Elastomer seal changes can be an indicator or potential pump problems.
Modification of pump use can often lead to problems that can damage the pump over time, even though it may not be immediately apparent.
Changes in the pump media, pressure, temperature and even the decision to opt for longer pump maintenance cycles can all affect pump performance. In these cases, physical changes to the pump's elastomer seals can be used as a diagnostic tool to provide early warning of problems.
The most common cause of pump problems is a failure of the shaft seal due to mechanical abrasion and heat damage, caused by the inability of the seal to allow free movement of the rotating shaft. Selecting an elastomer pump seal is based on a number of factors:
- Compatibility of the elastomer with chemicals, solvents and cleaning agents
- Temperature of the liquids
- Pressure
- Liquid characteristics such as viscosity and abrasiveness
- Fugitive emissions requirements
The mechanical stresses on the seal should also be considered. These can occur because of compression and dynamic rotational forces for pump sealing. A poorly fitting seal that has to be either stretched or squeezed into place will accentuate the effect of mechanical stresses, particularly when pump seals come into contact with process chemicals.
Forcing a seal into a groove creates stresses in the elastomer, increasing the risk of stress induced chemical attack (SICA). SICA is a result of stresses that make the polymer chains more vulnerable to chemical attack. As a consequence, the principal stresses that lead to seal failure may be significantly below the expected tensile strength of the material at the pump operating temperature.
Problems with pumps commonly happen when changes occur in the process. This might be a change of solvent or process temperature or the requirement to run the process for longer. The materials being pumped may be more viscous, contain particulates and react with the elastomer chemically, increasing friction and causing more heat to be generated.
Chemical Incompatibility
An early warning sign of chemical/steam incompatibility is extrusion (See Figure 1) arising from seal swelling. Chemical or fluid absorption is the primary cause of the swelling of elastomer seals, with the often-heard phrase ‘like-dissolves-like' providing an important guide.
Figure 1. Extrusion damage to a seal
For example, ethylene-propylene rubber (EPDM) is a non-polar elastomer. As such, it should not be used to seal a non-polar solvent such as hexane. However, EPDM can be used to seal against polar fluids, such as water.
Signs of chemical attack, on the other hand, are a hardening or softening of the seal. In extreme cases, the plasticisers and other process aids used to manufacture the elastomer may leach out producing an overall volume loss and less flexible seal.
Seal hardening or softening is the result of chemical attack that is progressively either increasing the cross-linking (vulcanization) of the polymer to produce a more rigid seal or unzipping the polymer's cross-links to produce a softer seal, as is the case with some grades of fluoropolymers (FKMs). Common FKM materials are the well-known A and B grades.
A-type fluoroelastomers are mostly cured by using a condensation reaction in which water is generated during the cure process. However when exposed to steam environments, the cure can be reversed, breaking down the cross links of the material. This can lead to premature failure.
An early sign of FKM incompatibility is surface cracking. Initially, this may seem as though the seal has become brittle. The seal surface cracks are not due to hardening but to a reduction in tensile strength of the elastomer (See Figure 2). It follows that in applications in which seals are likely to be exposed to steam, peroxide-cured fluoroelastomers should be used.
Figure 2. Chemical-attack surface cracking
Heat
Thermal damage is a common cause of pump seal failure (Figure 3). As a general rule, elastomer temperature resistance ranges from more than194 degrees F (90 degrees C) for natural rubber (NR) through more than 626 degrees F (330 degrees C) for perfluoroelastomers (FFKM). Other elastomers offer temperature resistances between these values.
Running a rotary pump longer, under greater working demands, creates increased thermal stresses on both the pump and seals. Localized heat generated due to friction expands the seal, increasing the sealing force which, in turn, further increases friction; temperature at contact faces in rotary pumps can easily exceed 752 degrees F (400 degrees C), especially with lubricating media. Signs of thermal attack are an increase in seal hardness. The seal will take on a ‘compression set', where the seal does not recover when it is removed from the application.
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.
It should also be noted that elastomer physical properties vary with temperature, so a material that has a tensile strength at ambient temperature of 1,450 psi, will be significantly weaker at more than 392 degrees F (200 degrees C).
For situations in which seals are continuously exposed to high temperatures, there must be sufficient free space in the seal groove to allow for thermal expansion of the seal. The typical coefficient of thermal expansion for an elastomer is 100 times that of its metal gland.
Failure to take into account elastomer expansion leads to extrusion of the elastomer from the groove. In extreme cases when extrusion isn't possible, the force generated by the elastomer will damage the pump. When extrusion occurs, it is best to seek professional assistance before changing the groove/elastomer profile.
Thermal attack is often considered to be a high temperature phenomenon, but the same problems can arise at low temperatures. The low temperature resistance of an elastomer is dependent upon its glass transition temperature, Tg. This is the point at which the elastomer changes from being a rubber-like material to a brittle material. (See Figure 3) An elastomer's low temperature performance is often overlooked when sealing problems occur.
Figure 3. Thermal embrittlement damage to a seal
Pressure
Sudden changes in pump pressure can occur in two cases: when a seal fails and when the pump stops unexpectedly. In both instances, the seal should be inspected for damage arising from explosive decompression or rapid gas decompression (RGD). RGD occurs when liquid permeates into an elastomer under high pressure. If the pressure is suddenly released, the liquid is vaporized, expanding and rupturing the elastomer. Signs of RGD damage are splits and blisters (See Figure 4).
Figure 4. Rapid Gas Decompression L-ring seal failure
A single incidence of RGD may result in catastrophic seal failure. Repeated RGD certainly will. Any incidence of explosive decompression, whether due to seal or pump failure, should be investigated immediately and action taken to either reduce the pump pressure or change the elastomer to a RGD resistant grade.
This article has discussed some of the ways in which pump seals can be used as part of the maintenance engineer's repertoire. By responding to the early signs of seal damage and taking preventative measures, unexpected pump failures can be avoided.
Pumps & Systems, April 2011
Figure 1. Extrusion damage to a seal