TANK LEVEL SYSTEMS
Tank level systems can be used simply to monitor the level in one or more tanks, or as part of a larger control system to collect and deliver real-time data about the tanks. In a stand-alone system, each tank would be equipped with a level transmitter and signals from those transmitters would be received at a central location for display. When used as part of a larger system, additional sensors might be installed on the tanks to collect temperatures, pressures, digital signals, etc., which would then be used by a control system as inputs for calculations and control logic.
For example, in automated blending systems prior to batch authorization, the tank level system is typically queried to verify sufficient available stocks in the raw material tanks and sufficient empty space in the destination tank. Tank temperatures and pressures can be used for real-time volume/mass calculations to fine-tune formulations on-the-fly during batch execution. High level switches on the tanks can be used to close valves and shut down pumps to prevent tanks from over-filling.
But regardless of its end-use, the heart of a tank level system is always its level sensors, and depending on the physical and/or chemical properties of the materials in the tanks; e.g., pH, temperature, vapor pressure, boiling point, flammability, etc, they could be any of several types. The most commonly used sensor in our systems is a simple head-pressure transmitter located at the bottom of the tank. Other types could include differential pressure, float & tape, ultrasonic, radar, capacitance, load cells on the tank legs, and others. Some applications use two pressure transmitters mounted at predetermined levels and an associated temperature transmitter so the computer can calculate the mass of the material in the tank instead of just the level. The particular sensor selected will typically depend on a combination of factors such as accuracy, precision, temperature range, electrical classification, and of course, cost. Some typical devices are shown in the following table; our level systems can utilize virtually any type.
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Signals from Wika or Rosemount (or equivalent) head pressure transmitters are combined with specific gravity data to determine liquid level. If the temperature of the tank is also collected, then adjustments for variations in temperature can be made on-the-fly. |
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Wika (or equivalent) ultrasonic level transmitters use sound waves to determine an ullage measurement, i.e. the distance from the surface of the liquid to the transmitter. This distance is subtracted from the tank height to yield the liquid level. |
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A Shand & Jurs (or equivalent) float and tape level gauge can be fitted with a transmitter to indicate the liquid level in the tank. This type of instrument utilizes a float riding on the surface of the liquid that is mechanically linked, via a metal tape, with a cylindrical indicator inside gauge unit. As the liquid level changes, the indicator in the gauge rotates to indicate the tank level. The transmitter is mounted on the side of the gauge and converts the mechanical rotary motion to an analog output signal |
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A Whitman float switch (or equivalent) can be mounted through the tank shell for point level detection. The float extends inside the tank and is physically lifted as the liquid level rises. The contact closure can be utilized to sound an alarm, start and stop pumps, close block valves, shut down power, or as an input in other control logic. |
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Capacitance probes like IFM Efector (or equivalent) can be used for point level detection. In these probes an electrode assembly is designed so that an electrostatic field is formed between the active electrode and the earth electrode. Any object entering this field will increase the capacitance. When the rising liquid level enters the field, a solid state switch changes state. |
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Programmable Logic Controllers (PLC) receive and manage all signals from the tank level system and provide communications with the operator interfaces, data networks, etc. PLC vendors and models are selected depending on the size and logic demands of the system. Vendors could include Allen Bradley, Siemens, PC-Direct, or others. | |
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Operator interfaces can vary widely depending on communication requirements and the amount/type of information to be displayed; such as text only, text and graphics, full interactive graphics, color, monotone, etc. Typical choices could include Allen Bradley Panelview, National Instruments - Lookout, Dell PC, and others |
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