Mechanical seals and packing are used to seal leakage along a rotating shaft; bearings are used to support radial and axial loads. Both are critical components that must be properly designed for the application to maximize operating reliably and efficiently. However, both are common failure points in pumps when they are not installed properly, when the pump is not operated properly and when support systems providing lubrication and cooling are not maintained.

Mechanical seals have two flat and smooth faces that contact axially to seal the process fluid from leaking to the atmosphere along the rotating shaft. To provide a seal, there is a contacting force that will generate heat due to friction and consume power. The seal faces are lubricated by the process fluid or another conditioned fluid, and along with the flat and smooth faces, they limit frictional power losses. Some leakage does occur with mechanical seals, but it is often not visible, with the liquid vaporizing as it exits the seal face to the atmosphere. For low-power pumps, the seal power requirement can be a significant portion of the pump input power, but for larger pumps, the seal power consumption is typically a small percent of pump input power. Improper selection, support systems or operations such as running dry or outside acceptable pressure and temperature limits will lead to excessive heat generation, leakage and premature failure.

Compression packing is made up of woven materials with good lubrication properties that are cut into rings. They wrap around the shaft and are compressed in a packing gland so the rings compress radially along the shaft. Typically, the shaft area is hardened where the packing contacts radially. Like mechanical seals, packing consumes power due to friction and requires lubrication and cooling from the process fluid or another conditioned fluid. As they are commonly used in water, wastewater and slurry applications, visible and controlled leakage is required to provide the proper cooling and lubrication, which is collected in a drain and diverted away. Improper adjustment (compression) can either increase energy losses and shaft wear (overtightening) or result in excessive leakage (undertightening). For low-power pumps, if compression packing is overtightened, the additional frictional power could overload the motor.

Bearings support the pump shaft and manage radial and axial loads. Bearings serve a different purpose than shaft seals, but similarly, they require lubrication and cooling to remove power loss due to friction. The amount of power loss for bearings depends on the load and bearing design. When pumps operate away from their best efficiency point (BEP), hydraulic forces often increase, leading to higher bearing loads and frictional power losses. From a reliability standpoint, the bearings will be designed to operate for a minimum design life within the pump’s allowable operating region (AOR), but operating outside AOR can overload the bearings, causing premature failure. Additionally, if lubrication is not sufficient or is contaminated, frictional power losses will increase and premature failure will occur.

From a system optimization perspective, these components perform best when installed per the manufacturer’s requirements, when lubrication is maintained and when the pump operates within its preferred operating region (POR). This minimizes loads, power losses and failure modes.

Refer to the Hydraulic Institute (HI)’s Introductory Pump System Training and American National Standards Institute (ANSI)/HI 9.6.3 Rotodynamic Pump Guidelines at pumps.org.