15-01-2013, 01:01 PM
PUMP DESIGN
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Classification of Water Pumps
Water pumps can be divided into three types: displacement, impulse and other types.
Positive displacement pumps can be of reciprocating and rotary type. In either case
liquid is displaced from the low pressure suction side to the high pressure discharge side
(the term positive refers to the direction of flow displacement related to the pressure
gradient). The geometry of the pump is changed periodically and determines the flow in
both supply and delivery system. In a positive displacement pump there is no direct
communication between the suction and discharge circuit. As a rule, a positive
displacement pump is self-priming.
Piston Pumps
A reciprocating piston pump basically consists of a piston, two valves and a suction and
a delivery pipe. Sometimes airchambers are applied to smooth the flow and reduce
shock forces. In the traditional piston pump the upper valve is usually situated in the
piston and known as the piston valve; the lower valve is called the foot valve. If the
upper valve is not integrated in the piston, the pump is usually called a plunger pump
(Figure 2).
Centrifugal Pumps
Centrifugal (or rotodynamic) pumps are based on the principle of imparting kinetic
energy to the water. In these pumps water enters axially and is discharged by the rotor
into a discharge pipe. They have an impeller which rotates in a casing of a special shape.
The impeller vanes accelerate the water, which is thrown out by the centrifugal force.
The shape of the casing is designed to effectively build up a high pressure at the pump
outlet. It is this pressure level that lifts the water against the pumping head. In Figure 5,
a single stage of a centrifugal pump is shown. This type of pumps are typically driven by
an electric motor or combustion engine and installed above ground level.
Torque and Flow
A single-acting piston pump without airchamber, directly coupled to a wind rotor, is
depicted in Figure 6. The rotor shaft drives the pump through a crank mechanism;
therefore the rotational speed of the pump is equal to that of the rotor and the pump
stroke dictated by the eccentricity of the crank mechanism.
Volumetric and Mechanical Efficiency
In a practical situation, the average output as found in (4) may be different due to
leakage, delayed valve response and inertia effects. We define the volumetric pump
efficiency h
vol as the ratio between the effective average output flow and the above
derived one, hence:
At low speeds of operation the volumetric efficiency is normally less than 100% due to
water leakage over the piston and the valves and to delayed closing of the valves. At
high speeds the volumetric efficiency may be above 100% due to the inertia of the water
flow: at the end of the upstroke the water has gained so much momentum that it
continues moving upward (through the valves which remain opened) during the
downward stroke. Volumetric efficiencies under design conditions are typically 80% to
90%.
Non-stationary Behaviour and Valve Response
The relations derived above only hold for a slow movement of the piston. At higher
pump speeds (above one stroke per second), acceleration effects become notable in the
water flow, causing a number of complications due to cavitation, pump rod buckling,
inertia flow, shock forces in the pump rod, and wave phenomena. In this section,
cavitation and buckling are shortly dealt with, while pump rod forces are more
extensively dealt with in section 4.
Coupling of a Pump to a Wind Rotor
A piston pump coupled to a windmill together form a system in which both components
influence each other. The behaviour of this system is complicated even more by external
parameters, such as the pumping head and the windmill safety system.
In general, by choosing a large pump high volumes of water are obtained but the
windmill will not run most of the time. In other words, the windpump has a high output
but a low availability. A small pump coupled to the same windmill will have a higher
availability but a lower output. In a practical situation, one tries to find the most
appropriate compromise between output and availability. This process of tuning the
system components to each other and to the prevailing wind regime, is called matching.
In a stationary situation, matching of a windpump system is a rather straightforward
procedure and formulas can be derived to determine the optimal combination. The
windspeed itself is a stochastic variable, but with the aid of wind regime (Weibull)
distributions, a mathematical expression can be obtained for the longterm water output
and availability. For the practice of windpumping, a first complication is the presence
and duration of lulls, i.e. consecutive days with low winds and no water. The occurrance
of periods with low windspeeds is not covered by the Weibull description, yet they
determine which size of storage tank is needed.
Controlling the Pump Rod Force
A reliable windpump will have a pump rod capable to cope with the occurring forces
under design conditions. For a given head and pump size, these forces vary in
congruence with the rotational speed of the pump and the rotor. The total fatigue load on
the pump rod depends on the distribution frequency of the pump rod loads, i.e. the load
spectrum, and the contribution of the corresponding stresses in the pump rod to the
cumulative fatigue.