Monday, October 1, 2012

Basic Pump Principles

Pumps are used to transfer liquids from low-pressure zones to highpressure zones:

Without a pump in this system, the liquid would move in the opposite direction because of the pressure differential. Pumps are also used to move liquids from a low elevation into a higher elevation, and to move liquids from one place to another. Pumps are also used to accelerate liquids through pipes.

How do pumps work?

The fluid arrives at the pump suction nozzle as it flows through the suction piping. The fluid must be available to the pump with sufficient energy so that the pump can work with the fluid’s energy. The pump cannot suck on or draw the liquid into the pump.

Positive displacement (I’D) pumps take the fluid at the suction nozzle and physically capture and contain the fluid in some kind of moveable enclosure. The enclosure may be a housing with a pulsing diaphragm, or between the teeth of rotating gears. There are many designs. The moveable enclosure expands and generates a low pressure zone, to take the fluid into the pump. The captured fluid is physically transported through the pump from the suction nozzle to the discharge nozzle. Inside the pump, the expanded moveable enclosure then contracts or the available space compresses. This generates a zone of high pressure inside the pump, and the fluid is expelled into the discharge piping, prepared to overcome the resistance or pressure in the system. The flow that a PD pump can generate is mostly a function of the size of the pump housing, the speed of the motor or driver, and the tolerances between the parts in relative motion. The pressure or head that a PD pump can develop is mostly a function of the thickness of the casing and the tolerances, and the strength of the pump components. 

As the pump performs its duty over time, and fluid passes through the pump, erosion and abrasive action will cause the close tolerance parts to wear. These parts may be piston rings, reciprocating rod seals, a flexing diaphragm, or meshed gear teeth. As these parts wear, the pump will lose its efficiency and ability to pump. These worn parts must be changed with a degree of frequency based on time and the abrasive and  ubricating nature of the fluid. Changing these parts should not beviewed as breakdown maintenance. Nothing is broken. This periodic servicing is actually a production function to return the pump to its best or original efficiency. 

Centrifugal pumps also require that the fluid be available to the pump’s suction nozzle with sufficient energy. Centrifugal pumps cannot suck or draw the liquid into the pump housing. The principal pumping unit of a centrifugal pump is the volute and impeller.

The impeller is attached to a shaft. The shaft spins and is powered by the motor or driver. We use the term driver because some pumps are attached to pulleys or transmissions. The fluid enters into the eye of the impeller and is trapped between the impeller blades. The impeller blades contain the liquid and impart speed to the liquid as it passes from the impeller eye toward the outside diameter of the impeller. As the fluid accelerates in velocity, a zone of low pressure is created in the eye of the impeller (the Bernoulli Principle, as velocity goes up, pressure goes down). This is another reason the liquid must enter into the pump with sufficient energy.

The liquid leaves the outside diameter of the impeller at a high rate of speed (the speed of the motor) and immediately slams into the internal casing wall of the volute. At this point the liquid’s centrifugal velocity comes to an abrupt halt and the velocity is converted into pressure (the Bernoulli Principle in reverse). Because the motor is spinning, there is also rotary velocity. The fluid is conducted from the cutwater around the internal volute housing in an ever-increasing escape channel. As the pathway increases, the rotary velocity decreases and even more energy and pressure is added to the liquid (again Bernoulli’s Principle). The liquid leaves the pump at discharge pressure, prepared to overcome the resistance in the system. 

The flow from a centrihgal pump is mostly governed by the speed of the driver and the height of the impeller blades. The pressure or head that the pump can generate is mostly governed by the speed of the motor and the diameter of the impeller. Other factors play a lesser role in the pump’s flow and pressure, like the number, pitch, and thickness of the impeller blades, the internal clearances, and the presence and condition of the wear bands.

In simple terms, we could say that PD pumps perform work bymanipulating the available space inside the pump. Centrihgal pumps perform work by manipulating the velocity of the fluid as it moves through the pump.


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