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Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

(OP)
Hi,

We are experiencing marginally high vibration at our FD Fan DE & NDE when the boiler load was increased from 80 T/hr to 105 T/hr. And the vibration reduced when the load is at 115 T/hr.

See below table for the measurements:

Fan DE Horiz Casing Vibration mm/sec 2 2 1.8 2 2.1 1.9
Fan DE Vertical Casing Vibration mm/sec 0.5 0.7 0.6 0.7 0.7 0.7
Fan DE Axial Casing Vibration mm/sec 0.8 0.7 0.7 0.8 0.8 1
Fan NDE Horiz Casing Vibration mm/sec 1.6 2.9 2 2.1 2.2 1.7
Fan NDE Vertical Casing Vibration mm/sec 0.4 0.6 0.6 0.4 0.5 0.5
Fan NDE Axial Casing Vibration mm/sec 1.8 4.3 3.8 3.7 3.8 1.3
Fan NDE Bearing Casing Temp Deg Cent 41 42 42 41 42 42
Fan DE Bearing Casing Temp Deg Cent 42 43 44 44 43 44
Flow / Load Ton/H 80 90 95 100 105 115
Fan Damper Opening % 36 38 40 41 41.5 44

It seems the flow dynamics of air through FD Fan when load is increased > 80 T/hr and Fan Damper starts to open > 36%, an excitation force is introduced which amplified the vibration (although still within acceptable limit). And when the boiler load is at 115 T/hr max, with Fan Damper is open at 44% --> the vibration reduced with the same amplitude as at load of 80 T/hr within acceptable limit.

Could this phenomenon related to a possible "resonance" of the adjacent structure? Or the dynamics of flow at both suction and discharge line? Or a possible relationship of both resonance <--> flow dynamics?

Thanks!
James

RE: Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

Yes. It is most likely that the flow dynamics are exciting some sort of resonance. The resonance could be the fan wheel, fan rotor, bearing housings, support structure or duct work. As a first step, you could use a motion amplifying camera to look for structural resonances. You could perform a full ODS on the fan and associated structure to check for resonance. If the resonance is the rotor or the fan wheel, these tests may not be conclusive. You could shut the fan down and do a shaker test of the rotor to determine how it responds to a range of frequencies.

Johnny Pellin

RE: Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

Does "vibration" = an overall reading made on the bearing housings, or perhaps the 1X vibration read by proximity probes, or something else altogether ?

RE: Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

FD Centrifugal fan vibration at 1xSS typically does not change with load. The filtered amplitude at blade pass frequency (#baldes x rpm) typically has lowest amplitude at BEP and gets high at both lower and higher flow rate. Other pulsation sound and vibrations can occur at low flow. What type of inlet dampers: vanes at inlet eye, parallel blade or opposed blade? What is the dominant frequency when the vibration level is high? Cannot speculate on resonance or flow dynamics without more information. What is fan rpm and hp? Can you provide a photo or drawing?

Walt

RE: Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

(OP)
Here are more info on the issue:

1. Noise can be heard at FD Fan inlet when the boiler load starts to be increased from 85 T/hr to 103 T/hr

2. During this time, overall vibration increased from 3.2 mm/s to 5.8 mm/s

3. Air intake into FD Fan is controlled via IGV Unit

4. Dominant peak at 1x when the vibration increased, with 3x & 6x also appear.

5. When the boiler load is at 115 T/hr, the vibration reduced to acceptable level of 3.4 mm/s; and the harmonics at 3x, 6x dissapear

RE: Forced Draft Fan vibration increased as boiler load increased from 80T/hr but reduced at 115 T/hr

I have worked on a lot of power plant fan vibration and sound problems. I suggest measuring sound pressure at the inlet along with vibrations. I am concerned with how you are describing the IGV opening, since the values are quite low relative to 100% open for a design flow rate above BEP. The dominant sound frequency may not match the 1xSs, 3xSS, and 6xSS vibration frequencies. A few more questions:

Fan HP?

Fan Speed and constant or variable?

Fan Design Flow Rate?

Fan Inlet single or double?

Fan Blades number and type?

Walt
w_f_stromng@msn.com

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