Measurement: automatic Level Gauges (ALGs) - technical background
ALGs are more commonly known as Automatic Tank Gauges (ATGs). Automatic Tank Gauges will not be covered by the Statutory Instruments implementing MID into UK law. However, Notice 179 at 4.7 says Automatic (Tank) Level Gauges (ALGs) must be installed, tested and adjusted as recommended by the Energy Institute (formerly the Petroleum Institute).
Whilst we prefer Automatic Tank Gauges to dipping and ullaging, ALGs are often relegated to stock control rather being used for revenue accounting, where meters are now more commonly used. We prefer ALGs to automatically measure and compensate for temperature.
Mechanical ALGs have a float attached to a wire that runs over a pulley and is kept taught by a counter weight. ALGs can not only measure and compensate for the temperature of the oil, but also the affects of oil temperature on the gauge itself. However, there are a range of other ALG technologies that involve probes with non-moving parts. They can be connected up to software that continuously monitors the ALG at the same time it monitors pumps and flow-meters, so detecting leaks or other problems.
ALGs can be fitted to all types of tanks including floating roof tanks and fixed in the tank top, bottom or indeed side.
Types of Automatic Tank Gauge in general use
The earliest tank level gauges consisted of a float on the surface of the liquid in the tank, connected by a wire or tape passing over a pulley to a counterweight or spring-loaded drum. The indicator at the side of the tank showed the distance of the float from the tank bottom.
Most tank gauges now in use in the UK are based on the same principle but progress in their design and in the techniques of installation has brought about a continuing improvement in accuracy and reliability. Some of the features which have brought about this improvement are as follows:
- Compensatory systems are incorporated to maintain the measuring tape under constant tension and to correct its length for ambient temperature changes.
- The apparatus is mounted so as to avoid deflections arising from distortion of the tank sides or bottom caused by the weight of oil or bending of the roof due to vapour pressure or persons moving on it. The gauge may be rigidly fixed to the bottom or to the top of the tank.
- Friction is as far as possible eliminated. Servo systems may be employed to provide power to the liquid level detector, and may cause the detector to “hunt” up and down for short distances around the surface level. In gauges which do not hunt, a device may be incorporated which allows an operator to raise or lower the detector to check that it returns to the same level.
The liquid detector may either:
- displace a constant volume, irrespective of the temperature and density of the oil; or
- ride in a vertical perforated tube and settle by electrostatic equilibrium at a pre-determined height above (usually 1mm) or depth below (usually 2mm) the liquid surface. The principle used to detect the oil-air interface can be used to detect an oil water interface and so provide a dip of water bottoms in tanks. (This is rarely used in practice).
- “Damping” arrangements are sometimes used to smooth out ripples on the liquid surface and give a steady reading at the mean level. Such arrangements can deal effectively with ripples up to about 25mm in amplitude but are only partially effective with surface waves having periods of oscillation exceeding 2 or 3 seconds.
ALGs fitted to ships tanks are in general similar to those installed in shore tanks. They have to withstand more mechanical stress and higher corrosion and usually achieve lower accuracies.
Other types of automatic tank level gauge
The following Automatic Level Gauges are based on different principles:
- Electronic gauges. The liquid level in wholly electrical gauges is determined by its effect on the resistance or capacity of electrical circuits.
- Ultrasonic devices. The depth is determined from the time taken by high frequency sound waves to pass through the liquid.
Although these gauges have the advantage of requiring no moving parts, they are affected by the density of the oil and in some cases by its composition.