Lubrication: general considerations

Compared with fluid-lubricated mechanical seals, unlubricated, dry-running seals have a much more restricted range of duty conditions (seal face velocities, pressures, product temperatures). they are subject to high wear, and the heat generated can be dissipated only to a limited extent. They can only be used for ordinary sealing requirements.

Lubricated mechanical seals are classified by lubricant: gas, product, or buffer fluid.

Gas-lubricated mechanical seals require gas pressure in the seal gap that causes the seal rings to part. This can be achieved by pressurization or, in the case of high velocities, with a suitable seal face design. Though the gas cushion provides contactless sealing, it also permits rather high and unpredictable leakage.

Product-lubricated mechanical seals can be used only under the following preconditions:

• Agitator turns only when the seal is effectively covered by product.
• Process parameters such as product temperature and viscosity and operating pressure are conducive to lubrication (liquids).
• Contained solids do not destroy the seal rings.
• Product cools the seal rings adequately.

 
This restricts their application to »submerged« mounting positions and makes proper sealing subject to the momentary filling level.

Normal agitator duty conditions (shaft speed, pressure, temperature) would overtax a dry-running seal but are not demanding enough to justify dynamic gas lubrication. Pressurization with gas is advantageous only in certain exceptional cases. The most common agitator mounting position, on top, precludes product lubrication. For these reasons, most agitator mechanical seals are operated with buffer fluid lubrication.

Seals lubricated with buffer fluid are filled with a suitable fluid, but one that is essentially freely selectable. Consequently, no duty preconditions (like those above) are imposed. Because the buffer fluid is usually circulated in a closed loop, it can be cooled as required. Significant differences arise, however, depending on the way the system is operated.

Pressureless buffer fluid
A pressureless buffer fluid reservoir makes refilling easy. the buffer liquid lubricates the seal faces by capillary action up to a counterpressure of about 7 bar (abs).

The fact that any leaked product enters the buffer fluid directly eliminates the possibility of unpleasant odors, environmental pollution, or injury to operators. Leaked product is removed simply by changing the buffer fluid, thus avoiding unsightly spillage or encrustation of leaked product on the impeller.

Pressurized buffer fluid
A suitable hydraulic component is used to set the buffer fluid pressure to a level higher than the operating pressure in the agitated vessel. This prevents product from penetrating into the mechanical seal. it also permits the use of materials not resistant to the product on the atmosphere side.

If pressurization is maintained at a constant level, the seal and the buffer fluid system have to be designed for the maximum vessel pressure for safety reasons. Inevitably, therefore, the normal operating conditions have negative effects on the mechanical seal with regard to wear, buffer fluid consumption, heat generation, and energy consumption.

By comparison, the use of a variable buffer fluid with its pressure always set to be slightly higher than the momentary vessel pressure, provides optimal safety, wear, and heat generation conditions. It shifts the pressure loading to the seal pair on the atmosphere side and relieves this pair of exposure to heat from the hot product - which now acts on the seal pair on the vessel side. this variable pressure system is achieved by employing suitable pressure boosters.

Thermosiphon effect
The mechanical seal and the necessary hydraulic components are incorporated in a circulation loop for the buffer fluid. The friction generated in the seal heats the fluid. This reduces the fluid's density, which creates natural circulation through the cooler (installed above the seal) and through a separate return line back to the buffer fluid inlet on the seal. the recirculation flow rate depends on the fluid\'s viscosity. If the fluid used has the right viscosity and the cooling surfaces and system tubing are adequately dimensioned, effective self-regulation cooling of the seal will be ensured by the thermosiphon effect.

Forced circulation
In cases where the volumetric flows produces by the thermosiphon effect just described are not sufficient to support the radiation- and convection-based dissipation of heat from the seal housing, it becomes necessary to apply forced circulation in the buffer fluid loop for external cooling.

To achieve forced circulation, it is often possible to integrate a pump impeller into the seal whose speed depends on the agitator speed. Because the amount of heat generated is also a function of the agitator speed, optimal operation is ensured. Besides, this arrangement makes use of the existing housing, which keeps costs down in cases involving high pressures.

If design constrictions preclude the integrating of a pump impeller, or if such an impeller would be rendered ineffective by low speeds at high pressures or high buffer fluid viscosity, it is necessary to install a stand-alone pump. In this case, the pumping and sealing functions can be fine-tuned separately.