Bearing and mechanical seal application in pumps

Centrifugal pumps require bearings to carry radial/axial loads. Bearing alternatives:  Anti-friction, hydrodynamic ring oil lubricated or hydrodynamic pressure lubricated. Oil lubricated (anti-friction) bearings are used in majority of process pumps to carry loads

• Pressure lubricated hydrodynamic bearings are normally used for high pressure, high horsepower, high speed applications

• Criteria in API Standard 610 for pressure lubrication: when product of pump rated horsepower and rated speed in rev/min is greater than 2.0 million
• Pressure lubrication systems can either be integral or separate, but should include, as a minimum, an oil pump, reservoir, filter, cooler, controls and instrumentation
• Ring oil lubrication may be applied to hydrodynamic journal bearings in less severe service (when dN factor is less than 300,000. A dN factor is the product of bearing, size (bore) in millimeters and the rated speed in revolutions per minute).

Mechanical seals are often used in pumps handling hazardous as well as non- hazardous liquids that must be contained within the unit.

• The single seal arrangement is most widely used in process industry.
• Single seal design consists of a rotary face in contact with a stationary face.
• For most services, a carbon face mating against tungsten carbide is satisfactory
• Seals offer the advantages of long life, low maintenance and high reliability.
• In general, seals handling light specific gravity liquids at low temperature and high  vapor pressure give most problems in the field.
• Materials for cold service seals must be suitable for temperatures of start-up, cool down and running ; the atmospheric side must be held above 0⁰C (32⁰F) to prevent ice formation; and there must be enough liquid at the seal surfaces
• Successful operation of any seal depends largely on correctly specifying liquid conditions of vapor pressure, temperature specific gravity, etc.
• API Standard 682 is an excellent resource for overall mechanical seal application guidelines.
• The pressure in the seal chamber (stuffing box) must be at least 25psig above the pump suction pressure.