Gaskets sealing lifetime in Plate Heat Exchangers

The correct choice of gasket characteristics is crucial to the effective operation of the Plate Heat Exchanger. Since the majority of Plate Heat Exchanger gaskets are manufactured from perishable rubber polymers, they require replacing as the gasket loses the ability to seal through hardening and deterioration.

Vital Properties

Incompressibility - The rubber compounds behaves as if it were a liquid that can change shape but cannot flow, so that after being squeezed it will return to its original vulcanised shape.

Strength - It is strong enough to withstand the forces normally incurred by squeezing into position and being acted upon by pressurised liquid.

Softness - This is adjusted by compounding so that the rubber is strong enough for the application but still soft enough to conform closely to the metal surfaces and provides a seal without undue stresses being applied

Resistance to Fluid - The high molecular weight, cross-linked polymer matrix is resistant to penetration by any fluid of a dissimilar chemical nature to itself.

Resistance to Temperature - One disadvantage of the desired 'rubberness' of the polymer material is the fact that all polymers will only behave as a rubber over a relatively short range of temperature. Thus accurate selection of the compound according to the required temperature range is crucial.

Longevity - Many modern synthetic polymers have a built in superiority in terms of useful lifetime over older materials, and modern additives can further enhance this property. It is an area that the correct compound selection is of vital importance: all rubber based compounds suffer from stress relaxation, i.e the initial sealing force slowly declines during use due to changes in the internal chemistry of the compound, and when the sealing force declines to below a value depending on the application, leaks will ensue.

Plate Heat Exchanger Gasket Materials

For plate (and frame) heat exchangers, the standard materials are EPDM and Nitrile rubber (NBR). EPDM is used normally for non-fatty/oily applications, and NBR for oil/fat applications. However, there are no rules without exceptions. An "oil" and fatty application may have a low fat content, making the application more suitable for EPDM instead of NBR.

Other rubbers used for PHE's are Hydrogenated Nitrile Rubber (HNBR), Fluorocarbon rubber (FKM), Butyl, Chloroprene Rubber (GR), Chlorsulphonated Polyethylene (CSM), Styrene-Rubber (SBR), Natural Rubber (NR) and Silicone Rubber (Q) are less common ones.

All additives matter for the product properties, and not only effect the quality level of the product, they also influence the consistency of the quality level.
 

Lifetime

Eventually all rubber materials will degrade, loosing their properties, making them unsuitable for further use. It is all a matter of time and the original quality of the product. The lifetime of rubber products is dependent on the environment (chemical types, concentrations) and temperature. Also, temperature changes and pressure changes effect the performance and lifetime of the rubber product.
 

Chemical Resistance

Each rubber material has its unique resistance towards various chemicals. The polymer used, as well as additives, effect which chemicals each rubber material is suitable for. 

High Temperature

At high temperatures the rubber will be attacked faster by the chemicals in use, and the surrounding oxygen. The oxidation process will either make the rubber harder (the most common one) OR make it softer (less common), OR both at the same time. Either way the change in properties will reduce the performance. All rubber materials have a maximum continuous temperature, which should not be exceeded to maintain good performance and to achieve a reasonable lifetime.
 

Low Temperature

At low temperatures the stiffness increases, and the elasticity decreases. This influences the sealing force of the gaskets. All rubber materials have a certain lowest temperature that have to be exceeded to be able to perform well enough as a gasket.
 

Food Applications

For food applications there are demands according to national standards. The usual one for PHE gaskets is FDA (USA). This standard regulates what additives are allowed to be used in the rubber and to what content. It also sets demands on what tests have to be done to ensure the quality level.

Rubber properties/Test Methods

Hardness: Measured normally as Shore A or IRH D. Both methods are based on the penetration of a needle into a rubber surface. The less the penetration is, the harder the rubber is. Tensile Strength: the force needed to break a piece of rubber, Measured in force/cross area unit

Elongation at break: How much a rubber can be stretched until breaking. Measured in % of original length. Tear strength: How much force is needed to break a rubber sample with a specified crack. Measured in force per thickness.

Compression set: The idea of measuring the compression set is that the lower the compression set is, the higher the sealing force is. It is measured in % of the deformation. Heat Resistance/Heat Ageing: is measured through measuring the change in various mechanical properties.

Chemical Resistance: Is measured through measuring the change in volume, weight, and various properties. Low temperature resistance: Is measured either through compression set of tensile retraction test, which is a test or elasticity at low temperature. Condition Audit Knowing the right time to replace gaskets for critical plate heat exchangers ensures plant reliability, productivity and uptime. That’s what one of the world’s largest ethylene producers in Saudi Arabia discovered after Condition Audit for the company’s 11-year-old gasketed plate heat exchangers. Many factors affect the operating condition of heat exchangers. Process conditions and heat transfer media, for instance, may change. Opening the heat exchanger frequently also influences heat exchanger performance. These have a direct effect on the mechanical condition of the heat exchanger. “Even if the gaskets are selected to meet design conditions, they can either last longer or require replacement earlier than expected depending on actual operating conditions,” “Gasket rubber also ages, which affects the sealing force as well as the service life of the plate heat exchanger.” Preventive maintenance for more uptime  Management at the plant agreed to conduct a Condition Audit on one of the 39 heat exchangers during scheduled maintenance. A Service Engineer was on site when the unit was opened. Condition-based auditing tools were used to analyze the current condition of gaskets and plates as well as actual operating conditions. The scope of work comprised internal and external inspection of the heat exchanger to determine the current condition of its:  • Channel plates, including detailed plate deformation analysis  • Gaskets, including a prediction of remaining lifetime  • Frame, including connection linings  • Bolts and bearing boxes  • Spare parts inventory Long-term service for a long-time customer   The data clearly indicated that the gaskets were approaching the end of their service life, and that reconditioning of the plates was necessary.  Environment affects performance Gaskets can lose elasticity and sealing capabilities at a faster rate due to operating conditions. For instance, gaskets in plate heat exchangers for applications involving water, especially at low temperatures, can be in service for decades without leaking. On the other hand, gaskets used in the process industry, where plate heat exchangers handle corrosive materials, such as lye and acids, at high temperatures are subject to high stress and wear more quickly. Choose the right material for the environment  Specific environments attack different types of rubber in different ways. The type of rubber polymer is the most important factor in determining the environmental resistance of the gasket. One formulation may be completely destroyed by a specific environment, while another of the same elastomer type may remain essentially unaffected. Therefore, compatibility predictions must be based on experience with actual plate heat exchanger formulations. Courtesy by Alfa Laval  It is important to know the capabilities and limitations of a gasket rubber compound. New gaskets are highly resilient, expanding to seal in the media and to direct flow. Maintaining this condition helps ensure optimum performance. To maximize gasket lifetime: (1) invest in the right type of gasket – one capable of compression in order to resist the stress and force generated by the flow of media, and (2) select the right gasket materials. Rule of thumb: 10 °C lower than max temperature ==> 2 years lifetime 10 °C above  the max temperature ==> 6 months lifetime