Q. Is it true that centrifugal pumps should be built with ball bearings that have greater than normal internal clearance?
A. It depends on the pump type and the conditions of service for the pump.
Bearing life depends on the operating speed and load, or force, on the bearings and factors such as good lubrication and minimum contamination from external solids or acids. The load on the bearing balls is usually a combination of the external force on the shaft and the internal force produced by the fit or clearance between the inner and outer raceways.
Ball bearings are built with a small clearance between the balls and the raceway. For example, a ball bearing with inside diameter of 50-mm has a normal clearance between the ball and raceway of between 8-μm and 28-μm (micrometers). When mounted on a shaft, an interference fit between the shaft and bearing inside diameter is usually used. This causes the bearing inner raceway to expand, which reduces or eliminates the original ball clearance. The elimination of the ball clearance eliminates any looseness or radial movement of the pump shaft, which maintains the close alignment good for proper operation of the pump seal.
This arrangement works well unless the pump is operating on high temperature liquids, like 750-deg F. At such temperatures, the shaft and inner race of the bearing expand further, putting an additional load on the bearing balls. At some point the additional load on the bearing will significantly reduce its life.
When this happens, a bearing with greater than normal internal clearance is appropriately used to compensate for the high temperature expansion of the shaft and bearing inner raceway. All bearing manufacturers produce a bearing with greater than normal clearance, which is identified as a C3 fit bearing. The same 50-mm bearing will then have an internal clearance of 23-μm to 43-μm, which will be reduced as the shaft and bearing expand.
Bearings with additional internal clearance can also be obtained. These are designated as C4 and C5 bearings. However, excessive internal clearance in the bearing could allow excessive radial movement at the shaft seal, reducing seal life and causing leakage.
Some pump users specify C3 fit as standard, but this allows for some radial movement of the shaft at ambient temperatures and may lead to some shaft seal problems.
Follow the OEM's recommendation regarding replacement bearings and internal clearances.
Q. I read that pumps with high suction energy level are more susceptible to cavitation damage than low suction energy level. Is this true and how is high suction energy defined?
A. Yes, pumps with high suction energy level are generally more likely to suffer from cavitation damage. However, there are many factors that contribute to this and no precise definition has been established. These factors typically include:
- The peripheral velocity at the outside diameter of the impeller eye. Values above approximately 120-ft/sec are considered high suction energy.
- The suction speed S of the pump. S = nQ2/(NPSH)0.75 where n = rpm, Q = gpm and NPSH = feet. Values above about 12,000 are considered high energy.
- The specific gravity of the liquid pumped. The higher the level, the higher the suction energy.
- Thermodynamic properties of the liquid. Cold water is one of the highest energy liquids, followed by high temperature water and hydrocarbons.
- The geometry of the pump inlet. Side suction pumps are considered higher suction energy than end suction.
- The overlap of the impeller vanes. Impellers with two or three vanes have higher suction energy than four or more vanes.
- The incidence angle between the inlet impeller vanes and the approaching liquid. The greater the angle, the greater the turbulence and suction energy level. This value may have to be obtained from the pump manufacturer.
- The geometry of the inlet piping to the pump. Piping turns and pipe size changes add to the suction energy of the pump.
- Operation away from the best efficiency point (BEP) of the pump. At reduced rate of flow, the pump may operate in its suction recirculation region, which considerably increases suction energy.
This is a complex situation and a single equation or relationship has not been developed which will accurately tie all of these factors together. Manufacturers can often provide installation lists of pumps that are operating successfully.
Q. What is the purpose of putting vacuum valves or vacuum breakers in pump discharge piping? How does this affect pump performance in a pump handling ambient temperature water?
A. The primary purpose for vacuum breakers is to minimize the risk of damage from water hammer. This is especially true when pumps are installed above an open source and discharging through pipe to a higher level. When the pump is stopped, the water in the discharge pipe may drain back to the source. Even with a check valve in the discharge pipe, the check may leak and the water in the pipe can slowly drain out.
When this happens, the falling water level in the discharge creates a partial vacuum which becomes filled with water vapor. When the pump is restarted, it quickly fills the empty pipe with water until the water meets the first obstacle in the pipe system. At that time, the rush of water is abruptly stopped and the power of the rushing water is converted to pressure energy, causing a high spike in pressure called water hammer. This pressure spike can and often does cause damage to the system. A longer discharge pipe will have a larger volume of moving water and result in a higher pressure spike.
If air is allowed into the pipe, it will cushion the flow of water and slow it down more gradually as the pressure builds. Water hammer and damage will be avoided. Vacuum release valves accomplish this. The basic design includes a float device, which is lowered when the water in the pipe is drained away. Air is then allowed into the pipe and damage is avoided.