Level instrumentation is required for a new vertical raw material storage tank that is 3 meters in diameter, 10 meters high, and open to the atmosphere. The specific gravity and viscosity of the raw material are 1.00 and 250 cP, respectively. 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?

1. 0 to 10 meters of water column
2. 0 to 9 meters of water column
3. 0 to 3 meters of water column
4. 1 to 10 meters of water column
5. 1 to 9 meters of water column

Differential pressure level transmitters for this type of application usually incorporate an integral diaphragm seal on the high-pressure side of the transmitter large enough to mitigate plugging. Sometimes, the diaphragm seals extend into the tank to eliminate voids that can solidify. Determining a reasonable calibration involves establishing the elevations of the level transmitter nozzle and hence, the transmitter.

Physically locating the level transmitter on a nozzle at the bottom of the tank is physically difficult, and can create a tripping hazard, but would 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, the tap is at 1 meter and results in similar physical difficulty in addition to possible plugging. Answer E suggests that the transmitter is at 1 meter and is connected to a nozzle at 2 meters, which is possible but, like Answer D, is not recommended. Answer C seems to reflect the tank diameter, which has no influence on the calibration.

In practice, the level transmitter nozzle would likely be located approximately 1 meter above the bottom of the tank, so the calibration would be 0 to 9 meters of water column (Answer B) which would be reasonable.

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.