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The Challenges of High-Level Measurement of Tanks With Floating Roofs

Mon, 10/01/2007 - 11:12am
In many cases, storage tanks containing volatile materials such as gasoline, naphtha, and organic chemicals are fitted with a roof that floats directly on the liquid. It prevents the buildup of vapors, reducing the risk of fire or explosion. The roof also limits evaporation, decreasing both product loss and air pollution.

A floating roof, however, does present some unique challenges for high-level and overfill prevention detection. Since the roof is a solid object, traditional level switches used to measure liquids, such as tuning forks or ultrasonic gap switches, are simply not applicable. Some creative approaches have been used to indicate when the roof reaches a high point.

Tilt, Mercury, & Micro
Mechanical tilt switches have been tried, but these often hang up or are damaged by contact with the roof. Mercury switches can be used but regulations against mercury are making them less common, and the outlook for the future of these devices is even less promising. In addition, since mercury switches are mechanical devices, they are subject to hang-up or wear-out. Another once-common approach is the use of a micro switch with a long arm attached to a cam that senses the rising of the roof. This approach also is subject to hang-ups and is, thus, rarely used today.
Cable & Take-Up Reel
Another method uses a cable and take-up reel. The cable is attached to the roof or a float just below the roof. The reel takes up the cable as the roof rises and falls. Typically, the cable runs up and over the top of the vessel and is connected to a visual indicator on the outside. The concept is simple in theory. However, in practice, there is a great deal of difficulty in getting a reliable indication with all the moving parts associated with this type of device. Take-up reels hang up and cables kink. If the roof or float sinks, it can take the measurement down as well. While this type of level indicator can perform satisfactorily when first installed, it is inevitable that problems occur within several months to a year, depending on the installation and its surrounding environment.
Radar & Ultrasonic
Another favored approach is to mount a continuous non-contact level instrument, such as a radar or ultrasonic device atop the vessel to track the roof's movement. Although this approach is reliable and works well, it can be a fairly expensive solution if all that is required is a high-level stop-fill or shut-down alarm. In addition, continuous ultrasonic transmitters tend to have problems with vapors if the roof leaks or sinks.
RF Admittance


Instruments for high-level indication of floating roofs include the IntelliPoint and ThePoint no-calibration RF point level switches.
One cost-effective and reliable way to perform floating roof high-level measurement is an RF admittance switch with a flexible sensing element that can be mounted in the structure at the top of the vessel. When the roof rises to the point where it comes in contact with the sensor, a high-level alarm output is generated. A small weight at the end of the flexible sensor, which is made of brass to prevent sparking, makes contact with the rising roof.

The RF admittance switch also can be calibrated so that it alarms on the material in the vessel in the event the roof sinks, preventing an overfill condition. RF admittance switches rely on the electrical properties of the material under measurement — in this case, the floating roof. The switch is calibrated so that the capacitance generated by the sensor hanging in the air is balanced across a bridge circuit. When the roof reaches the sensor, the switch detects an increase in capacitance. That unbalances the bridge circuit and produces an alarm output.

There are no mechanical components to hang up or wear out. Sensors can be made to any length to accommodate a wide range of installation requirements. Also, RF admittance sensors can be modified in the field for higher or lower control points. And, since an RF switch does not rely on mechanical parts or piezoelectrical components, it is significantly more rugged that other technologies and can operate under a wider range of operating conditions. The result is a cost-effective solution that has no moving parts to hang up or wear out, provides years of maintenance-free operation, and is available with simple testing capability to ensure proper operation.

Technical information for this article was provided by Ametek Drexelbrook in Horsham, PA, a leader in RF/capacitance level measurement instruments including those used for high-level indication of floating roofs. More information is available by calling 215-674-1234 or visiting www.drexelbrook.com.

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