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Associate

Join Date: Sep 2009
Posts: 36

# Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/02/2009 10:17 AM

Dear all,

With reference to the below pasted pic i have a case in my plant where the construction has been done for impulse tubing similar to this , please note that it is actually a closed vessel and not open. Now i can't seem to put my finger on the URV and LRV since it is a dry leg orientation and not a wet leg and the transmitter is between the HP and LP flange.

Any suggestions in this particular case???,if i am not wrong usually zero suppression is done for dry leg type dp transmitters where it is below the HP flange, and elevation in the wet leg case, how about this case ???

Thnx in advance

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Commentator

Join Date: Jun 2008
Location: India
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#1

### Re: Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/02/2009 6:38 PM

What I think in the closed vessel the liquid vapors are going to condenste on the LP side of the transmitter which will give errors in the reading if it is dry leg orientation.

This type of installation is mostly preferred with capillary seal type transmitters.

your second question is regarding zero suppression or elevation. It is to be done where you start to measure as zero of the vessel between the tappings which further corelates with LRV of the transmitter . LRV + span willbe the URV of the transmitter .

Associate

Join Date: Sep 2009
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#2
In reply to #1

### (PIC) Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/03/2009 2:54 AM

The pic below is the actual example i am talking about, any suggestions ??? primary suggestion can be to bring the instrument below the HP line but lets say if we can't then how to adjust the URV and LRV ??? Is it possible ???

Guru

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#4
In reply to #2

### Re: (PIC) Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/03/2009 10:20 PM

Hello Usman Nazar Rathore,

From the illustration in your original post, I had thought the application was liquid level, via differential pressure, in a tank/reactor. Your photo in Post #2 however seems to suggest a flow rate application. Is this true?

Mike

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Commentator

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#5
In reply to #2

### Re: (PIC) Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/05/2009 8:08 AM

From the picture it looks like they tried to create a standoff measurement column but if the tank in the background is the one being measured, the low pressure side will be filled up when the tank is half full. In addition, when the tank level gets below the level of the d/p transmitter, it will become dry, therefore it will never give you a correct reading of tank level. You are correct that the d/p transmitter should be either at the same level as the hp tap or lower.

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Commentator

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#6
In reply to #2

### Re: (PIC) Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/06/2009 10:34 AM

As per the photo you have shown there is a condensate pot which shows that it is wet leg measurement .ie LP side should be full of condensate.

Now on the HP side check the vessel design where the outlet is orientated .

if outlet of vessel above the HP tap than the HP tap will never go dry, so no need to change the Installation . if HP tap goes dry than there is a possibility of gas trap which can give errors in the readings, here you have to take the transmitter down or in line with HP tap

Also it is possible to set the LRV and URV by taking the oulet of the vessel as reference ie ;the zero of the vessel .

as I stated in above discussion

point to be measure zero of the tank say x mmwc as head pressure on HP side than

LRV= x-LP = -Xmmwc ( here you can set LRV by setting x)

so URV= -X+ span = -Y mmwc (span should be between the zero(x) and the upper tap)

hope you have clear picture of what I stated in this discussion

regards

Adil

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#3

### Re: Calculation of LRV&URV for a dry leg D/P Level transmitter above HP flange

10/03/2009 7:06 PM

It looks as though you have been plunked down into an instrumentation nightmare reminiscent of Hieronymus Bosch, or maybe the Three Stooges. (I apologize if you are too young to know of these references.) In this picture, it looks as though there is a vertical 2" (50mm) line with 1" (25mm) branches through some large-handwheel valves, reduced by ridiculous 2-bolt circular flanges to about 3/8" (10mm) tubing. I don't know what might be concealed near to the weld in the vertical pipe.

One item in this photo looks like a device meant to vent vapor, but of the four connections shown, only two are used (the others being plugged). Please review this.

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