Tank Gauging Technologies

Changes in the liquid level inside the tank raise or lower a large stainless steel float. The float is attached to a powerful negator spring via a perforated tape. The negator spring provides constant tension, which balances the float on the liquid level. The perforated tape engages pins on a sprocket wheel that, in turn, drives the counter assembly.

When a gauge board is used to display level in a float & tape system, the negator spring is replaced by a counter weight system. The liquid level in feet and inches or meters and decimeters is displayed on the gauge counter or indicated on a gauge board. This simple design and operation allows the gauge to perform with negligible maintenance throughout its working life.

A range of analog and digital tank gauge transmitters are available that mount directly to mechanical tank gauges. Level measurement data is encoded by the transmitter and output via industry standard communications to the control room.

Some transmitters also offer spot temperature measurement integration that can be used for inventory control applications. When a tank gauge transmitter is used, communications and power are required at the gauge head. Varec transmitters do not require an adaptor flange. When connecting third party equipment, a specific adaptor flange, depending on the transmitter, is often required.

Simplify the process of negator motor replacement and improve the performance of your mechanical tank gauge with a removable Negator Cassette with the negator motor enclosed.

Varec servo tank gauges operate on the principle of displacement measurement. A small displacer on a measuring wire is unwound from a drum and accurately positioned in the liquid medium using a servo motor. The weight of the displacer is precisely balanced against a magnetic coupling and the wire drum. When the displacer is lowered and touches the liquid, the weight of the displacer is reduced because of the buoyant force of the liquid. As a result, the torque in the magnetic coupling is changed and this change is measured by 5 sets of Hall sensors (U.S. patent). The signal, an indication of the position of the displacer, is sent to the motor control circuit. As the liquid level rises and falls, the position of the displacer is automatically adjusted by the drive motor. This system provides an outstanding accuracy of +/- 0.7 mm. Servo gauges are ideally suited for light and middle distillates of petroleum.

Principle of Measurement

Radar tank gauges are “downward-looking” measuring system installed on the tank roof. Operating on the time-of-flight method, they measure the distance from the reference point (process connection) to the product surface. Radar impulses (FMCW principle) are emitted by an antenna, reflected off the product surface and received again by the radar system. The distance to the product surface is proportional to the travel time of the impulse. Due to the nature of the microwave, radar tank gauges need to be equipped with functions to suppress interference echoes (e.g. from edges and weld seams) in the tank so they are not interpreted as level measurement. Radar technology is suitable for measuring a wide range of petroleum products.

Process Conditions

Radar gauge measurement is independent of the process conditions inside the tank. Temperature, pressure and gas vapors have minimal affect on “radar beams”. The dielectric constant of the liquid being measured is a critical factor when considering a radar tank gauging solution.

Radar Antennae

It is essential for specific application and installation requirements to be evaluated and the correct antenna type selected. The Varec range of radar antenna include:

• Horn and Parabolic antennae – suited for free space applications
• Planar antenna – specifically suited for stilling well applications
• Small horn antenna – suitable for installation on existing tank fittings

Transmitters convert the measurement parameter (rotation, resistance, etc.) to an electrical signal for transmission over an instrumentation field bus or communications loop. Most transmitters use an incremental or absolute encoder with either brush, optical, magnetic or the latest capacitive sensors.

Encoders

Incremental encoders count steps up or down to find level and then store the information in memory. A power loss can causes the transmitter to forget where it is, so recalibration or a battery backup may be required. Absolute encoders count the absolute position on the encoder disc to find level. After a power loss the encoder knows where it is, so no battery backup and no recalibration is required.

Sensors

Brush sensors make contact with the encoder disc causing the encoder to wear down over time and brushes may “feather”, which will affect the accuracy or reading. Brush sensors need replacing every 5-8 years. Optical sensors, either linear or reflective, do not make contact with the encoder disc. Sunlight may affect calibration (except with infrared sensors) and photo-etched (painted) discs may fade, affecting the reliability and accuracy of the reading. Varec’s 2920 FTT uses the latest capacitive sensors with gold plated etched discs to provide maintenance free operation with the most accurate output to on the market to date.

Temperature Measurement Integration

Temperature sensors connected to the transmitter allow the inventory system, such as FuelsManager^®, to calculate volumes that are temperature corrected. Spot temperature sensors are appropriate for float & tape tank gauging, while average temperature devices are generally associated with radar or servo tank gauges due to the increased accuracy obtained from the level device.

In order to determine net standard volume in the oil and gas industry, it is necessary to measure the temperature of the product. Once the product temperature is known, it is possible to calculate the volume at an internationally agreed upon ‘standard’ temperature – typically 15°C or 60°F.

A temperature change of 1°C causes a change in volume of about 0.1%. The American Petroleum Institute (API) and American Society for Testing and Materials (ASTM) recommend a minimum of three temperature measurement devices for inventory control applications and a minimum of six points for custody transfer applications.

The more elements used, the more accurate the measurement. In practice, between one and sixteen temperature elements are used in a tank.

Hydrostatic tank gauging utilizes up to three pressure sensors and one or more temperature elements. The hydrostatic pressure (liquid head) is measured; as the height increases the pressure increases as well. If a single pressure sensor is used, a manual density measurement is required for an inventory calculation. A second pressure sensor is used to enable automatic density calculations. A third pressure sensor is generally used to measure vapor pressure and increase the overall accuracy of the system. Combined with the tank strapping table, this technique provides an excellent method to measure mass, but liquid level, volume and density are only indirectly inferred. A 0.02% full scale accuracy, which is dependent on the pressure sensors installed, is normally desired to provide reasonable inventory management.

Hybrid tank gauging combines an accurate level gauge, temperature sensor(s) and pressure transmitter(s). By utilizing the best of both level based and mass based systems, hybrid tank gauging obtains level, temperature compensated volumes, mass and density measurements.