There are many factors that affect the accuracy, reliability and repeatability of a tank gauging measurement, as each application, even on similar tanks, may be different. The considerations mentioned below indicate that tank gauging for bulk storage tanks demands qualified and experienced engineering to achieve highly accurate tank volume and mass measurement. At Varec, we have experienced personnel that can identify sources of errors and assist in overcoming and negating these errors as much as possible.
Tank Shape and Deformation
The tank shape itself can cause errors. The weight of the product against the side of the tank wall causes the wall to bow (deform). A vertical cylindrical tank suffers more deformation and errors than a spherical or bullet shaped tank.
Large tanks tend to have rounded corners to transition from vertical side walls to bottom profiles to help withstand hydraulic hydrostatically induced pressure of contained liquid. Measuring close to the tank wall is not recommended and most tanks do not have entries or stilling wells for instrumentation close to the tank wall.
Tank Capacity Table (Strapping Table)
The error in the tank capacity table is caused by the errors made with the proving equipment. Over time, the capacity of the tank will also change, so most companies recalibrate the tank every five to ten years. If errors do exist in the Tank Capacity Table, they are generally consistent and repeatable and hence appear on every measurement. Varec Tank gauging systems (FuelsManager® and 8130 RTU) are able to incorporate tank strapping tables into volumetric calculations to improve the accuracy of the inventory measurement.
Tank Bottom Movements
The tank bottom always moves in a different way when the tank is filled. Therefore, at the bottom of each tank, generally below the data plate, there is a "no gauge" zone (no measuring allowed). For instance, this occurs when the roof is not floating, but resting on its supports.
Tank Shell Thermal Expansion
Due to temperature changes, the tank characteristics and capacity may change. A tank with a stored product of 200°C has a different capacity and will deform more than at ambient temperatures. However, predictions and corrections can be made to overcome these differences.
Roof Weight Changes
With floating roof tanks, the actual roof weight also changes. Every time the roof weight or position changes, the product level will. For example, with heavy winds the roof can possibly oscillate and reduce in weight. Also, the rubber seals at the side of a floating roof can cause friction, so the roof does not move equally (or level) with the product. In cold climates there can be a large amount of snow on the roof, which also causes substantial changes in the roof weight.
Datum Plate Movements
Due to the tank deformation, the datum plate will move. This datum plate is a physical plate welded in the tank, which represents the official zero level in the tank. Measurement below this level is not permitted. It may not even be possible to get product out of the tanks due to the mounting position of the inlet and outlet pipes.
Right above the datum plate in a tank is the only official measuring point in the tank to measure the correct level. However, in some cases it is not possible to mount the level gauge at this particular point, so the gauge is mounted elsewhere. A correction is then made for the tilt of the tank. In some areas, the ground is quite weak and the tank will move slightly over the years. If a new Tank Calibration Table (TCT) or strapping table is made, these errors can be eliminated.
Stability of the Gauging Platform
Not all mounting positions are always stable. For instance, the roof of a fixed roof tank is only a few millimeters thick. Walking on such a roof will cause roof movements resulting in errors in the measurement. Tank wall deformation also affects the tank roof or a gauging platform. In some cases, this will cause permanent changes in the position of the gauge in relation to the product level.