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Commentator

Join Date: Nov 2007
Posts: 88

# Level Measurement Under Vacuum

03/20/2012 2:41 AM

Hello every one,

I am facing a problem with calendria level measurement. We have a effluent plant where in we have calendria where effluent is concentrated using multi effect evaporation. We need to monitor and maintain level in it. During operation calendria is under a vacuum of 100mmHg absolute. vaccuum applied from top of calendria throuhg a condenser. and an axial flow pump continuosly recirculated the effluent in the calendria.

I have installed a DP level transmitter with remote seal on HP side and impulse tube on LP side. I have connected LP to top most part of calendria to measure vapour pressure. and HP to bottom of calendria. and calibrated and done ranging under running condition.

My problem the level transmitter is not displaying correct value of level it varies with different values of vaccuum applied and different levels.

I am unable to find what is wrong with this setup and what could be the right method.

Thanks

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Guru

Join Date: May 2009
Location: Richland, WA, USA
Posts: 20064
#1

### Re: level measurement under vacuum

03/20/2012 3:52 AM

If both sensor are not submerged in liquid, of course it will give incorrect values.

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Guru

Join Date: Mar 2007
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#3

### Re: level measurement under vacuum

03/20/2012 10:29 AM

I am not sure you are right and for a simple reason:

The difference in pressure is proportional to the liquid level over the bottom sensor.

Which means that one sensor should measure pressure over the fluid and the second at the bottom of the tank. The difference is proportional to liquid hight over the sensor at bottom since pressure will be same all over. What I fear is that the sensors are not ABSOLUTE pressure sensors but relative and this could have an influence in the zero shift and in the difference value. If both sensors are in the fluid the difference will be constant as long as the space between sensors will be filled with fluid.

If I am not clmear enough let me know and I shall make a sketch.

Guru

Join Date: Jul 2010
Posts: 651
#2

### Re: Level Measurement Under Vacuum

03/20/2012 9:27 AM

Please provide some examples of level and vacuum values for what you expect and what you are getting at various vacuums.

What temperature does the process run at?

Does the level read correctly at atmospheric pressure or at any particular level?

What is the LRV and URV setting on the DP transmitter?

What is the expected level maximum?

Does the process change (extract or add a component, perhaps) or chemically change the medium so that the specific gravity of the solution changes during the course of the process, so that when you start, the SG = X, but as the process continues, the SG changes to Y?

Commentator

Join Date: Nov 2007
Posts: 88
#4

### Re: Level Measurement Under Vacuum

03/20/2012 12:47 PM

Hi guys please find the sketch of the calendria.

Commentator

Join Date: Nov 2007
Posts: 88
#5

### Re: Level Measurement Under Vacuum

03/20/2012 12:49 PM

I am unable to comprehend, how come vaccuum is only effecting the HP side, and why should it be that both HP and LP should be under fluid?

Specific gravity in this case changes as steam is applied to the calendria, but this case i have taken trial with out applying steam, so specific gravity effect i guess is negated.Still it doesnt work

Guru

Join Date: Jul 2010
Posts: 651
#6

### Re: Level Measurement Under Vacuum

03/20/2012 1:01 PM

Is there any particular reason you don't want to answer my other questions?

Power-User

Join Date: Mar 2011
Location: oz
Posts: 208
#7

### Re: Level Measurement Under Vacuum

03/20/2012 9:18 PM

Your HP should be at the top of tank, this should be filled with effluent.LP at bottom.The problem with you current set up is because it is under vacuum your LP is filling with effuent and causing the reading to be wrong.

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Commentator

Join Date: Aug 2010
Location: Venezuela
Posts: 91
#8

### Re: Level Measurement Under Vacuum

03/20/2012 9:54 PM

Friends,

I agree a lot with Irish

How about the temperature? How much is variance the temperature during runing condition?

Maybe, you will need use other level measurement technology, other that is not affected by the pressure/temperature changes.

Is this a new plant?

Regards

JP

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Guru

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#9

### Re: Level Measurement Under Vacuum

03/21/2012 12:12 AM

Is your DP transmitter similer to Rosemount 1199 Seal Systems?

If yes, please note the 3rd point in perticuler:

A seal system consists of a pressure transmitter, one or two seals, a fill fluid, and either a direct mount or capillary style connection. Seal systems provide a reliable process pressure measurement and prevent the process medium from contacting the transmitter diaphragm. Transmitter/diaphragm seal systems should be considered when:

• The process temperature is outside of the operating ranges of the transmitter.

• The process is corrosive and/ or requires specific exotic materials of construction.

• The process contains suspended solids or is viscous and is prone to plugging of connections.

• The application requires the use of flush-mount hygienic connections that facilitates CIP/SIP service.

• There is a requirement for easier cleaning of the process from the connections to avoid contamination

between batches.

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Power-User

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#10

### Re: Level Measurement Under Vacuum

03/21/2012 12:51 AM

Measuring in a closed vessel especially one under vacuum requires a reference leg to measure against

You need to put the LP side of the transmitter to the process and the HP side to the top of the vessel. The HP leg must be kept full of process material so the transmitter has a reference point to measure against.

Calibrate your transmitter in the negative region ie: -x place measurement type here (-height of your reference leg x S.G. of liquid) = 0% = 4.00mA or 0.00mA depending on your choice of transmitter range.

Your 100% = 0 place your measurement type here (0 x S.G. of liquid)

or if your 100% level is below the height of the reference leg it will be ( 0- difference in height x S.G. of liquid. ie you may end up with to negative measures as your 0 and 100% of range.

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Commentator

Join Date: Nov 2007
Posts: 88
#11

### Re: Level Measurement Under Vacuum

03/21/2012 1:14 AM

Will try the recommendations. But

HP stands for high pressure side, so why should it be connected to top of tank? i guess pressure is high at bottom?

effluent is a high TDS liquid with almost 30% TDS, but not corrosive.

In my case i am using emersion transmitter with remote seal on HP side and impulse tube on LP side. and i have connected remote seal that is HP to the bottom side which is filled with liquid and impulse tube which is dry is connected to LP.

How can i fill LP side with liquid because it is intended to measure only vapour ?pressure

Power-User

Join Date: Mar 2011
Location: oz
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#12

### Re: Level Measurement Under Vacuum

03/21/2012 2:15 AM

As i Stated earlier you have the HP and LP incorrect, I did not give much details as the person above which is a very good explanation but If you fill the leg to the top to 100% it now becomes the HP because more solution, it will now measure the the difference in level to the tank.

You have to possibly install a T piece so you can fill the HP{not LP} side to the100 % mark.

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Commentator

Join Date: Aug 2010
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#13

### Re: Level Measurement Under Vacuum

03/21/2012 8:36 AM

It is very probable that the problem is caused by the LP side tubing.

impulse tube which is dry is connected to LP <---- sometimes this is possible and sometimes this is not correct, because some vapour return to liquid state creating a little hydrostatic pressure on the LP side producing some error in the level measurement, this error is going to increase due the vapour return to liquid in the LP side.

You must choose some liquid compatible with the process condition in order to put on the LP side (wet leg reference) and recalculate a new range having the HP side fill fluid on the seal as HP side minus the pressure created by the reference LP side, you know

I hope this help

JP

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#15

### Re: Level Measurement Under Vacuum

03/22/2012 2:51 AM

In this case the HP is not being used as the measurement side but as a reference that the changing LP side can use. This is why your range will be in the negative region of the transmitter.

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#16

### Re: Level Measurement Under Vacuum

03/22/2012 2:58 AM

Unfortunately your selection of transmitter will not work for the vessel enviroment you have. You will require a standard DP/GP transmitter with impulse lines/product lines to each side. The common mistake people make is not making sure that the HP connection to the top of the tank is kept full.

This arrangement will also work on high pressure vessels. The common mistake was people draining the HP impulse line instead of keeping it full.

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#14

### Re: Level Measurement Under Vacuum

03/21/2012 11:06 PM

I have tried below things mean while.

Have interchanged the HP and LP directions, now HP(Remote seal) is connected to top side or vapour side. and LP that is impulse tube line is connected to the process side.

Recalibrated the instrument for zero span, but after the starting the bottom axial pump and vacuum pump to suck vapours from top, the reading in the transmitter is gradually reducing and showing 7% reading in level, where actual level is 50%.

Below is the additional info on this.

the vertical length between top level(LP) and bottom level(HP) is 240cm. process fluid feed is at 20% TDS which is enriched to 40% by heating it......

One technique a friend suggested is to put a pressure transmitter on the top and bottom the calendria, and the difference between the pressure readings will indicate the true level of liquid, but i suspect that is what happens in a DPT.

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#17

### Re: Level Measurement Under Vacuum

03/22/2012 3:06 AM

You are on the right track but just putting the remote seal to the top of the tank does not give the transmitter its reference point of 100%. On further thought what you could do is place the HP remote seal at the same level as your LP point and run a tube back to where the HP seal is mounted at the moment. This tube must be kept full of process to your 100% level. This point I can not stress enough. Hope this helps.

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Commentator

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#18

### Re: Level Measurement Under Vacuum

03/22/2012 8:31 AM

but after the starting the bottom axial pump and vacuum pump to suck vapours from top, the reading in the transmitter is gradually reducing and showing 7% reading in level, where actual level is 50%. <-- Before to start both pumps is correct the level indicated by the transmitter?

process fluid feed is at 20% TDS which is enriched to 40% by heating it.....<-- Is this happen inside the calendria vessel? How much is the initial and final temperature in order to enriched the TDS?

How are you doing the calculus for the range of transmitter?

Initially, I can recommend you that use the PV value showed by the field communicator for calculus the range, I mean use the real value sensing by the transmitter instead of apply the formule LRV = 240cm x HP seal S.G., of course these two values must be very similars but one of this is theorical and the other is real.

Is the LP tubing at the same level or below of the process tap? Where is located? Is there some height difference between the LP side tubing and the transmitter capsule?

Regards

JP

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#20

### Re: Level Measurement Under Vacuum

03/25/2012 3:28 AM

dear friend,

one most effective level measurement technique in your case is " displacer type level transmitter" . you can use this. and it will work surely. all the best.

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#19

### Re: Level Measurement Under Vacuum

03/22/2012 3:15 PM

It is very important that you understand what you are doing, if not you are going to run into many problems.There are some good literature if you google level meters for boilers.

Participant

Join Date: Mar 2012
Posts: 3
#21

### Re: Level Measurement Under Vacuum

03/29/2012 9:53 AM

Hi Dear,

we have same problem in multi effect desalination package and all of Dp level transmitter in brine & product water display wrong level. can you inform me or send any reference that we can solve the problem?

thanks

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Commentator

Join Date: Nov 2007
Posts: 88
#22

### Re: Level Measurement Under Vacuum

04/01/2012 1:30 AM

Finally, i have replaced the DP transmitter with magnetic float type level transmitter, its working fine.

But the thing that still haunts me is that can a DP type transmitter be used for level measurement under vacuum in multi effect distillation column, has any one tried this ? or any reference data available on this.

Thanks

Participant

Join Date: Jun 2016
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#23

### Re: Level Measurement Under Vacuum

06/15/2016 8:46 PM

I have personally tried it a while ago (sealed type rosemount dp transmitter in an almost full vacuum pressure condenser tank) and it didn't work.

Though it works when I re-range it during vacuum condition but once the vacuum is gone or deteriorates, the transmitter becomes erroneous.

And I agree, displacer type really works.

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