vibrations on economiser and boiler smoke chamber 2

[Gregor77]

Hello guys, i need some help. We installed a three-pass 6 MW hot water boiler 90/70°C with an economizer and a combined low NOx natural gas burner (class III.) and light gasoil (class II.) . The economizer shakes and vibrates during operation. No particularly loud noise. It vibrates towards the chimney and back towards the smoke chamber of the boiler. The economizer is fixed to the boiler with a flange, behind the economizer there is a compensator, which connects it to a 5 m long flue pipe with a 40° slope towards the chimney. The chimney is 20m high. The vibration occurs over the entire working field of the burner from 20% to 100% and increases linearly. The vibration is similar when using either of the two fuels. The vibration acts as a pulse, and when observing the gas pressure gauge (when burning with natural gas), a visible movement of the pointer in the area of 5-7% of the scale is consistent with the vibration. The burner in the front does not shake and neither does the boiler in the front. The vibration seems to be happening between the smoke chamber and the economizer inlet. The flue connection does not vibrate as the economizer is connected to the compensator. It looks like the economizer is stretching and contracting. We additionally reinforced the smoke chamber internally and reinforced the base of the economizer, but without any effect on vibrations. Burner combustion appears normal, NOx emissions are relatively low (NOx gas 80 and oil about 150), and oxygen is constant. The chimney was recalculated twice and the result was positive. Three draft regulators are built into the flue pipe towards the chimney; we performed a test with closed and open but it has no effect on vibrations/pulsations. However, the smoke draft is more stable with closed draft regulators. Does anyone have an idea how to tackle the issue and determine the cause?

Video links:

https://streamable.com/a1x6zx

https://streamable.com/etklzq

https://streamable.com/0bfaw1

 

[FacEngrPE]

Your boiler has a bad case of "rumble"

Flynn T, Timothy A, Fuller T, Rufener S Finney C, and Daw C. 2017. AFRC Industrial Combustion Symposium.
Thermoacoustic Vibrations in Industrial Furnaces and Boilers
Industrial boilers and furnaces occasionally suffer low-frequency vibrations generated by a dynamic feedback process between the burner (or burners) and acoustic modes in adjacent gas-filled cavities in the main combustion chamber or connecting ductwork. ...

The resonance is most often between the wind box and the firebox, forced by the energy provided by the flame in the burner. Possible mitigations described in the following webpage include adjusting the air / fuel tuning, increasing pressure loss at the combustion air damper (and speeding up the associated VFD), adjusting burner components, changing the shape of the burner tile, adding sound damping material (usually in the windbox).
Six Boiler Combustion Rumble Solutions, Engineered Systems

 

[Gregor77]

Thank you for your opinion. Do you think the vibration on the economizer piping would be reduced if there was no compensator behind the economizer and it was fixed at both ends? The biggest problem currently are the two water pipes connected to the economizer and their movement in the longitudinal direction of the flue gases. It looks like the economizer is loose like a garden water hose without control taking all the vibrations.

 

[saplanti]

There might be two separate reasons on the economiser side;

1. Resonance of tubes on the flue side
2. Two phase flow on the water side.

Both of them might be sourced by the problems of control systems or the lack of design.

In case the system was operating smoothly earlier you need to check for non-functioning control equipment.

3. I am not sure from the photos or videos that both side of economiser have kind of expansion bellow. If that is the case under the pulsative flow the holding structure seems to be very slander. This might cause additional vibration or it is the cause of the vibration.

 

[Dhurjati Sen]

Can you perform a cold run (i.e simply circulating water without turning on the burners as if you are going for a hydro-test) ?

If there is no significant observation, then you can be assured that this is a draft related issue.

As long as you are getting the desired steam output, there should be no issue with the burners.

DHURJATI SEN
Kolkata, India

 

[Gregor77]

Hello Dhurjati,
The burner is giving out the requested output. The water in boiler is for district heating...it is their only source now and we cannot open the ECO for inspection because it is running 24/7. The water seems to be flowing because the smokes temperature drops from 150 to 86°C. What i am not sure (because i lack the experience and examples) how much force should be normally felt on an ECO during normal operating conditions on a similar boiler? How strong should the support structure of ECO be at 6MW and how much to they play; are they only menat to carry the weight are also the vibrations?! The support system here is relatively weak it seems (support beams are 40x40mm, thickness 2mm) The draft problem is not excluded but the chimney calculations are positive (H=20,5m d800mm, L=6m d800mm + curve 40°), ... and there seems to be a 0,3 - 0,5mbar of draft what we measured with combustion instrument.

 

[davefitz]

This problem was tackled in the 1980's in the USA and most of the solutions were posted in ASME tech papers authorred by dr Frank Eisinger. The forcing function can be either fluid elastic vibration of the tubes or burner pulsations, both of which can be addressed in the field. The problem becomes noticeable or damaging when the vibration frequency matches one of the natural frequencies of the structure it self, or one of its several eigenmodes.Changing that natrual frequency is another solution.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[Chen Chen]

Hello Gregor77,

Did you find the real cause and solve it?
Basically, I agree FacEngrPE's opinion. His "resonance" pdf has well explained the reason and performance of this phenomenon. From the video you posted, I believe it is something called rumble by FacEngrPE, which is quite common among FGR burners. So the key I think is whether the burner is an FGR burner. I see pipe comes out from the middle of the econmizer and somke chamber but I don't see any picture about the burner, so I'm not sure what the burner type actually is.
Let's assume it's an FGR burner and the burner can be well adapted to the boiler, which means the boiler can satisfy all dimensions, pressures and/or other technical requirement of the burner, it's sure that a detailed combustion data including all setpoints (pressures, comubstuion parameters, flue/atmos temp etc.) of the whole curve is necessary before any workable advice, I'd like to share my experience about FGR burners: TURN DOWN the FGR a little and see if there is any improvement. Typically, turning down the FGR ratio can give you a lower vibration, even make it disappear, and the price is you get a higher NOx level. So my suggestion about FGR burners is, try to keep the FGR as low as possible while the NOx is just meet the local emission regulation.