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Table showing generic pump types |
Note: Links to more detailed information are included in the rows below..
Single Pump Systems
The system head generally consists of two parts:-
The difference between the suction and final discharge static levels in an open system. This is usually independent of flow.
Energy Loss in the Pipework.
This is essentially the friction head loss in the piping and is related to the flow . Under turbulent flow
operating conditions the friction loss may be assumed to be related to the flow velocity 2. Therefore
a friction loss plot is a parabola passing through the origin.
It is often the case that the static head is small compared to the friction loss e.g. in
Centrifugal pump curve.
A typical pump operating with a set impeller diameter and at a set rotational speed
delivers a head related to the liquid flow rate. The head is normally
highest when no flow is delivered (dead headed) and lowest when there is no restriction to
the flow and it is consequently at its maximum flow. Both these extreme conditions are
very undesirable. A dead headed pump will result in all of the energy supplied
at the pump shaft being dissipated in the small closed volume of liquid. -The fluid will rapidly
heat up An open discharge pump is operating with very
poor efficiency with a risk of overloading the drive motor.
A pump is generally selected for a duty such that when the pump curve is superimposed on the system characteristic curve the curves cross over at the pump best efficiency point (BEP) at the desired flow rate.
It is impractical to obtain a perfect match of the best efficiency point of the pump with the system characteristic curve and generally the flow will be higher or lower than desired. This may result in the pump not providing the desired performance and may sometimes result in pump instability. Methods of achieving a match include:
There is a modern trend to provide electrical variable speed drives to pump to enable the performance of the pump to be adjusted - on the run. The modern AC inverter controlled motor is a very efficient low cost method of varying the motor speed.
Centrifugal pumps in parallel provide additive flow conditions and when mounted in series provide additive head conditions. Therefore three matched pumps mounted in series will provide the same flow at three times the head (aprox.) compared to one pump. Three matched pumps in parallel will provide the same head at three times the flow compared to one pump. A very popular variation on this theme is to provide multi-stage pumps with up to twenty stages mounted on a common shaft to provide a high head from a single pump.
The energy imparted to a fluid by a pump is measured as the head (in metres)per unit weight of fluid.
dH = (Z2 - Z1) + ( P2 -P1 )/ (ρ g ) - (v22 -v12)/ (2 g )
Normally the velocity head and the height head is ignored for initial selection of the
p (N/m2 ) = dH * ρ g
As liquid flows from the pump inlet flange into the energy conversion
region e.g.impellor or piston chamber, the head initially falls as the velocity of
the fluid is increased.
If the liquid is near its vapour pressure the drop in head may be sufficient to
cause the liquid to boil. This results in "cavitation". Cavitation is detectable as a rattling noise and
results in low efficiency, and high risk of damage to the pump.
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