Chemical feed systems inject precise amounts of chemicals into processes to produce water fit for a specific purpose, maintain process efficiency, protect equipment and minimize costs. Chemical feed is separated into three main application segments including cooling, boiler and water treatment. Each requires a chemical feed system that basically includes a metering pump, storage tank and level sensor. Chemical feed tanks are small and often stand alone. They’re typically plastic mini-bulk tanks around 6’ in height, or IBC totes between 3’-6’ in height, or drums or pails that are 3’ or less in height. While purification requirements and processes differ in industrial, municipal and commercial applications, chemical feed plays a common role in water treatment. The primary requirement for this application is to monitor the liquid level and notify the operator when additional chemical supply is needed prior to running out, thus maintaining water integrity. Mini-bulk tanks are typically refilled from tanker trucks, while IBC totes, drums or pails are exchanged.
Technology
A short duration, high frequency ultrasonic sound wave is pulsed up to four times per second from the face of the transducer. The sound wave reflects off the surface of the liquid and returns to the transducer. The level sensor measures the time of flight between the sound generation and receipt, and translates this into the distance between the transducer face 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
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 or walls inside the tank. The level sensor must be located above the highest liquid level and never be submersed during normal operation. Water treatment systems may be located indoors or outdoors. If outdoors, the chemical feed tanks likely have no environmental protection, and will be directly influenced by heat from the sun or cold from the weather. Where so, temperature differentials may generate condensation within the tank. This more often occurs at the high end of the temperature range, but may also occur at the low end, resulting in the formation of water droplets or ice on the transducer, which can affect acoustic transmission and receipt. Where significant condensation exists, select a level sensor with a longer more powerful measurement range, and apply a very thin layer of Rain-X or petroleum jelly water repellant across the transducer face. Liquid is normally dispensed into chemical feed tanks from the top and removed from the bottom. This typically creates an agitated liquid surface during filling and a smooth liquid surface during emptying. Depending upon the liquid type and amount of agitation, foam may be generated. Foam absorbs the acoustic signal and can reduce the effective measurement range of the level sensor. Where foam exists, the level sensor should be installed in a stand-pipe to separate out the surface foam from the point of measurement. Another solution is to extend the fill pipe to the bottom of the tank and eliminate the agitation. Vapor is normally associated with corrosive liquids in higher concentrations, but may also be found in non-corrosive liquids at higher temperatures. Vapor absorbs the acoustic signal and can reduce the effective measurement range of the level sensor. Where significant vapor exists, select a level sensor with a longer more powerful measurement range, and properly vent the tank. With strong vaporous acids such as hydrochloric or nitric, the level sensor should also be installed in a stand-pipe to maximize the acoustic signal strength.
Installation
There are several ways to mount a level sensor in this application. The inside of a chemical feed tank normally has few obstructions or equipment. The tank top may be flat, domed, round or angled. Find a mounting location where the level sensor has a direct view of the liquid throughout the entire measurement span. The location must be flat, level to the liquid and accessible. The following equipment can be used to install the sensor.
Coupling
A shorter half coupling is preferred over a taller full coupling. Use a coupling that is slip x thread, and avoid thread x thread couplings. If you use a full coupling, it must follow the height and diameter restrictions described under Riser with Flange.