Diaphragm seals, also known as isolators, are commonly used to help safeguard the wetted parts of pressure measurement instruments, such as sensors (transmitters and transducers), gauges and switches, from corrosive chemicals in your system's pipes. In some instances, however, diaphragms are also used to protect the instrument from large particles that could potentially obstruct the process connection.
As a business development leader at Ashcroft – a leader in pressure instrumentation – I frequently address these concerns in low-pressure applications. Read this article to learn about the factors to consider for using a diaphragm seal in your operation and to use them on different types of instruments (mechanical or sensor) to keep your system flowing. You will also see additional resources you can use as a reference to deepen your knowledge of these isolating instruments.
Ensuring diaphragm seal compatibility
Following ASME B40.100 guidelines, it is important to inform your pressure instrument supplier about any conditions that may be relevant to the application and environment. This will help them determine the most appropriate gauge for the situation and whether or not a diaphragm seal would be required protection for your operation.
If a diaphragm seal is recommended, the wetted materials must be compatible with the process media. For more information, view our compatibility guide.
Why compatibility is important
The elastic element on a diaphragm seal is generally a thin-walled member, which operates under high-stress conditions and must be carefully selected for compatibility with the pressure medium being measured. No common material is immune to all types of chemicals. Corrosion potential is influenced by various factors, such as the concentration, temperature, and contamination of the medium.
Figure 1: Diaphragm seal components
Also, the pressure element's ability is affected by how it's designed, the materials used, and how its parts are connected.
Although diaphragm seals are effective protection devices, there are circumstances where they can create problems if not configured properly. One of the most common conflicts deals with low pressure spans on pressure indicators or low set points on pressure switches.
To address these concerns I'm going to break the considerations into two categories: mechanical instruments and sensor-based instruments.
Protecting mechanical instruments in low-pressure applications
Mechanical pressure switches operate using a piston and spring mechanism that moves slightly back and forth as pressure increases or decreases. When a certain pressure level is reached, the piston activates a micro switch, either closing or opening a circuit. Whether the circuit is open or closed determines if power flows through the device. This mechanism is typically used to switch equipment on or off based on the application.
Figure 2. Mechanical pressure switch components
Considerations for attaching mechanical switch to a diaphragm seal
In applications with corrosive media, it's essential to protect the wetted components of the pressure switch. Devices like diaphragm seals or isolation rings, which contain a sensing element tailored to the application, exert pressure on a transfer fluid that, in turn, activates the switch's actuator.
The physics of this process demands sufficient force from the process pressure to act on the isolator's sensing element, which must then move enough of the transfer fluid (with viscosities ranging from water to glycerin at 1300 cSt) to ensure the instrument functions accurately. Due to the extra resistance introduced by the diaphragm seal, there are limitations on how low your set point can be.
For example, a switch that could see a set point of 2 inches of water, or even a 2 psi set point as a direct connection may be limited to a setpoint of 6 psi when installed on an isolator.
Considerations for attaching mechanical gauges to a diaphragm seal
Traditional Bourdon tube pressure gauges, typically used for pressure ranges of 15 psi and above, feature a metallic tube that is welded to the pressure connection. When pressure increases, it causes the tube to straighten, which then activates a series of gears that move the gauge's pointer. This design allows the gauge to withstand high burst pressures and maintain reliable accuracy even in harsh environments.
To support this robust design, sufficient displacement from the diaphragm seal is essential for precise gauge movement. Pressure gauges with smaller spans, ranging from 10 inches of water to 10 psi, utilize a bellows system rather than a Bourdon tube. When these gauges are used with an isolator, they require specific size and material configurations to ensure adequate displacement for gauge operation.
Strategies for achieving higher displacement
Whether we're discussing lower set points (less than 6 psi) or lower spans on gauges (less than 30 psi) we typically need to look at high displacement options for diaphragm seals. There are several ways to accomplish this.
1. Use a Viton™ or Kalrez™ diaphragm.
This can facilitate precise calibration on very low-pressure spans and enable low set points (as low as 10 inches of water). The flexibility of these diaphragms reduces the resistance of the process on the transfer fluid. However, the selection of these materials is limited and may not be suitable for all media. If they are compatible, this solution can be applied to threaded, flanged, and various inline designs.
2. Use a larger diaphragm when metallic seals are required.
Certain seal designs utilize a larger diaphragm to expand the surface area exposed to the process. This enhances the transfer of force compared to a traditional metallic diaphragm. Designs accommodating a larger diaphragm can be either threaded or flanged. Flanged options may include:
- A high displacement design like the Ashcroft® 702/703 Flanged High Displacement Diaphragm Seal with a large diaphragm and lower housing (with or without a flush port).
- A flush design like the Ashcroft® DF Flushed Flanged Diaphragm Seal, which does not require a lower housing because the diaphragm extends over the raised face of the flange making it the only wetted component.
The requirement for flushing is typically minimized due to the design, however, if flushing is required the DF seal can be used in conjunction with a flushing ring.
For the DF seal to be effective on lower pressure spans or lower set points, it requires a larger diaphragm. The typical design for a DF seal relies on the pipe connection to determine the size of the diaphragm. For example:
- A 1-inch pipe would have a 1-inch diaphragm
- A 2-inch pipe would have a 2-inch diaphragm and so on
For lower spans, 3-inch diaphragms or larger ones would be required. In instances where smaller pipe sizes are driving the size of the DF seal, adaptors can be used to mate smaller pipe sizes with a 3-inch DF seal.
Protecting sensor-based instruments in low-pressure applications
Where mechanical instruments tend to be concerned with displacement, sensor-based products are sensitive enough to avoid that dynamic. However, being so sensitive presents other issues. Because sensor-based instruments on isolators can respond accurately to lower pressure spans, any temperature fluctuation or thermal expansion of the fill fluid can lead to a pressure shift, which becomes more pronounced with lower pressure spans.
Considerations for attaching any instrument to a diaphragm seal
1. Limit the volume of fill fluid above the seal as much as possible. Using low-volume fittings, using instrumentation that achieves multiple functions, and using options on the instrument that minimize the accessories are all strategies to limit the fill fluid.
2. Use instruments that can be easily adjustable. Any instrument installed on a diaphragm seal, particularly with pressure spans less than 200 psi, should have some form of zero/span adjustment. This can be a necessary step once the seal-filling process is completed.
Although following these guidelines can help performance it is important to be aware of the dynamics that could affect low pressure spans or low set points in use with a diaphragm seal. Always consult the manufacturer of the assembly to verify if a particular configuration will work effectively.
Ready to learn more?
Using a diaphragm seal in low-pressure applications is a highly effective way to protect your pressure instruments and keep your system running efficiently. 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.
For more information on diaphragm seals and other isolators, like isolation rings, here are a few relevant 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.
