Understanding Reverse Running Speed in a Deep Well Vertical Turbine Pump: Causes, Risks, and Solutions
The deep well vertical turbine pump is widely used in applications requiring high-lift water transport such as irrigation, municipal water supply, and industrial operations. While it is engineered for reliable performance under normal conditions, one critical issue that must not be overlooked is reverse running speed—the speed at which the pump shaft rotates in the opposite direction due to reverse fluid flow.
This phenomenon can lead to severe mechanical damage if not properly understood and prevented. In this article, we explore what reverse running speed is, when it can occur, the dangers it presents, and how to effectively protect deep well vertical turbine pump systems against it.
What Is Reverse Running Speed?
Reverse running speed (also referred to as reverse speed or return speed) describes the rotational speed of the pump shaft when fluid flows backward through a deep well vertical turbine pump under a certain pressure head. This typically happens when:
The system experiences a sudden shutdown,
The outlet check valve fails,
Or a surge pressure event causes backflow through the pump.
In such cases, fluid from the discharge side flows back into the suction side, reversing the flow direction and driving the pump impeller in the opposite direction.
When and Why Reverse Running Occurs
Reverse rotation is often triggered in systems with:
A high static system head (H_sys,0),
Parallel pump operation, or
Sudden power failure without a reliable check valve.
For example, when a deep well vertical turbine pump operating in parallel with another unit is suddenly shut down, and the outlet check valve fails to close, the high-pressure fluid from the running pump can push backward through the idle pump, causing reverse rotation.
Additionally, surge control mechanisms like slow-closing valves may not respond quickly enough to prevent this backflow. The situation becomes more dangerous if the returning fluid is close to its boiling point, as depressurization can lead to vaporization and rapid acceleration of reverse flow.
How Fast Can a Deep Well Vertical Turbine Pump Run in Reverse?
Reverse running speed depends on two main factors:
System pressure/head at the time of shutdown or reversal,
Specific speed (nₛ) of the pump design.
For radial flow pumps (nₛ ≈ 40), the reverse speed may reach up to 25% higher than the rated forward speed.
For axial flow pumps (nₛ ≥ 100), reverse speeds can be up to 100% higher, effectively doubling the normal operational speed.
This extreme speed increase can severely damage components like:
Plain bearings, which are direction-sensitive,
Mechanical seals, which may only seal in the designed rotation direction,
Threaded connections on the rotating shaft, which may loosen under reverse torque.
Additional Hazards of Reverse Running
Motor damage: If the motor is accidentally started during reverse pump rotation, asynchronous motors may experience prolonged startup times and excessive heat buildup.
Cavitation risks: If returning fluid vaporizes during depressurization, it may cause cavitation and lead to serious impeller erosion or seal failure.
System instability: Reverse rotation can introduce pressure fluctuations and hydraulic instability that compromise the entire system.
How to Prevent Excessive Reverse Running Speed
Preventive strategies are essential to protect a deep well vertical turbine pump from damage due to reverse operation. Recommended countermeasures include:
Install a mechanical anti-reverse device
A backflow locking mechanism installed on the pump shaft can immediately stop rotation if the shaft begins to reverse. It operates freely in the forward direction but engages automatically in reverse.
Use a reliable self-closing check valve
Installing a high-quality swing check valve or spring-loaded check valve on the pump outlet pipe helps ensure that fluid cannot flow backward in the event of a shutdown.
These safeguards help eliminate the risk of backflow-induced shaft reversal, even during emergency shutdowns or power failures.
Conclusion
Understanding the concept of reverse running speed is crucial for the safe and reliable operation of any deep well vertical turbine pump. Left unmitigated, reverse flow can drastically accelerate shaft rotation beyond design limits, damaging internal components and compromising system performance.
To avoid costly failures, it’s essential to implement robust anti-reverse mechanisms such as mechanical locks and high-performance check valves. With the right protections in place, deep well vertical turbine pumps can continue to deliver dependable performance, even under unpredictable system conditions.
