What’s different about a submersible pump? Apart from the fact that, like fish, they mostly stay underwater…
There’s a few things that work differently in submersible pumps from any other kind of pump. The most important ones, that this post is going to discuss, are the motor cooling, the suction, and the control.
The majority of pump motors are air cooled by a fan that is connected to the top of the motor. The fan spins with the motor, drawing air in through the grill protecting it and pushing it past the motor to remove the heat. As submersible pumps don’t have air around the motor, and water in the motor housing would damage it, it isn’t an option to have a fan on the end of the motor. Instead, the motor housing is designed to lose the heat into the surrounding water. This means that submersible pumps should always be under water when operating, so that the motor doesn’t overheat.
Instead of having the pumped medium piped to the suction of the pump, it’s all around. For this reason submersible pumps don’t usually have a flange on the suction, but just an open hole for the water to enter the impeller housing. The impeller designs are often unique to submersible pumps for this reason, and because they can be required to handle solids (such as in sewage).
Most pumps are required for their output, such as sending heated water around a building to warm it. However, many submersible pumps are actually required for their input, such as sucking in storm water and sending it away to drain: the storm water isn’t needed at the discharge point, but it is required to be taken away from the suction point. The control systems used reflect this fact. The most common method of control is to measure the level of the water in the tank and turn the pump on when that level rises to a pre-determined point. Importantly, this system also turns the pump off again when the water level falls, to avoid the motor overheating. Floating tilt switches, that float on the surface of the water and send a signal to the control panel when the water rises high enough to tip them over, are used for this.