Boiler Drum Level

could it have an air lock?

As far as i consider we have given proper venting n drum vent was closed only aftr free flow of water was ensured coming out frm vent...at around 5kg/cm2

First, did you fill up the reference leg before start up? Second, if you did not, and if you are using a 2oo3 voting, then give each transmitter a good blowdown. It will take about an hour for the signal to settle down. Surprised you said you had water coming out the vent. On start up the vent valve at the top of the drum is left open until about 2 Bar pressure then it is closed, but that has nothing to do with this problem. If the level signal is fluctuating then Auto mode makes the control worse. Is the feed water pressure constant downstream of the control valve? How many transmitters do you have and where are they mounted?

Drum vent was open during boiler hydro test...to ensure proper venting. Total there r three transmitters n arrangement is like one set consists of one frm steam side at top of drum n one fr water side bottom side of drum. we have flushed the transmitters yet its not helpd much.

Are all three transmitters fluctuating?

All the three transmitters r fluctuating. We have done flushing twice yet its reading are +22mm; -11mm; -48mm. The average value is taken as median for reference and its fluctuating coz of all three values varying.

The boiler might be priming. How close to the blowdown current set point is the current total dissolved solids level?

Do agree with the previous post. If you had just overhauled the boiler there must be a lot of grease and oil in the boiler drum water. Normally one does a boiling out and an additional input of anti- foaming and Alkalinity booster chemicals. Your pH within the boiler drum should be gradually built up to 10.0 - 11.5. I suppose this has been done.

The other interesting issue is the discussion of blowing down/flushing the transmitters. You should be having a HP and LP impulse piping from your Steam Drum coming down to connect with the Drum level transmitter. At the top of this impulse piping you should have a refill plug. Once you have flushed this piping, you should refill the entire impulse piping with water and then close the plugs. Has this been done or is the impulse piping empty?

You could have fluctuating levels due to both these reasons .

Well during shutdown we had replaced the entire eco tubes coils with new ones. Though acid or alkaline cleaning must hv been done but due to some reasons it did not happen. Aftr v did hydro test i gues the iron particles got settld in drum though v did 3 times flushing entire water frm eco. The iron content is high means conductivity is highr than permissble levels whn we were doing light up but due to load schedule tie up we went ahead finally synchronisng machine with grid aftr gvg an entire blowdown..again via emptyng hotwell n gvg make up at same time. regarding hp ip impulse line i wud find out as i am not sure about it.

Is the water quality too high? We have seen times when we needed to add ammonia to the feedwater in order to get a reading. I wouldn't think that this would be the problem as you just recently brought the unit back up. The water should have high silica for a while. There is still the possibility that the conductivity of the water (it is demineralized) is too low to get a stable reading. Perhaps your ammonia injection system is not pumping very well. (Just a suggestion.)