Level instrument on vacuum tower

[petrotech50]

Just wanted your opinions on what works best for you in a vacuum tower for MVGO. We are cosidering a guided wave radar and also a Displacer. What are your suggestions?

 

[vpl]

MVGO?

Patricia Lougheed

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[HEC]

What are you trying to measure, fluid type and to what accuracy do you need it. Try not to use TLA's without explaination!

Mark Hutton

 

[SeanB]

MVGO = medium or mid vacuum gas oil. We use displacer type level instruments on our vacuum towers for our draw offs. I am by no means an instrument specialist but I don't see how a guided wave radar would work in a distillation column?

 

[HEC]

Basic options for level measurement are;
1. hydrostatic (pressure at the bottom of the liquid column)
This will be sensitive to headspace pressure and density of the liquid. Headspace pressure can be accounted for by measuring it and using the differential pressure to imply the liquid level, taking into account the density. If density varies then a third pressure sensor a known height above the bottom sensor is needed. The fluid density can be derived from this pressure differential.
2. Float and position measurement (I think this is a displacer type you refer to?)
A physical float and LVDT will provide a reliable level independent of vessel pressure. This system will be sensitive to density variations due to the float floating higher or lower.
3. Ultrasonic or Radar (distance measurement based on time of flight for a sound or radio wave)
Time of flight for reflected waves is independent of vessel pressure and fluid density. Ultrasonic (sound) can be affected by headspace density and other sources of reflections in the tank eg agitator blades and baffles. For a vacuum vessel application where variable vapour levels exist there may be an un reliable measurement. Radar is immune to headspace density variations, but more sensitive to tank internal reflections. This has been solved by the use of a guide (aerial) to reduce the false echoes.
4. Discrete level switches.
Simple robust switch at the level the process needs to be controlled to. If the control requirement is on at level 1 and off at level 2 then 2 switches can be used to reliably solve the problem.
The answer depends upon process requirements – accuracy, switching range, fulid and process conditions etc.



Mark Hutton

 

[HEC]

Sorry in short the guided radar will work in your application, but is it most appropriate form.

Mark Hutton

 

[SeanB]

Where do you mount the probe in a distillation column?

 

[doct9960]

petrotech50,

I am not an instrumentation specialist, but i have noticed that modern vacuum towers use nucleonic level transmitters. Two of the recent vacuum towers I've worked on used this type of level control. I was asked to design the vessel clips for the nuclear source and detector.

Browsing through the internet, I have read that radar level detectors does not work well in vacuum towers because of interference from inlet vapor or steam. (sorry forgot the website)

Here's a website for an overview of different level instruments

http://www.iceweb.com.au/Technical/LevelTechnologies.html

 

[StoneCold]

A lot of times on a distillation column or any large vessel you are better off to measure the liquid level in a pipe attached to the side of the column. With an inlet and outlet but really no flow other than level changes. Then you could put a guided wave radar in the top of the pipe and still have a sight glass or magnetic displacement float on the side of the pipe to either field verify the readings or just give you a second reading.

Regards
StoneCold

 

[doct9960]

petrotech50,

The nucleonic level transmitter I mentioned in my previous post was used at the bottom of the tower. At the MVGO tray, I normally see differential pressure level transmitters and magnetic level gauges used on them.