Deaerator Level Measurement

[chili1961]

All,

We have two rosemount transmitters that are averaged to provide an output to a control valve. They are DP cell which are connected to a close tank. Our Deaerator is mounted outside and are reference and measuring sensing line are heat traces. It seem when with get changing in ambient temperature or rain it results in inaccurate level control. The transmitter have not indication although we have 2 other transmitters that sense level and are experiences the same conditions. We dont have condensing pots installed. What do you believe the cause is?

 

[HEC]

How old is the deaerator, what condition is the insulation in? What measurement method are the other two level transmitters based on? With this additional information we may be able to provide a better answer to the probable cause.

Mark Hutton

 

[chili1961]

1. How old is the deaerator? 20 Years old
2. what condition is the insulation in? Good condition
3. What measurement method are the other two level transmitters based on?
Two transmitter monitor the deaerators and their outputs are averaged. The transmtters are rosemount 1151DP with a filled reference leg. We do not have condensate pots installed.

We seem to only experience level indication issue when either we have:
1. a heavy rain
2. While shutting down
3. Starting up
4. Or an issue like a feedpump trip

 

[HEC]

What style of dearetor is it? What is the operating pressure?
Part of your problem could be associated with shrink/swell of the liquid in the deaerator. As the pressure/flow/temperature change you may be experiencing some local boliing or condensing wihtin the deaerator drum.


Mark Hutton

 

[gwmontrose]

If the tank is closed, you can use a differential pressure transmitter with the top of the tank connected to the low side and the bottom to the high side.

Greg

 

[roydm]

You have no condensate pot. I suspect the level in the Low Pressure leg is fluctuating. When it rains the LP leg cools down and the level in leg goes up indicating a lower level in the deaerator, To test this out you could bleed in a small amount of cold water, that should keep the level in leg constant.
a proper fix would be to add the condensate pot.

Roy

 

[glowing]

good points roydm
This makes sense. Also may want to consider capillary DP cells.

One other question...how much cable for the transmitter is exposed to the elements?? I have seen people leave cable in a matter the will expose them to pooling water or snow, and this will thought out your signal big time. Are they in a propper cable tray? You may want to consider putting the cable in some extra shielding (ie pvc conduit or liquid tight flex.