Stilling Well for Tank Gauging
Is your stilling well ready for tank gauging?

For most tank gauging applications, it is recommended to use a so-called "stilling well" for an Automatic Tank Gauge, Radar or Servo. This is because most tanks show hydrostatic tank deformation, resulting in the undesirable vertical movement of the tank roof and, hence, a change in the gauge reference height (GRH). Installing the gauge on the stilling well, which is more stable, improves the accuracy of the gauge by eliminating the GRH change. Unfortunately, not all stilling wells are suitable for an ATG platform. There are basically two 'must meet' criteria:

The first and most important is the fact that stilling wells must be perforated. Holes (or slots) allow free movement of product through the stilling well.

The second is that the stilling well should be supported from the tank bottom or from the lowest coarse of the tank shell to provide that extra stability. Local ground or tank foundation must be sufficiently stable.

A stilling well that is not perforated makes the measurement system susceptible to external influences and increases the chances of errors in your level measurements, such as product stratification or free water at the bottom of a tank.

A result of product stratification is that the product within the stilling well will not be representative of the tank contents. A small temperature difference of 1°C will cause a 0.1% change in density in most hydrocarbons. For a 12-meter high tank with 10-meter product, this means a level difference of 10 mm!

Assume a tank with free water at the bottom and the situation where the un-slotted stilling well extends into the water. If the tank is filled, the product flowing into the tank will displace the water and push the water into the stilling well. For a normal hydrocarbon, this will result in a 'product' level 20% lower than the 'actual' level in the tank.

For both examples, stratification and free water, even more extreme and possibly even more dangerous cases can occur, such as an overfill.

Stratification	Free Water	Slotted

The Solution is Simple
Try to use stilling wells with perforations only. API MPMS Chapter 3.1B contains recommendations. A good guideline is to have holes at least every foot, roughly one inch in diameter and drilled on opposite sides in a 'staggered' pattern.

For Very Viscous Product
Larger slots can be considered; one inch wide should be enough for 8", 10" and 12". For 4" and 6" stilling wells, smaller diameters should be considered, especially when used in combination with a radar device.

For Very Light Product
For LPG's and additives often stored in bullets or cigar shaped tanks, it is now possible to use Radar gauges with a 'wave-guide'. This acts as a dedicated stilling well and for these applications even smaller diameter holes can be used (5-8 mm diameter for every 15 cm).