Bubble Tube (Purge) Systems level calibration
The bubble tube system continuously bubbles air or an inert purge gas through a tube that extends to nearly bottom of the vessel at low flow rate. As showing in figure 20, the back-pressure in the bubble tube will be a function of the hydrostatic pressure or head of the liquid in the vessel. The lowest point of the purge tube determines the zero point reading; therefore any liquid below it cannot be detected.
The bottom of the purge tube is notched to keep:
•The bubble size small
•Allow the bubbles to escape easily from the tube.
•Take care to minimise the back-pressure pulses.
A Clearance gap has to be maintained between the bottom of the vessel and the tip of the purge pipe so that sediment does not block the tube. A blocked tube will result in a false reading (reads maximum level).
This type of system is susceptible to freezing or blocking/plugging by process fluid.
Care has to be taken to ensure that the purge gas does not cause a chemical reaction with the liquid in the vessel. Air must not be used where it is likely to cause a highly combustible mixture.
For this system to operate correctly there must be a constant airflow through the purge tube. Regulated pressure should be slightly higher than the maximum head pressure of liquid in the tank. The air pressure in the system will be equal to the hydrostatic head of the tank liquid at any point because any excess pressure will bubble out of the bottom of the tube. If the purge pressure is regulated at a value lower than this, then eventually a point will be reached where the bubbles will not escape from the tube leading to an incorrect measurement of the liquid level. Devices such as pneumerstats and constant differential pressure relays can be used to carry out the pressure adjustment automatically so that this problem will not occur.
Density variations of the liquid being measured will affect the reading.
Purge systems are particularly suited to measuring the level of:
•Corrosive liquids (brines)
•Viscous liquids
•Liquids containing entrained solids (slurry)