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Vacuum condenser level xmtr
How we can solve the problems in vacuum condenser level transmitter.

Hi every bode

we have a steam condenser below low pressure steam turbine in our power plant...OK

the problem with me that there is a problem installing and calibrating the Honeywell smart DP transmitter to monitor the condenser level throw the DCS. given that i'm configure the xmtr with the URV&LRV and the span and we filled the condensing chamber well with water and venting the air very well.

NOW
the problem is
1.the xmtr reads bellow the actual level by about 10%

2.every time we check the condensing pot we find it empty.

can you help me please!!!!!!!!!!!!!!!!!!!!!!!!!!!!

If you check your steam tables you will see that under a vacuum water boils off at a low temperature so you need to cool down the leg or provide a source of cold water.

Is your condensate pot insulated, it shouldn't be, You might even have to provide a cooling coil.

Hope this helps
Roy

I think the condensate pot take the function of keeping the water in the high side cool but i want to add some thing to my question sir that XMTR gives the real level for about 5 MIN ok;

after that the level begins to decrease until 10% bellow the actual level..

how can i solve this.
Given that before we install the Honeywell transmitter that level was sensed using foxboro DP transmitter and gives the right reading.

please replay me for importance

Hi there,

You have a leak somewhere.

Look again at all the connections, disconnect from the vessel and do some pressure testing.

Use Snoopy (soapy water) to test for leaks while pressurizing each line.

Also test the equalizing valve for leaks. Replace the complete manifold if possible, otherwise do proper testing on it since the manifold have vents and equalizing valve that can be the cause of this. Very difficult with vacuum to pick up a leak so the best is to use pressure to find it.

Once all this is done and you still having problems send us some details of how the calibration have been done, the exact position of the catch pots, distances from transmitter to Z/S points and so on.

By a.eldessuki on 24 October, 2010 - 11:59 am

OK sir how can i calculate the upper range value and lower one in your point of view. please don't ignore my question

Ditto on the probability of the leak. That can drive you nuts if you're boiling off the reference leg.

Our condensers have 10 year old ABB delta P's with remote sealed diaphragms on both legs, a Sergeant and Lundy design I really appreciate because it completely eliminates the reference leg and minimizes the leak issues, which is a good thing for vacuum systems.

Hi again,

Fully agree, diagram remote seal is the best to use in a vacuum application.

a.eldessuki
I can guide you through the calibration if you send me some details of what the various distances between you tapping points are and also the distance from the bottom tapping to where the transmitter is situated. Also where should z/s be?

Have you found a leak or are you now sure there is no leak and that it could be a calibration problem? I also need some details on where and how the catch pots are installed.

By a.eldessuki on 26 October, 2010 - 2:05 pm

No we still have the problem in that transmitter but i can say that iam sure there is no leakage in the system at all i think the problem in the calibrating values (URV & LRV).

Hi again,

I agree with R Monge, especially a slow leak. Have you done all I suggested or why do you say you are now sure there is no leak. How and what have you tested and done and what was the result. a Wrong calibration will give a constant error from scratch and you say the installation is working ok for the first 5 minutes. We will go through the calibration later but the leak needs to be eliminated first.

Please put in some effort to give proper feedback and information about the installation and your efforts so far. If not we will just lose interest and move on to the next question.

You have given two clues:
1. Every time you check the condensate pot that is empty.
2. At the start the level reading is correct and after 5min the reading gives 10% lower value.

You have also told that the condensate pot is connected at the high side port of the transmitter.

Just for a fictitious calculation I am assuming that the condensate pot is at height of 1200 mm from vessel zero reference. Say your measuring range is 1000 mm wc. When the tank is empty you get 1200 mm wc differential pressure across the transmitter and it gives you 20 mA. When the tank is full at height of 1000 mm, you get a differential pressure of 200 mm and this should be set at 4 mA.

Now you must have reversed your transmitter output so that when tank is empty with differential pressure 1200 mm you get 4 mA. When tank is full with differential pressure 200 mm you get 20 mA, In this way transmitter out put increases with increase in level in the tank i.e decrease in differential pressure across the transmitter.

Now suppose when the tank is full your condensate level at the wet leg (condensate leg) has reduced 100 mm due to vapourisation of water. Therefore differential pressure has reduced and the transmitter should show you higher value of level.

Similarly if there is any leak in the transmitter valve manifold, then water will pass from higher elevation wet leg to the low pressure port at vessel side. Eventually the differential pressure across the transmitter will reduce and you will get high level reading.

But your feedback is you are getting 10% lower level reading. This is not matching with common understanding. Could you pl. tell us in detail how you are setting the LRV and URV of the transmitter?

You definitely have a leak somewhere in the LP leg to the transmitter. We have measured hotwell level with DP transmitters for over 40 years. On our oldest running unit we started having the same problem you are describing and the technician troubleshooting it installed condensing pots that we did not have before. He was going nuts and could not find the leak, but somehow in the process of installing the condensing pots and modifying the piping to fit the pots, the leak was fixed, and it was believed that the condensing pot solved the problem. Some time later another unit starting have the same problem, but this time installation of condensing pots made no difference. After some head scratching, it was determined that the isolation valve for the transmitter was leaking. At the next outage the leaky valve was replaced and the problem solved. I agree, finding leaks in vacuum systems is hard, specially slow leaks, transmitters with remote seals avoid this problem, but if your piping is done properly, any DP transmitter will work well regardless of brand name.

Hi,

The problem is with the ref column. Whatever level you filled in normal atmospheric condition, it starts evaporating when you bring it under vacuum which will have lower ref column. To counter this, add a small condensate supply line from Valve gland seal in water header or from Condensate header with a needle valve. Throttle the needle valve to deliver few drops of water into the condensing pot which will compensate the evaporation losses.

Do not admit water when the isolation valve of the condensing pot is isolated from the hotwell.

This will solve your problem.