If you experiencing issues with diaphragm seal failure in your process, you are not alone. In harsh applications that involve corrosive media and other challenging conditions, diaphragm seals are your primary line of defense in protecting your pressure instruments from becoming damaged.
At Ashcroft, we've seen practically every scenario that can cause a diaphragm seal to fail, leading to inaccurate readings and costly downtime. In this article, we will review how a diaphragm seal works, explore the three conditions that account for the majority of diaphragm seal failure — corrosion, high temperature, and fluid fill leakage — and offer some corrective measures to fix these issues.
When you are done, you will have a better understanding of why diaphragm seals can fail and be able to identify and prevent potential issues in your own process. You will also find links to additional resources that will help address other diaphragm-related questions you may have.
How a diaphragm protects your pressure instruments.
The diaphragm seal's job is to isolate the pressure-sensing element of a pressure measurement instrument from the process fluid and simultaneously communicate process pressure changes through its diaphragm.
Here's how it works:
The diaphragm seal assembly and measurement instrument are filled with a fluid that transfers pressure. The diaphragm flexes when there is a pressure change at the process connection, which causes the transfer fluid to move. That movement is how the pressure change is communicated to the instrument's pressure-sensing element.
Figure 1. Components of a diaphragm seal.
Three common reasons diaphragm seals can fail.
Diaphragm seals seldom fail due to a mechanical issue. That's because in most cases the diaphragm deflection is only a few thousandths of an inch.
The typical culprits for diaphragm seal issues are corrosion, high temperatures and liquid fill leaks. Let's take a closer look at each one.
Problem 1: Corrosion (Material Compatibility).
Corrosion of your diaphragm seal typically occurs due to incorrect material selection. The wetted materials of the seal must be compatible with the process media. For example, if the process involves pumping sulfuric acid, the diaphragm seal wetted materials you select (i.e. Hastelloy® B, Tantalum, PTFE or Kalrez®) need to withstand the corrosive liquid.
The chemical concentration is also an important factor to consider with material selection. A change in concentration or temperature of a chemical will cause corrosion which will not occur with the same chemical at a different concentration or temperature.
Finally, dissimilar metals that come in contact with one another can also cause galvanic corrosion in your process. NASA describes galvanic corrosion as "an electrochemical action of two dissimilar metals in the presence of an electrolyte and an electron conductive path." In a diaphragm seal, if Stainless steel or carbon steel comes in contact with an alloy (magnesium or aluminum) galvanic corrosion can accelerate the corrosion of the alloy material.
Ensure material compatibility.
Before selecting the diaphragm seal for a corrosive process, ensure the seal and wetted materials are compatible. You can do this by using the online Ashcroft® Material Selection Tool. This interactive resource enables you to input your specifications and receive detailed information about material compatibility for your application. You can also watch our Corrosion and Material Compatibility Webinar for more information or check out the Ashcroft® Corrosion Guide.
Figure 2. Material Selection Chart from the Ashcroft® Material Selection & Corrosion Guide.
Problem 2: Temperature Error.
Temperature errors in diaphragm seals stem from fluid expansion or contraction. Understanding factors like fill volume, coefficients of expansion, assembly characteristics, and temperature variations can help predict and mitigate errors. While the internal fill volume change is independent of the instrument pressure range, the final error percentage is influenced by the range. Because of this, a high-pressure gauge will have fewer errors than low-pressure gauges.
Figure 3. Liquid Fill Expansion Coefficient.
When the diaphragm seal/gauge assembly is filled correctly, calculating the temperature error becomes a straightforward process. The total fill volume includes the internal volume of the pressure instrument (like the Bourdon tube or bellows) and the diaphragm seal. Ashcroft® 4 1/2 inch pressure gauges and diaphragm seals, for example, have specific internal volumes as listed in Figure 1 and Figure 2, respectively.
Figure 4. Gauge Internal Volume.
Note: The temperature error for all 2½ and 3½ gauges (and for 4½ gauges with pressure ranges
above 200 psi) will be relatively small and need not be considered.
Figure 5. Seal Internal Volume.
For the best results, it is important to use the right temperature range fill fluid for your application. Use Figure 6 below as a reference for the temperature specifications of each fill fluid.
Figure 6: System Fill Fluid Temperature Limits.
Problem 3: Liquid Fill Leaks.
Fill fluid leakage is another common cause of potential diaphragm seal failure. There are three primary ways this can occur.
- Nicks on the gasket surfaces. For example, leaks can occur if there are nicks on the diaphragm capsule stud of the O-ring. If this is the case, a new O-ring or new top housing can address the issue.
- Loose gauge connection. If you have a leak, check and tighten the connection between the seal and the attachment.
- Installing incorrectly. One example of improper installation is turning the gauge while installing the assembly into the process instead of the diaphragm seal. Always be sure to follow installation instructions provided by the manufacturere.
Ready to learn more?
Now that you know the three most common reasons why diaphragm seals fail and the preventative steps you can take to help ensure these failures do not occur in your process, you may still have questions. Here are a couple of related articles that may interest you.
• Best Practices for Installing Flanged Diaphragm Seals
• How Does Temperature Affect Pressure Gauge Performance?
Or, to speak with someone directly, feel free to contact one of our product experts with any questions you have.
In the meantime, download our guide to learn more about pressure instrument assembly.
