System fan Study

System fan # 4 High Vibration Junaid Mazhar Fan Trouble shootingCase Study

Simple Vibration causeVibration problems in centrifugal fans 1- Balancing ( Dynamic balancing done upt0 1.0 mm/sec)2- Miss-alignment (Alignment of fan checked)Fan should have this normal vibration trend other wise there are complex reason for fan vibration

Normal vibration Trend of System fanComplex Vibration Causes

Acoustic & Flow EffectsFoundation Natural FrequencyCase ResonanceDisc Wobble Resonance

Irregular vibration Trend observed at system fan # 4

Acoustic & Flow Effects

Acoustic & Flow Effects

Comparison of fan Inlet & outlet1-Inlet condition2-Outlet conditionElbows to close to fan inletAbrupt duct transitionInlet spin due to duct designDamper not fully openDamper locationInlet boxElbows too close to fan outletAbrupt transitionFree dischargeDamper locationDischarge too close too wall

1-Inlet Condition RequirementNo bend at fan Inlet:

62- Outlet Condition RequiredNo bend at Fan Outlet

3- Installation of Suction DamperInstallation of suction damper

Direction of inlet Damper before Direction of inlet Damper after changing ContinueDamper vanes effectively guide the flow in the rotating direction of the impeller.Miss installation of inlet damper causes gas to flow against the rotating direction of the impeller, and abnormal high shaft power may be drawn, large pressure pulsation occur resulting in unstable fan operation.

CM #1CM # 2CM # 3CM# 4Motor Ampere (Shaft power drawn)27252738Vibration TrendNormal

Normal

Normal3.5~8.5 (Abnormal)% damper opening set point90~100%30~35%70~80%System fan picture (Air opposes direction of revolution)

Direction of rotationSystem fan Damper changed (Air enters in the direction of revolution)

Direction of rotationConclusionHigh vibration of system fan was due to counter rotating swirl that was striking the impeller at opposite direction of rotation.Gases were entering the impeller at an incident angle mismatch the blades and result in rotation stall condition. That is why vibration change of fan from mean value is 3.4~4mm/sec.This swirls created additional load on impeller and increased its amperes up to 38.Results obtainedFollowing positive results were obtained.Mill started at 1400 hrs and system fan vibration remained from 1.2 ~ 1.4 mm/sec with 6 hours of operation. (Mill stopped due to feeding elevator coupling fault)Smooth vibration trend was observed with no abnormality as shown in figure.Fan motor amperes greatly reduced from 38 amperes to 23 amperes with a decrease of 14~15 amp. Increasing fan performance and optimizing energy consumption.

Further recommendationRecommendationsChange the damper opening position of CM# 3 system fan as per requirement, it will help to decrease shaft power and vibration.Inspection of CM # 2 damper opening position.Modification of inlet box in order to destroy any vortex formation tendencies. Splitter plates may be installed in inlet box directly opposite to the inlet damper. This would further enhance fan performance by reducing fan vibration, pulsation and noise level.

Inlet flow splitter used to reduce Inlet vortexComparison of Inlet BoxSystem fansHot gas fan at coal Mill