Bubbler tubes provide a simple and inexpensive but less accurate (±1-2%) level measurement system for corrosive or slurry-type applications or in open or vented containers, especially those in harsh environments such as cooling tower sumps, swimming pools, reservoirs, vented fuel tanks, drain sumps, air washers, etc
In the bubble tube system, liquid level is determined by measuring the pressure required to force a gas into a liquid at a point beneath the surface as shown below:
Operating Principle
Bubblers use compressed air or an inert gas (usually nitrogen) introduced through a dip pipe called the bubble or sensing tube (which has an air flow restrictor at its end) immersed at a fixed depth into the vessel. The air flow restrictor reduces the airflow to a very small amount. As the pressure builds, bubbles are released from the end of the bubble tube. Pressure is maintained as air bubbles escape through the liquid. Changes in the liquid level cause the air pressure in the bubble tube to vary. At the top of the bubble tube is where a pressure sensor (transmitter) detects differences in pressure as the level changes. Most tubes use a small V-notch at the bottom to assist with the release of a constant stream of bubbles. This is preferable for consistent measurement rather than intermittent large bubbles.
Although reasonably accurate level measurement can be obtained without liquid entering the bubbler tube, bubble tube blockages still occur. Blockages can be minimized by keeping the pipe tip about 3inches from the bottom of the tank.
Merits of a Bubbler Tube System
- Equipment setup is relatively simple
- It is suitable for use with corrosive fluids.
- It is intrinsically safe
- It can be used for high temperature applications
Demerits of a Bubbler Tube System
- Requires compressed air and installation of air lines
- Build-up of material on bubble tube not permissible but does occur
- High level of Mechanical wear necessitating constant maintenance
Application limitations
Bubble tube devices are susceptible to density variations, freezing and plugging or coating by the process fluid or debris. The gas that is used can introduce unwanted materials into the process as it is purged. Also the device must be capable to withstand the maximum air pressure imposed if the pipe becomes blocked. When all the aforementioned factors are considered, d/p cells typically are preferred to bubblers in the majority of level measurement applications.
