Using Ultrasonic Level Sensors for Bulk Storage Tanks

Above ground bulk storage tanks are vessels which can hold large volumes of liquid and commonly range from 10 to 32 feet in height. They operate under zero pressure, are available in various shapes, and are widely composed of plastic, fiberglass or metal. The most common types are vertical cylindrical tanks. Depending upon the liquid being stored, they may also possess a chemically resistant inner lining. There are various environmental regulations that the design and operation of bulk storage tanks must adhere to, with above ground tanks having different regulations than their below ground counterparts. Bulk storage tanks have two basic control systems – one to introduce liquid into the tank and another to expel liquid from the tank. Some tanks also possess secondary containment walls to prevent spills or leakages from occurring, and all have excellent transportation access to aid with deliveries. The main requirement for this application is to monitor the fluid level, automatically refill the vessel, and prevent it from overflowing or becoming dry.

Technology

Ultrasonic sound waves with a high frequency but a short duration, are pulsed up to four times per second from the face of the transducer. The sound waves reflect off the liquid’s surface and return to the transducer. The time of flight between the sound generation and receipt are measured by the level sensor and translated into the distance between the face of the transduce and surface of the liquid. The distance is then deduced into a percentage of measured span and outputted as a proportional 4 to 20 mA signal.

Best Practices

The installed level sensor requires a clear view of the surface of the liquid, meaning that there should be no obstructions in measurement space below the level sensor, including pipes, apparatus or walls inside the tank. The level sensor needs to be positioned above the highest liquid level and should under no circumstances be submersed during normal operation. Bulk storage tanks are often found outside with no environmental protection. This means that are directly influenced by heat from the sun or cold from the weather, leading to temperature differentials which can generate condensation within the tank. This normally occurs at the higher end of the temperature scale but can also occur at the lower end. This sometimes results in the formation of water droplets or ice on the transducer, affecting the acoustic transmission and receipt. Where there is significant condensation, you should choose a level sensor with a greater measurement range and coat a very thin layer of Rain-X or petroleum jelly water repellent across the face of the transducer. Liquid is often dispensed into bulk storage tanks from above and removed from below. This can create an agitated liquid surface upon filling and a smooth liquid surface whilst emptying. Depending upon the type of liquid used, and the amount of induced agitation, the generation of foam is possible. The foam absorbs the acoustic vibrations and can reduce the effectiveness of the level sensor. It is recommended that the level sensor be installed in a stand-pipe when foam is present, as to isolate the surface foam from the measurement point. An alternative solution is to increase the length of the fill pipe so that it reaches the bottom of the tank and thus eliminates the agitation. At high concentration, vapor is usually associated with corrosive liquids but can also be found in non-corrosive liquids at elevated temperatures. The vapor absorbs the acoustic vibrations and reduces the range effectiveness of the level sensor. If significant vapor exists in your tank, it is advised to select a level sensor with a greater measurement range, and efficiently vent the tank. When using strong vaporous acids such as hydrochloric or nitric, the level sensor should be installed in a stand-pipe to maintain an effective acoustic signal strength.

Installation

There are multiple ways to mount a level sensor for this application. The bulk storage tank usually possesses few obstructions. The tank top can either be flat, domed, round or angled. Locate a mounting position where the level sensor has a clear view of the liquid throughout the whole measurement span. The location must be flat, at liquid level and easily accessible. The level sensor can be installed using the following equipment.

Tank Adapter

Tank adapters are recommended for use when the tank’s mounting position is level and not on an incline. It is recommended that a tank adapter used should be slip x thread, and thread x thread adapters should be avoided. Do not mount tank adapters upside down.

Coupling

Shorter half couplings are preferred over taller full couplings. It is recommended to use a coupling that is slip x thread, thread x thread couplings should be avoided. If you choose to use a full coupling, it must adhere to the height and diameter restrictions described under Riser with Flange.

Stand Pipe

You can install level sensors in a stand-pipe to separate surface foam, dampen turbulence or maximize the acoustic signal strength of the level sensor. Never use a stand-pipe in applications where dirty, coating or scaling liquids are present, as it will leave build-up of matter on the inner pipe wall. The inner diameter of the pipe needs to be equal to or greater than the beam width of the level, with larger diameter pipes being recommended. Installation of the level sensor requires the mounting of a low-profile threaded coupling onto the top of the pipe. Under the coupling, and within the dead band of the level sensor, drill two quarter-inch vent holes on opposite sides of the pipe. The pipe should reach the bottom of the tank, if not, it is required to be below the level sensor’s measurement span. Cut a 45° angle on the bottom of the pipe. Finally, maintain the level above the 45° cut, ensuring that there’s always liquid in the pipe.

Riser with Flange

Long, narrow risers, that extend within fiberglass tanks by a few inches from the top can affect the acoustic transmission and receipt. The internal surface of the riser needs be smooth in nature and free from ridges, especially in the region below the face of the transducer. It is highly recommended that risers with 3” diameters or greater should be used. If your only option is a 2” diameter riser, then the height of the riser and any mounting connections must not exceed 5”. Caution should be ensured with any riser heights that are greater than 8”, and it is also not permissible to use tee connections within the riser structure.

Center of Dome Top Tank

Whenever possible, you should avoid installing a level sensor at the center of a dome top tank. The dome top behaves like a parabolic reflector and can amplify acoustic energy that may cause the level sensor to become ineffective at certain tank levels.

Electrical

Bulk storage tanks are sometimes located near large pumps, motors or variable frequency drives which are known to potentially produce substantial EMI or RFI noise. You need to ensure that such devices are grounded to earth, and that the level sensor and associated electrical equipment are grounded to the same earth-ground connection. Some areas may be susceptible to regular lightning strikes or possess unreliable power sources. Whenever this is the case, it is recommended that efficient surge protection and filtering is in place.

Span

The level sensor outputs a 4-20 mA current signal that’s proportional to the measurement span within a bulk storage tank. Users normally set the 4 mA to either empty or the lowest level measured; and 20 mA to either full or the highest level measured. The placement of the 4 mA or 20 mA span setpoints at or near levels where pumps, valves or alarms may actuate should be avoided.

Interface

The 4-20 mA current signal in a level sensor is often connected to a local controller or centralized control system. These devices can include a PLC, SCADA, DSC or stand-alone level controller. Either is usable, so long as it accepts a 4-20 mA current signal. The operational range of the controller is then required to be programmed as to match the measurement span of the level sensor, while taking into account that the level sensors 4 mA setpoint is usually placed above the empty tank condition. Once the operational range of the controller has been set to the correct levels and engineering units, the relay setpoints are then applied to the pump, valve or alarm automation. You should always use an independent high-level alarm or safety shut off system with the primary system, and an independent low-level alarm or safety shut-off system is recommended for the protection of the pump and the process.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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