower response than direct mounting

• Fill fluid temperature effects

• More careful engineering selection

• Potential calibration changes if components are mismatched

Proper specification minimizes these limitations.

14. Merit Instruments Diaphragm Seal Solutions

Merit Instruments offers diaphragm seal solutions designed for demanding industrial applications where standard pressure instruments require protection from aggressive process conditions.

Whether measuring corrosive chemicals, viscous fluids, sanitary products, or high-temperature media, properly selected diaphragm seals help improve measurement reliability while extending instrument service life.

Our technical team can assist customers in selecting:

• Appropriate diaphragm seal styles

• Compatible diaphragm materials

• Correct fill fluids

• Process connections

• Capillary configurations

• Pressure gauges and transmitters matched to the application

  
  

By considering the complete measurement system—not just the pressure instrument—Merit Instruments helps customers achieve accurate, dependable pressure measurement across a wide range of industrial processes.

Conclusion

Diaphragm (chemical) seals are among the most effective methods of protecting pressure instrumentation in demanding industrial environments. By isolating the sensing element from corrosive, abrasive, viscous, sanitary, or high-temperature process media, they improve reliability, reduce maintenance, and extend instrument life.

Successful diaphragm seal installations depend on selecting the correct combination of seal type, diaphragm material, fill fluid, and pressure instrument for the specific application. Understanding these factors allows engineers and maintenance professionals to optimize performance while protecting both equipment and personnel.

For assistance selecting the right diaphragm seal assembly for your application, contact the technical specialists at Merit Instruments.