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Velocity of sound

Velocity of sound (a), or, to use the correct physical term, pressure wave propagation velocity, is the velocity with which a weak, isentropic (of equal entropy) pressure disturbance is propagated in a fluid or solid object with the following relationship:


Schallgeschwindigkeit_Formel_1

Velocity of sound plays an important part in centrifugal pump technology, particularly in the case of surge pressures in piping systems, but also when a mixture of liquid and vapour is pumped (in cavitation conditions). In the latter case, an extremely low velocity of sound of the order of
magnitude of the absolute velocity (v) can occur (sound limit v/a = 1, compression (shock) wave with sudden and abrupt disappearance of the vapour bubbles).

The low sound velocities in a mixture of liquid and vapour are explained by the fact that considerable changes in density (dρ) of the mixture occur in relation to the change in pressure (dp), because of evaporation or condensation (particularly in the region of low pressure and low vapour contents).

Unexpectedly low sound velocities also occur in mixtures of liquid and gas, such as inside a waste water pump or pulp pump.

In fluid-filled pipelines the velocity of sound is also influenced by the material of the pipe and the ratio between the pipeline diameter and the wall thickness. The velocity of sound for water in pipelines made of steel, cast iron or concrete is approx. 1000 m/s.