TFW-9000CP Series Servo Tank Gauges

FW-9000 SUPER INTELLIGENT SERVO TANK GAUGE has been developed based on the technical and field application know-how obtained through Tokyo Keiso’s long time activity in level measurement field.

The mounted microprocessor for servo tank gauge offers high speed control/monitoring and remarkable improvement of function. In addition to normal measurement and transmission of liquid level and temperature, interface of two different liquids (i.e. Oil/Water), sludge height (or tank bottom height) and liquid density are measured and transmitted.

Two different sizes of wire winding drum are ready to cover from small process tanks to large storage tanks of max. height of 60m. Thanks to the improved and strengthened data communication function, maximum 8 points 2-way contact transmission from control room to field and vice versa and digitalized data transmission of 1 point of field analog signal (4~20mA). It is possible to offer combined monitoring and control of tank related instruments, i.e. pumps, valves, actuator, etc.

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Servo Tank Gauging

Tank gauging is essential for the assessment of tank contents, tank inventory control and tank farm management. Our servo tank gauges help in accurate level gauging for refineries, tank terminals and petrochemical industries. Our servo tank gauges are suitable for custody transfer as well as inventory control and are designed to be used in SIL-3 loops to prevent spillage.​​​​​​​​

How Does It Work?

​In the most challenging circumstances such as dome roofs and bad stilling wells, servo tank gauge maintains its ultra-high performance level on a wide range of applications and various tank conditions. The Enhanced Performance Signal processing enables the radar tank gauge to provide an accurate level reading even in difficult applications (close to the tank shell) and prevents the need for a costly stilling well. The servo tank gauge has been certified for use in SIL-2 and SIL-3 applications by the TÜV which allows it to be used in overfill protection applications. Additional diagnostics have been added to detect hardware failures, while a 2004D voting algorithm will detect, report and isolate failures on the boards without interrupting the loading or unloading of a tank.

What Problems Does It Solve?

​The servo tank gauge offers high flexibility that can be used for dedicated tank applications such as independent overfill protection, floating roof monitoring, rim fire detection, roof immersion compensation, etc. The servo tank gauge wireless option reduces the cost for wiring infrastructure and increases flexibility as various instruments can be added to the network without adding cables. The radar tank gauge is also certified for use in custody transfer applications.

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Selecting servo tank gauge technology for custody transfer considerations

Working principles of servo tank gauge

The principle of servo tank gauge measurement is based on Archimedes law, which states “any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.”

Typical servo tank gauges consist of three compartments: drum compartment, drive compartment, and power supply compartment. The drum compartment contains a precise machined drum on which the measuring wire is wound. The displacer is suspended by this wire into the tank. The drive compartment contains the drive train (servo tank gauge motor and electronics) and the measurement electronics.

A magnet coupling is used to convey torque between the drive train and the drum and forms the isolation between the process and the electronics and environment. The displacer is moved by a stepper motor in the drive compartment.

As soon as the displacer reaches the surface of the liquid in a tank, buoyancy reduces the apparent weight of the displacer. This is measured by the servo tank gauge electronics using the force transducer. By also keeping track of the exact position of drum and number of revolutions, the length of the expanded wire can be measured. In combination with the known position of the gauge (gauge reference height, GRH), the exact product level can be calculated. This level is then corrected for several typical tank uncertainties, such as product temperature, tank shell effects, changes in GRH due to hydrostatic tank bulging, etc.

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Four basic types of servo tank gauge

First, is the low-accuracy pulse radar units, now these are only “low accuracy” when compared to units designed for servo tank gauge. These are excellent general purpose radar units, but they are limited in use to materials with dielectric coefficients greater than about 8 or so. It generally rules them out for low viscosity hydrocarbons.

Second is the various radar-by-wire designs, such as Alliant(Bindicator-Celtek) and Magnetrol, etc. These are mostly used for solids, but could be used in hydrocarbons. They are also relatively inexpensive.

Next are the FMCW units, these are moderately high accuracy, and can be used in gauging fluids with a dielectric of 3-5 or so. The compromises made in the designs of these units in order to make them look through small (4″<100 mm> to 6″ <150 mm>) vessel ports reduces the accuracy and the ability of these units to see lower dielectric constants. These are designed as process servo tank gauges, not tank farm designs.

The highest level of accuracy are the units designed specifically for Tank Farm Gauging, they were designed to compete specifically with servo tank gauges and with hydrostatic level gauging and are designed to be used specifically for tank farms. These servo tank gauges consistently outperform their competition but are very expensive.

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Servo tank gauge instrument

The intelligent servo tank gauge Proservo NMS81 is designed for high accuracy liquid level measurement in custody transfer and inventory control applications with NMi- and PTB-approvals. It meets the relevant requirements according to OIML R85 and API 3.1B. It fulfills the exact demands of tank inventory management and loss control and is optimized in regards of total cost saving and safe operation.

  • Hardware and software developed according to IEC 61508 up to SIL3 (in homogeneous redundancy) for high level of safety
  • Maximum reliability through accuracy up to ±0.4mm (± 0.02″)
  • Developed according to international metrology recommendations such as OIML R85 and API MPMS
  • Local and country-specific certifications like NMi or PTB for custody transfer applications
  • Superior corrosion resistance with 316L materials in the process compatible with sour applications such as crude oil
  • Measurement of interfaces between up to three liquid layers, tank bottom, spot, and profile densities
  • Best fit for LNG/LPG applications without influence of dc value or Boil Off Gas

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Principle of Operation for Servo Tank Gauge

Our servo tank gauge operates 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. 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 our servo tank gauges. Servo tank gauges are ideally suited for light and middle distillates of petroleum.

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Servo Tank Gauge Datasheet

Servo tank gauge operates using the principle of fluid displacement to provide continuous level measurement to an accuracy of ±1mm. A highly sensitive torque measurement system continually measures the effective weight of a displacer and maintains the displacer in equilibrium at the interface between different fluids. The displacer is suspended on a stainless steel wire and is raised or lowered from a wire drum housed in the Gauge Head.

The drum is driven by a servomotor control mechanism via a magnetic coupling, thereby reducing friction to a minimum and isolating the control mechanism from the servo tank gauge environment.

Servo tank gauge functions can be accessed locally by a pushbutton located on the Gauge Display Unit or remotely using servo tank gauge software such as MTG 5000 Inventory Management System. Servo tank gauge functions can be controlled either locally from the Gauge Display Unit or remotely from a host computer. Current data for Level, Temperature and Gauge Status are stored locally in the Gauge and can be retrieved by the host computer or displayed on the servo tank gauge Display Unit local to the Gauge head.

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