Understand tank level correction

For most tanks, the level height is not measured directly. Instead, the tank gauge is measuring the weight of the contents of the tank. Floating roofs, which may have their full weight plus the weight of any snow on the roof resting on the fluid contents, are commonly used. The gauge must therefore be corrected for the effects of this extra weight.

In addition, if the temperature of the container itself can be determined, the gauge can be corrected for this effect. This is referred to as the shell correction. Both internal fluid temperature and ambient temperature must be known to determine this value. The shell correction factor is typically quite small.

To calculate the tank level correction

Note: If the polled level has returned a volume (as a Gross Observed Volume [GOV] or a Gross Standard Volume [GSV]), no level correction is required, and it is not necessary to complete this procedure.

1. If there is snow on the roof, the roof is floating, and the Controller has entered this quantity as a thickness:

   1. Correct the level height by subtracting the snow thickness.

   2. Look up a new Total Observed Volume (TOV) from the strapping table using the corrected height.

2. Determine the Floating Roof Adjustment (FRA). If the roof is a fixed roof, or if it is a floating roof but it is currently resting on a tank leg, the FRA is zero.

   Otherwise:

   3. If the volume configured in the strapping table has already been adjusted for the floating roof effect:

      1. If the reference density used when configuring the strapping table is the same as observed density in the tank at current temperature, no correction is needed, and the FRA is zero.

      2. Get the observed density at the current temperature. If only the corrected density is available (that is, a product table density), then observed density for the current temperature can be derived from the appropriate density correction table.

      3. The FRA is the volume correction per density unit x (observed density – reference density).

         Where:

         — volume correction per density unit is a configured value.

         Note: The FRA in this case can be a positive or a negative number.

   4. If the strapping table volume is not adjusted for the floating roof effect:

      • The FRA is the weight of roof / (density x volume correction factor for temperature [CTL])

        Where:

      • Density units are consistent to both CTL and units of roof weight. That is:

        — Density is a corrected density, corrected to the same base temperature used for the CTL.

        — Density is expressed as weight per unit volume where the weight units are the same as the weight of the roof.

   5. If snow is present on the roof and has not already been applied as a correction to the level (refer to step 2), then the volume correction for the snow is determined as an addition to the FRA. The snow addition is zero otherwise.

      • Snow addition is snow weight / (density x CTL).

        Where:

        — The density unit consistency applies as detailed in step 3b.

   6. The FRA is the FRA + snow addition.

3. Determine the tank shell temperature correction (CTSh). This calculation is done if the ambient temperature is available in addition to the temperature of the liquid in the tank. Otherwise, the CTSh is 1.

   

   Where:

   • α is the Linear coefficient of expansion of the tank shell material.

   • This is hard coded as the value for Mild Carbon type steel and degrees Fahrenheit (0.0000062).

   • ΔT is the Tank Shell Temperature (TSh) – Base Temperature (Tb).

     Where:

     • Tb = 60°F

     • TSh = ([7 x Tl] + Ta) / 8

       Where:

       — Tl is the liquid temperature (provided by the polled analog).

       — Ta is ambient temperature (provided by the polled analog).

4. Apply the factors determined above to get the GOV.