Water Level Sensor Placement Guide
Water level sensor placement is the choice of where each sensing point sits in a tank, basin, sump or reservoir, and it decides when filling starts and stops, when alarms activate and whether pumps are protected from low water. A water level control system can only respond to the information it receives, so a sensor in the wrong place can make a good controller ineffective.
Proper positioning is a vital part of designing or upgrading an automatic water level control system. The sensing points should reflect the actual operating range of the equipment.
Start With the Normal Operating Water Level
Before deciding where sensors belong, determine the normal operating level for the application. A cooling tower basin, fire protection tank, wastewater sump and storage tank do not necessarily require the same control range.
In a fill application, the fill-start sensing point identifies when additional water is needed. The fill-stop sensing point determines when filling should stop. These points are separated to form an operating range, instead of switching the equipment on and off as the water surface moves.
The fill-start point must allow enough water for normal operation, and the stop point must leave enough space below the overflow level.
Keep Sensors Away From Misleading Water Movement
Water inside a tank is not always calm. Incoming water, pump suction, recirculation, turbulence and splashing can cause the level around one part of a vessel to behave differently from the overall water level.
Installing a sensor directly beside a forceful inlet can cause unnecessary switching because the sensor may repeatedly become wet and dry. Similar problems can occur where strong turbulence affects the sensing area.
The best location is one that represents the true level of the vessel as consistently as possible. This matters particularly where equipment cycles frequently or water is continuously moving.
Protect Pumps From Low Water Conditions
Sensor placement also plays a role in pump protection.
Once the level has fallen below a safe point, pumps that need a sufficient water supply should not be allowed to run. Operating a pump below its required water level can damage components such as bearings and shaft seals, so use the selected pump manufacturer’s minimum level and dry running limits.
A correctly positioned low level sensor can provide the signal needed to stop equipment, activate an alarm or initiate another programmed response before the water reaches a damaging level.
Its exact position depends on the system, including the pump intake, required reserve volume and the level at which continued operation becomes undesirable.
Leave Space for High Level and Overflow Protection
The upper sensing point should not simply be placed as high as physically possible. Tanks and basins need room between their normal full level and the point where water can escape.
The U.S. Department of Energy states that properly operated cooling towers should not have leaks or overflows, and recommends checking float control equipment and system valves to confirm the basin level is being maintained.
Where high level alarms are used, their sensing point should provide useful warning before an overflow occurs. The normal fill stop point and the high level alarm should perform separate jobs rather than respond at essentially the same height. On the WLC-5000 water level controller, for example, fill start, fill stop, high alarm and low alarm each have their own probe length. The difference between a high water alarm and an overflow alarm is covered in high water alarm vs overflow alarm for level controls.
Review Sensor Position When Conditions Change
A system that worked well when first installed may need review after equipment changes. A replacement pump, altered flow rate, new basin configuration, different operating level or changing water demand can affect the most useful sensing positions.
That makes sensor placement worth checking during upgrades and recurring maintenance. Operators should also look for physical changes around sensors that could interfere with consistent detection.
Controllers and Sensors for Defined Sensing Points
Reliable automatic control begins with accurate detection of the water level. Waterline Controls manufactures frequency-based level controllers and stainless steel sensors for cooling towers, tanks, sump systems, fire protection applications, HVAC equipment and other commercial uses. For help selecting a water level control system and setting sensing points for your application, call Waterline Controls at 480-905-1892.
References
U.S. Department of Energy: Best Management Practice #10, Cooling Tower Management
Waterline Controls water level control systems