Pressure gauges, switches and transducers all have important roles in the safe and efficient operation of process piping, skid systems and other applications found in many industries. However, in certain scenarios, these sensitive instruments require protection from a range of challenging conditions, including extreme temperatures, potential clogging from particulates, or corrosive environments that could compromise their ability to function properly.
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 ensure the longevity and accuracy of your pressure instruments.
In this article, you will learn more about the three biggest challenges of these systems and the reasons to use a diaphragm seal or isolation ring. You will also learn about material traceability and certain certifications that may be required for these devices.
How diaphragm seals protect against corrosion.
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, and
- A lower housing that connects the diaphragm/top housing assembly to the process piping
Both the diaphragm and the 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 process media for its composition, particularly the concentration of harsh chemicals, as well as the temperature, to determine the appropriate materials for the wetted parts.
How diaphragm seals protect against clogging.
The sensing components of pressure instruments often contain cavities and dead ends where particulates can accumulate. When the process media includes consistent, sustainable, or sizeable particles, this accumulation 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.
Additionally, diaphragm seals can be engineered to align more closely with the process, thereby eliminating dead ends and cavities and reducing the potential for significant buildup. This alignment can be achieved through inline flanged, inline threaded, or inline welded assemblies, or by directly welding the seal to the pipe using a saddle seal, where the lower housing is welded to the top of the pipe. In all these configurations, the instrument component can be removed without disassembling the piping.
Figure 1: Expanded view of diaphragm seal with flushing port.
Why isolation rings may be a better solution for low-pressure, high-particulate applications.
Isolation rings are often used in water/wastewater and mining industries for applications with lower pressure (under 400 psi), and normal temperature conditions, which often involve thicker slurry and larger particles that can clog instruments.
Isolation rings function similarly to diaphragm seals but are installed in line with the piping and typically positioned between two pipe flanges. Material options for isolation rings are more limited than those available with diaphragm seals, so they are less suitable for corrosive applications. However, wetted parts like PTFE, EPDM, and CVPC are available when both slurry and harsh chemicals are present.
Although the end plates can be made from various materials, including metallic and non-metallic options, the sensing elements of isolation rings are always non-metallic, necessitating careful consideration of temperature compatibility.
Figure 2: Isolation ring assembly example.
How extreme process temperature can affect your decision to use an isolator.
Challenges arise when process media temperatures exceed the 50 °F - 90 °F 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's ability:
- Capillary Line or Micro-Tube Siphon: Reduces thermal expansion effects by allowing quick temperature dissipation.
- Standard Coils or Pig Tail Siphons: Suitable only for steam applications, these devices block steam with a condensate barrier. However, 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 reaching the instrument assembly, avoiding thermal expansion issues. Having a direct connection instead of a diaphragm seal will also help avoid thermal expansion concerns.
Why material traceability is important for isolators.
In the past decade, the demand for documenting the origin of wetted materials in instrumentation has significantly increased. This traceability is crucial for maintaining quality and compliance in manufacturing processes. Here are the two primary standards 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: Offers 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 need to ensure traceable material sourcing and tracking throughout the manufacturing process to provide this certificate.
Both reports are essential for the wetted components of an instrument. However, many instruments have sensing assemblies that may lack traceable materials, making documentation difficult. In such cases, using diaphragm seals with traceable materials is an effective solution. The straightforward construction of a diaphragm seal facilitates the use of traceable materials, ensuring compliance and reliability.
Certifications for diaphragm seals and isolation rings that ensure safety and compliance.
Here are two important certifications to look for when choosing an isolator like a diaphragm seal or isolation ring for your system:
NACE Certification
Instruments that are used in hydrogen sulfide (H2S) applications typically require NACE-compliant wetted components. The materials to use will often depend on which standard is required (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 capable of having the appropriate wetted materials to meet the NACE requirements. Often materials like Hastelloy C276 and Monel are preferred. If the instruments are not available in these materials it is important to use a diaphragm seal which tends to have many configurations available for various materials.
NSF Certification
NSF (National Sanitation Foundation) offers certification for materials that may come in contact with potable water to ensure that the water is not contaminated by the material. Although there are a variety of instruments that carry this certification not all of them do so. In this event, using a diaphragm seal that does carry the certification will help protect the water from uncertified (and potentially harmful) materials.
Ashcroft offers diaphragm seals in 316LSS, as well as isolation rings with EPDM/316LSS materials that are certified by NSF. Both configurations can be used to help install an instrument that is not verified as safe. See a complete list of Ashcroft NSF 61-certified diaphragm seals and isolation rings.
Ready to learn more?
For more information on the minimum spans and maximum allowable working pressure (MAWP) of certain pressure instrument configurations, you can refer to the Ashcroft Diaphragm Seal Pressure and Temperature Min/Max Guide. Plus, here are a few more related articles that you may find helpful.
- What factors affect a diaphragm seal's performance?
- How do I safely select diaphragm seals for high-temperature applications?
- When to use a diaphragm seal vs. and isolation ring
- Why use a flushing connection on a diaphragm seal?
Feel free to contact us with any questions or concerns. In the meantime, download our guide to learn more about selecting an instrument assembly solution for your specific needs.
