Thermal System of a Mechanical Seal

A mechanical seal works under a combination of the various phenomenon. Thermal condition is one of the aspects if avoided can eventually lead to seal failure. Proper monitoring and ways of playing with the thermal nature of mechanical seal can increase the seal life. We will try to know the relation of mechanical seal and heat, its causes, effects and techniques/provisions of handling it.

Let's take first on our list, Seal Interface. Heat can be generated between the seal interface due to many reasons viz. frictional heat by dry running or high drag forces due to the viscosity of the fluid, heated fluid, tribological properties of the seal faces, etc. Some of the causes are neutralized right away if the proper and optimised film thickness is maintained between the seal faces.

If not optimised it is a huge compromise with one of the eminent functions of the seal. Therefore, it is said that the service life of seals working with full film lubrication is more at the expense of leakage. Calculations are made to balance these two properties by making adjustments in force balance based on pressure, face width, design of faces, material to be used for seal faces and mode of sealing. Manufacturers have been experimenting and controlling the coefficient of friction by using special materials and coatings. Diamond-like coating DLC, molybdenum disulphide, Stellite and other seal face coating technologies are incorporated in the design to reduce friction resulting in reduction of frictional heat.

Due to this, the chances of various other outcomes like thermal coning, distortion, waviness, vaporisation of fluid between seal faces are reduced. The stiffness and stability of the fluid are reduced when the interfacial temperature increases. Extreme lower and upper temperature both have their own kind of effect on the fluid handled by the seal making it prone to Coking, Scaling, Crystallization and solidification of the fluid.

Similarly, another issue of a mechanical seal is the heat soak. Heat can propagate from any source. It can propagate from shaft, seal chamber, heated fluid, etc. This heat flow can create rise in temperature of the parts thereby leading to thermal expansion. As we all know that mechanical seal is a critical functioning device in which, slight deflection or change in the dimension of the parts can reflect on the performance of the mechanical seal at scaled up level. Therefore, thermal behaviour of the components is to be considered while designing the seal. Sometimes, due to the thermal expansion of the shaft, special arrangements are to be made in the seal in order to compensate with this working environment.

Since we have seen how the heat management is another criterion for better performance of the seal, various design provisions like creation of cooling holes, provision of hydro pads on seal faces, flushing and quenching ports are some of the techniques used to cool and maintain temperature of the seal faces. Hydro pads are used for both lubrication and cooling between the seal faces. As a result, wear rate, heat generation, seal failure, vaporisation of fluid, is how ever reduced. Flushing ports are created by keeping in mind the flow should be near the interface of the seal faces. Special ports are created over the mating ring in order to maintain its temperature. This cooling fluid is however sealed from all areas so that it doesn't get mixed with other process fluids. Overheating might affect the secondary seals adjacent to it. Another consideration I taken care of that the seal exposed as heat is dissipated to the surrounding.

Studies are done on the behavioural model of heat mechanisms in a mechanical seal. Proven efforts are made to reduce undesirable result in the performance of the seal. Proper prediction and monitoring are necessary while designing any seal.