A centrifugal pump transfers input power to kinetic energy of the fluid being pumped. This energy, through the specifics of the pump design, is converted to pressure energy that causes the fluid to flow. The most common type of centrifugal pump is termed the “volute pump”. In this type of pump, fluid enters the pump at the center of a rotating impeller. The rotating impeller causes a rapid radial acceleration of the fluid from the eye of the impeller to the pumps circumferential casing. This creates a vacuum at the center of the impeller, resulting in continual entry of more process fluid. The fluid exits the pump through a discharge port located at the outer perimeter of the casing.
The energy from the pumps motor is converted to kinetic energy of the fluid according to the Bernoulli equation. The energy ultimately transferred to the liquid depends on many factors, including the diameter of the impeller and the speed at which the motor turns the impeller. The larger the impeller diameter and the faster it rotates; the more kinetic energy is transferred to the liquid. This relationship is described by the Affinity laws.
Pressure and Head
Imagine that the discharge of the pump points straight up as in the diagram above. Then, depending on the energy of the fluid at the discharge, or how much kinetic energy it has, the fluid will rise a certain distance into the air and is measured in meters or feet and can be converted into common pressure units. This height is known as head – and the height equal to the height that the fluid rose is called the shut off head. This maximum head is primarily determined by the speed of the impeller rotation and its diameter. Head is variable and will change as the capacity of the pump is altered.
The kinetic energy of the fluid as it exits the impeller is high, but as soon as the fluid encounters an obstruction, or resistance to flow, the fluid slows down, converting its kinetic energy into pressure energy. The main resistance that the fluid encounters is the pump casing. It is this resistance to flow that is read as a pressure on a pressure gauge attached to the discharge line of a pump. Confusingly though, a pump cannot create pressure, it can only create flow. The pressure is a measure of the resistance to this flow.
Head is the term used to designate a pump’s energy rather than pressure, because the pressure is dependent on the specific gravity of the fluid being pumped, whereas head is an inherent characteristic of the pump. Remember, head is the height that a column of fluid would rise from the pump’s discharge port due to the kinetic energy transfer from pump to fluid as the fluid traverses the pump. As long as the fluid being pumped is Newtonian (fluids whose shear strain responds linearly to shear stress), the pump’s performance is best described by the term head.
Different Types of Pump Head
Total Static Head – Total head when the pump is not running
Total Dynamic Head – Total head when the pump is running
Static Suction Head – Head on the suction side, with the pump off, if the discharge is higher than the pump’s impeller
Static Suction Lift – Head on the suction side, with the pump off, if the discharge is lower than the pump’s impeller
Dynamic Suction Head/lift – Head on the suction side of the pump with the pump running
Dynamic Discharge Head – Head on the discharge side of the pump with the pump running
Because head is independent of a liquid’s specific gravity, a pump will pump all liquids to the same head, as long as the impeller is rotating at the same rpm. The higher the specific gravity of a liquid, the more input energy will have to be applied to the impeller so that it can reach the same rpm as that when it is pumping a liquid with a low specific gravity. This concept can be best described by thinking of pumps as constant head machines. They are not constant pressure machines as the pressure is a function of both head and the density (specific gravity) of the liquid being pumped. The head of a pump can be expressed as: (in metric units)
where:
= total head developed (m)
= pressure at the pump’s discharge (N/m2)
= pressure at the pump’s suction (N/m2)
= density (kg/m3)
= acceleration of gravity (9.81 m/s2)
= velocity at the outlet (m/s)
The head of a pump can be described in simple terms. It is the vertical lift – in feet or meters – of water at which a pump can no longer exert enough force to lift the water. This height is known as the pump’s shut-off head. The shut off head, in a pump curve that plots head versus flow, is the point on the curve where the flow is 0.
Pump Efficiency
The pump efficiency is a measure of the fraction (in %) of useful work that the pump transfers to the process fluid. It can be calculated as a fraction where the power input is the numerator and the power output is in the denominator. This equation will yield the pump’s efficiency, as a fraction, usually indicated with an n.
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