Q. What information is available regarding bearing minimum loads for pumps? A. All rolling element bearings require a minimum applied load. This ensures that normal elasto-hydrodynamic operation occurs. If too small a load is applied, then the bearing rolling elements (balls or rollers) can skid as opposed to rolling. When skidding occurs, wiping of the lubrication film typically results because of the less favorable surface motion. This can cause metal-to-metal contact and reduction of bearing life. The minimum load requirements vary depending on the bearing design. As a general rule, the larger the contact area between the rolling element and the raceway, the greater the minimum load required. Therefore, a cylindrical roller bearing (which has a line contact) has a higher minimum load requirement than a deep-groove ball bearing (which has a point contact). Variations in bearing internal geometry, tolerances and surface finish can also affect the minimum load requirements to a lesser extent. Rolling element bearing manufacturers normally provide data tables or equations to allow the designer to calculate the minimum load required. The practical consequence of bearing minimum load requirements is that if the result of the L10 life calculation of a bearing is very high, the bearing may fail prematurely because of insufficient bearing load to prevent skidding. This stands in contradiction to the tendency of users to specify high L10 life requirements. As a general guide, it is recommended that bearing life requirements not be set higher than 100,000 hours under the worst-case combination of loading. Pumps with single volutes have a large variation between the hydraulic radial thrust at minimum flow compared to BEP flow. There is a considerable risk that in designing for a high L10 life at minimum flow, the minimum load requirements are not met at BEP. These pumps should always be checked against the minimum bearing load requirements. If the bearing minimum load requirement cannot be met, the designer can either select a bearing with a lower minimum load requirement or determine a way to increase the load on the bearing. For example, the bearing load can be increased by changing the pump hydraulic design (increased hydraulic load) or by applying mechanical loading (often using springs to generate preload). For more information on applications for rotodynamic pumps, see ANSI/HI 1.3 Rotodynamic Centrifugal Pumps for Design and Application. Q. As an alternative to mechanical seals, what kinds of stuffing box configurations are available for rotodynamic vertical pumps? A. The two most common packed stuffing box configurations for vertical pumps are those with and without lantern rings. Both arrangements have a bushing below the packing. The two figures show these two constructions: The construction in Figure 2.3.3.7.4a is used when the pump discharge pressure is not high and when pumped fluid is clean and its leakage to atmosphere is acceptable.
Figure 2.3.3.7.4a. Stuffing box for low to intermediate pressure service
Figure 2.3.3.7.4b. Stuffing box with injection- Low viscosity – In most fluids, viscosity decreases with increasing temperature. Maximum allowable pump or fluid temperature set point can prevent a pump from operating beyond minimum allowable conditions.
- Low volumetric efficiency – When internal recirculation or slip in a rotary pump approaches 50 percent of its displacement (typically from low-speed operation, internal wear or both), the pump differential temperature will not stabilize, and the pumped liquid temperature can exceed maximum allowable pump or fluid temperature.
- Vapor pressure – Vapor pressure increases with increasing temperature. For volatile fluids, increasing temperature may cause the pumped fluid to vaporize in the pump inlet and cause the pump to cavitate.
- Relief valve, system recirculation or bypass valve open – Rotary pumps require relief valves for overpressure protection. Recirculation valves are used to maintain fluid system pressure. Bypass valves are often used to control flow to the system. If the relief valve or bypass valve is partially or fully open, then the temperature of the exhaust increases proportionately to the oil horsepower bypassed. If the exhaust recirculates to the inlet side of the pump, then the pumped liquid temperature can quickly exceed the maximum allowable pump or fluid temperature, resulting in catastrophic pump failure.
- Dry run – With no liquid to dissipate heat, temperature will rise rapidly. Pump failure is likely.
