Lift Station Guardian Controls In Depth Guide

Go deeper into the Lift Station Guardian controls with a look at instrumentation, control logic, and special features.

The bar chart below shows the instrumentation and setpoints described in this guide.

Instrumentation

Level Transducer

The level transducer is the primary level for a Lift Station Guardian equipped lift station.

A level transducer is installed as close as possible to the bottom of the well. It measures the hydrostatic head pressure (pressure from the weight of water column above its sensor).

Analog output from this transducer gives the level of the well, indicated by a black arrow on the chart. When the Lift Station Guardian controller known as “DPO” is in Auto Mode (i.e., the DPO HOA, Float Override, and Local HOA are all toggled to “Auto”), the Lift Station Guardian is using the level transducer output to control operation of the pumps based on their start/stop levels.

High Float Switch/Low Float Switch

Float switches are the secondary or back-up level control system within a Lift Station Guardian enabled lift station. Float switches mark the “High” and “Low” float levels within the wet well. In the event that the level transducer malfunctions, the float switches will turn the pumps on and off.

The floats are mercury-activated, narrow-angle control switches that automatically turn pumps on or off based on the water level inside the well. These tilt switches use a small amount of mercury to open or close a circuit to operate or shut off pumps.

Lift Station Guardian uses the following control switch configuration:

High level configuration (normally open) – when the water level reaches the high level setpoint, the float switch tilts to close the circuit, which tells all available pumps to start, preventing an SSO from occurring.

Low level configuration (normally open) – when the water level reaches the low level setpoint, the float switch tilts to close the circuit, which stops the pump(s) and prevents pumps from running dry.

Click here for more on level sensing instrumentation: https://romtecutilities.com/understanding-level-sensing

Lift Station Guardian DPO Control Narrative

Pump Start Level/Pump Stop Level

The Pump Start Level is shown to the left of the bar graph, in blue. The Pump Stop Level is shown to the right of the bar graph, in gray. These start/stop setpoints define the range of wet well level operation and are within the level range of the float switches. This is the primary level control scheme for the wet well.

High Float Level/Low Float Level

High and Low Float Level setpoints are set at the physical level of the High Float Switch and the Low Float Switch within the wet well. Upon commissioning of the wet well, these levels are determined and this defines the setpoints. Wet well High Float Level and Low Float Level are indicated by orange arrows on the bar chart.

Float switches can move around within a wet well, so Lift Station Guardian provides an automated way to test float switches and reset the levels within the wet well. Skip to Maintenance Features for more information.

Control Hierarchy for Operation

The Lift Station Guardian is a full-featured Lift Station Controller including the primary control mode known as “Lift Station Guardian DPO Control” which will smoothly cycle the wet well based on the Pump Start Level and Pump Stop Level and uses the High Float and Low Float as a back-up safety system for normal lift station operation.

Lift Station Guardian “DPO” Control

For the Lift Station Guardian Controller to be active, the physical Tagger device must be powered on and connected via cellular modem to the internet, and the following must be true:

• DPO HOA must be in Auto,

• Float Override in Auto

• HOA (Local) in Auto

When these prerequisites are met, the Lift Station Guardian controller will use the control narrative described above using the level sensor as primary level and the float switches for back-up.

Redundant Control Logic

If the Lift Station Guardian Controller is unavailable, the high float switch and low level float switch will be the default control system and the station will operate based on physical float switches.

The Lift Station Guardian Controller could be offline if the Tagger device is not powered on or if the cellular connection is lost. Also, if the level sensor fails, this back-up control logic is in the PLC and will locally control the pump station if the Tagger device is offline due to one of the above mentioned reasons.

Performing regular float tests is critical to ensuring the back-up system level control system is available if the primary controller is unavailable. Lift Station Guardian is equipped with automated float tests that can be easily scheduled, see Maintenance Features below.

Local Control (Human in the Loop)

In an emergency or in a maintenance situation, operators may use the physical HOA, which overrides both the Lift Station Guardian and PLC control to run pumps as needed manually on site. This will disable any ability to remotely control pumps at site via SCADA.

Maintenance Features

Lift Station Guardian Maintenance Features are located in the “Maintenance” tab of the Lift Station Guardian User Interface. Maintenance can be configured entirely through the UI and maintenance can either be scheduled to run automatically or run instantaneously using the “Queue Maintenance” button.

Float Test

A Float Test will verify that both the high and low floats are functioning properly. The High and Low float levels will be adjusted based on the Level Transducer reading when each float trips. The Float Test Mode will bypass the normal start and stop levels until the test is complete.

The “Highest Search Level” is the level at which if the High Float Switch has not be activated or “found”, the test will exit and normal operation will resume. Timeouts can be configured to determine how long Lift Station Guardian Float Test Mode is allowed to search for each float before the test is aborted. The Lift Station Guardian will send a text message alert to operations staff that the test was aborted and a workorder should be written to fix the High Float Switch immediately.

The Highest Search Level and Timeouts are redundant safety measures to prevent overflows during float tests. It is recommended to schedule these tests or run them manually with “Queue Maintenance” during normal working hours where operators and maintenance staff are available to take action to fix a float switch or sensor if the test is aborted.

Pump Snore

The pump snore setting establishes a period of time to run the pumps once the low float level has been reached. This feature allows lift station operators to bring the surface level of the well to the pump inlet, clearing floatable material.

Force Main Scour

The force main scour uses a flow setpoint to establish a high enough velocity in the force main to convey solids down the line, protecting from capacity-reducing build up.

Odor Control

This feature is especially helpful for lift stations operated on an intermittent basis, as it can be used to ensure the lift station pumps are run on some scheduled frequency. Doing so helps prevent the wet well from going septic, reducing unnecessary odor.

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