Specifications

155
Suction Lift
Suction conditions are some of the most important factors affecting centrifugal pump
operation. A pump cannot pull or “suck” a liquid up its suction pipe because liquids do not have
tensile strength. When a pump creates suction, it is simply reducing local pressure by creating
a partial vacuum (Sucks out the air above the liquid). Atmospheric or some other external pressure
acting on the surface of the liquid pushes the liquid up the suction pipe into the pump.
In feet, the head is given as: Head = PSI X 2.31 / Specific Gravity
For Water it is: Head = 14.7 X 2.31 / 1.0 = 34 ft.
34 feet is the theoretical maximum suction lift for any pump pumping cold water. No pump
can attain a suction lift of 34 ft; however, well designed ones can reach 25 ft quite easily. From
the equation above, it is clear that specific gravity can have a major effect on suction lift. The
suction lift of a centrifugal pump also varies inversely with pump capacity.
14.6 Static suction head:
Static suction head is the vertical height through which the water has to be lifted from the
well or underground tank i.e., the height from the water level to the centre line of the pump. The
head is always the vertical height and not along the pipe line. It is from the water level to the
pump centre and not from the foot valve or the bottom of the well.
14.7 Static delivery head:
This is the vertical height through which the water is lifted. It is from the centre line of the
pump upto the level where the water is discharged into the over head tank. The head is always
the vertical height and not along the pipe line.
The friction head as much as possible in relation to the total head should be reduce from all
causes. This can be achieved by reducing the velocity or providing a larger diameter of pipe.
14.8 A centrifugal pump should never be allowed to run dry.
The pump gland packings are water lubricated and water cooled. Water also acts as a
barrier between the atmosphere and the interior of the pump. If the pump is allowed to run dry,
excessive heat will be generated resulting in possible seizure of the spindle and will result in
burn out of the motor. No pump should, therefore, be allowed to run dry or started unless it is
fully primed.
14.9 Priming of pump
Priming means filling the pump and the suction pipe completely with water so that all air is
expelled from the system. Not only should the pump be fully primed when it is started, but it
should remain fully primed through out the period it is running.
14.10 Self-priming pump
The self priming centrifugal pump is designed to lift water from some level below the pump
suction without having to fill the suction piping with liquid. It accomplishes this by creating a
partial vacuum at the pump suction which removes the air from the suction line. The pump then
releases the entraped air through it’s discharge while retaining the initial fill of water in the pump