Centrifugal pumps are hydraulically powered machines that transport energy to fluids (most notably liquids) via the action of a field of centrifugal forces. Their primary function is to move fluids by increasing pressure. Centrifugal pumps come in a variety of shapes and sizes, but its operating principle and fluid dynamic characteristics stay unchanged.
Centrifugal pumps are composed of an impeller that revolves within a casing. The impeller is made up of a set of blades, preferably radial in shape, that deliver kinetic energy to the pumped fluid. Suction and discharge nozzles are provided on the casing for the fluid being pumped. Suction nozzles have an axis that is parallel to the impeller’s rotational axis, but discharge nozzles have an axis that is perpendicular to the impeller’s axis, but still lies on the plane passing through the axis.
The fluid reaches the suction nozzle of the pump via the suction pipe. The fluid must provide sufficient energy to the pump so that the pump can operate on the fluid’s energy, as the pump cannot sucking on the liquid or drawing it into the pump. This is why most of the centrifugal pumps onboard vessels are located on the bottom plates, under sea water level.
A shaft connects the impeller to the shaft. The motor or any other type of driver (pulley, any other kind of transmissions) spins the shaft. The fluid enters the impeller’s eye and becomes caught between the impeller blades. The impeller blades hold the liquid and accelerate it as it passes from the impeller eye to the impeller’s outer diameter. As the fluid accelerates, a zone of low pressure forms in the impeller’s eye (the Bernoulli Principle, as velocity goes up, pressure goes down) and this is one of the reasons why the liquid must have adequate energy to enter the pump.
The liquid exits the impeller’s outer diameter at a high rate of speed (the motor’s speed) and quickly bangs into the volute’s internal casing wall. At this time, the centrifugal velocity of the liquid comes to a standstill and the velocity is changed to pressure (the reverse Bernoulli Principle). There is also rotary velocity present because the motor is spinning. The fluid is directed from the cutwater through an ever-increasing escape channel around the internal volute housing. As the channel lengthens, the rotating velocity drops and the liquid absorbs even more energy and pressure (again, Bernoulli’s Principle). The liquid exits the pump under discharge pressure, ready to overcome the system’s resistance.
The flow rate of a centrifugal pump is largely determined by the driver’s speed and the impeller blade height. The pressure or head that the pump can create is mostly determined by the motor’s speed and the impeller’s diameter. Here down below is an excellent video animation with explanation on how the centrifugal pump works:
Suction pressure is the pressure measured at the suction nozzle of the pump and is most likely the most critical pressure within the pump. The suction pressure determines the output of the pump in its entirety, as the pump transforms suction pressure to discharge pressure and if the suction pressure is insufficient, cavitation occurs. Discharge pressure is the pressure measured at the pump’s discharge nozzle and it is equal to the suction pressure plus the total pressure developed by the pump.
Cavitation occurs when vapor bubbles (hole in the liquid which occupies space inside the pump and affect the pump’s pressure and flow) form and then collapse or implode within the pump, because the liquid’s absolute pressure falls below its vapor pressure. When the vapor bubbles collide with sufficient force , the pump makes the sound of pebbles and rocks moving through it, and they can dislodge metal from the impeller and internal casing wall, leaving impression marks resembling strikes from ball pen hammer.
When cavitation occurs in a pump, its efficiency is reduced, causing sudden surges in flow and pressure at the discharge nozzle and is accompanied by noise and vibration. Cavitation can occur due, generally to the following factors:
- increase of the temperature of the pumped liquid
- a reduction or frequent oscillation in suction pressure
- increase or reduction in the velocity or flow of the fluid due change in the fluid viscosity
- undesirable flow conditions caused by obstructions or sharp elbows in the suction piping
If the pump operates under cavitation conditions for long period of time, the following can occur:
- premature bearing failure
- pitting or erosion marks on the impeller and casing wall of the pump
- premature mechanical seal failure
- failure of the pump shaft and other fatigue failures in the pump components.
For starting and operating centrifugal pumps the following procedure and precautions can generally be applied:
- before starting, the engineer must ensure that the pump volute casing is filled with the liquid to be conveyed by opening the vent plug until the fluid escapes at the vent hole. Some of the centrifugal pumps are self-priming type equipped with priming units driven by the pump itself or self driven. Never start the pump without priming it first.
- make sure that suction filter is cleaned (sea chest filter for sea water in engine room or suction filter for emergency fire pump usually located inside bow thruster room).
- make sure that pressure gauges are functional and calibrated.
- engineer must ensure that suction valves is open. Dry running of the pump can damage its components within minutes.
- adequate suction pressure must be always available for proper function of the pump, especially in case of the ballast pump system. Hydraulic hammer can cause serious damage and must be prevented.
- the pump must be protected against back flow (non-return flaps on the suction and delivery side) and pressure shocks.
- shortly start the motor to check its rotation, especially for newly installed pump or after a major overhaul. If the direction is wrong, change the motor phase connections and try it again.
- start up the pump against the closed delivery valve.
- as soon as the pump commences delivery with its full discharge power (pressure increase on delivery side pressure gauge), slowly open the delivery valve. A centrifugal pump should never be operated continuously at or near the fully closed delivery valve. This normally happens when a tank or vessel is near the maximum capacity and an operator starts closing the discharge valve while the pump is running. All this wasted energy is transferred to the fluid being pumped and this will shorten the life of the pump. This energy is converted into heat and vibration raising the fluid temperature.
- engineers should continuously supervise the operational discharge pressure of the pump
- regularly check the running noise of the bearings. An uniform, purring sound is audible in the event of an unobjectionable bearing contact. Damaged bearings cause a loud, irregular, chattering noise. It is very important that leakage holes at bearing seat must be always open in case mechanical seal is leaking – so that no water can run into the bearings.
- shaft seals must never run dry , not even for short periods. See explanation about it in here. In operation, mechanical gland seals are running free of leakages which means that the low quantity of conveying medium required for lubrication evaporates when escaping from the sealing gap. Supervise the escape of leaking water in the range of the seal or at the checking outlet. If a drop by drop escape of fluid is exceeded, then examine the mechanical gland seal and replace it if necessary.
- the pump should not be operated outside its designed specifications.
Engineer’s proper care and regular service maintenance as per manufacturer’s instruction manual will ensure a long service life of the pump without being necessary to intervene for repair and troubleshooting. Engineers must be familiar with pump’s operation and monitoring and during their regular engine room rounds must ensure that:
- pressure gauges are functional.
- pumps are running within designed parameters.
- priming device is functional if installed.
- there are not abnormal sounds or vibrations during operation.
- the pump is not overheated.
- there are no abnormal leaks present.
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