The bubbler system measures the liquid level by measuring the pressure required to force air or gas into a liquid at a point beneath the surface of the liquid.
The air purge system, also known as the bubbler tube system is one of the most popular hydrostatic pressure types of liquid level measurement systems. The bubbler tube system of liquid level measurement is suitable for all types of liquids.
Methods of Liquid Level Measurement
- Direct Methods
- Physical indication methods
- Sight Glass Indicator
- Float-type Indicator
- Displacer Level Indicators
- Indirect Method
- Hydrostatic Pressure type
- Electrical Method
Let us assume that a liquid is contained in a tank. At rest condition, the liquid will exert some force on the walls of the tank. The force in the liquid at rest is called hydrostatic pressure. The hydrostatic pressure is proportional to the height of the liquid (i.e., depth) in the tank.
The hydrostatic pressure methods used for liquid level measurement are:
- Pressure Gauge Method
- Air Bellows
- Air Purge System
- Liquid Purge System
In this article, we will discuss air purge / bubbler system
Air Purge / Bubbler System
Construction of Bubbler System
An air purge system or bubbler system contains a hollow tube that is inserted into the liquid in the tank. The hollow tube is called the bubbler tube. The bubble tube has two connections made to it. One connection is made with the regulated air supply and the other connection is made with a pressure transmitter. The pressure transmitter is then calibrated in terms of liquid level. The bubbler tube can be made of any material which can withstand the properties of the process fluid. There is a rotameter connected to the air supply so as to keep a check on the flow rate.
Working of Bubbler System
Initially, when the tank is empty, the air will flow out of the bottom of the bubble tube. Thus there will be no back pressure and the pressure transmitter will indicate zero.
As the liquid level rises in the tank, the flow of air will be restricted by the depth of the liquid. So, the air pressure that is acting against the liquid head will appear as the back pressure to the pressure transmitter. Due to this back pressure, the pressure transmitter will start showing the value which is calibrated in terms of liquid level.
To register the full range of head pressure, the air pressure is kept fed to the bubbler tube slightly above the maximum head pressure in the tank. The range of the bubbler system is determined by the length of the tube. When the level goes down and is below the tube, at that time also air can escape out and hence result in zero reading.
Due to the fact that the air is continuously bubbling from the bottom of the tube, the tube is said to be purged. The purging fluid is usually air. But in certain cases where air reacts with the fluid in the tank or gets absorbed, some other gases like nitrogen or carbon can also be used.
In this system, a pressure gauge as shown in the figure can also be used to show the level in the field.
Criterion for Liquid Level Measurement
- Reliability of purge gas supply- the supply of air must not stop or else, the level measurement will be inaccurate.
- The supply pressure of purge gas should never be more than the hydrostatic pressure. If the pressure of purge gas is higher, the level of liquid will fall below the actual liquid level.
- The flow of purge gas should be maintained at a low rate so as to avoid pressure drop errors. Pressure drop error is the excess pressure that is measured because of the friction of the purge gas through the tube.
- The gas used for purging must not react with the process.
- The gas used for purging should not contaminate the process.
- The purge gas will be consumed continuously; hence it should be economically feasible.
Advantages of Bubbler System
- The pressure transmitter or the pressure gauge can be mounted above or below the tank level. The pressure transmitter or the pressure gauge can also be mounted away from the tank with the help of piping.
- An air purge system or bubbler system is very useful in measuring the level of corrosive liquids.
- The construction of an air purge system or bubbler system is easy.
- An air purge system or bubbler system has a low cost of installation.
- The maintenance cost of the air purge system or bubbler system is also low.
- The measurement in an air purge system or bubbler system is not affected by turbulence in liquid.
Disadvantages of Bubbler System
- An air purge system or bubbler system is not suitable when there are suspended solids in the liquid whose level is to be measured.
- The inlet at the end of the bubbler tube is very tiny. Hence, there are high chance of it getting clogged.
- An air purge system or bubbler system is generally not used where biological growth is possible in the process.