Reverse running speed refers to the speed (also called return speed, reverse speed) of adeep well vertical turbine pumpwhen fluid flows through the pump in the reverse direction under a certain head (that is, the total head difference between the pump outlet pipe and the suction pipe).

This situation can occur in systems with a system characteristic curve with a high static head (Hsys, 0), but also in deep well vertical turbine pumps operating in parallel. 

When the pump unit unexpectedly shuts down, the outlet check valve fails, and the outlet pipeline is open, the direction of the fluid through the pump will be reversed, and the pump rotor will rotate at the reverse operating speed after the flow direction changes.

Reverse operating speed is usually significantly higher than normal operating speed and depends on system conditions (especially current pressure) and the specific speed of the pump (ns). The maximum reverse operating speed of the radial flow pump (ns ≈ 40 r/min) is approximately 25% higher than the normal operating speed of the pump, while the maximum reverse operating speed of the axial flow pump (ns ≥ 100 r/min) is higher than the normal operating speed of the pump. Runs 100% faster.

These operating conditions may also occur if the closing element used to protect against surge pressure (water hammer) is not a check valve but a slow-closing closing element. Most of the returned fluid can flow out through the deep well vertical turbine pump.

If the surge pressure is caused by a power failure in the drive unit and no check valve is installed, the pump shaft will also rotate in the opposite direction. During this process, close attention must also be paid to the risks involved with plain bearings and mechanical seals that only work in one direction of rotation, as well as the possible loosening of threaded fasteners on rotating shafts.

If the returning medium happens to be in a state close to the boiling point, the medium may vaporize when the pump or pressure side throttling device depressurizes.

The reverse operating speed of the vapor-containing (return) flow versus the liquid return flow, as a function of the square root of the liquid/vapor density ratio, can rise to dangerously high values.

If the drive motor is switched on in a deep well vertical turbine pump that rotates in the opposite direction to the normal direction of rotation, the start-up time of the pump set will be significantly longer. In this operating condition, for asynchronous motors, the additional temperature rise of the motor must also be noted.

Only appropriate measures can be taken to prevent damage to the pump set caused by excessive reverse running speed.

Countermeasures to prevent reverse running speed from being too high include:

1) Install a mechanical anti-reverse device (such as a backflow locking device) on the pump shaft;

2) Install a reliable self-closing one-way check valve (such as a swing check valve) on the pump outlet pipe.

Note: The anti-reverse device is used to prevent the pump from reversing. Among them, the backflow blocking device works according to the principle of forward rotation without hindrance. Once the rotation direction of the shaft is reversed, the rotor rotation will be stopped immediately.