What are the potential causes of a submersible pump short-cycling

As a critical fluid-handling device, submersible pumps' stable operation is crucial to many systems. However, a common fault phenomenon is short-cycling, whereby submersible pumps frequently start and stop within extremely short intervals. This abnormal behavior not only significantly increases the motor's energy consumption but also causes severe mechanical and electrical stress, ultimately leading to premature device failure.

1. Pressure Tank or Bladder Failure
In submersible pump applications with pressure control systems, such as domestic water supply or small booster systems, the pressure tank plays a crucial role.
Bladder rupture or insufficient pre-charge pressure: A ruptured or under-inflated bladder inside the pressure tank significantly reduces the effective water storage capacity within the tank. When the pump starts, water quickly fills the tank, causing the pressure to rise rapidly to the cutoff point, causing the pump to stop. Once a small amount of water is used, the pressure drops rapidly back to the starting point, forcing the pump to start again. These small pressure fluctuations force the pump to start and stop frequently, resulting in typical short-cycling.
Inappropriate pressure tank capacity: A similar problem can occur if the selected pressure tank is too small and fails to provide sufficient buffering for the system. Even if the air bladder is intact, its limited water storage capacity cannot effectively balance low-flow water demands, forcing the pump to cycle frequently.

2. Improper Control System Settings
Submersible pumps are typically started and stopped by a pressure switch or level controller.
The pressure switch differential is set too low: A pressure switch typically has two adjustment points: the start-in pressure (cut-in) and the stop-out pressure (cut-out). If the difference between these two pressures (i.e., the differential) is set too small, for example, to just a few psi, even small pressure changes in the system can trigger the pump to start and stop, causing short cycling. The correct differential pressure setting should be determined based on the pump head and system requirements to ensure sufficient pump run time.
Faulty or improperly positioned level controller: In sewage or drainage pump applications, a level float switch is used to control the pump's start and stop. If the float switch's connecting wire is too short or stuck, or if the two floats are set too close together, the water level can fluctuate within a very small range, causing the pump to start and stop frequently.

3. System Leaks or Backflow
Minor leaks in the system are a common, hidden cause of short cycling.
Leaks in pipes, valves, or fittings: Small cracks in the water supply pipes, or leaks from valves or fittings, can cause a slow drop in system pressure. When the pressure drops to the trigger point, the pump starts to replenish water. Once the pressure reaches the cut-off point, the pump stops, but the leak continues, causing the pressure to drop again. This continuous leak cycle forces the pump to frequently short-cycle.
Check Valve Failure: A check valve prevents liquid from flowing back from the pump's outlet piping into the pump body or the water source. If a check valve is damaged, stuck, or leaking, some liquid can flow back, causing the outlet pressure to drop rapidly. This immediately triggers the pressure switch, causing the pump to restart, creating a vicious cycle.

4. Pump Casing or Impeller Wear
Although less common, wear inside the pump body can also indirectly cause short cycling. Impeller or Pump Casing Wear: In applications involving long-term handling of fluids containing abrasive particles, the impeller and pump casing of a submersible pump will gradually wear, resulting in decreased pump performance. This manifests as the pump being unable to achieve the rated flow and head at the same speed. The pump may need to run longer to reach the set pressure, but as the pressure approaches the cutoff point, the rate of increase becomes slower, making it more sensitive to pressure fluctuations and inducing short cycling.

5. Electrical Issues
Failure of electrical components within the control cabinet can also cause erratic behavior in a submersible pump.
Sticked Relays or Contactors: After extended use, relays or contactors in the control cabinet may experience arcing or sticking, preventing them from opening or closing properly. This unstable electrical connection can cause false start and stop signals to the pump.
Sensor Failure: If the pressure or level sensor itself is faulty, such as an unstable or drifting output signal, the controller will receive erroneous pressure or level information, triggering unnecessary starts and stops.