Gasoline/Water Interface Level


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We have a gasoline/water interface to be measured.

We first had a displacer many moons ago that measured this level. It has been cast away due to the many complications associated with them (If you work with them you know). Fastforward, we have tried Ohmart Vega guided wave with little success due to emulsion.

We now have a EH cap probe installed that works ok. But we will have process changes and my clear gasoline/water interface becomes very murky on the bottom (water side) and develops a emulsion layer about 3-4inch thick. My gasoline is staying the same fared value so no troubles there. When this happens we have to re-calibrate the meter, and then calibrate it back when it clears up. The changing of the media is inevitable due too the process. Looking for other suggestion what might work better. 100% full chamber, 36" span, let me know if there are any other details I might be able to provide. I do know why the water is murking up just can't do anything about it.
Yes I know what you are saying, I hate displacers as well. You were however on the right track before. You should use a GWR in this application. The GWR have a rule that you should never use the first 100mm part of the probe (bottom of the probe) for measurements and this is called the bottom dead zone. It also have a top dead zone. What we normally do is to set the zero to about 50 to 100 mm above the bottom dead zone depending on how long the probe or measurement is. In your case set your zero to about 50mm above the bottom dead zone and yu will never have the emulsion at the bottom bother you again. I am not familiar with the probes from Vega but the Khrone BM100 with twin rods should be able to solve this problem for you. Other makes and models with a similar feature should do the trick as well I imagine.

You should also look into getting a auto drain that runs on a timer installed or interlock the drain solenoid with the changes in the process that causes the build-up at the bottom.
If GWR isn't working, you should also look at using DP measurement since the chamber is always full.

It works based on variations in DP due to the changes in the overall SG of the liquid in the tank as the proportion of gasoline/water changes.

Distance between the taps x SG = DP
The distance is fixed, you're measuring the DP, so you can work out the actual SG of the liquid. If you know the SG of the water and the SG of the gasoline, then you can work out the proportions of each between the taps which gives you the interface level.

Some limits:
Both the upper and lower tap need to be covered at all times
You can only do interface measurement
There is a minimum recommened DP range (can't remember from memory - perhaps someone else could help on that)

On the plus side it's relatively cheap and it isn't affected by emulsions.

thank you for your input but I have to disagree with your suggestion. In theory you are right in that the only time a DP transmitter have a very slight chance of measuring an interface level is when it is installed on a permanently flooded vessel. In other words on a vessel or chamber where the gasoline/top product will always fill the chamber completely.

You can even do the calibration calculation which for interest sake is about two A4 pages long since you have to take into account that the two products have different densities and the pressure measured by the DP transmitter will vary depending on how much of each product is in the chamber. You will eventually come up with a very small calibration if you have done the calculations correctly. Something like -117mmH2O at the LRV and -121mmH2O as the URV. Obviously these values will depend on the size of the chamber or vessel you are working on.

We have actually done this before and when we put the transmitter on line everything was fine for the first 24 hours and the reading varied and it seems like it was working at first and then everything went all haywire. Now the reason why it did not last could be various reasons like small sg changes in either the water or product on top, but in our case we thought that since we worked with a product that is pumped from below the seabed we have found in the past that the actual composition of the product changes from time to time and that will mean we will have to go and recalculate the calibration of the DP transmitter again every other day.

Our conclusion at the end of the day was that unless you have a constant and calm flooded vessel, a very stable process and are very sure that there is no chance of changes to either the densities or compositions of either product, don’t try doing interface measurements with a DP transmitter.
An emulsion layer starts off as drops of organic in water
Moving up the drops get larger until suddenly the solution flips to drops of water in organic.

So it goes from conductive to insulating.
It's quite easy to track this in an unconventional way