Quiz Corner: Differential pressure level calibration (water storage tank)

Level instrumentation is required for a new vertical clean water storage tank that is 3 meters in diameter, 10 meters high and open to the atmosphere. The tank sits on a concrete slab and its top is flat. Which of the following are reasonable calibrations for the new differential pressure level transmitter?

A. 0 to 10 meters of water column
B. 0 to 9 meters of water column
C. 0 to 3 meters of water column
D. 1 to 10 meters of water column
E. 1 to 9 meters of water column

Determining reasonable calibrations involves establishing the elevations of the level transmitter nozzle and transmitter. Physically locating the level transmitter on a nozzle at the bottom of the tank would be difficult, potentially create a tripping hazard, and result in a calibration of 0 to 10 meters of water column (Answer A). Answer D implies that the transmitter is at the bottom of the tank and results in similar physical difficulty. Answer C seems to reflect the tank diameter, which has no influence on the calibration.

In practice, the level transmitter nozzle would reasonably be located between 0.5 and 1.5 meters above the bottom of the tank. If located 1 meter above the bottom of the tank, the calibration would be 0 to 9 meters of water column (Answer B). Answer E suggests that the transmitter is at 1 meter and is connected to a nozzle at 2 meters, which is possible.

Answer B and Answer E are reasonable calibrations, but the calibration is dependent on establishing the actual elevations of the transmitter and its nozzle.

Additional complicating factors

The problem statement states that this tank requires level instrumentation. Even though the level transmitter can generate a high-level alarm, regulations and good engineering practice often dictate the installation of an independent high-level switch to help prevent the tank from overflowing by sounding an alarm and sometimes taking action, such as closing a fill valve or turning off a pump. The high-level switch generally should not utilize the same measurement technology as the level transmitter.