Understanding High and Low-Level Float Switches

Sensor liquid level control is one of the most common process functions in everyday life, with the most frequently used type being float switches. These simple yet effective devices play a vital role in managing liquid levels in various applications, including stormwater pump stations and sewer pump stations. They ensure that pumps operate automatically, preventing issues such as overflows or pump damage, and can operate alarms when levels are too low or too high. Incorrect settings can lead to functional inefficiencies of the pump/s and operational problems. Let’s explore how float switches work, why their settings are important, and what can happen if they are not positioned correctly.

 

High and Low-Level Float Switches

High (Start) and low-level (Stop) float switches can be used together to maintain the optimal liquid levels in a pump station. Where the distance between pump on and pump off is less than 400mm, a double acting float switch can be used to perform both functions. 

High-Level and Low-Level Alarms

A high-level float switch is designed to activate the pump when the liquid reaches a maximum level, alerting to the threat of overflow. This could indicate that there is excessive inflow into the station as in the case of an excessive rain event, or that the pumps are not operating correctly. A low-level alarm, if fitted, activates when the liquid drops below the normal minimum switch off level, potentially protecting the pump from running dry. This may occur when the stop float is jammed up preventing the pump from turning off. Provided the float switch is set correctly, and regular preventative maintenance carried out on the pump station, low-level alarm protection is often not required.

The Importance of Proper Float Switch Settings

Correctly setting the high and low-level float switches is crucial for the efficient operation of any pump station. If these switches are set too close together, various issues can arise, potentially affecting the system's performance and longevity. Note that the start and stop float switches need to be far enough apart to prevent the pump cycling as this can cause premature failure of the pumps. 

What Happens if the Float Level is Set Too Low?

It is important that the low-level stop float is positioned low enough to ensure the pump station is empty at the end of each pumping cycle, but setting it too low can result in a number of operational problems including:

• Liquid creating a vortex – The liquid in the pump station can vortex resulting in the pump sucking air. This can cause the pump to lose its prime and cease to operate.
• Jammed stop float - Increased possibility of the stop float being jammed in the on position with rubbish, causing the pump to run continuously. This can cause excessive electricity consumption and even result in the pump burning out.
• Increased wear and tear - Frequent start-stop cycles, known as short cycling, can cause excessive wear on the pump, reducing its lifespan and increasing maintenance costs.
• False alarms - Setting the low-level float switch too low can result in frequent, unnecessary alarms, potentially leading to alarm fatigue where genuine issues might be overlooked.

 

Problems with float level set too high

Setting the high-level start float level is important to ensure that the pump station is kept as empty as possible. Setting the level too high can also have adverse effects, such as:

• Delayed pump activation - The backup pump may only start operating when the liquid reaches a critically high level, which might be too late to manage heavy inflows effectively, which is a common functionality in stormwater pump stations. This could lead to flooding or environmental contamination.
• Excessive liquid retention In a wastewater or sewerage application it is ideal that the pump station pumps out regularly. If excessive volume is allowed to accumulate, the liquid can go septic which results in increased odours and component corrosion.

 

How to set Your Float Switches 

To avoid these issues, it’s essential to set your float switches correctly. Here are some tips for ensuring your high and low-level float switches are optimally configured:

Low-Level Alarm for Stormwater and Sewer Applications (if required)

• For small pumps: Set the low-level alarm to reliably operate 50mm above the pump's suction.
• For larger pumps: Set it 100mm above the pump's suction.

High-Level Alarm

• Stormwater Applications: The high-level alarm can be set as high as possible to eliminate nuisance tripping in a heavy downpour situation i.e. at either the station overflow level or the lowest inlet drain—whichever is lower.
• Sewer Applications: Set the high-level alarm 100 mm below the invert level to prevent flooding of the pipework, or 100 mm above the start float, whichever is the lowest.

Start/Stop Float Settings

• Single Pump Systems: The start/stop floats come preset on single or 1-Phase pump stations.
• Twin Pump Systems: If the start/stop floats are supplied separate, they will need to be set manually. 

Note: For Stormwater applications the pumps should not exceed 15 starts per hour. For Sewer applications the pumps should not exceed 7.5 starts per hour. This involves setting the float switches so that the volume between start and stop operation is equal to 120 seconds of pump capacity.

High and low-level float switches as well as start/stop float switches are critical components in stormwater and sewer pump stations. By ensuring these float switches are properly set and maintained, you can prevent common issues like pump short cycling, unnecessary alarms, and the risk of overflow. Taking the time to calibrate and set your float switches will keep your pump station running effectively, protecting both your equipment and the environment. For more information, get in touch with a Pump Stations Direct team member for assistance.