Highs and Lows of Pump Sealing

High and low temperature pumping environments have long posed particular problems for pump engineers. The default position for many is to opt for either a metal ring seal or even seal-less pump design. Yet there’s no getting away from the benefits of elastomers seals in reducing pump wear and operational performance. John Kerwin, Materials Technology Manager with Precision Polymer Engineering reports on how new elastomer seals are coming to the aid of pump manufacturers and engineers facing high and low temperature pumping.

The elasticity of elastomers sets them apart from all other types of pump sealing material. When a deforming force is applied, they retract to their original shape when the force is removed. It is this property of the elastomer to return to its original, undeformed shape that gives the seal its sealing force.



Naturally the way in which a seal performs is an important factor in pump efficiency.

Another characteristic of elastomers is the ease with which their physical properties can be modified to better suit the working environment. By making changes to the filler system it is possible to optimise the physical properties of a particular grade of elastomeric material when compared to others within the same grade. The reinforcement effect of a filler is complex and dependent upon its structure, particle size and chemical make-up of the particles themselves. 

Carbon black, for instance, has a very irregular surface, which makes the reinforcement particularly effective. However, some synthetic silicas are perfectly spherical, offering very little in terms of reinforcement. In order to achieve specific physical properties from a material the correct combination of reinforcing and non-reinforcing fillers must be selected.

Fillers can be classed as reinforcing or non-reinforcing, depending upon whether they arrest crack propagation to a greater extent than they raise stresses, or vice versa. Within a polymer the filler can have two effects, they may act as stress raisers, reducing the energy at break, or may arrest crack propagation to increase the energy required for breakage.

A simple summary of the requirements of physical properties that may be optimised through the use of fillers is shown by Figure 2.

Pump manufacturers can take advantage of the ‘filler effect’ to select the quadrant of the ‘Physical Properties Box’ best suited to their pumping equipment. The modulus of a material is related to its hardness. As the modulus increases then so does the hardness; O-rings of high hardness are more capable of withstanding extrusion to higher pressures. The ultimate tensile strength, elongation to break and hysteresis loss of a material are useful indicators of the elasticity of an elastomer. Although seals by their very nature are typically used in compression, their elastic properties can result in the development of tensile stresses within the body of the seal when subjected to compression or shear stress.

Typical dynamic applications include pump sealing against a reciprocating or rotating shaft or bore. The compression of the material, combined with shear frictional forces, can result in tensile stresses that exceed the ultimate tensile strength of the material causing a tensile failure. In this case using a large surface area, small particle size filler would be a better choice than the standard reinforcing filler.

Taking the Heat

Heat damage is a common cause of pump seal failure. Running a rotary pump or compressor longer, under greater working demands, creates increased thermal stresses on both the pump and seals.



Although pumps should be designed with sufficient cooling in mind, the heat-soak during shutdown can damage the seal or at least reduce the seal life. For this reason, it should be borne in mind that the temperature of the process is not necessarily what the seal will actually have to withstand. It could be lower or higher depending upon the location of the seal and the effectiveness of the cooling system. Here it is critical that the correct sealing material is selected at the outset.

As a general rule, elastomer temperature resistance varies from +90oC for natural rubber (NR) through to + 330oC for perfluoroelastomers (FFKM), with other elastomers offering temperature resistance in between these values.  If the problem continues, it may be necessary to either modify the design of the seal or change its location. It should also be noted that elastomer physical properties vary with temperature, so a material that has a tensile strength at ambient temperature of 10Mpa, will be significantly weaker at 200oC.

It should be noted that while standardisation to ASTM testing is useful for comparison purposes during seal selection, it is more important for the pump manufacturer to take into account the actual conditions that will be experienced by the seal. How a material performs under laboratory conditions at 70o°C from a data sheet or at 180 Deg °C which is where the seal actually has to perform?

By applying the principles of seal customisation described earlier, new grades of high temperature resistant fluoroelastomers (FKM) have been developed. These exhibit lower compression set and ageing in  high temperature pump applications up to 200oC. With a compression set of just 5% the effect of high temperature ageing is much reduced. Moreover, the FKMs have been engineered such that they retain there their mechanical properties significantly longer than conventional FKM grades, allowing extended seal service life, and increased efficiency.

Low temperature applications of fluoroelastomers has until now been limited to -30oC. By similarly modifying the  polymer compounding  it is now possible for fluoroelastomers to work effectively down to -4051oC.

Using the new FKM grades presents pump engineers, manufacturers and service organisations with the opportunity to increase service intervals, prevent leaks and generally increase the efficiency of their pumps. Moreover, the improved sealing efficiency provided by the FKMs allow pumps to run quieter, an important consideration in enclosed environments. Both fluoroelastomers exhibit the same chemical and mechanical properties as conventional FKM grades.

New developments in elastomer seals offer pump engineers a practical solution to meeting their sealing needs. All that remains is for them to take the initiative and work more closely with seal developers and manufacturers to ‘fine tune’ the performance of the seal, and in so doing enhance the performance of their pumps.


Figure 1: Elastomer seals can be optimised for pump sealing.

Figure 2: Elastomer Properties Table.

Figure 3: Surface cracking – a sure sign that the seal is feeling the hear.

Figure 4: New temperature resistant FKM elastomer seals come in all shapes and sizes.

Precision Polymer Engineering Ltd
Blackburn
Lancs

Can be contacted on

Tel: 01254 295400