Mechanical Seals with Fluilift Technology (“Gas Lift-Off”): Ensuring a Perfect Seal on Process Fluids with Minimal Energy Consumption and Zero Atmospheric Emissions
Introduction
The mechanical seal serves to contain the process fluid in machines with rotating shafts. In the case of a single seal, one side of the seal faces the process fluid, while the other side is exposed to the atmosphere. A pressurized double seal, however, consists of two mechanical seals with an intermediate chamber between them. One side is in contact with the process fluid, while the intermediate chamber contains a pressurized fluid (either liquid or gas) known as the barrier fluid, and the other side faces the atmosphere.
In the realm of pressurized double seals, two types are possible depending on the technology used:
- Pressurized Double Seal ‘Contacting Wet’ with liquid barrier fluid
- Pressurized Double Seal ‘Non-Contacting’ with gas barrier fluid
Pressurized Double Seals ‘Contacting Wet (CW)
This type of mechanical seal is used in process pumps and agitators. A liquid film between the rotating and stationary rings must be maintained to provide cooling and lubrication to the rings.
The heat generated by friction between the seal ring faces must be removed via external flushing to maintain proper flatness and ensure that the lubricating film remains in a liquid phase. The API Plan 53B is an external flushing system designed to supply the barrier fluid to the seal. Plan 53B operates using a diaphragm accumulator connected to the double seal. The barrier fluid circulates thanks to a pumping device typically connected to the rotating seal, passing through a water or air heat exchanger to dissipate the heat generated by the system. The cooling water used to remove the heat produced by the seal can consume significant amounts of energy.
Pressurized Double Seals with Gas ‘Non-Contacting (NC)’
The use of pressurized double seals with gas, typically nitrogen, is designed for a fundamentally different operating concept. Contact between the rotating and stationary rings is avoided during operation due to the barrier gas, which enters specially sized grooves on the surface of the rotating ring, creating a separating effect between the two rings. The leakage of the barrier gas is minimal, and continuous flushing to the seal is not required. Under these conditions, the friction generated is negligible and significantly lower than that of a traditional seal with a liquid film. The absence of overheating prevents ring deformation, providing substantial performance benefits.
MINIMAL POWER CONSUMPTION, MINIMAL HEAT GENERATION – MAXIMUM ENERGY EFFICIENCY
The power consumption generated by the viscous friction of the gas between the seal rings is negligible, keeping the temperature increase of the rings within 5°C. This condition provides a significant energy advantage for two main reasons:
- Reduction in power consumption by the seal
- Elimination of cooling through auxiliary systems required by traditional seals
The API Plan 74 is a system designed to ensure the presence of barrier gas between the seal faces. Plan 74 allows for the supply of barrier gas from an external source through an instrumented panel that monitors the necessary parameters for the correct operation of the double seal, namely the pressure and flow rate of the barrier gas. The barrier gas pressure must be higher than the process fluid pressure; otherwise, an alarm will be triggered. Conversely, an excessive flow rate would indicate a seal leak.
Ring with Hydrodynamic Grooves
The following graph shows theoretical values of the power absorbed due to friction between the seal faces, comparing a double pressurized CW seal and a double pressurized gas NC seal, with values based on the seal diameter.
ENERGY BALANCE DIAGRAM IN A CENTRIFUGAL PUMP
- The diagram does not represent other energy losses due to various frictions in the pump-motor system.
- The diagram does not account for the thermal energy transmitted by the pumped fluid (heat soak) and dissipated by the machine.
Conclusions
In conclusion, the use of pressurized double seals with gas, such as the Fluiten LL6D and LL8F models, although requiring careful consideration during the selection phase, offer compelling advantages over traditional seals where installation is possible, given the low power requirements of the flushing system, reduced barrier leakages of barrier gas and zero process emissions into the atmosphere
