Working principle of centrifugal and reciprocating pumps

Pumps convert mechanical energy into pressure energy of a liquid. A centrifugal pump does this by a rotating impeller (rotodynamic action); a reciprocating pump does it by a piston moving to and fro in a cylinder (positive displacement).

Centrifugal pump

        delivery pipe
             ^
             |
     +-------+-------+
    /   volute casing \
   |    +---------+    |
   |    | impeller|<---- shaft (motor)
   |    +----^----+    |
    \        |        /
     +-------+-------+
             |  eye
        suction pipe
             |
        foot valve + strainer

1. Priming: the casing and suction pipe are first filled with liquid so that no air remains (air cannot create enough suction).
2. The motor rotates the impeller. The liquid in the impeller is forced outward by centrifugal force, which creates a low pressure at the eye (centre).
3. Atmospheric pressure on the sump surface pushes liquid up the suction pipe into the eye.
4. The vanes give the liquid high velocity and pressure. In the volute casing the area increases gradually, so kinetic energy is changed into pressure energy.
5. The liquid leaves through the delivery pipe at high pressure. Flow is continuous and smooth.

The head developed is the forced-vortex rise $\frac{\omega^2 r_2^2 - \omega^2 r_1^2}{2g}$, so a centrifugal pump needs a certain minimum speed to start delivering (minimum starting speed).

Reciprocating pump

   delivery valve      suction valve
        |                  |
   +----+------------------+----+
   |  cylinder  <==piston==>    |---- crank & connecting rod
   +----------------------------+

1. A crank driven by a motor moves the piston to and fro through a connecting rod.
2. Suction stroke: the piston moves outward, the cylinder volume rises and pressure falls. The suction valve opens and liquid enters; the delivery valve stays closed.
3. Delivery stroke: the piston moves inward and pushes the liquid out. Pressure rises, the suction valve closes and the delivery valve opens.
4. Discharge per revolution (single acting) is $Q = \frac{ALN}{60}$, where $A$ is piston area, $L$ stroke length and $N$ rpm. Flow is pulsating; air vessels are fitted to smooth it.

Use: centrifugal pumps suit large discharge at low to medium head (water supply, irrigation, dewatering of powerhouses); reciprocating pumps suit small discharge at high head (boiler feed, hydraulic testing).