The task of any centrifugal pump is to move liquid, requiring “work.” To do this, the pump must equip the fluid with a suitable form of energy. Why do we say the suitable form of energy? Because for work to be done, we need the right type or form of energy to do the task. For example, we may have access to thermal energy, but if we heat the water it will not move from point A to point B, it will boil. But that same thermal energy in a power plant is converted to electrical energy, which rotates the motor and the pump shaft, and in turn converts the energy to kinetic and pressure energy, which is just the form we need for fluid transportation.
A centrifugal pump is an energy conversion machine. The mechanical energy of the shaft is converted to kinetic energy by increasing the velocity of the fluid leaving the impeller. Then most of this energy is converted to pressure energy in the pump volute or diffuser. How? According to Bernoulli’s principle, put simply, for incompressible flow. The total energy of any arbitrary point along the streamline is constant. An increase or decrease in one form of energy results in a decrease or increase in other forms, but the total energy remains the same. In our case when we talk about the total energy at any point in the pumping system (pumps, pipes, valves...) we mean the kinetic, potential, and pressure energies. But in a pump volute or diffuser, changes in potential energy (height) is negligible, therefore the total energy of concern in pump volute consists of the kinetic and pressure energies.
Looking at the sectional view of a pump volute we see that the impeller is offset to the casing. The fluid passage between the impeller and the casing increases as it gets closer to the discharge port. We know that the flow rate (Q) is equal to velocity times the area (Q=VxA). Therefore, for a fixed flow rate (Q), the increase in the cross-section area results in a decrease in velocity (kinetic energy.) Since the total energy should remain constant, there is no choice but for an increase in pressure energy of the fluid. The fluid expends the gained energy to move in the pipeline and overcome the static and dynamic friction forces in the system and perform the “work” of moving through the pipeline.