Level measurement using dP cell transmitter

[zrzr]

Hey guys,

I'm wondering if anyone can refer me to the correct ways of dP cell transmitter installation. The scenario is that I have a vent stack which has a glass type level gauge to monitor the level of water in the vent stack. (Water is used to isolate the process line to atmospheric)

I'm planning to remove the existing level gauge and replace it with a dP type level transmitter. (Or can anyone come up with a better method for this installation?). Using the existing available nozzles (2 nos), what would be the best practice to install this dP cell level transmitter.

My main concern is on the elevation of the dP transmitter.

What is the best elevation for it to be installed based on the vent stack level diagram? How is it calibrated?

How does a dP cell determine the level based on pressure differential? (Based on wet leg installation, etc)?


Hope you guys can help me on this. Thank you in advance.

 

[JLSeagull]

Start here:

http://www.engineeringtoolbox.com/static-pressure-head-d_610.html

http://www.engineersedge.com/instrumentation/differential_pressure_level_detector.htm

 

[djs]

The dP transmitter needs to be mounted at or below the level of the lower nozzle. You will also need to add provisions for purging the impulse lines.

 

[JLSeagull]

Cryogenic fluids permit a trick that is an exception to the installation location with the tranmitter below both taps. For cryogenic fluids in a much warmer atmosphere (such as liquid ethylene in Saudi Arabia) you can mount the transmitter above both taps using surplus impulse line, perhaps five foot. The fluid flashes in the impulse lines thus functions as a self bubbler.

 

[amptramp]

Here is a link to an appnote on tank level measurement using hydrostatic tank gauging.

http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00816-0100-3206.pdf

 

[roydm]

The second link from JLSeagull is the best
I like to modify figure A2 by running the dry leg up above the top connection then back down again, this eliminates the possibility of water running into the dry leg if the level reaches the top connection.
We often add a small flow of dry gas to the dry leg if moisture builds up.
If you use a wet leg (figure A3) add a large tee or condensate pot at the top valve this ensures the level stays constant with temperature change.
Finally you need to worry about a wet leg freezing if exposed to low temperatures.
Roy

 

[Standingback]

I agree with Roydm/LJseagull, but remember that if you use a wet leg reference (figure A3)on the HP and connect the LP to the stack, the output of the transmitter will be inverse to the level.

I trust this helps,
Mlv

http://www.coulton.com

 

[zrzr]

Guys, thank you so much for the useful answers and information.

 

[Magnetostrictive]

You don't say why you are replacing the glass level gauge but I assume it's due to a remote readout requirement.

Anyway, a good solution for this would be a Magnetic level gauge, the advantage being you have a clean and visible display, you can fit it to the existing connections and you can also install an external transmitter and/or switchs to give you a remote signal.

See the link below for a typical unit.

http://www.tc-fluidcontrol.com/Products.aspx?thirdPage=86

Good luck with what you decide.