Industrial lift stations collect wastewater streams from processes and transfer their contents into wastewater treatment. Lift stations are normally below grade, gravity fed cylindrical or rectangular sumps, constructed of concrete or fiberglass with depths ranging from 6 to 30 feet. There are three main application segments in the lift station market including industrial wastewater, municipal sewer and municipal storm water. Wastewater enters lift stations from the top and is removed from the bottom using one or two transfer pumps. The pumps can be located above grade near the lift station or submersed within the sump. Lift stations are typically crowded, dynamic environments with ladders, pipes, pumps, rails, electrical, sensors and waste streams pouring in from various sources. Wastewater can be corrosive, dirty, foaming, viscous, oily or vaporous, making it difficult to measure reliably. The primary requirement for this application is to monitor the liquid level, periodically transfer the wastewater from the lift station to another location, and prevent it from overflowing or running dry.
Technology
A pulse of microwave RF energy is emitted from the base of the probe antenna at the speed of light. The pulse reflects off the dielectric material in the surface of the liquid and returns to the antenna. The level sensor measures the time of flight between the pulse generation and receipt, and translates this into the distance between the antenna and liquid surface. The distance is then converted into a percentage of measured span and output as a proportional 4-20 mA signal.
Best Practices
Lift stations can be open, covered with safety grating or enclosed with a top. The installed level sensor must have a clear view of the liquid surface. This means that the measurement space beneath the level sensor should be free of any obstructions such as pipes, fittings, ladders, submersible pumps, waste streams or apparatus. The level sensor must be located above the highest liquid level and never be submersed during normal operation.
Installation
There are two primary locations and several ways to mount a level sensor in this application. Select the LR30 level sensor for below grade installation, bracket mounted to the sidewall or an aperture inside the lift station. Select the LR15 level sensor for above grade installation, tank adapter or flange mounted on the lift station grating or sump top. Find a mounting location where the level sensor has a direct, unobstructed view of the liquid throughout the entire measurement span when the sump is empty. The location must be flat, level to the liquid and accessible. The following equipment can be used to install the level sensor.
Bracket
The LR30 level sensor comes with an adjustable stainless steel bracket that’s typically bolted to the sidewall or an aperture within the lift station.
Tank Adapter
Tank adapters 3” or larger are recommended to install the LR15 level sensor where the mounting location is level and not on a slope. Do not use tank adapters that are mounted upside down.
Riser with Flange
Flanges 3” or larger with short risers and riser diameters of 3” or more are recommended to install the LR15 level sensor where the mounting location is level and not on a slope. The end of the installed antenna should protrude through the riser or be minimally recessed.
Electrical
Lift stations may be located near large pumps, motors or variable frequency drives that can generate substantial EMI or RFI noise. Make sure that such devices are grounded to earth, and then ground the level sensor and associated electrical equipment to the same earth-ground as these devices. Some areas maybe subject to frequent lightning strikes or have un-reliable power. Where so, proper surge protection and filtering is recommended.
Span
The level sensor outputs a 4-20 mA current signal that’s proportionate to the measurement span within the lift station. Users typically set the 4 mA to empty or the lowest measured level, and 20 mA to full or the highest measured level. Avoid placing the 4mA or 20mA span set points at or near levels where pumps or alarms may actuate.
Interface
The sensors 4-20 mA current signal is normally connected to a local controller or centralized control system. These devices may include a PLC, SCADA, DSC or stand-alone level controller. Either control device is fine as long as it accepts a 4-20 mA current signal. The operational range of the controller must then be programmed to match the measurement span of the level sensor, and take into account that the sensors 4 mA set point is normally placed above the empty sump condition. Remember, the primary requirement for this application is to monitor the liquid level, periodically pump down or transfer the wastewater from the lift station to another location, and prevent it from overflowing or running dry. Where two pumps are involved, users typically apply lead-lag and alternating pump control logic to maximize pump efficiency and life. Where two pumps are involved, users typically use lead-lag and alternating pump control logic to maximize pump efficiency and life. An independent high and low level alarm or safety shut off system should always be used in addition to the primary system.
