A pumping system can never be operated in steady-state condition all the time, since starting up and stopping the pump alone will change the duty conditions.

Generally speaking, every change in operating conditions and every disturbance cause pressure and flow variations or, put differently, cause the flow conditions to change with time. Flow conditions of this kind are commonly referred to as unsteady or transient. Referring specifically to pressures, they are sometimes called dynamic pressure changes or pressure transients. The main causes of transient flow conditions are:

• Pump trip as a result of switching off the power supply or a power failure.
• Starting or stopping up one or more pumps whilst other pumps   are in operation.
• Closing or opening of shut-off valves in the piping system.
• Excitation of resonant vibrations by pumps with an unstable H/Q curve.
• Variations of the inlet water level.