Applying Ultrasonic Level Sensors In Above Ground Bulk Storage Tanks

Above ground bulk storage tanks are containers that hold large volumes of liquid and typically range from 10 to 32 feet in height. They operate under no pressure, are available in many shapes, and are widely manufactured out of plastic, fiberglass or metal. Vertical cylindrical tanks are the most common. Depending upon the liquid, they may also have a chemically resistant inner lining. There are many environmental regulations associated with the design and operation of bulk storage tanks, and above ground tanks have different regulations than below ground tanks. Bulk storage tanks have two basic control systems, one to add liquid into the tank and one to remove liquid from the tank. Some also have secondary containment walls to prevent spills or leakage, and all have good transportation access to support deliveries. The primary requirement for this application is to monitor the liquid level, enabling operators to know when it’s time to refill the tank, and prevent it from overflowing or running dry.

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.

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 or walls inside the tank. The level sensor must be located above the highest liquid level and never be submersed during normal operation. Bulk storage tanks are often located outdoors without any environmental protection, and are directly influenced by heat from the sun or cold from the weather. As such, 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 bulk storage 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.

There are several ways to mount a level sensor in this application. The inside of a bulk storage 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.

Tank adapters are recommended where the tank’s mounting location is level and not on a slope. Use a tank adapter that is slip x thread, and avoid thread x thread adapters. Do not use tank adapters that are mounted upside down.