Level problems in a shell boiler 2

[daylami]

I'm working at a site which is currently operating it's shell boilers at around 80% of the gauge glass. The level moves up and down rapidly from 60% to 90% of the glass. I am convinced that this is due to high TDS but the results of the water treatment says that TDS is within spec. The reason why the boiler is at 80% of the glass 'is to prevent low level trips when a high load commences'.

If the TDS is in spec, I have suggested to the site that they go back to NWL to allow a greater surface area for steam to form. If that doesn't work they should mve from single term level control to three term control. Any thoughts?

 

[pmover]

what about other boilerwater qualities, not just tds?

i've encountered the same problem years ago. boiler water level in sight glass was rapidly changing significantly (fixed foundation, not shipboard installation). the chemical sales guy told operators to activate the chemical injection pumps for 2 hours a day. i took a sample of boilerwater and the lab determined the amount of solid material within sample. we roughly determined that there was 300 lbs of solid matter within the boiler. the boiler water test program was not in practice and test equipment was not kept clean. boiler was shutdown, given a good blowdown, cleaned test equipment, and commenced a regular test program. problem corrected. salesman was not too pleased - too bad. . .

perhaps, if possible, question the tds data and sample from boiler. also, shutdown the boiler, conduct a good blowdown, refill, and restart boiler and observe any differences.

confirm with OEM the design normal water level in boiler before changing. lastly, boiling mud or thick water will result in abnormal water level in sight glass. oh, double check sight glass operation and piping.

hope this helps.

good lick!
-pmover

 

[quark]

Stars to both of you. I am just wondering why I didn't get even hint of it nor read anywhere about this issue, though I have been working with shell boilers for past 10years. The capacities were not more than 10TPH and pressures are 10kg/cm². We do annual cleaning of shell and biweekly cleaning of fire tubes.

What capacities generally fall prey to this type of problem and which mechanism causes this problem?

PS: I couldn't resist myself from confessing. I silently laughed when I saw the post before the first reply. Besides excellent engineering help, this site, definitely, is an ego suppressor.

Thanks once again.

 

[25362]

Some more comments on the subjects in hand.

1. Gauge glasses generally show "false" levels due to the density difference between the water in the glass and the bubbling water inside the drum, sometimes in the ratio of 1:0.5. Thus, high gauge glass levels should be carefully considered when trying to avoid entrainment of feedwater (BFW) into the steam, even when having "good" demisters.

2. Recommended TDS values are generally maintained by the blowdown (BD), and are allowed to be higher in lower pressure steam rising. They also are a function of the BFW quality and treatment. Deionized or demineralized BFW may require a BD of 1-2%, while lime-softened BFW may need BD values in the range of 10-20%.

3. The response of the internal boiler level to steam demands may show what is called a reverse response of short duration. For example, upon a sudden increase in demand the added (cooler) BFW may condense rising steam bubbles, actually dropping the level until the added inventory raises it again.

4. Levels may also show transient changes due to pressure variations. They may swell by a sudden increase in demand (lower steam pressure) due to water flashing into steam. Or they may shrink upon a pressure increase.

These situations are more frequent in low-pressure operations due to the larger difference in density between liquid and vapor. These short-lived effects disappear when the incoming BFW input is in equilibrium with steam withdrawal in steady-state operation.

5. From the control point of view these level variations are ignored, letting the forward control loop carry the load, while maintaining loose settings on level control. LLC are customarily kept at a proportional band of [≈]100% and several minutes' integral time.

 

[TBP]

It doesn't take very much contamination in the form of oil or fat in boiler water to make the water in the glass bounce, even with the TDS right where it's supposed to be. This contamination can be from a leaking HX bundle or tank jacket, residue from the boiler manufacturing process itself, threaded pipe installation where the cutting oil wasn't flushed out prior to the line going into service, etc.

What about the feedwater pumps - are they on/off or do the boilers have level control valves?

 

[GenB]

Depending on the zise nof the boiler, you may have a lot of water in the steam piping or very wet steam!
You are experiencing the so called foaming and very easy to correct,
contact your chemical supplier!
GB