HI,

how i can calculate the URV and LRV for measuring of interface level of water and oil (s.g. 0.92) where as distance between lower and upper level is 2.5 meter (98.42 inch).

what will be the LRV and URV values for above data.

Regards
ahfaq

 

Hi there,

I can show you how to do the calculations but it will never work since you cannot use a DP transmitter of any kind to measure oil/water interface levels. We have tried, believe me, and our final conclusion was that there is only one application where it have a very small chance to work and that is if you have a vessel with a constant overflow of the oil, in other words a flooded vessel. Even then the conditions must be very stable for it to work and as you know not any process is completely stable.

If you still want to give it a try let me know and I will show you how the calculations are done and why I say it cannot work.

If you want me to do the calculations for your application, send me the distance between the tapping points as well as the distance between the bottom tapping to where the transmitters is installed. Also tell me where Z and S should be and what setup you want to use, capillary type or pipes transmitter. Also details of the transmitter you want to use.

My suggestion to you is to install a Guide Wave Radar, a displacer or a float type transmitter. The GWR are the best of the three for oil water I/f levels so go that route if at all possible.

 

Actually Sam is not quite correct, the company I work for measure Oil Water interface using differential bubble tubes and a DP cell in some very nasty fluids.

We usually get flack from a new client for this but eventually when they explore all the other methods they agree ours works, the others don't.

Guided wave radar makes sense for a simple oil water interface but sometimes the oil based solution has a higher SG than the water based solution so the signal cannot penetrate to the interface using a top mounted GW radar. Sometimes it comes down to a materials issue, the radars are simply not available in materials that will withstand the conditions.

Another example is where you get an emulsion layer at the interface, there is no clearly defined change in dielectric constant that radar relies on.

I see no reason why a low range DP cell could not be made to work provided you use the correct installation method you have a range 0-200 mmWC but you have to have some method of preventing the water entering the LP tap replacing the oil if the interface goes over the upper tapping point.

If the tank contents and compensating leg are at different or changing temperature this will also be a problem.

 

Hi again,

Just my response to Roy's post.
I agree with Roy that the GWR is obviously not suitable for all I/F measurements of all kinds of products, that is why I suggested to have a look at the dielectric constants first to see how suitable the GWR will be. The bigger the difference in DC the more suitable they are. It is also important to have the top product DC lower than the bottom product.

The original question from "ashfaq ahamd kahn" was for a oil/water interface, and not for all products, so we are slightly off the topic here.

Anyway, the emulsion layer is also taken into account in the GWR and compensated for as a constant 100mm. In a oil/water interface the emulsion layer is rarely more than that. So again for oil/water interface they work like a charm but like Roy said, not so for all I/F applications.

About Roy's opinions on using the DP cell,

Roy, if you find a DP cell to be a suitable instrument to use on interface measurements, good for you. I have spend considerable time on this issue in the past and my opinions is based on the results I obtained from this research and calculations. I have also seen the results with my own eyes when someone also said it can be done and went ahead and did it with a capillary DP cell with the LP connection on top of the tank, hoping this will eliminate the oil getting to the LP tapping, as you also said. The funny thing was that directly after the installation the DP cell starts indicating a level close to the sight glass and everyone agreed that it works and a letter is send to head office with a pat on the back, saying that the I/F level problem has now been resolved.
Two days later the problems started and the technicians start getting call outs two three times a day to go and try and get the reading accurate since it went completely out of bounds. They would set it up again and a couple of hours later if is off again in the opposite direction and so on. This went on for weeks until everyone had to agree eventually that it just does not work.

We then start looking at it mathematically and saw why this was happening and also proofed it mathematically why it is imposable to do, for this oil/water application anyway. If you use different products with different conditions and it works, like I said good for you. Personally I will never again try and use a DP cell on a I/F application of any sort.

If you are interested in the mathematical proof, do the calculations first for perfect conditions and you will find that it can actually work, but that is only theoretically and on paper. Once you start incorporating actual and realistic process conditions in the calculations, you will see why it is imposable to do. What I saw what was happening with the installation and the mathematical proof why this was happening was a perfect match.
The calculations also proof that the more stable the process conditions are, the slower and smaller the errors will be, giving the illusion initially that it is actually working. This will only be so until you get the first process or condition upset in your otherwise perfectly stable plant.

Keep in mind having a process measurement running exactly on set point does not mean you have control. Getting a measurement back to set point after a upset or condition change is control.

 

for example, my data was wrong for the actual range. how can i make a new calibration range in online dp type transmitter?

pls. help me!!thanks