Why Should You Use a Diaphragm Seal or Isolation Ring?

Diaphragm seals and isolation rings are used to protect pressure gauges, switches and transducers from challenging process conditions such as corrosion, clogging and extreme temperatures. Pressure instruments play important roles in the safe and efficient operation of process piping, skid systems and other applications across many industries. However, their sensing elements can be vulnerable when exposed directly to demanding process media.

To address these challenges, the American Society of Mechanical Engineers (ASME) created industry standards and recommends strategies to protect instruments from damage in demanding applications. For example, ASME B40.100 recommends the use of isolation devices, widely recognized as diaphragm seals or isolation rings, as protective barriers that can help ensure the longevity and accuracy of pressure instruments.

In this article, you will learn about three major process challenges that may require a diaphragm seal or isolation ring:

• Corrosion
• Clogging from particulates
• Extreme process temperatures

You will also learn about material traceability and certifications that may be required for these devices.

How diaphragm seals protect pressure instruments from corrosion

Diaphragm seals protect pressure instruments from corrosion by isolating their sensitive sensing elements from process media that may be incompatible with the instrument's wetted materials.

The instruments used for measuring, monitoring and controlling pressure all rely on a sensing element that is vulnerable to corrosion. For example, a Bourdon tube on a pressure gauge, an actuator seal on a pressure switch or a diaphragm on a transducer are all elements that sense pressure changes in a system.

These components, often referred to as "wetted" parts, are typically made from materials such as 316L stainless steel, brass/bronze, PTFE or elastomers like Viton™ or Buna and are in direct contact with the process media. If the process media is a chemical that is not compatible with these materials, it could lead to corrosion and potentially irreparable damage to the instrument.

Diaphragm seals can be manufactured from a wide variety of compatible materials and typically consist of two main wetted components:

• A diaphragm that deflects a transfer fluid to drive the instrument.
• A lower housing that connects the diaphragm/top housing assembly to the process piping.

Both the diaphragm and lower housing can be made from various metallic and non-metallic materials, including 316L stainless steel, Hastelloy C276, Hastelloy B, Monel®, Duplex, PTFE, Viton™, PVC and PVDF, among others.

Figure 1: Components of a diaphragm seal

It is crucial to evaluate the composition of the process media, including the concentration of harsh chemicals and the process temperature, to determine the appropriate materials for the wetted parts.

How diaphragm seals protect pressure instruments from clogging?

Diaphragm seals can protect pressure instruments from clogging by preventing particulate-filled process media from accumulating within small cavities and dead ends in the instrument's sensing assembly.

The sensing components of pressure instruments often contain cavities and dead ends where particulates can accumulate. When process media contains a consistent concentration or significant size of particles, this buildup within the sensing assembly can pose significant challenges. To mitigate this issue, isolators such as diaphragm seals or isolation rings are used.

Common diaphragm seal designs may have cavities and dead ends, but they are typically larger and easier to clean. These seals can be equipped with single or double flushing ports, allowing them to remain installed during the cleaning process.

Diaphragm seals can also be engineered to align more closely with the process, eliminating dead ends and cavities and reducing the potential for significant buildup. This alignment can be achieved through:

• Inline flanged assemblies
• Inline threaded assemblies
• Inline welded assemblies
• Saddle seals, where the lower housing is welded directly to the top of the pipe

In all these configurations, the instrument component can be removed without disassembling the piping.

Figure 2: Expanded view of diaphragm seal with flushing port

When is an isolation ring a better choice than a diaphragm seal?

Isolation rings may be a better solution for low-pressure applications with thick slurry or large particles that could clog a pressure instrument.

Isolation rings are often used in water/wastewater and mining applications with lower pressures, typically under 400 psi, and normal temperature conditions. These processes often involve thicker slurry and larger particles that can clog instruments.

Isolation rings function similarly to diaphragm seals but are installed inline with the piping and are typically positioned between two pipe flanges.

Material options for isolation rings are more limited than those available with diaphragm seals, making them less suitable for some corrosive applications. However, wetted materials such as PTFE, EPDM and CPVC are available when both slurry and harsh chemicals are present.

Although the end plates can be made from various metallic and non-metallic materials, the sensing elements of isolation rings are always non-metallic. This makes process temperature compatibility an important consideration when selecting an isolation ring.

Figure 3: Isolation ring assembly example

How does extreme process temperature affect isolator selection?

Extreme process temperatures can affect pressure instrument accuracy and material compatibility, making temperature an important factor when deciding whether and how to use a diaphragm seal or other isolation device.

Challenges can arise when process media temperatures exceed the instrument's range, leading to decreased accuracy in pressure instruments. Seals should only be used with elevated temperatures if clogging or corrosion is also a concern. If clogging or corrosion is not an issue, seals may be problematic.

Extreme temperatures can surpass the instrument's temperature rating, causing containment issues. Diaphragm seals can help by acting as heat sinks but may also cause thermal expansion in the system fill, resulting in inaccurate readings.

To address these challenges, consider using the following devices to supplement the isolator:

• Capillary Line or Micro-Tube Siphon: Reduces thermal expansion effects by allowing quick temperature dissipation.
• Standard Coils or Pigtail Siphons: Suitable only for steam applications, these devices block steam with a condensate barrier. They should only be used with diaphragm seals if chemical exposure is a risk. In these cases, the seal is placed above the siphon.
• Uninsulated Stand-Off Piping: Lowers process temperatures before they reach the instrument assembly, helping avoid thermal expansion issues. A direct connection instead of a diaphragm seal can also help avoid thermal expansion concerns.

Why Is material traceability important for diaphragm seals and isolation rings?

Material traceability helps verify the origin and composition of wetted materials used in diaphragm seals and isolation rings, supporting quality and compliance requirements.

In the past decade, demand for documenting the origin of wetted materials in instrumentation has significantly increased. Two primary standards are used for material traceability on wetted parts:

• EN 10204 2.2: Provides a sample document indicating the typical origin of the material and includes a compliance statement confirming that the material meets the manufacturer's specifications. This certificate is generally easier to supply for products that have already been manufactured, sourced, shipped or installed, even when direct traceability is challenging.
• EN 10204 3.1: Provides a more detailed traceability process involving a serial number or heat number that links the material to a documentation package. This package traces the metal back to its original melting point. Manufacturers must ensure traceable material sourcing and tracking throughout the manufacturing process to provide this certificate.

Both reports are important for the wetted components of an instrument. However, many instruments have sensing assemblies that may lack traceable materials, making documentation difficult.

In these cases, diaphragm seals manufactured with traceable materials can provide an effective solution. Their relatively straightforward construction can make it easier to provide traceable wetted materials while supporting compliance and reliability requirements.

What certifications are important for diaphragm seals and isolation rings?

NACE compliance and NSF certification are two important considerations when selecting diaphragm seals or isolation rings for applications with specific safety, material or regulatory requirements.

When Is National Association of Corrosion Engineers (NACE) compliance required?

NACE-compliant wetted components are typically required for instruments used in hydrogen sulfide (H2S) applications. The appropriate materials often depend on which standard is required, such as MR0175:2021/ISO 15156:2020 or MR0103/ISO 17945:2015, as well as the sulfide concentration, pressure and temperature of the media.

Not all instruments are available with wetted materials that meet NACE requirements. Materials such as Hastelloy C276 and Monel are often preferred. If the required instrument is not available in these materials, a diaphragm seal may provide a solution because seals are available in many different material configurations.

When is NSF certification important?

National Sanitation Foundation (NSF) certification is important when wetted materials may come into contact with potable water and must meet requirements intended to prevent contamination from unsuitable materials.

Although various pressure instruments carry NSF certification, not all do. In these cases, using a diaphragm seal that carries the appropriate certification can help isolate the water from uncertified instrument materials.

Ashcroft offers diaphragm seals in 316L stainless steel as well as isolation rings with EPDM/316L stainless steel materials that are NSF certified. Both configurations can be used to help install an instrument that is not verified as safe for direct contact with potable water.

How do you decide between a diaphragm seal and an isolation ring?

The choice between a diaphragm seal and an isolation ring depends primarily on the process media, pressure, temperature, potential for clogging, material compatibility and application requirements.

A diaphragm seal may be appropriate when:

• Corrosive process media could damage the instrument.
• Material compatibility requires specialized wetted materials.
• Particulates could clog the instrument's sensing element.
• Flushing or cleaning access is needed.
• Material traceability is required.
• NACE compliance or NSF certification is needed.

An isolation ring may be a better option when:

• The application operates at lower pressures.
• Thick slurry or large particulates are present.
• An inline design is preferred.
• Process temperature and media are compatible with the available non-metallic sensing materials.

Evaluating these conditions before selecting an isolator can help protect the pressure instrument while supporting accurate and reliable measurement.

Ready to learn more?

Check out the related resources below for more information about isolation instruments. If you have specific questions about your application, you can contact us and speak to a product expert. In the meantime, download our instrument assembly guide for help choosing the right solution for your needs.