Draft Fan vibration problems

[gregdachs]

We have a boiler with a 10' Clarage draft fan. Right now, the fan is throttled primarily by use of the inlet dampers. We have noticed that the fan has vibration that increases at night. The shaft is balanced and well mounted and the rest of the system seems ok, so we have concluded that the problem is possibly caused by an aerodynamic instability caused by the denser air.

The proposed solution to this is to switch to throttling with the outlet dampers. I am about to argue against this, but given that I hadn’t seen a fan curve until Friday I thought I would make sure I was on the right track. Wouldn’t throttling with the outlet dampers serve to increase the density of the air contained within the fan body because it is pressurizing it slightly against the outlet dampers?

Any suggestions would be greatly appreciated. Also, if anyone can think of a more logical reason for our problem I’d love to hear it.

-Greg

 

[rmw]

Greg,

Give us some more particulars, such as performance conditions for the fan, actual operating conditions.

You use the term "draft" fan, which to some of us might connote Induced Draft fan, while you specifically mention it handling "air", which, then would mean that it is a Forced Draft fan.

Inlet vortex damper control is used on either.

How much is the load varying on this boiler at night??

What is the ambient temperature swing between day and night operation???

If it is a FD fan, the inlet guide vanes give the incoming air a 'swirl' aiding the fan in its quest to do work on the air to increase the pressure, and 'pump' the air to the boiler.

I can tell you this at this point, there are lots and lots of FD fans varying loads all over the map through out the day/night, summer/winter cycle, without signficant vibration problems. We will need more information in order to diagnose what might be going on with yours.

rmw

 

[pmover]

interesting that a similar situation exist elsewhere...

gregdachs,

clarify what fluid the fan is moving - ambient air or flue gas?

clarify whether inlet dampers or an inlet box damper is installed on fan.

if ambient air is the fluid, request the fan mfg to provide you another curve(s) for the ACTUAL process conditions at night and to clearly identify minimum flow required to maintain stable operation.

controlling process flow/pressure using outlet dampers and inlet dampers should not be done. it will further magnify the problem of unstability. this leads to the clarification question of inlet dampers or inlet box dampers.

inlet box dampers do provide a degree of pre-swirl of the fluid into the wheel, thus allowing for a greater turndown operation. whereas inlet dampers do not create pre-swirl and location installation of dampers is critical in that the process stream into the fan should be uniform. if not, then one may experience unstable operation at a desired operating point. this is where design assistance from fan mfg can be helpful.

i could ramble on, but i would definitely get a set of curves from mfg for the actual process conditions and proceed from there.

i gather that fan speed is constant (not variable)?

lastly, i will send sufficient theoretical textbook information upon request. too bad attachments can not be part of the posting.

good luck!
-pmover

 

[gregdachs]

I apologize for my lack of clarity.

The fan is a forced draft fan, thus moving ambient air. The inlet dampers are inlet box dampers.

The load and fan speed remain constant, or at least approximately constant 24 hours a day. The vibration readings double at night, going past our alarm points.

The temperature fluctuation is around 20 degrees F (~95 daytime, ~75 nighttime). Last week is was cool and raining all week, and the alarms went off non-stop.

My original question asked how the density behaved as related to the position of the dampers and whether that would lead to the vibration. I understand that there are numerous explanations to this and with the limited information that i am able to get from a fan that cannot be turned off analysis of the problem might be impossible.

So, another more easily answered question is are outlet dampers inherently more stable than inlet dampers? I know that they're generally less efficient, but if you blow against an almost closed outlet damper is the situation better than if you try to pull against an almost closed inlet box damper?

pmover, the theoretical info would be greatly appreciated.
gregory.dachsii@equistar.com

Thanks

 

[pmover]

gregdachs,

any fan operating at a given speed has a minimum flow value based on inlet conditions and the fan will generate a given head (or delta p) at that given speed. this is the situation regardless whether or not inlet box dampers, inlet dampers, inlet guide vanes, or outlet dampers are installed. dampers, themselves, whether or not in inlet or outlet, provide a degree of turndown, but not as great as inlet box dampers or inlet guide vanes (these devices create pre-swirl into the wheel).

therefore, are outlet dampers inherently more stable, it is difficult, if not impossible, to answer that question as the situation depends upon the operating range in which these dampers (inlet or outlet) are utilized. creating too much head across the wheel (too much flow resistance; i.e. damper position) will eventually lead to unstable operation of a fan.

i trust that the fan load/flow is constant throughout the day and the only variable changing is ambient temperature? is there a flow measurement device for the fan?

i again stress the importance of obtaining fan performance data from fan mfg at the actual process conditions, including minimum recommended flow.

be glad to continue the discussion, but please read the information forwarded.

good luck!
-pmover

 

[pmover]

gregdachs,

thinking about this matter some more, the fan is existing and has been operating satisfactory (?) for a period of time - please confirm.

if this problem recently developed, what event transpired (if any) that may have caused higher vibrations?

the case may be in which, unknown to operators, mechanical damage occurred to the dampers themselves. this would disrupt the pre-swirl and cause high vibrations.

is it possible to conduct a boroscope inspection to check the dampers without impacting operation?

it will take a little sluething/detective work on your part, but just providing you some thoughts to ponder.

good luck!
-pmover

 

[rmw]

Have you had our vibration problem long enough to have observed what happens during winter or cooler spring and/or fall conditions???

rmw

 

[gregdachs]

I did some more serious detective work today and finally got some answers. They neglected to tell me a crucial little piece of history.

Apparently a year or two ago a 50 lb piece of plate steel fell off the inside of the fan enclosure into the rotating wheel and was launched through the outlet dampers into an elbow about 30 feet away. Remarkably, the dampers were not touched. One of the bearings, however, completely crashed. The bearing was replaced, and because the unit is critical to operation, there wasnt time to do a detailed analysis of the wheel. Now it looks like the foundation that bearing was on might be damaged too.

So, to sum up, the way it looks to us right now is that the temperature induced vibration is because the denser air makes the van work harder and thus causes higher vibrations in the foundation and possibly the wheel itself. Now we know why the vibrations are there. Based on all the info we could dig up from Clarage and this forum thread, the inlet dampers are likely the best way to run the fan, as is the way it has been run. The bigger problems with the wheel and foundation need to be fixed.

Thanks to you guys who helped me out. The insight greatly helped in figuring out what to look for.

 

[rmw]

As I pondered this thread, mechanical problems was screaming at me. It was the reason I questioned vibration problems in other than current ambient temperature ranges.

Your additional information confirms my gut reaction.

I think when you can bring this fan down, and repair the wheel (I have a mental picture of what the blade(s) that did the launching of the foreign object must look like, and/or the bearing alignment, and/or the dynamic balance, or all of the above, I think your problem will be solved.

Good luck, and post back to this thread when you get results.

rmw

 

[pmover]

gregdachs,

very good!

yes, mechanical damage to any rotating or fixed device in a fan can change the aerodynamics (geometry), thus the operating characteristics of the fan.

i definitely check the entire rotating assembly and any fixed device as well to ensure the fan is in original design condition. any deviation or change may cause a change in fan operation.

good luck!
-pmover

 

[rmw]

Further, I suspect that if you can get some more in depth vibration analysis done, you would find some real anomolies with the blade pass frequency readings, especially during this period of higher vibration.

This would confirm mechanical damage to the rotating assembly (blades at a minimum) as the culprit.

You could have a serious ding in the shroud, as well, and maybe even the back plate.

Take photos and post them when you get in there.

Caution!!!!!!!!!

In a wheel this size, eventhough you did not state the rpm, you could be facing catastrophic failure of the wheel. If this foreign object shook this wheel enough to wreck a bearing, it can well have ripped blades, shrouds, or other parts of the fan rotating assembly loose, which could subject the wheel to imminent failure.

This has potential for safety issues as well as just wrecking your fan. Put some red tape around this fan immediately until you can determine its exact condition. Keep personnel away from the immediate area of the fan.

Should this wheel come apart, it could well scatter itself beyond the envelope dimensions of the fan housing.

I have been to a few fan wrecks, they are not pretty.

rmw

 

[gregdachs]

We are working on trucking in some temporary boilers now so we can keep the plant alive during the inspection.

If i am still here when they get it open, i'll gladly let you all know what is going on. I am just a co-op for the summer, so they'll have to work pretty quick.

Thanks again for the help.

 

[GenB]

I see a draft pressure change in the exhaust chimney,
something is slowing down the fan slightly, have you check voltage changes?
ER