Q - How frequent should the lube oil in the bearing housing be changed?
A - A typical recommendation from a pump vendor is to replace the oil in the bearing housing once a year for light service, once every six months in medium service, or once every three months in heavy (or severe) service.
For this purpose, a light service may be defined as the continuous, or intermittent, operation of small size (or low horsepower) pump, at low running speed (4-pole motor speed, or less), in low operating temperature. A heavy service may be defined as the continuous operation of large size (or high horsepower) pump, at high speed (2-pole speed, or faster) in ambient, or high operating temperature. Those in between may be considered to be in medium service.
The problem with this approach is that there is substantial overlap in what may be considered as light, medium, or heavy service; the decision when to change oil becomes subjective. This approach is also too simplistic and ignores the thrust loads on the bearings, and the effects of worn-out running clearances, and that of the rotor that can go out of balance.
The ideal solution is to implement a monitoring program whereby oil changes are done at regular intervals, say once every three or six months initially, and carefully monitoring for premature oil discoloration, or signs of viscosity breakdown, and then adjusting the oil change interval accordingly. The need for more frequent oil changes is an early indication of potential trouble, and the pump can then be scheduled for preventive maintenance to avoid costly breakdown.
When doing oil change, drain the bearing housing completely, flush with suitable solvent, and refill with new oil. Use recommended lubricants only, and maintain correct oil level - never overfill.