Views:2 Author:Site Editor Publish Time: 2020-03-16 Origin:Site
When it comes to replacing a Plate Heat Exchanger gasket, Guphe warns that buying on price alone can be risky. If a failure occurs during operation, any savings made by purchasing a cheaper gasket could pale into insignificance when set against the value of lost production. Here, the company suggests the factors that need to be considered when selecting a replacement gasket that you can rely on science in itself.
To compound the right rubber mixture can involve the selection of anywhere from five to fifteen different substances from around 1700 of different grades of polymer, vulcanising chemicals and processing materials currently available. And that selection process is, itself, influenced by a host of other parameters that could affect the eventual performance of the gasket. The working environment, the service temperature and pressure, the actual nature of the duty and the interreaction between the gasket material and the plates all, individually and collectively, help to determine the final choice of materials used.
The working environment
The nature of the duty and the location in which the heat exchanger is used can bear heavily on the choice of gasket material. The most important factor is, of course, to ensure full compatibility between the gasket material and the chemical profile of the product being heated or cooled. Almost compatible simply won’t do since even minute traces of other chemicals can change the equation quite significantly. As an example, while EPDM rubber is ideal for duties involving liquid or gaseous ammonia it is unsuitable for refrigeration systems because traces of compressor oil in the ammonia can cause the elastomer to swell. Equally, while Fluorocarbon elastomers are generally compatible with an oil contaminated mixture of glycol and water, the picture changes radically with the addition of just 100ppm of amine type corrosion inhibitor into the mix. The alien component causes the fluorocarbon polymer to dehydrofluorinate and leads, ultimately, to gasket failure. To ensure peak gasket performance it is essential to take into account every potential chemical reaction and evaluate – and more importantly, understand – its possible effects on the gasket material.
Temperature & pressure
Likewise, it is equally important to have a totally accurate picture of the temperatures and pressures involved in a specific duty. Without both of these vital pieces of information, the efficiency and working life of the gasket cannot be guaranteed. The ideal gasket working environment, of course, is a steady-state operation with no pressure or temperature fluctuations such as those found in air conditioning and other comfort applications. However, these are unlikely to be the prevailing conditions in most process applications, particularly with so many processes operating on a batch-wise basis. Unplanned changes in temperature and pressure, caused by frequent shutdowns for media changes and cleaning, can physically alter the gasket material and lead to a much shorter service life. In the worst case scenario, severe deterioration of the elastomer can lead to catastrophic failure.
Rubber’s natural characteristics are altered or enhanced by the addition of other substances to provide a gasket’s desired sealing properties. Consequently, to select the right gasket it is also necessary to understand the kinds of reaction that can take place between the gasket material and that used to manufacture the PHE plate. For instance, stainless steel and nickel alloys all react badly with chlorine and chloride ions. So, using them with something like chlorophene rubber which, under the right circumstances, can release chloride ions could be asking for trouble. The same is true of other gasket materials such as resin–cured compounds; poor quality graphite and compressed fibre materials. All of them can create localised corrosion, including stress corrosion cracking, where they come into direct contact with the plate material. More exotic materials such as titanium, tantalum, niobium, zirconium and their alloys are also very susceptible to fluorine and fluoride ions. Using gaskets made from fluorocarbon elastomers with these materials has to be undertaken very carefully.
Some fluorine might be released during the vulcanisation stage; more can be released as a result of polymer reaction to temperature extremes or the presence of phenols, alcohols, amines or ammonia. Irrespective of its origin, the presence of fluorine traces can lead to localised corrosion including stress corrosion cracking. As if chemical reactions and potential changes due to temperature and pressure cycling weren’t enough to worry about, there are also the geometry and physical properties of the actual gasket to take into account. Rubber is subject to thermal expansion and this natural property has to be allowed for from the outset; at the point where the gasket mould is being designed.