Fan Troubleshooting – Balance & Vibration

Published 23 Apr 2020

Portable Fans | ATEX & IECEx Certified | Hazardous Area & Explosive Atmospheres

Article Written By Woodcock & Wilson | 5 of 6 Articles

Woodcock & Wilson are specialist manufacturers of independently certified ATEX fans and IECEx industrial fans for the ventilation of hazardous area locations and potentially explosive atmospheres.

Below is Part 5 in a series of 6 Articles by Woodcock & Wilson discussing fans, the laws, common problems, troubleshooting the main performance issues and getting your fan started again if you encounter any issues.

Fan Mechanical, Balance & Vibration

A) Balance Grades & Categories
B) Possible reasons for excessive vibration during commissioning
C) Possible reasons for vibration experienced after successful commissioning

A) Balance Grades & Categories

Fan balancing and vibration is in accordance with BS848-7 BS ISO 14694. Woodcock & Wilson Ltd have selected “Fan application category: BV-3 for fans ≤ 37KW & BV-4 for fans > 37 kW.

The balance quality grade for these categories are G 6.3 & G 2.5 respectively. The tables below are taken from the above standard. They give vibration limits for tests in manufacturers workshop & recommended vibration limits for fans operating in situ.

Maximum Vibration Levels Limits for Factory Tested Fan

Fan Category Application Category	Rigidly Mounted mm/s		Flexibly Mounted (AV Mounts) mm/s
Fan Category Application Category	Peak	r.m.s	Peak	r.m.s
BV-3	3.8	2.8	5.1	3.5
BV-4	2.5	1.8	3.8	2.8

Fan Vibration Limits For Fans Operating In Situ

Condition	Fan Application Category	Rigidly Mounted mm/s		Flexibly Mounted mm/s
Condition	Fan Application Category	Peak	r.m.s	Peak	r.m.s
Start-up	BV-3	6.4	4.5	8.8	6.3
	BV-4	4.1	2.8	6.4	4.5
Alarm	BV-3	10.2	7.1	16.5	11.8
	BV-4	6.4	4.5	10.2	7.1
Shut-down	BV-3	12.7	9	17.8	12.5
	BV-4	10.2	7.1	15.2	11.2

The vibration levels on the table before are guidelines for acceptable operation of fans. The vibration level of newly commissioned fans should be at or below the “start up” level.

Historical data is an important factor when considering vibration severity. A sudden & maintained change in the vibration level may indicate the need for prompt inspection or maintenance.

B) Possible Reasons for Vibration Experienced During Commissioning

Fans test run and balanced in the manufacturers works and delivered as a complete, fully assembled fan.

Mechanical Problems

Damage to fan during transport causing out of balance or misalignment problems
Foreign bodies on impeller causing out of balance
Fan pedestal distorted during installation causing drive chain misalignment
Incorrect selection or fitting of anti-vibration mounts
Loose holding down bolts
Defective foundations

II) System or System Effect Problems:-

The system has greater resistance to flow than anticipated resulting in the fan operating further up the characteristic in a stalled section. Depending on fan type, vibration may be experienced when operating in the stalled area
On fans fitted with inlet vanes or inlet louvers on an inlet box, vibration may be experienced at lower vane angles due to a condition known as inlet cone vortex
Bands or other obstacles fitted to the fan inlets may cause airborne instabilities resulting in vibration (see “V fan systems” in “underperforming” section

Fans not previously run:

All previous possible reasons + the following:-

Impeller out of balance
Faulty or wrongly fitted bearings
Motor out of balance
Shaft misalignment in bearings
Coupling misalignment (on direct coupled fan)
Pulley misalignment or loose belts (on v belt driven fan)
Bent shaft
Components Looseness (impellers, pulleys, couplings etc.)
Failed impeller
Shaft flexural critical speed
Shaft torsion critical speed

C) Possible reasons for increased vibration levels after successful commissioning:

Dust of product build up on the impeller
Impeller deterioration due to wear or corrosion
Fan bearing deterioration
Components looseness (impellers, couplings, pulleys, etc.)

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