Reverse running speed
The reverse running speed is the speed that occurs in a centrifugal pump when the fluid flows in reverse direction through the pump at a certain head (i.e. difference in total head between discharge nozzle and suction nozzle).
This situation can arise in systems whose system characteristic curves have high static heads (Hsys,0), but it can also be observed in centrifugal pumps operating in parallel. Following a drive failure and when the discharge line is open, the fluid will reverse its direction through the pump, and the pump rotor will rotate at reverse running speed following this change of flow direction (see Turbine mode). The reverse running speed is generally markedly higher than the normal operating speed and is dependent on the system conditions (in particular the current pressure head) and on the pump's specific speed (ns). The max. reverse running speed for radial pumps (ns ≈ 40 rpm) is approximately 25 % higher than the operating speed of the pump, and for axial pumps (ns ≥ 100 rpm) it is up to 100 % higher.
These operating conditions can also occur if a slow-closing shut-off element – rather than a check valve (see Valve) – is used for protection against surge pressures. This allows the back-flowing fluid to run through the centrifugal pump.
If the surge pressure is caused by a power failure of the drive and a backstop has not been installed, the pump shaft will also rotate in reverse. The risk involved for plain bearings and mechanical seals which can only be operated in one direction of rotation must also be considered.
If the back-flowing fluid is near its boiling point (see Vapour pressure) it can evaporate in the pump or in the discharge-side throttle device.
The ratio of reverse running speed when operating with steam and liquids can rise to dangerously high values, as a function of the square root of the ratio of the density of liquid/vapour.
If the motor is switched on when the centrifugal pump is running in the reserve direction, the run-up time of the pump set (see Start-up process) is considerably extended. With asynchronous motors it is advisable to observe the additional temperature rise of the motor under such conditions.
Excessively high reverse running speeds that would lead to damage to the pump set can only be prevented by taking the appropriate measures.
Measures to prevent high reverse running speeds
- Fitting a mechanical backstop on the pump shaft
- Installation of a reliable, automatically closing check valve (e.g. swing check valve) in the piping
