Because the failure of a lift station could result in damage to homes, property and the environment all lift stations should be equipped with some type of audible and visual alarm at a minimum. For large installations a computer will automatically contact operators in the event of a failure or the system will be monitored through telemetry. A “high water alarm” should be included for every installation.
Motor Control Center (MCC) The pump motor controls are located in a motor control center. This is typically an electrical box with switches for operating the pumps in either hand (manual) mode, in automatic mode or for turning them off. For this reason these are known a H-OA switches (Hand-Off-Auto). MCC panels usually include pump starter coils, which allow the 110 V switch to engage the higher voltage motor circuit and a system for alternating lead and lag pumps. Alarm lights and reset buttons will also be located on the MCC panel.
The pumps in a lift station should be equipped with equipment hours recorders so that the total run time of each pump can be compared. If one pump in an automatic alternating duplex lift station runs much more than the other it is a indication that something is wrong, such as a leaking check valve. Also, flow through the lift station can be established if the pumping rate and operating hours are known.
All lift station pump controls are similar in that they control the pumps based on the level of water in the wet well. Controls vary greatly in how they accomplish the task. Common lift station control systems include:
- Float controllers. Floats are one of the oldest pump control devices. As the water in the wet well rises, the float (a ball attached to a rod) rises with it until a switch is triggered which turns the pump on. When the pump lowers the level to a pre-set point an actuator shuts the pump off.
- Electrode Controllers (probes). Electrode controllers utilize electrode probes or leads that are an open circuit until the water level rises, wetting both electrodes and allowing current to flow which enacts the control circuit on the pump to turn it on. When the water level drops and the upper probe becomes exposed, the circuit opens and the pump shuts off. This type of control is subject to problems caused by grease and rags.
- Pneumatic Controllers (bubblers). Pneumatic controllers work by sensing the pressure required to force air bubbles out of a tube located near the bottom of the wet well. When the wet well is full, it takes more pressure to force bubbles out of the tube (due to the higher head created by the standing water). The on off set points for the pumps on this type of controller are actually pressure set points. Bubblers rely on an air pump, similar to a fish aquarium pump, to provide air so that the pressure can be measured as bubbles are forced out the tube near the bottom of the wet well. If the tube becomes clogged by a rag or grease, or the air pump fails the system will stop working. If the length of the bubbler line changes, (accidental damage while removing a pump for example), the system will function incorrectly, if at all.
- Mercury Float Switches. Mercury float switches are one of the most common types of lift station controls. They consist of a float that contains a sealed chamber with two electrodes and a small amount of mercury (quicksilver). When the float is in one position (vertical, while hanging for example) the mercury is pooled away from the electrodes and the circuit is open. When the float is tilted to the other position (horizontal, while floating) the mercury flows down to the electrodes and the circuit is completed which triggers the pump control to turn the pump on. Mercury float switches can be purchased as either normally open (on switch) or normally closed (off switch) when in the vertical position. The floats must be kept clean so that they will tilt when floating and they must be lowered carefully back down into the wet well after they have been cleaned and checked.