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Turbine pumps typically are used in low flow, high-pressure applications. The high pressure is created by the rotating impeller. As the water passes from the suction of the pump to the discharge, it continuously circulates around each blade of the impeller in a helical path -- the axis of which coincides with the water channel. Each blade of the impeller imparts additional energy to the water building up the pressure as the water is propelled to the discharge of the pump. The water acquires its discharge pressure in less than one revolution around the impeller.
While the turbine can be more efficient for low flow, high-pressure applications than a comparably-sized centrifugal, its design requires very close tolerances between the impeller blades and the pump case to achieve this efficiency. Consequently, any debris, even microscopic material in high enough concentrations can damage the pump. Therefore, it is critical that no debris in the system (e.g., pipes, radiators, receivers, etc.) that is larger than 25 microns be allowed to pass through the pump. Large particles, weld spatter, and other material typically found in new piping systems will likely bend the impeller vanes and can sometimes lock up the pump.
Therefore, suction strainers are typically used with turbine pumps to help screen out debris. This is in contrast to centrifugal pumps where suction strainer are not recommended nor provided. Another drawback of the turbine pumps is that they typically require high maintenance because of the close tolerance design. Turbines also often produce a high pitch whine sound that increases in intensity as the differential pressure produced in the pump increases.
On initial startup, the chambers inside the pump must be filled with water before the motor is turned on. These chambers include the suction and discharge cavities of the pump, along with the area in and around the impeller up to the mechanical seal. As water fills the pump suction housing, the air inside the pump is pushed out through the discharge.
The pump is turned on by an electrical signal sent by the pump controller. The type of pump controller depends on the particular application. For example, on a condensate unit the controller is typically either a float switch or mechanical alternator. On a boiler feed unit, the controller typically is the water level controller on the boiler.
The mechanical seal (located on the motor shaft between the impeller and pump head) is designed to prevent water from seeping into and damaging the motor. The water slinger (a rubber disk that rotates on the motor shaft) provides additional protection to the motor if small amounts of water should leak through the pump head.
The suction pressure created by the weight of the body of the water (typically in a receiver attached to the pump) pushes the water through the pump suction housing and into the eye of the impeller. The water flows enter the pump through a suction port in the raceway channel and is directed to the rotating impeller by means of channels in the raceway. (Note: the direction of rotation can be either clockwise or counter-clockwise depending on the manufacturer.)
The water is picked up by the blades of the rotating impeller and is literally pushed in a spiraling motion through the raceway channel. Each impeller blade imparts additional energy to the water as it is propelled to the raceway cutoff. Different designs of impellers are used on the pump to create the required discharge pressure for the specified flow capacity (i.e., expressed in gallon per minute).
The energized fluid is then discharged through the raceway by means of a cut-off (i.e., water block) that is built into the raceway. The discharge is directed into a built-in vortex pocket in the raceway that separates air from the liquid being pumped.
The pressure on water in the suction housing must exceed water vapor pressure to prevent cavitation (or steam bubbles) from forming within the eye of the impeller, the point were pressure is lowest.
If steam bubbles form, they are swept along the vanes of the impeller. At some point the pressure upon the water once will again exceed the water vapor pressure causing the steam bubbles to implode (or collapse) which is known as cavitation. These implosions sound like marbles or rocks rattling within the pump. Also commonly known as water hammer. The force of shockwaves created by implosions WILL DAMAGE THE PUMP!
After the pump has satisfied the operating conditions of the pump controller, the controller sends an electrical signal to turn off the pump motor.
When the motor is turned off, the pull (or flow) of water generated by the impeller stops. The pump case should remain full of water and therefore is primed for its next cycle. The cycle will then repeat.