Stilling Wells for Tank Gauging
Is your stilling well ready for tank gauging? Varec recommends the use of a "stilling well" for a radar or servo tank gauging application. The stilling well is designed to provide a stable gauging platform that is able to offset hydrostatic tank deformation. The deformation would otherwise result in 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 fact is 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 portion of the tank shell to provide that extra stability. The local ground or tank foundation must be sufficiently stable.
A stilling well that is not perforated makes the measurement system susceptible to external influences , such as product stratification or free water at the bottom of a tank and increases the chances of errors in your level measurements.
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.
In the example for stratification and free water, even more extreme and possibly even more dangerous cases can occur, such as an overfill.
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 Products -
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 Products -
For LPGs 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).
For additional information, review our "Stilling wells for radar tank gauging" and "Servo density profiling" white papers or contact a local Varec representative.