density or interface/interphase level by bubbler
For measuring the interphase DP has been callibrated in the following way Density 1= 0.08, Density 2 = 1.12, height diffrence between dip tube 500mm.
URV(Upper range value)= 1.12*500 = 560mmWC
LRV(lower range value)= 0.8*500 = 400mmWC
In this case both LP and HP side of DP is connected to two diffrent rotameters.
interphase tanks having two rotameters each
A couple things could be going on here.
The use of 2 bubblers could mean the use of a measurement technique known as HTG or Hydrostatic Tank Gauging. HTG calculates the density of the liquid using the principle that the difference in head pressure readings for a given material (not two materials) between two elevations is proportional to the density (specfic gravity) of material.
The other thing that might be done is interphase measurement using a single head pressure measurement system and some calculations to indicate the interface level, or as you call it, interphase level.
Without knowing how the system is plumbed and what calculations are being done, it's hard to say. I would appreciate feedback from your colleague when he's available as to the actual setup & performance of bubblers in your plant.
1) HTG
The graphic below shows an open tank with two dip tubes, where each tube is one one side of a DP transmitter.
If the elevation difference (H) between the dip tubes is 500mm, then the specific gravity of the liquid is:
DP (mm w.c.) divided by 500 mm
If the liquid were water which has a S.G. of 1.000, the DP at H would always be 500.0 mm w.c.
For a chemical liquid with a specific gravity of 1.12, the DP would be 560 mm w.c.
Back calculating with a reading of 560 mm w.c.:
560 mm w.c. divided by 500 mm = 1.12 specific gravity for that liquid.
For a liquid less dense than water, the reading at H might be 400 mm w.c.
Back calculating with a reading of 400 mm w.c.
400 mm w.c. divided by 500 mm = 0.80 specific gravity
If the two tubes are connected to both ports of DP transmitter, then the measurement is most likely density in specific gravity. Given that you've provided the calculations for the elevation difference betweent the two tubes, it seems likely that the dual tube system is measuring density.
If the transmitter is a multivariable transmitter, one that provides
- DP
- high side AP
- temperature from a temp sensor
then other calculations are doable, usually in the control system.
With multivariables, typically a barometric pressure is either assumed or supplied from a barometric sensor, and the barometric pressure is subtracted from the high side absolute pressure value to provide a gauge pressure reading for the lowest elevation dip tube. That is then the liquid level height L (in the graphic) in water column units.
If the tank has only one material in it, then the physical liquid level can be calculated from the density and the L calculation.
2) There's another technique that calculates the interface/interphase level with a single point hydrostatic head pressure measurement. But it has RULES.
Rules for interface level by single point head pressure measurement:
1) Maximum liquid level must be held constant.
2) Densities of the two liquids must remain constant
If you break the rules the measurement is out the window.
Let's use the example you gave with your specific gravities:
liquid 1 = Density 1 = 0.80 (assuming a typing error for 0.08)
liquid 2 = Density 2 = 1.12
and a couple values for an example:
tank height = 4200 mm
constant liquid level = 4000 mm
At either extreme, the tank can be either full of liquid 1or liquid 2.
If full of liquid 1, the head pressure level would be 4000*0.80 = 3200 mm w.c.
If full of liquid 2, the head pressure level would be 4000*1.12 = 4480 mm w.c.
When there is a combination of both liquids, then each liquid will contribute a head
pressure in proportion to that liquid's level compared to the maintained constant level.
The table below shows 8 points, from zero to full scale, as an example.
Notice that the level of the heavy liquid is the interface/interphase level.
When the heavy liquid 2 is at 1000 mm, the remaing 3000 mm must be liquid 1.
The interface level is 1000 mm.
When the heavy liquid 2 is at 3000 mm, the remaing 1000 mm must be liquid 1.
The interface level is 3000 mm.
Since the rule is that constant level must be maintained, the values must range between the two extremes mentioned above.
The span between the extremes is 4480 minus 3200 = 1280 mm w.c.
This is the range of the DP readings. When the level is maintained with 2 liquids of the specified specific gravities, then the DP must range between these limits.
The ratio of the extreme span to the height of the tank is the scaling factor.
To calculate the interface level from the maintained pressure level:
maintained level minus [(max DP reading possible minus DP reading) times scaling factor ]
Example:
maintained level = 4000 mm.
max DP reading possible = 4480
DP reading = 3,520 mm
scaling factor 3.125
4000 - [(4480 - 3520) * 3.125]
4000 - [960 * 3.125]
4000 - 3000 = 1000
Interface level for a DP reading of 3520 = 1000 mm
3) If the vessel had a means of detecting actual level, then the rule 1 would not be needed.
The actual level could be substituted for maintained level, and be used in additional calculations used to re-calculate max possible DP reading, and the 'extremes span', all of which will dynamically change with a change in maintained column height level.
An example might be where a single bubbler is used to determine interface level, with a top mounted ultrasonic or radar unit to determine actual maintained level.
Dan
