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Silicone Diaphragm vs. Sodium Hydroxide (NaOH)

Diaphragms are the high-wear component of most valves, so we thought it would be useful to show some of the common failures that occur with diaphragms. In this article, we’ll discuss the most common failures we have observed, and we’ll show an example of a specific case and how the problem was resolved.

Over the years we have seen or heard about various types of diaphragm failures. Most of them fall into one of two categories:

  • Chemical incompatibility
  • Incorrect assembly

Here is an example of chemical incompatibility:

Damaged silicone diaphragm

close-up

The diaphragm shown here was exposed to a 10% (2.5M) solution of NaOH overnight. This failure occurred sometime during the night.

The system where this diaphragm was installed was set up with a downstream valve that was closed, and then the tank valve (where this diaphragm was installed) was opened. The piping from the downstream valve through the tank valve and into and including the vessel were flooded with NaOH. This solution was left in place overnight, with the tank valve left open during this time as well. In the morning the NaOH was found on the floor and, as you can see, the diaphragm was destroyed.

In this case, the customer chose silicone because it was already approved in their system and, according to most chemical compatibility/resistance charts (example chart), silicone is rated as being affected in a minor (0-5% swell) to moderate (5-10% swell) level by sodium hydroxide. While these guides are useful starting points for choosing an elastomer, they are generally limited by type of exposure, time of exposure, and the elastomer generally being static and not dynamic. These additional factors can have a significant affect on elastomer performance.

The major contributing factor to the destruction of this diaphragm was the high stress point combined with the level of compatibility of the silicone with NaOH.

How can this be avoided? There were two options that were available to this customer:

  • Process change
  • Material change

Because there were factors that prevented a material change, the customer made a process change. After flooding the line, tank valve, and vessel, the customer closed the tank valve, relieving the high stress point and preventing premature material failure.

Have you experienced problems with a diaphragm? Let us know and we can work with you to resolve the problem. Send us an email or post a comment.

Chemical incompatibility can be a difficult problem to resolve. In most cases you’re left with an increase in diaphragm replacement or a change in materials, and if you don’t have a certain material approved for use in your system it can be difficult to change to a more compatible material.


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Comments

  1. When cleaning your in-line radial diaphragm valve is a 1% Liquinox/5% Bleach cleaning solution detrimental on a silicone diaphragm?

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