If you work in a production plant or facility that uses highly corrosive process media, you likely own diaphragm seals to protect the sensing element of your pressure instrumentation from getting damaged. Without that protection, your whole operation can be compromised. But how can you be sure your diaphragm seal is performing properly?
Ashcroft, a leader in pressure and temperature measurement, provides training on different aspects of these devices. In fact, as a territory manager, I have been training product distributors on this very topic for years. Now I want to share this information with you. In this article, I will cover how diaphragm seals are used in different applications and the steps you can take to ensure these instruments perform as you need them to.
What is the primary purpose of a diaphragm seal?
There are many reasons to use a diaphragm seal. Primarily, diaphragm seals (also known as isolators) are used to protect pressure instruments from damage due to corrosive process media and clogging. For instance, in water/wastewater plants, chemical feed systems and pump systems, there is a risk that harsh chemicals, solids or other abrasive substances can come in contact with the pressure-sensing element. If that happens, the pressure instrument will not be able to accurately measure the pressure flow of the operation.
Diaphragm seals are also used to protect instruments from extreme temperature applications. However, in these cases, the diaphragm seal is typically used in conjunction with a temperature dissipation device. This is because diaphragm seals have certain challenges and limitations relating to high-heat processes.
How do I ensure my diaphragm seal is performing properly?
If the design, form, fit and function of the diaphragm assembly are not executed properly, they may not perform as needed. Here are steps you can take to ensure the proper assembly for your application:
Choose the right configuration.
The first step in ensuring the performance of your diaphragm seals is to select the right style for the job. For instance, if the seal needs to protect the process against clogging, you will choose a design that is conducive to preventing the process from blocking the sensing element. For traditional diaphragm seals (versus isolation rings) some specific designs and features are less prone to clogging. These include:
• In-line constructions. Mounted in line with the process piping.
• Saddle seals. Welded directly onto the process piping.
• Flush diaphragm seals. Welded directly to the assembly flange.
Standard threaded and flanged seals, like the Ashcroft® 200-201 Threaded Diaphragm Seal and the Ashcroft® 202-203 Flanged Diaphragm Seal and the seals typically have a dead leg in the lower housing that would accumulate particulates. If these are being used, it is important to configure them with one or two flushing ports for ease of maintenance.
In industries such as water/wastewater, pulp/paper and mining, isolators, such as isolation rings are designed to both withstand the impact of large particulates in a process and prevent clogging as the construction is in line with the piping system. Isolation rings can be flanged (either with bolt-through designs or wafer style) or threaded into the piping system.
Whether you use a traditional diaphragm seal or an isolation ring depends on the process media. Traditional diaphragm seals tend to have a better selection of materials allowing more media compatibility options. In other words, if the application has both clogging and corrosion concerns then it is necessary to choose an isolator that can address both conditions. My article, When to Use a Diaphragm Seal vs. an Isolation Ring goes into more detail for your reference.
Choose the right material.
There are two considerations for material selection:
1. Choose a material that is compatible with the process.
2. Choose a material that works within the constraints of your operating pressure and temperature.
For low-pressure applications, you will want to either select a very flexible diaphragm material or one that has a large surface area. Either of these characteristics is necessary to handle mechanical products with low-pressure ranges or mechanical-pressure switches with low set points. This is mainly a concern if the pressure range is less than 30 psi.
The Ashcroft Diaphragm Seal Pressure and Temperature Min/Max Guide provides the minimum spans for certain pressure instrument configurations.
Evaluate the operating temperature.
If the temperature is too high, it can impact the accuracy of the instrument. An elevated temperature can cause thermal expansion in the fill fluid causing a pressure shift reading that is not caused by the process pressure. It can also affect the instrument directly, causing temperature errors or even damaging components of the instrument. That’s why choosing a diaphragm seal and fill fluid that is rated to the operating temperature is so important. In these cases, it may be necessary to use a heat dissipation device to further protect the attached instrument. Read our blog, How do I safely select diaphragm seals for high-temperature applications, to learn more.
Practice the correct filling process.
Diaphragm seal assemblies have a very specific and methodical filling procedure. It is critical to have the right equipment and follow the specified process. Then, once the seal is assembled and filled, note that adjustments may be necessary.
Seal filling process:
o Pull a full vacuum on the diaphragm seal assembly to remove as much air as possible, and
o Pump the system fill into the assembly so air cannot get in. Any air left in the system will impact instrument performance.
Calibrate your instruments.
Once all the products are assembled and filled the last step is calibration. Even though the instruments may have been calibrated prior to assembly, further adjustments may be necessary after installed on a diaphragm seal. Here’s why:
- The dynamic of having a diaphragm seal typically degrades the accuracy of the instrument(s) and it can be helpful to recalibrate once assembled.
- Mechanical gauges designed for the process market typically have a micrometer pointer which can help adjust the zero/span back within tolerance.
- When specifying a pressure gauge, consider whether there is easy access to a zero/span adjust and, also that the case is not filled with liquid.
- Cases are sometimes filled with liquid (such as glycerin or silicone) to protect against pulsation/vibration. However, if the case is filled with liquid, accessing a micrometer pointer can be tricky.
- There are other ways of protecting against pulsation/vibration that do not involve a liquid-filled case. Using a dampened movement (like Ashcroft’s PLUS! Performance) in conjunction with a pressure snubber or throttle valve can protect the instrument from pulsation/vibration and still allow easy access to the micrometer pointer
Use adjustable instruments.
Sensor-based products such as pressure transducers and transmitters also require adjustments. So, select a sensor that has a zero/span adjustment and can also be accessed after the assembly is complete.
Many options for industrial sensors have zero and span adjustments when they are constructed for general-purpose applications. However, there are not many that can be adjusted when the housing is constructed for hazardous area locations. Typically, those housings are well sealed off. The Ashcroft® E2F & E2X Explosion-Proof Transducer is one exception. A magnet is used for zero/span adjustments.
Use available accessories.
Accessories such as pressure snubbers, throttle valves or capillaries are often used to help against harsh conditions. It is common for these accessories to be installed above a diaphragm seal. There are several considerations to make when selecting accessories.
1. Minimize the fill fluid above a diaphragm seal. The more fill fluid above the seal, the less responsive the instruments will be, sometimes to the point where the instruments will not function properly. Also, the more fill fluid in a system the higher the impact temperature affect will have on the assembly.
2. Choosing accessories that can serve multiple purposes or are designed to limit the fill fluid is critical. For example, when there are multiple instruments in an assembly using a low-volume T-fitting versus a goal post design is helpful.
Other considerations for ensuring diaphragm seal performance.
Make sure all orifices are large enough to allow the fill fluid to pass through. For one, filling an assembly that involves too small of an orifice can be troublesome. It is critical to effectively transmit the pressure to the instruments. For example, when using a pressure snubber be sure to select a configuration with larger pores designed for oils.
Capillaries are often used for temperature dissipation or to remotely mount the instrument away from severe vibration or pulsation. Capillaries are also required when differential pressure instruments are installed on diaphragm seals. Selecting a less viscous fill fluid (i.e. 10cTs Silicone) is helpful.
It is not uncommon to see valves installed above diaphragm seals. Valves for the purpose of isolating the instrument is not recommended. It may send the wrong message to operators by insinuating that the instrument can be isolated and removed. When in fact if this happened the system fill would be compromised, and the assembly would need to be refilled/calibrated by the factory. Some valves are designed for throttling. Although this is an acceptable practice one must also be careful when dealing with lower pressures. Opening and closing a throttle valve changes the volume capacity within the seal. If the volume changes, it can impact the accuracy of the instruments – especially when the pressure spans are low.
Finally, leave it be.
Once an assembly is filled and calibrated be sure to install without tampering with the assembly. Do not attempt to tighten or loosen any of the connections above the seal. As mentioned previously, any adjustments that result in changing the volume capacity can impact performance.
Do you want to know more?
The importance of using a diaphragm seal in harsh applications to protect your pressure instruments cannot be understated. Following these steps can help ensure this valuable instrument always performs as intended. However, there is so much more to understand. For your reference, here are a few more articles related to this topic that you may find useful.
- How to Optimize Water & Wastewater Monitoring Instruments
- 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.
