vibrations 2

[ww2]

With our new HDS/SAACkE 30ton/hour double fired fire tube boiler we have severe vibration problems. The vibrations are occurring at relatively low, about 1/3, load and at the change over from negative to positive combustion chamber pressure. We’ve managed to improve it by altering the combustion air/gas mixture ratio. But still not to satisfaction.
Does anyone on the forum as experience with this phenomenon. Can anyone suggest me any literature on the matter
Please help
koekoek

 

[abeltio]

seems like you have a flow induced vibration problem... i.e. the strouhal number of your tubes is in the range that gets excited with the flow... too bad... a possible solution is to add extra supports to reduce the un-supported length of the tubes... you mentioned that the boiler is NEW... your best bet is to raise the issue with the supplier, because if you modify it yourself you may void the warranty.
HTH

saludos.
a.

 

[smckennz]

I have experienced pulsation (as opposed to vibration) problems when trying to run boilers under suction but too close to atmospheric. In that case I figured the problem was due to tramp air being sucked into the furnace, expanding, making the furnace pressure go positive thereby stopping the tramp air flow and causing a subsequent reduction in furnace pressure. Slightly increasing the draft set point (more suck) and a serious attempt to minimise tramp air ingress did the trick.
Another problem I have heard about but not experienced is vibration caused by the flame partly lifting off the burner head. This can be caused by poor (primary)air fuel regulation causing a reduction in flame speed. You may want to try a bit more primary air and see what happens.
Sounds like your boiler is designed for positive furnace pressure; unfortunately I cannot help you much on this as my experience is limited to balanced draft and slughtly negative furnace pressure. However if you do have uncontrolled air flow, it will give you stability problems.
I would take a long careful look at the burner flame; it will often give a clue, if the vibration is combustion induced, and not caused by inadequate tube support.

Cheers

Steve

 

[ww2]

Mid of march we will shut down the boiler for control of the super heater weldings. I will also control the supports and materials own frequency at the first fluegas cross over. During a vibration test to locate its centre. It showed this centre occurred to be very near this fluegas crossover.
But first I will study the behaviour of the flames as you guy’s suggested
Thank you for that
koekoek

 

[hacksaw]

with a fire tube boiler it sounds like you are having flow distribution problems rather than combustion or FIV, although at low fire you can have an unsteady burner operation which can cause the problems you describe

 

[ww2]

Hachsaw, I agree
Problem is how to detect the real thing and last but certainly not in the least how to solve the problem in the most economical way.
koekoek

 

[daisyleng]

i thind this problem is caused by the fluid induced vibration. Because it is a Steam Boiler. when the load reduced the flow rate will do. and perhaps in some area, the pulsate of the flow will as the same as the inherent frequency of the pipe. so the Boiler will vibrate.
I think the most perfect measure is to imporve the inherent frequency of the boiler.

 

[hacksaw]

daisy,
this is a fire tube boiler. how will turndown increase the chance for vibration?

 

[ww2]

The vibrations starting point is near the jump over from negative to positive combustion chamber pressure. Still being slightly negative.