The following table shows that similar chemical injection flowmeters that typically operate at higher flow rates exhibit similar potential savings when comparing flowmeters with 0.5% and 0.3% of rate accuracy.

Excerpted from Measuring Difficult Flow Streams and More Accurate Flow Control Can Improve Oil and Gas Well Profitability in Processing magazine.

Changing the "clean" water outlet flow on the interface level would easily take tens of minutes to affect the interface level at the opposite end of the extractor. Even though the remainder of the unit had electronic controls operated from a central control room, this extractor was controlled with a local pneumatic displacer level transmitter. Therefore, operators did not regularly monitor the extractor level nor did they really care about the extractor level unless an excessive amount of (valuable) product was found the daily “clean” water analysis. Needless to say, better extractor control was part of the justification for upgrading the instrumentation in this unit.

There was no question that the new instrumentation would be monitored and controlled in the distributed control system. Replacing the existing pneumatic interface level controller with its electronic / distributed control system counterpart is one approach to upgrade control of the extractor. However the time delay between "clean" water adjustments and interface level was excessive so it was decided to control the interface level directly above the "clean" water outlet to ensure a suitable separation zone and reduce the possibility of product leaving with the "clean" water.

Once the control strategy was decided, the instrument used to measure the interface level directly above the "clean" water outlet had to be developed. The extractor was designed and installed in the UK and it was not known if its manufacturer was still in existence. Mechanical drawings that came with the extractor were found in the mechanical maintenance files. The drawings and physical inspection revealed that the extractor did not have any nozzles in the separation zone near the "clean" water outlet (except the nozzles for the actual inlet and outlet flows).

How would you tackle this problem? Which interface level measurement technology would you use?

... more next month.

This article originally appeared in Flow Control magazine.

For flow measurement purposes, Boyles’ Law is often used to calculate the change in volume that occurs when pressure changes:

V / Vo = Po / P

where V is volume and P is the absolute pressure. In this problem, 1 kg of air initially occupies the entire volume of the tank (V0). There are 3 kg of air in the tank after it is pressurized with the additional air. Therefore, 1 kg of air now occupies only one-third of the tank volume. Estimating atmospheric pressure to be 1 bar (for ease of calculation), substituting into the above equation (where p is gauge pressure), and solving for p yields:

(1/3 V) / V = 1 / (p +1)

p = 2 bar

Therefore, the correct answer is Answer A.

Additional Complicating Factors

It is good practice to check the answer. A pressure of 2 bar gauge is an absolute pressure of 3 bar absolute. Boyles’ Law states that the volume of an ideal gas is inversely proportional to its absolute pressure, so air at 3 bar absolute pressure will occupy one-third of the volume that it occupies at 1 bar absolute pressure. Therefore, the air that originally filled the tank will now occupy one-third of the tank and the additional air will occupy the remaining two-thirds of the tank.

This article originally appeared in Flow Control magazine.