Article Source:Dr. Rosdiazli Ibrahim,Universiti Teknologi Petronas (EEB5223/EAB4223 Industrial Automation & Control Systems)
Thursday, February 23, 2012
If the process liquid contains suspended solids or is chemically corrosive or radioactive, it is desirable to prevent it from coming into direct contact with the level transmitter. In these cases, a bubbler level measurement system, which utilize a purge gas , can be used.
Purge-type liquid-level measurement:
Usage: for corrosive liquids, liquids that contain suspended objects, high viscosity liquids, and underground tanks.
Principle of operation: A bubble tube is inserted into a tank, and a fixed flow of air is forced through the tube such that bubbles emerge from the end of the tube. If the air flow rate is not extremely large, the air pressure in the bubble tube can be considered to be the same as the pressure of the liquid at the end of the tube. The liquid level can be measured by measuring the air pressure.
Open tank application for bubbler system:
As shown, a bubbler tube is immersed to the bottom of the vessel in which the liquid level is to be measured. A gas (called purge gas) is allowed to pass through the bubbler tube.
Consider that the tank is empty. In this case, the gas will escape freely at the end of the tube and therefore the gas pressure inside the bubbler tube (called back pressure) will be at atmospheric pressure. However, as the liquid level inside the tank increases, pressure exerted by the liquid at the base of the tank (and at the opening of the bubbler tube) increases.
The hydrostatic pressure of the liquid in effect acts as a seal, which restricts the escape of, purge gas from the bubbler tube.
As a result, the gas pressure in the bubbler tube will continue to increase until it just balances the hydrostatic pressure (P=S.H) of the liquid. At this point the backpressure in the bubbler tube is exactly the same as the hydrostatic pressure of the liquid and it will remain constant until any changes in the liquid level occurs. Any excess supply will escape as bubbles through the liquid. As the liquid level rises, the backpressure in the bubbler tube increases proportionally, since the density of the liquid is constant. A level transmitter (DP cell) can be used to monitor this backpressure. In an open tank installation, the bubbler tube is connected to the high-pressure side of the transmitter, while the low pressure side is vented to atmosphere. The output of the transmitter will be proportional to the tank level.
A constant differential pressure relay is often used in the purge gas line to ensure that constant bubbling action occurs at all tank levels. The relay maintains a constant flow rate of purge gas in the bubbler tube regardless of tank variations or supply fluctuations. This ensures that bubbling will occur to maximum tank level and the flow rate does not increase at low tank level in such a way as to cause excessive disturbances at the surface of the liquid. Note that the bubbling action has to be continuous or the measurement signal will not be accurate.
An additional advantage of the bubbler system is that, since it measures only the backpressure of the purge gas, the exact location of the level transmitter is not important. The transmitter can be mounted some distance from the process.
Closed tank application for bubbler system:
If the bubbler system is to be applied to measure level in a closed tank, some pressure-regulating scheme must be provided for the gas space in the tank. Otherwise, the gas bubbling through the liquid will pressurize the gas space to a point where bubbler supply pressure cannot overcome the static pressure it acts against. The result would be no bubble flow and, therefore, inaccurate measurement signal. Also, as in the case of a closed tank inferential level measurement system, the low-pressure side of the level transmitter has to be connected to the gas space in order to compensate for the effect of gas pressure.
Basics of Instrumentation & Control
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