Boiler Harmonics

Hi whezmabeer, Sounds similar to a problem during the commissioning of a couple of 660MW coal-fired boiler some years ago although some sounds have different causes on these plants. In this case the cause was a flow-induced vibration and a standing wave in the economiser banks located in the backpass. It took longer to identify the cause than to do the fix. The fix was a couple of baffle plates between banks of economiser sections for the depth of the bank to change the flow width and not allow the standing wave to initiate or carry.

Apart from the annoying environmental noise you need to check the vibration frequency is not a damaging one to cause fatigue failure of attachments - or you may hear a louder noise.

A few searches on standing wave, heat exchanger, natural frequency, von Karman vortices should drag up some preliminary helpful info.

Good luck.

The reason for this sound is the vibration of the fire tubes. It only happens when the flame is on and is the result of the water boiling off on the waterside of the tubes. The only way to reduce or avoid this noise is to reduce your max setpoint.1050 kpa reduced to 1000 kpa will still work very well. (This working pressure would have had a test pressure of 1365 kpa applied to it). You may also reset your PLC, if it is used, to start slowing down later, but quicker, or start slowing down sooner and "slower" increments. The air temp is only a coincidence. However bear in mind that not all the tubes will be vibrating, but only those that get the hottest air, or most heat. Since this vibration will eventually result in minute stress cracks, it may be wise to eliminate the noise altogether by reducing the working pressure to a lower setpoint, as suggested.

Hi jvrj, it is a water tube boiler not a fire tube Boiler. does that make any difference?

Aussie Bob, thanks for your comments. learn'tr a bit about votex's etc from your things to follow. Think this will be a bigger job than I expected if jvrj's comments don't work.

Hi,

The same principles apply to watertube as well. The vibration is caused by the rapid expansion (heat) of tube while the water tries to contract the tube. The noise mainly comes from the water, but the vibration can cause havoc, eventually. This laymans explanation is the simplest i can think of, plus of course if you observe an aluminium pot on the stove whilst heating to boil. At a certain point you will hear the "squeaking" sound from the pot and sometimes also when water got between the pot and stoveplate. Many years ago I simply reduced operating pressure and all is still well. In my case i went down to 975 kpa and our process was not affected in any way.. If your boiler was adequatly specced, you wont have problems either.

Simply reduce the max operating pressure and observe, if i am right, bingo, problem solved for free. Of course if i'm wrong, simply up pressure again, nothing lost either.

i agree with JVRJ, no funnies here, just plain heat and water boiling into a harmony. BUT it may become destructive.

Comparison to a boiler of the scale in my experience is almost silly, but I did work around a 50 hp, gas fired, fire tube Cleaver Brooks package boiler running at 15 psig. that did something like that. It seemed to be at only certain firing rates, and my boiler had modulatiing controls. I always just thought it had something to do with the harmonics of flame propagation. The supply air temperature was pretty constant within a range of 20 degrees or so.

The cleaver package will have very short tubes, nevertheless the noise will come and go depending on boiler design. i.e. If you have long tubes, unsupported, the noise will be audible and worse than if the tubes were short or supported at the required intervals.

Sounds similar to an overfire condition I ran into on a few boilers back in the day. These units were also of coal-stoker configuration in a steam plant, the steam from the boilers feeding the stokers, throwers, grate shakers and such.

We ran into a noise/vibration issue when the control system would cycle between a running, stop, and then running condition. Our plant had to deal with a bunch of labs in the science wing on the other side of the complex - and they would go from using two to three hundred cubic feet per minute to none, and then back to high demand again.

We came to realize the problem was in the water feed rate to the boiler - the plant would be chugging along under load and all was well - we could even run under full steam without a problem. Someone would change their demand and we would loop our steam back into the local traps to return to condensate (just dump the head). At this point our control system instructed the control valves to throttle back to prevent too much waste of heat - but this allowed the boiler tubes to warm beyond a safe condition. The water that was entering the boiler tube was unable to cool the tube sufficiently, so the water would boil long before it reached the "hottest" point in the tube. When demand went back up the control system would again flood the boiler, dumping water into the tubes and causing a rapid contraction of the tubes (quenching the steel). Not a good situation at all.

Our solution was to re-educate the PLC as to proper boiler procedure. When the call for heat dropped we would shut down the stokers and dump steam head, but at the same time not reduce the water flow until the temp of the boiler tubes decreased. Then we would modulate the water inlet to the boiler and the steam head according to both tube temperature and current demand. It was wasteful to say the least - but when our demand was fluctuating so wildly it was the only option we could come up with beyond cutting pipe to lay in new components.

Thanks peeople. This gives me something to go on.

Does any think that maybe we need to alter the PLC program to maybe slow down the stoker and fans earlier to stop the steam pressure going over the set point which is where the vibration occurs. Our steam flow varies a lot and doesn't following any pattern or timing.

The other thing would it be usefull to put the overfire air onto variable speed drive as well? Connected to an oxygen sensor of course

I might sound stupid but what the heck!

Check your feed pump for irregular feed due pump cavitation , and do you have very wide pressure drop during peak demand, if so how frequent/24 hrs? Maybe without a modulating feed water control your on/off feed supply via the water tubes may not be synchronizing via the PLC with reference to feed water demand(slow due pump cavitation) resulting in tubes having irregularties in thermal expansion & contraction.

I apologize beforehand if this is purely off-the-topic thought. It just came in my mind.

Whezmabeer, Your question ties in with the suggestions I offered in an earlier reponse. To alter the PLC to either lenghten or shorten the grate timing/speed.You may also alter the fans to extend low fire condition sooner for longer, this mar then reduce the intensity of the heat at a certain period. You will have to spend a little time to balance everything to still run efficiently and enough steam and above all, SILENCE

It's often an indication of a hole in the fire. It can affect the structure of the <...boiler...> if not immediately corrected.