PUMPS I. Practical definitions.

The use of pumps requires knowledge of practical definitions. Of essential aspects such as capacity or flow rate, head, efficiency and power absorbed.

• The capacity or flow rate of a pump is referred to the volume of water pumped per unit of time. It is actually the transfer flow rate and is measured in litres per second (l/s) or cubic metres per hour (m3/h).
• The height expresses the vertical distance between two reference points. It should be noted that the head is not the vertical distance between the level of the liquid to be pumped and the discharge point of the delivery pipe, but that the head is made up of various terms which are described below. The head can also be related to the difference in energy between the pump inlet and outlet.
• The geometric suction lift (hs) is the difference in elevation between the surface of the suction liquid and the pump impeller, and the geometric height of elevation (hd) is the difference in elevation between the liquid level at the discharge and the pump shaft. The total geometric head is the difference in height between the liquid level at the discharge and the liquid level at the suction.

hg = hd -hs

• The power of a pump is the work per unit time taken to lift a given flow at a given manometric head. This would be the useful power.

Useful P= ϒ-Q-Hm

where

ϒ = Specific gravity of the liquid

Q = Flow rate

Hm= Height

Thus, depending on the units we would have:

• P = (Q-H-ϒ)/270 =C.V; where Q=m3/h ; H=m ; ϒ=Kg/dm3=1
• P = (Q-H-ϒ)/75= C.V; where Q=l/s; H=m

The efficiency would be the quotient between the power applied to the pump shaft and the useful power actually used, μ = Useful P / P axis

P shaft = Torque shaft ω; Torque at Nw m; ω angular speed

Frictional losses are the energy losses used to overcome the friction produced by the fluid as it moves through the pipe. They are usually calculated by Darcy-Weisbach or Hazen Williams.

There will also be localised pressure losses, to overcome the resistance that special parts exert on the liquid, these are expressed as fractions of pipe length:

hm = K -V²/2g, the value of the coefficient K can be easily found in hydraulics manuals.

Do you know Rigometer?

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