Critical Zone: The Facts

Having visited many sites where tank operators have their own opinions & interpretations of the operational requirements of tank critical zones (aka zone of partial displacement), this article’s intent is to reinforce the facts of this complex item as it has a very real effect on the operational performance of tanks with internal floating appurtenances.

Before we commence with the facts on this matter, let us first familiarize ourselves in laymen’s terms per the following:

•  Before you run your daily bath at home you plug the drain and run the required water level to approximately half full anticipating that once you submerge your body the water level will rise to approximately three quarter full essentially giving you enough open room to clean and rinse your body without making a mess because the water level ended up to close to the rim.
•  With this in mind, after filling the water to half, and before entering the bath, take a height measurement of the water level. Remember this height.
• Enter the bath and take a measurement of this new height. Is the water level higher? Yes! But why?..

This is referred to as displacement – your body, being more dense than water, has displaced the water within the confines of the bath.

With your body you have created a zone of partial displacement in a bath knowing you will float if the same exercise was performed inside a larger body of water ie. a swimming pool.

Now with these 3 easy steps the same considerations, to a larger extent, can be made when considering the critical zone of a tank with an internal floating mechanism such as an IFR / IFB..

An IFR / IFB will displace a certain volume of liquid when it is in the free-floating position.

The weight of the liquid displaced will be equal to the weight of the roof and attached deadwood.

Therefore, the roof weight, liquid temperature, and observed density has to be considered when calculating the roof displacement.

The roof displacement is used to correct the tank capacity table volumes when the liquid height in the tank is at, or above the point or elevation where the roof floats freely (fully buoyant).

When the floating roof is resting on any of its supports (legs), the correction for roof displacement does not apply.

The liquid is partially displaced by the roof between the point or elevation where the liquid just touches the lowest section of the roof and the point or elevation where the roof floats freely.

This partial displacement area is referred to as the “Critical Zone” aka the zone of partial displacement.

The tank volume in this partial displacement area may, to a limited amount of accuracy, be computed using the formula indicated in our tank calibration tables.

However, the only accurate way to obtain volumetric data for a tank capacity table in the “Critical Zone” is by a liquid calibration procedure as per API 4269, but site constraints very rarely allow for this measuring exercise.

Computing the tank volume in the “Critical Zone” is subject to considerable error!

It is essential, therefore, that the opening and closing gauges be taken with the roof floating freely or with the roof resting on its normal supports and with the liquid height below the lowest section of the roof.

For the closest approach to accuracy, the roof should be floating freely for both opening and closing gauges.

The position of the critical zone is dependent upon the length of leg (low or high) in use.

Also, caution should be exercised if there is sludge and/or sediment in the tank as this can raise the level of the critical zone as indicated on the tank’s capacity table.

Should the displacement of the floating roof be increased due to accumulations of water it will be necessary to remove or estimate the additional weight in order to compute the roof displacement.

During custody transfer operations involving tank gauges, if water cannot be removed from a floating roof, it is best to keep the same conditions for both opening and closing gauges if possible.

So to conclude, the critical zone is a very complex item of measure when considering today’s value on hydrocarbons therefore as a whole, SCMS Africa does not recommend operating tanks within this zone as the inaccuracies partied to it are in excess of the accuracy range of 0,02%.