SubMonitor: What it CAN’T do

In the last few Franklin AID posts, we’ve discussed how SubMonitor protects a 3-phase submersible installation against a wide variety of potentially harmful conditions. It does this by continuously monitoring voltage, current, and a heat sensor in Subtrol-equipped motors.

On the mechanical side, however, there are a few things that SubMonitor just can’t protect against. One of these is water hammer. Water hammer most often occurs when a check valve is not used, has been improperly installed, or leaks. As a result, when the pump stops, the water drains back down through the pump inlet and creates a vacuum in the discharge piping. When the pump restarts, water rushes to fill that vacuum at a high velocity. When it strikes the hardware or stationary water above, it causes a hydraulic shock that can split pipes, break joints, and damage the pump/motor. The picture below shows a thrust bearing that was destroyed by water hammer.

Water Hammer

Another mechanical condition that SubMonitor won’t protect against is radial side loads. The most common cause of side loading is misalignment of the pump and motor. This creates a side which, depending on the how extreme that load is, can cause a rapid failure in the radial motor bearing near the top of the pump.

Radial bearing

Finally, another condition that SubMonitor won’t protect against is upthrust. As the name implies, upthrust occurs when a large volume of water pulls the impellers upward. This carries across the pump coupling/motor shaft assembly and pulls the shaft up with it. This generally occurs at start-up, and the pump and motor are designed to handle this on a momentary basis. However, if the pump is operating on the far right-hand side of the curve for long periods of time, the resulting upthrust can damage the motor and pump.

When looking at these three mechanical failure modes, it may seem obvious that no electronic protection device could guard against these types of failures. These situations underscore the importance of using good installation procedures. Combined with the protection that SubMonitor offers against events that you can’t control, your submersible installation will reliably deliver water for years to come.