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Drum level Transmitter
one of the three drum level transmitter behaves abnormally

We are using three differential pressure drum level transmitter for drum level control with 2 out of 3 logic. But one Transmitter gives lower reading than other two with a variation of 5 to 20% of the drum level. We replaced the transmitter with a new calibrated one but problem is still there. Can anyone help me out for this problem? The transmitter range is from -610 to 0 mmWC.

A couple of questions:

1. Is this a problem on a new installation, has it cropped up recently following a period of correct operation, or have you been living with it and now you are trying to find a solution to your problem?

2. Is the transmitter that is 'faulty' connected right next to one of the other two transmitters or standing on its own on the other side of the drum to the other two transmitters?

3. Is the high leg (reference leg) at the same conditions as those for the other two transmitters? Is there an undesired heat source next to this leg?

4. Do different firing configurations (operating some burners and not the others) in the furnace change the response of your 'faulty' transmitter with respect to the others?

5. Is the problem present at all boiler loads or at a particular boiler load?

6. Do you have physical level indicators on the drum, like the traditional glass 'tube' level indicators with which you can see the actual drum water level?

7. In blaming the transmitter, are you sure that the water level in the drum is perfectly horizontal and not sloping to one side?

8. When was the last time that the drum internals were inspected?

You need to be sure that the drum water level is truly horizontal, before you start blaming the transmitter, especially in view of the fact that you have changed the transmitter with a new one and got the same results. There are cases where the drum level is not perfectly horizontal, these mainly being caused by uneven heat flux in the furnace, causing higher water/steam flows in the furnace tubes that are exposed to the higher heat flux.

Obviously there could be other causes, but my point here is to highlight the fact that before blaming the transmitter for faulty operation, make sure that the water level being measured is truly what you are expecting rather than what the transmitter is showing.

Just to reinforce the point - I was once involved with a boiler with redundant transmitters, one at each end of the drum. The boiler was a compact type with the burners at one end of the firebox and the drum running parallel to the flame.

In practice, the bulk of the water rose in the risers at the far end of the firebox from the burners, and went down at the burner end. When first commissioned, we had a high alarm from the transmitter and switches at the far end, and a low alarm an indication at the burner end - all due to water circulating and flowing from hot end to burner end.

Bruce

With refer to your reply I am again explaining my problem. We have three differential pressure transmitter installed at 170TPH, 87bar Bagasse fired boiler.

Boiler Drum Level transmitter which is installed at Right side of the boiler steam drum is giving high deviation upto 30% in level than other two transmitters installed at Left side of the boiler. We installed a new transmitter on the same side at different tappings for observation but it is showing same. We have checked calibration, Transmitter, and also the impulse tubing but the problem is still there.

The main problem is this that this deviation is not permanent it varies from +5% to -30% than other two transmitters. Sometimes it becomes normal and sometimes a high deviation in few minutes at constant load.

We have also compared with hydrastep the level is actually coming down in one side only. Due to this drum level control has become difficult. After all our effort we are unable to solve this.

Please suggest. How can we find out the uneven fuel feeding as all feeders are running at same speed. Also the drum is horizontal. Drum internals are checked and they ok. This is a new boiler and is a new intallation.

Please suggest what we can do.

Naresh Kumar,
nkmaan05jh [at] yahoo [dot] co [dot] in

I sympathise with your problem. In my early days of commissioning boilers I had my first dog of a drum in 1973. Drum level would sit at half glass, steam flow would go up about 10%, feed valve would open slightly, boiler would trip on low water!!

Spoke to many gurus about drum level transmitter problems. They all said the same thing -- it is the boiler not the transmitter. There were a couple of things causing that problem.

1. The water treatment was so bad, the TDS very high causing a lot of foaming in the drum.

2. The boiler design was such that risers were changing to downcomers as the combustion conditions changes.

Again, had a similar problem on a recovery boiler. Under certain load conditions, a downcomer would change to a riser. We solved that by shutting the boiler down, and welding a deflector plate on that tube to minimize the turbulence. It worked.

There are occasions when you can get an imbalance of level in the drum. This is usually on wood waste or coal fired stoker units when the fuel has settled such that you have a large amount of fines on one side that burn higher up in the combustion chamber creating greater volatility on one side of the drum.

Bagasse does not suffer from this grading problem. Boilers that burn this fuel are usually ones that have been designed for coal firing with the feeders modified for this fine black fuel. Main difference though is the fire ball is higher in the combustion chamber. This also means a smaller superheater if you have one.

So, bottom line, in my opinion, is you have a circulation problem that varies as the heat intensity changes in the furnace. I am also assuming that your TDS is good. What does the boiler manufacturer say. Are there similar units that you can talk to the users about.

Who is the manufacturer? Tell us also what your boiler chemistry is.

Dear Naresh,

Sounds like you've hit a stone wall. About 10 years ago i commissioned a number of very large water tube boilers and in the process encountered a number of unique problems. While the event that i am about to describe did not occur in the boilers that i worked on, one of the vendor experts i was working with described an incident that might be of use in this situation.

In one of the boilers, there were issues with the way the inlet distributor was connected and this resulted in an uneven flow of water into the steam drum. This actually generated a wavefront within the boiler drum which became even more significant during large upsets. Because the drums were large the wave front moved relatively slowly. Interestingly enough this caused more fluctuation on one side of the boiler as opposed to the other because of the location of some of baffles within the boiler internals. With some rectification of the control system first (to stabilise inlet flow) followed later on with the rectification of the inlet distributors, they were able to fix the issue. Have no idea if this helps.

Regards,
PKumar

For me its due to improper blow down. one leg of the transmitter was filled by dissolved solids; Try to give pulse blowdown on the side of leg.