Examination of Critical Centrifugal Fans and Blowers

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-2220

Examination of Critical Centrifugal
Fans and Blowers

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

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Engineering Design Guide:

Examination of
Critical Centrifugal Fans
And Blowers

CONTENTS

0

SECTION

INTRODUCTION/PURPOSE
0.1
0.2
0.3
0.4
0.5

Reasons for Critical Classification of Arduous Duty
Fans < 500 kW
Reasons for Critical Classification of Arduous Duty
Fans > 500 kW
Critical Classification for High Substance Hazard
Reasons
Centrifugal Blowers
General Purpose Fans

2

2
2

2
3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

PRINCIPLES

4

5

CONDITION MONITORING

4


6

4

6.1

Impeller

5

6.2

Shaft and Sleeves

5

6.3

7

EXAMINATION

Bearings

5

ADDITIONAL EXAMINATION REQUIREMENTS FOR
ALL FANS DEEMED CRITICAL FOR HIGH PROCESS
SUBSTANCE HAZARD REASONS

7

7.1

Casing

7

7.2

Dynamic Seals

7


APPENDICES

A

EXAMINATION LOGIC DIAGRAM

DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE


0

INTRODUCTION/PURPOSE

GBH Enterprises categorizes fans Into Arduous and General Purpose (nonarduous) duties according to various criteria as defined in the appropriate
specifications below:
GBHE-EDS-MAC-1809:

The Design & Construction of Steel Centrifugal
Fans Operating on Arduous Duties.

GBHE-EDS-MAC-1810:


GBHE-EDP-MAC-6601 requires the Identification and registration of Critical
Machines Systems. Included amongst these are Arduous Duty fans as defined in
GBHE-EDS-MAC-1809 and also those fans and blowers deemed Critical for high
substance hazard reasons. This Guide is intended to assist the Specialist
Engineer In the verification of the design of Critical fans and blowers as required
by GBHE-EDP-MAC-3604 and in the preparation of the necessary Schemes of
Examination.

0.1

Reasons for Critical Classification of Arduous Duty Fans < 500 kW

In this category, the concern is failure of the Impeller due to overstressing. The
boundary between arduous duty and non-arduous duty is defined by a line on a
graph of impeller tip speed v. Impeller shape factor (see Figure 1 in GBHE-EDSMAC-1809. This approximates to the maximum stress levels acceptable In
normal' carbon steel materials.
For other criteria for Arduous Duty Fans see GBHE-EDS-MAC-1809, Clause F.2.
Other possible failure modes. E.g. shaft couplings, etc, would not result In a
Critical classification.
Design Verification in this case will confine itself to ensuring that the Impeller is
adequately designed for the duty and that manufacture and testing were in
accordance With GBHE-EDS-MAC-1809. Similarly the subsequent Scheme of
Examination will also confine itself to maintaining the Integrity of the Impeller.


0.2

Reasons for Critical Classification of Arduous Duty Fans > 500 kW

In this category, the concern is both the failure of the impeller as in 0.1, and any
other failure that could release significant kinetic energy. As a result, the Design
Verification Will need to look at the wider mechanical issues Including drives,
couplings, bearings, shafts, etc, as well as those specified In 0.1, above.
In either of these cases, any Scheme of Examination which required the routine
stripping and detailed Inspection of parts would seem to be inappropriate unless
deterioration is expected.
Instrument protection (e.g. vibration, bearing temperatures) and routine condition
monitoring would be more appropriate.
0.3

Critical Classification for High Substance Hazard Reasons

Arduous Duty fans and General Purpose fans are considered during
classification according to GBHE-EDP-MAC-3602 against the various criteria.
Sub clauses 0.1 and 0.2 above relate to mechanical hazards. If, however, the
concern is loss of containment of a high hazard substance, then Design
Verification needs to address additional aspects, such as Integrity of the casing
welds and shaft dynamic seals (see 6.3 (b) and 6.5).

0.4

Centrifugal Blowers

Centrifugal blowers should be considered as Arduous Duty fans for the purpose
of this Guide.

0.5


General Purpose fans cannot be > 500 kW. They can only be deemed Critical for
high substance hazard reasons. In the future it is envisaged, however that for all
such duties Arduous Duty fans designed and constructed to GBHE-EDS-MAC1809 would be specified. Where steel construction is adequate (see 0.3 above).


1

SCOPE

This Guide addresses itself to the mechanical integrity of the fan/blower, in
particular of the impeller, for Arduous Duty fans < 500 kW as 0.1 above, and of
additional items for Arduous Duty fans > 500 kW as 0.2 above.
Also additional requirements are necessary for all fans deemed Critical for high
substance hazard reasons as 0.3 above. The manufacturer's maintenance
instructions should be adhered to In the renewing, rebalancing, reassembling
and setting up of all components within the complete fan/blower system and that
such work will be carried out by suitably trained personnel.

2

FIELD OF APPLICATION

This Engineering Guide is for use by Specialist Engineers involved In the Design
Verification of Critical centrifugal fans and blowers, and for general reference by
machines engineers and also for use by works based engineers. Specific
Schemes of Examination may be produced from the information presented.

3

DEFINITIONS

For the purposes of this Guide. the following definitions apply:

Critical Machine
System

A Critical Machine System IS a machine system which has
been assessed to present an unacceptable hazard if the
machine or its protective system should fall. and where
failure or malfunction could lead to danger of life.
Significant harm to any person or to the environment.

Examination

Examination means a careful and critical scrutiny of a
Registered Critical Machine by an authorized person to
assess its actual condition and whether it is safe to use for
the period up to the next Examination. An Examination
may be 'thorough' or 'intermediate'. All examinations are
'thorough' unless specifically described as 'intermediate'.


Level 1 audits

These are regular routine workplace audits undertaken by
local plant teams.

Scheme of
Examination

A Scheme of Examination defines the extent, nature
preparatory work and frequency of Examinations.

With the exception of terms used as proper nouns or titles. those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.

4

PRINCIPLES

As a general principle, it IS recommended that the frequency of thorough
Examinations is minimized commensurate with maintaining the integrity of the
machine system. To this end, the use of correctly targeted conditioning
monitoring is of prime importance. In addition work associated with intermediate
Examinations should be minimized, again commensurate with maintaining the
integrity of the machine system.
If deterioration in service is not expected, e.g. due to erosion/corrosion, then the
period between thorough Examinations could be fairly long. The Implementation
of condition monitoring is key In this respect (see Clause 6 for more details).

5

CONDITION MONITORING

Either continuous or routine periodic condition monitoring as appropriate, should
be instigated for all Critical fans and blowers.
The following key Items should be considered for monitoring, but (a) should be
monitored as a minimum:"
(a)

Bearing vibration; horizontal, vertical and axial;

(b)

Bearing temperature;

(c)

Bearing lube oil pressure, if applicable.


Log sheets should be provided to operating personnel which clearly indicate the
normal reading as a standard together with all maximum and minimum values.
The sheets should also provide guidance on actions to be taken at certain levels.
A lubrication schedule should be available and adhered to rigorously. An oil
analysis program should be considered.
Level l audits should be In place to provide the necessary check that the system
is working.

6

EXAMINATION

A Scheme of Examination for a specific critical fan/blower should be based on
the following guidelines. and the approach to be adopted should be In stages.
(a)

Stage 1

Condition Monitoring

Implementation of condition monitoring should commence within 3 months of the
fan being classified as a Critical Machine System. The Initial readings should be
compared with the expected normal readings.
As an example, for vibration levels, the manufacturer's data should be checked,
but for general guidance, fans should conform to as 4675 Class A quality
Judgment or the Good Quality Judgment of ISO 3945 (I.e. 2.8 mm/s RMS).
Corresponding alarm and trip settings are suggested:Normal

2.8 mm/s RMS

Alarm

10 mm/s RMS

Trip

18 mm/s RMS

Depending how the various readings compare then different lines of action may
be necessary. See the Examination Logic Diagram (Appendix A).


Further information on Vibration Monitoring can be obtained from GBHE-EDGMAC-4506.
(b)

Stage 2

Intermediate Examination

With the fan shut down and isolated the unit should be disassembled, as
appropriate, cleaned and inspected in-situ as thoroughly as possible In
accordance with the relevant sub-clauses, 6.1 to 6.3 inclusive.
For example, all that may be required to be done is an in-situ rebalancing to the
specified quality grade of the rotating assembly after cleaning. Checks after start
up should then establish that vibration levels are satisfactory. If so, the machine
can run and condition monitoring and trend analysis should continue.
(c)

Stage 3

Thorough Examination

If possible this should be carried out in-situ but if this is not possible, the rotating
Assembly should be removed from the casing and transported to a workshop
environment. The impeller should only be withdrawn from the shaft and any keys
and half coupling removed when deemed necessary by examination. If this is
necessary all items should be thoroughly cleaned to allow examination which
should include:

6.1

Impeller

This sub clause applies to all Critical Arduous Duty fans and Critical General
Purpose fans.
(a)

Visual inspection and dimensional checking of the backplate. blades, eyering, shrouds for signs of erosion/corrosion, and surface cracks,
deformation, etc.

(b)

100% MPT (or DPT if non-magnetic) of all fillet and butt welds.
Radiography of butt welds should be considered if the MPT or DPT
indicate possible defects.

(c)

Ultrasonic thickness checks of hollow section blades, on fans on
erosive/abrasive duties.

The following sub-clauses (6.2 and 6.3) apply to Arduous Duty fans > 500 kW.


6.2

Shaft and Sleeves

(a)

Visual inspection and dimensional checking of the shaft, including the
journal areas or sleeves for bearings and seals, and changes in section,
etc.

(b)

100% MPT (or DPT if non-magnetic), to stress raisers. E.g. keyways,
thread undercuts, changes of section, dowel holes etc.

6.3

Bearings

During service just prior to shutdown, the condition of the bearings may be
ascertained by audible means, by measuring the temperature and by examining
the lubricant. In addition portable vibration monitors may be used where
permanent monitors are not fitted. (See Clauses 4 and 5 above.)

6.3.1 Rolling Element Bearings I Radial and Thrust)
(a)

The rings, cage, rolling elements and other parts of the bearing
arrangement should be thoroughly cleaned using a suitable solvent.

(b)

They should be then visually inspected for evidence of wear or fatigue of
the tracks, elements and cages and dimensionally checked including
Internal clearance, if practicable.
Comparing the bearing 1.10 life with the known service period should
assist in the decision whether to re-use the existing bearings or to replace
by new ones.

Note:
It is imperative that any new bearings fitted are to the correct specification with
The appropriate internal clearance and they are from an approved manufacturer
(see GBHE-EDS-MAC-1806).

(c)

After cleaning and inspection, bearings to be re-used should be oiled or
greased immediately using the appropriate in-service lubricant to prevent
corrosion which can be rapid.


6.3.2 Plain Journal Bearings
(a)

These should be removed cleaned and visually inspected for signs of
wear or particle embedment.

(b)

The bores of the Journals should be dimensionally checked to ensure they
are within design limits.

(c)

Bearings should be replaced as deemed necessary from the above
actions. Bearings to be re-used should be Oiled or greased using the
appropriate in-service lubricant.

6.3.3 Tilting Pad Radial and Thrust Bearings
(a)

These should be removed, cleaned and Visually inspected.

(b)

The tilting pads should be individually checked for freedom of movement.

(c)

The pads/other components or whole units should be replaced as deemed
necessary from the above actions. Items to be re-used should be re-oiled
using the in-service lubricant. Individual pads should not be changed.

6.3.4 Fixed Taper Land Thrust Bearings
(a)

These should be removed and cleaned.

(b)

By visual inspection the taper lands should be checked for loss of metal
due to wear for signs of surface corrosion/erosion and for deposition.

(c)

The oil gullies between the taper land wedges and the oil flow control
weirs should be checked.

(d)

The shaft mating collar should also be Visually examined to check its
condition.

(e)

The bearings should be replaced as deemed necessary from the visual
inspection.

For further detailed information on bearings see GBHE-EDS-MAC-1806. Bearing
a arrangements for Machines.

7

ADDITIONAL EXAMINATION REQUIREMENTS FOR ALL FANS
DEEMED CRITICAL FOR HIGH PROCESS SUBSTANCE HAZARD
REASONS

7.1

Casing

(a)

After removal of the fan rotating assembly. the Interior of the casing
should be thoroughly cleaned and visually Inspected.

(b)

All important structural and sealing welds should be subjected to 100%
MPT (or DPT If non-magnetic).

7.2

Dynamic Seals

During service just prior to shutdown the condition of the seal should be
assessed for ingress/egress of gas, oil, moisture etc. where practicable.
Seals should be removed thoroughly cleaned and visually Inspected with
particular attention being paid to mechanical seal rotating and stationary faces,
condition of springs, other components, labyrinths, Integrity of buffer systems,
and all auxiliary dynamic and static seals.


APPENDIX A

EXAMINATION LOGIC DIAGRAM


Examination of Critical Centrifugal Fans and Blowers

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