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I am having a problem with a vessel that has two interface level transmitters, one of them a GWR unit the other a float style with 2-20ma output.

They are mounted in stilling wells off the side of the vessel two tap units with the bottom wet the top dry.

The problem is when the vessel is put in service the float style moves up to setpoint(12.5%) and the radar only goes about half as high.

After running at sepoint for a few hours if the radar is isolated and drained then put back in service it will shoot up to 46-48%. If the control valve is then put in manual and the float is isolated drained and put back in service it will also shoot up to 46-48%. Then when the valve is put in automatic both drop very quickly to zero then the process starts all over again.

The process is spent caustic/strong kero, the vessel has a weir that will allow the kero to overflow to the other side and the spent caustic is supposed to stay in it's side and drain trough the bottom of the tank.

I have about four members of management screaming that i need to get my instruments working right.

I'm fairly certain that there is nothing wrong with them and it is an application problem, that the wells are not giving an accurate representation of what is going on in the vessel.

Anybody have any ideas?

Hi

I have may be some idea but first is it possible to send us a drawing of your vessel with the tapping points of the level stilling well?
Do you have the values of the density?

Can you confirm that the top of the stilling well is in dry position?
regards
Bruno

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Taps are on the side of the vessel 30 inches center to center.
I believe that when the taps are drained and refilled I am only getting the bottom fluid and that is rising close to the overall level of the vessel.We are now trying to keep the interface level in the bottom tap but controlling at 5% is not really something I want to do.

The density of the upper is .85 and the lower is 1.2.
I really think the vessel should have been setup with stilling wells inside the vessel instead of on the side external to the vessel.
Now management wants me to get the factory techs for the transmitters out here because it has to be the instruments.

I guess I'll spend their money.

I think that taps on the side are not a "problem" but the situation with the bottom wet and the top dry is not designed for interface level measurement.

Is this situation (top dry) normal or just a start up situation for test only ? I agree with you: you have only a little more that the bottom fluid in your stand pipe on side (level of bottom fluid + a part of level due to pressure of the top fluid on the bottom fluid inside the vessel).

I saw many installations with taps on side of the vessel for level interface but with bottom and top wet.

Ken,

this is a fairly common issue when trying to measure interface in a stilling well (only bottom fluid entering the chamber). Is there any reason you are using one in the first place? If you need to use one and interface is the PV you want to measure, put it directly in the vessel. Make sure you drill plenty of holes or slots throughout the stilling well.

Chances are you could use a Rosemount GWR without a stilling well and you'd be fine.

You cannot measure interface with a dry top connection, the interface will be different to whats in the vessel and will never change. Your top connection needs to be below the weir so the fluid can circulate and the interface find it's correct level.

Is the velocity too great to put the GWR directly in the vessel without a stilling well?

Roy

Thanks for all the Input guys. We have three vessels that we measure interface on, two have the flooded tap design that has been mentioned above and work fine. Then we have the one I've been asking about. I had the GWR setup for level, NOT interface, because i did not see how it was going to work. I figured they were going to use the float style for the interface and the GWR for overall indication of level.....then they told me that both had to be measuring interface and my problems began.
I currently have a Rosemount application specialist writing me up something that will explain to my engineering department why it won't work....hopefully he will use small words.