Bubbler system level measurement
A bubbler system is one of the hydrostatic type level measurement technique . Level measurement is done by measuring the pressure required to supply air throughout a dip tube . This dip tube is fitted vertically in the tank whose liquid level need to be measured . . The method is simple and inexpensive one . The other names of bubbler system level measurement are purge system ,dip tube system etc .In this article we will learn how such systems works????
Method
A pipe called dip tube with proper length is fitted vertically in a tank . Air is supplied through it It is allowed to escape from the bottom of the dip tube ..The pressure inside the dip tube is a measure of level in the tank .This pressure is also called as back pressure . Actually this pressure is a net balanced pressure of air supplied in to the dip tube against the head pressure due to liquid in the tank.Means it is the net effect of these two opposing pressure .
See above figure .A dip tube is provided in a tank .And air is supplied through this dip tube . Air flow is adjusted so that sufficient air bubbles always escaping from the bottom of the dip tube .A pressure sensing device is connected to the dip tube as shown .It reads back pressure created or the net pressure existing in the dip tube .
When the tank is empty there is no opposing head pressure due to the liquid level .And the air supplied to the dip tube bubbles out freely from the bottom of the dip tube .So the net pressure inside the dip tube drops to minimum. See figure below .
See below figure .In this situation back pressure is minimum because level is zero and there is no opposing liquid head . Pressure gauge reads zero which means level is zero .
And when the level rises these back pressure builds up because of the opposition offered by the liquid head .And this back pressure is maximum when the level is maximum .
See below figure .In this situation back pressure is maximum because level is maximum . Pressure gauge reads a pressure equivalent to the maximum level in the tank .see below
So by measuring the back pressure we can easily find the level .These back pressure is fed in to the pressure transmitter or differential pressure transmitter to read the level.
Arrangement -Bubbler system level measurement
A diagram shown below represents a typical bubbler arrangements
1.Dip tube
2.pressure measuring device .
3.Rotameter
4.Air pressure regulator
5.Isolation valve
Range calculations of transmitter for bubbler system level measurement.
Suppose we are planning to monitor a tank level .And we are using bubbler system for this . We have the data like tank height, maximum and minimum level positions, liquid density etc .A transmitter is used for measuring the back pressure thereby finding level . But we need transmitter with a proper range .Or the transmitter must have calibrated or ranged to suit for the given data .
So we have to find the range of the transmitter .For this we need to do some calculations .How will you do that? Before get in to that we need to learn some basic of hydrostatic type level measurement .
Basic principle -Hydrostatic type level measurement
Pressure due to a height of liquid column is proportional to the height of the liquid column provided that density is invariant .This is the basic principle in hydrostatic type level measurement .
In the above figure a liquid filled in a tank up to a height h will exerts a pressure ,
P = hρg .
Or
P = h × S .G
So if we are measuring pressure, we are measuring level , provided density of the liquid is invariant .This is the principle behind hydrostatic type level measurement .
Practical example -Range calculation
Let us calculate the range of a transmitter shown below diagram .
What is range of measurement of bubbler systems ??? .
It is from the dip tube bottom to the maximum limit of level measurement .
Why we measure from dip tube bottom?? Why we don’t taking it to the tank bottom???.
This is because back pressure variations happens when the level is in this range .when the level varies below the bottom end of dip tube,the back pressure not varies .This is because all the air bleeds out and back pressure drops to minimum or zero at any level below this end of dip tube
So at minimum level back pressure P = 0 mmwc .
At maximum level,100% ,back pressure P = h × S.G .
Here h is the height from the bottom of the dip tube .
So at maximum level ,back pressure,= 3000× 1.2 .
So The transmitter range 0 – 3600 mmwc .
0 mmwc…..4 mA (level in the tank is 1m because dip tube bottom is 1 m from the bottom of the tank).
3600mmwc ….20mA….. level in the tank is 1+3= 4 metres .
Note here , up to 1m from the bottom of the tank we couldn’t get the level variations because dip tube end is 1m away from the bottom of the tank and the level variations in that range will not produce any back pressure .So the 4 mA actually says level is 1 m or below 1 metre .
