We are using Honeywell ST3000 DP transmitter to measure liquid level in a tank in which the pressure is less than atmospheric pressure. It is wet leg DP type measurement. The problem we are facing is that the transmitter reads level less than the actual level in the tank. First we thought this might be due to lowering of level in the wet leg, but this would result in measured level being higher than the actual level in the tank. Any suggestions, please.

Regards,
singhbvirk

 

You need to check the following:

a. is the wet leg of the DP transmitter connected to the tank top part also? I presume yes, so this will not be considered further.

b. what is the level of vacuum you have in the tank? Keep in mind that the air part of the tank actually contains air (or for all that matters a gas derived from the liquid in the tank). This exerts pressure on the liquid. When you bench calibrate the instrument (or apply the factory calibration), you have air at atmospheric pressure on the dry leg which is there and you are automatically including in the calibration. When you apply vacuum, the density of that air (or gas for all that matters) is reduced, and so the pressure exerted is reduced. This results in a lower net pressure on the dry leg, and thus the lower level reading. So you need to compensate for this variance in you calibration of the transmitter.

c. is the density of the liquid you have in the tank changing with the applied vacuum? A lower density will result in a lower reading level, again when compared to atmospheric pressure.

 

Can you please elaborate the situation?

What is the liquid in question?

Is, it boiling inside the vessel?

What is the temperature inside?

You are right in assuming the vacuum sucking the liquid of the low side will no doubt raise the indicated level by Dp tx.
For the solution to the problem one possibility is that the temperature inside the vessel is higher and in the condensed wet legs is less typically ambient. This will make the densities of the liquid different and the one inside the vessel less. So as the temperature inside the vessel increases indicated level will decrease.
Recall that this is also the case in pressurized steam boilers where actually 3 densities are acting on the DP transmitter and density correction is done through the pressure transmitter.

Regards
NIK

 

Jojo,

With the increase in pressure or vacuum in the vessel it will act on both the legs of the transmitter. So the net DP because of the increased or decreased vacuum will be zero that is why we use DP transmitter.

The only DP shall be acting on the transmitter will be because of the increased or decreased level. So if we assume that because of the vacuum the level inside the wet leg (LP side) is sucked then there will be erroneous reading.

Consider the case when the actual level inside the vessel is decreasing and simultaneously vacuum is increasing this may suck the liquid from the LP leg, DP will be lowered and will result in higher level reading.

Please correct if I am wrong.

There should be some other reason for that. Any other suggestions, please...

 

Dear singhbvirk,

1 - When we use a DPT for level measurement in an open vessel, we don't need a dry leg at all. We just open the -ve side oif DPT to atmosphere.

2 - When we use it in a closed vessel, we connect the dry leg (the low pressure side) to the vessel so that all vapour pressure is compensated in the reading because:

Hydrostatic pressure due to level = wet leg pressure - vapour pressure

In this case we have a positive pressure in the dry leg.

3 - For your application, we have a -ve pressure or Vacuum in the dry leg and since your transmitter is a DPT (2 gauge pressure transmitter in one package with output equal to the difference of two), I fear that it is not capable of handling this situation. Probably its -ve leg has already been destroyed.

If you have a vacuum controller for the vessel then you can add absolute values of both pressures in your DCS or PLC and get the right level reading according to:

Pressure wet leg < Pressure actual
Pwl = ( density x 9.8 x height of liquid ) - vacuum )
so Pwl + vacuum = P actual

If you don't have a vacuum transmitter, get one. If you have very stable vacuum, note down the correction value and use it for reference.

Further questions can be sent to nadeemaziz_ei at hotmail. com

 

NG,

No, I'm sorry but you are not 100% correct. The vapour pressure above the liquid in the tank exerts a pressure also on the dry leg. As I said in my previous post, under atmospheric conditions this is inherent in the calibration and so is ignored. When you move off atmospheric pressure, in either direction, you need to compensate for this change in vapour pressure, either through an offset in the calibration, or else through additional hardware in the control setup.

 

dear NIK,
Reply to your queries is: as liquid under question is water, it is not boiling but being condensed. Temperature of the liquid is about 45 deg. C.

Regards
singhbvirk

 

Dear jojo,
Thanks for your inputs.
Actually,when the DP transmitter is first 'charged' i.e. put into service, it gives correct reading for about first 2-3 hours. The problem starts thereafter, i.e. it is then that the transmitter starts recording the level in the tank (under vacuum -0.94 kg/cm2) less than the actual level by about 200 mmwc.

Regards
singhbvirk

 

I have been watching this post, waiting for the answer.

I couldn't figure it out until I was reminded by another post that in the past DP transmitters were sometimes installed backwards, with the HP side to the wet leg and the LP side to the bottom connection. If this is the case as the wet leg looses water the signal will go down.
If the DP transmitter is installed as normal with the HP side to the bottom of tank and the wet leg evaporates the signal will go up as you pointed out.

My guess is you have the earlier method and the upper connection is quite hot so the water is evaporates for the first few inches. A quick way to check this would be to add a small flow of cold water to the wet leg. If that helps you could try adding a condensate pot at the upper connection to help cool it down.

BTW what is the absolute pressure?

Hope this helps
Roy

 

Roy,
this doesn't explain behavior. If water is lost in wet leg connected to the + side of transmitter, reading will be higher. This is due to lack of water column pressure in the high pressure side. This kind of installation has reverse characteristic. When there is no deltaP, level is high and tank is full

 

Mikas,
I have been really puzzled over this, it seems to me that if you reverse the transmitter loss of wet leg should also have the reverse effect.

With the DP in normal as level goes up so does mA
With the DP backwards as level goes up mA goes down.

In both cases loss of wet leg causes the DP to go down.
It's something obvious and simple we are missing.

Perhaps it's as simple as where the poster is looking at the signal, before or after the inversion, at the transmitter or in the DCS.

Regards
Roy

 

Mikas,
I read the manual, it recommends connecting HP to wet leg but says it will work the other way but won't guarantee the accuracy.
I'm starting to think that the DP cell doesn't like to operate at near a perfect vacuum, perhaps something is flashing off inside the cell so that the diaphragms are loosing contact with the measuring element

Hopefully the poster will try my suggestion of adding a small water purge to the wet leg to see what effect it has. I'm sure the water is vaporizing at thet pressure and temperature
-0.94 kg/cm2 45° C

Please keep us updated Sinhbvirk
Regards
Roy

 

hello all,
did every one miss the important information that have been given by our friend sinhbvrik?

he says that after "2-3 hours" the level reading became inaccurate. Is this mean something? the liquid was water and it evaporated and the temperature are 47 degrees celcius. my opinion is that the difference between the transmitter reading and the actual level inside the tank are mostly caused by the temperature that incrising due to vacuum pressure, altough it supposed to be at same between the wet leg for the transmitter and the actual level inside the tank.

But the liquid inside the wet leg piping will evaporated more easily because the connection are above the actual liquid inside the tank. indeed some method are used to anticipate this by filling the wet leg continuisly with cold liquid with the same specific gravity and mass, or venting the wet leg to the atmosphere so only the high side of the transmitter that have the actual level inside the tank liniear to the pressure, but for me you must do counter measure to the vacuum pressure instead.

That means the vacuum pressure condition have to be controlled integrated with level measurement and temperature reading at the tank system such as cascade or feedback combining with feed forward. and the most important thing you must observe very carefuly to the process. is something is missed or unnoticed and don't forget to take notes for every parameters that you think are connected to the problems and last please give some updated to us here, this are very interesting problems that need solved together.

thank you

 

singhbvirk, a couple of questions:

1. when you stated that 'when the DP transmitter is first charged' does it mean that the main tank is already under vacuum, and you are simply putting the DP transmitter in service, or when the DP transmitter is put in service the tank is at atmospheric pressure, and the vacuum is established within the 2hrs you are indicating?

2. what is the fluid in the main tank and what is its temperature?

3. is the DP transmitter being exposed to heat either from the main tank, or from some other process close by? Is the DP transmitter body operating temperature within its operating limits or heating up?

4. are the connection pipes between the main tank and the DP transmitter at the same temperature as the DP transmitter, and colder than the main tank? Is there the possibility that the fluid in the dry connection leg boiling out, and thus lowering the effective head?

 

Singhbvirk,
Can you tell us please what is the purpose of the tank? surge tank or what?

Is it Inside the tank is only water?

Do you have level glass installed at the tank? I think this answer is yes

Do you have any TT or PT installed in the tank in order to measure the
temperature and pressure changes? the presure and temperature is steady or not!

Is it the HP side on the top of the tank?

Which is the measuring range actually?

I was facing years ago similar problem with a honeywell level transmitter installed a vacuum tower, and this transmitter was loosing the wet leg because a leak, of course the wet leg was sucking into the tower.

Regards
J.P

 

Dear Roy,
The absolute pressure inside the tank is about 0.16 kg/cm2.

singhbvirk

 

Ultima,
No, several of us commented on that. I'm sure it is evaporating but we are puzzled as to why the indicated level goes down and not up.

Sighbvirk - a couple of questions,
A. Where do you see the level go down, DCS, local indicator, HART Communicator?
B. What is your calibration, is it 4 mA with tank full or empty?
C. Which connects to wet leg HP or LP?
D. Are you inverting the 4=20 mA signal anywhere?

I suggest you try adding some cold water to the wet leg.
My guess is the Wet leg doesnt have a condensate pot, just a tee that won't get rid of enough heat. I think this application would run with a dry leg, not as it is now of course.

Regards
Roy

 

I'm curious about this too. When you say that transmitter readings are lower than actual level, what do you mean? How do you know that?
Are you comparing level reading to the glass guage or what?

 

Singhbvirk

There have been so many discussions already on this post to find out the ultimate solution information is required from you.

1. How is the transmitter connected. Is the HP side to the bottom of the tank and LP to the top of the tank.

2. Is there monitoring of pressure and temperature inside the tank.

3. from the previous posts it is established that the temp is not creating the problem as the liquid is water. only thing which is happening is sucking of liquid inside the LP (wet) leg as the vacuum is applied. so please clarify

1. where exactly you see the actual level is different from the indicated level.

2. Are you doing any mA signal inversion in the DCS.

3. How is the Tx calibrated. Hope reversed for analog i.e. for 500 mmWC calibration: 0 mmWC corresponding to 20mA and 500 mmWC corresponding to 4 mA.

4. If HART what is the URV and LRV.

Regards
NG

 

Dear Jojo,
reply to your queries is as under please: -

1.when I state that 'when the DP transmitter is first charged' it
means that the main tank is already under vacuum and I am simply putting the DP transmitter in service.

2. The fluid in the tank is water and its temperature is about 40 deg. C.

3. DP transmitter is not being exposed to any heat and its body temp. is well within limits.

4. We have checked the level in the wet leg by isolating it from the stand pipe connected to the tank and found the level to be OK.

Thank you,
singhbvirk