ISO 21940-13:2012

INTERNATIONAL ISO
STANDARD 21940-13
First edition
2012-03-15
Mechanical vibration — Rotor
balancing —
Part 13:
Criteria and safeguards for the in-situ
balancing of medium and large rotors
Vibrations mécaniques — Équilibrage des rotors —
Partie 13: Critères et sauvegardes relatifs à l'équilibrage in situ des
rotors moyens et grands
Reference number
©
ISO 2012
©  ISO 2012
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ii © ISO 2012 – All rights reserved

Contents Page
Foreword . iv
Introduction . vi
1  Scope . 1
2  Normative references . 1
3  Terms and definitions . 2
4  In-situ balancing . 2
4.1  General . 2
4.2  Reasons for in-situ balancing . 2
4.3  Objectives of in-situ balancing . 3
5  Criteria for performing in-situ balancing . 3
6  Safeguards . 4
6.1  Safety of personnel while operating close to a rotating shaft . 4
6.2  Special operating envelope for in-situ balancing . 4
6.3  Integrity and design of the correction masses and their attachments . 4
6.4  Machinery-specific safety implications . 4
7  Measurements . 5
7.1  Vibration measurement equipment . 5
7.2  Measurement errors . 5
7.3  Phase reference signals . 6
8  Operational conditions . 7
9  Reporting . 8
9.1  General . 8
9.2  Report introduction . 9
9.3  Vibration measurement equipment . 9
9.4  Results . 9
9.5  Text information . 10
Annex A (normative) Precautions and safeguards for specific machine types during in-situ
balancing . 12
Annex B (informative) Example of an in-situ balancing report for a boiler fan 1 MW . 13
Annex C (informative) Example of an in-situ balancing report for a large 50 MW turbine
generator set . 17
Bibliography . 23

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 21940-13 was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and condition
monitoring, Subcommittee SC 2, Measurement and evaluation of mechanical vibration and shock as applied
to machines, vehicles and structures.
This first edition of ISO 21940-13 cancels and replaces ISO 20806:2009, of which it constitutes a minor
editorial revision.
ISO 21940 consists of the following parts, under the general title Mechanical vibration — Rotor balancing:
1)
 Part 1: Introduction
2)
 Part 2: Vocabulary
3)
 Part 11: Procedures and tolerances for rotors with rigid behaviour

4)
 Part 12: Procedures and tolerances for rotors with flexible behaviour
5)
 Part 13: Criteria and safeguards for the in-situ balancing of medium and large rotors
6)
 Part 14: Procedures for assessing balance errors

1) Revision of ISO 19499:2007, Mechanical vibration — Balancing — Guidance on the use and application of balancing
standards
2) Revision of ISO 1925:2001, Mechanical vibration — Balancing — Vocabulary
3) Revision of ISO 1940-1:2003, Mechanical vibration — Balance quality requirements for rotors in a constant (rigid)
state — Part 1: Specification and verification of balance tolerances (+ Cor.1:2005)
4) Revision of ISO 11342:1998, Mechanical vibration — Methods and criteria for the mechanical balancing of flexible
rotors (+ Cor.1:2000)
5) Revision of ISO 20806:2009, Mechanical vibration — Criteria and safeguards for the in-situ balancing of medium and
large rotors
6) Revision of ISO 1940-2:1997, Mechanical vibration — Balance quality requirements of rigid rotors — Part 2: Balance
errors
iv © ISO 2012 – All rights reserved

7)
 Part 21: Description and evaluation of balancing machines
8)
 Part 23: Enclosures and other protective measures for balancing machines
9)
 Part 31: Susceptibility and sensitivity of machines to unbalance
10)
 Part 32: Shaft and fitment key convention

7) Revision of ISO 2953:1999, Mechanical vibration — Balancing machines — Description and evaluation
8) Revision of ISO 7475:2002, Mechanical vibration — Balancing machines — Enclosures and other protective
measures for the measuring station
9) Revision of ISO 10814:1996, Mechanical vibration — Susceptibility and sensitivity of machines to unbalance
10) Revision of ISO 8821:1989, Mechanical vibration — Balancing — Shaft and fitment key convention
Introduction
Balancing is the process by which the mass distribution of a rotor is checked and, if necessary, adjusted to
ensure that the residual unbalance or the vibrations of the journals or bearing supports and/or the forces at the
bearings are within specified limits. Many rotors are balanced in specially designed balancing facilities prior to
installation into their bearings on site. However, if remedial work is carried out locally or a balancing machine
is not available, it is common to balance the rotor in situ.
Unlike balancing in a specially designed balancing machine, in-situ balancing has the advantage that the rotor
is installed in its working environment. Therefore, there is no compromise with regard to the dynamic
properties of its bearings and support structure, nor from the influence of other elements in the complete rotor
train. However, it has the large disadvantage of restricted access and the need to operate the whole machine.
Restricted access can limit the planes at which correction masses can be added, and using the whole
machine has commercial penalties of both downtime and running costs. Where gross unbalance exists, it may
not be possible to balance a rotor in situ due to limited access to correction planes and the size of correction
masses available.
vi © ISO 2012 – All rights reserved

INTERNATIONAL STANDARD ISO 21940-13:2012(E)

Mechanical vibration — Rotor balancing —
Part 13:
Criteria and safeguards for the in-situ balancing of medium and
large rotors
1 Scope
This part of ISO 21940 specifies procedures to be adopted when balancing medium and large rotors installed
in their own bearings on site. It addresses the conditions under which it is appropriate to undertake in-situ
balancing, the instrumentation required, the safety implications and the requirements for reporting and
maintaining records.
This part of ISO 21940 can be used as a basis for a contract to undertake in-situ balancing.
It does not provide guidance on the methods used to calculate the correction masses from measured vibration
data.
NOTE The procedures covered in this part of ISO 21940 are suitable for medium and large machines. However,
many of the principles are equally applicable to machines of a smaller size, where it is necessary to maintain good records
of the vibration behaviour and the correction mass configurations.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
11)
ISO 1925, Mechanical vibration — Balancing — Vocabulary
ISO 1940-1, Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state —
12)
Part 1: Specification and verification of balance tolerances
ISO 2954, Mechanical vibration of rotating and reciprocating machinery — Requirements for instruments for
measuring vibration severity
ISO 7919 (all parts), Mechanical vibration — Evaluation of machine vibration by measurements on rotating
shafts
ISO 10816 (all parts), Mechanical vibration — Evaluation of machine vibration by measurements on non-
rotating parts
11) To become ISO 21940-2 when revised.
12) To become ISO 21940-11 when revised.
ISO 10817-1, Rotating shaft vibration measuring systems — Part 1: Relative and absolute sensing of radial
vibration
13)
ISO 11342, Mechanical vibration — Methods and criteria for the mechanical balancing of flexible rotors
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1925 apply.
4 In-situ balancing
4.1 General
For in-situ balancing, correction masses are added to the rotor at a limited number of conveniently engineered
and accessible locations along the rotor. By doing this, the magnitude of shaft and/or pedestal vibrations
and/or unbalance is reduced to within acceptable values, so that the machine can operate safely throughout
its whole operating envelope. As part of a successful balance, transient-speed vibration might be comprom-
ised to some degree to obtain acceptable normal running speed vibration on a fixed-speed machinery train.
NOTE In certain cases, machines that are very sensitive to unbalance cannot be successfully balanced over the
complete operating envelope. This usually occurs when a machine is operating at a speed close to a lightly damped
system mode (see ISO 10814 to become ISO 21940-31 when revised) and has load-dependent unbalance.
Most sites have limited instrumentation and data-processing capabilities, when compared to a balancing
machine, and additional instrumentation is required to undertake in-situ balancing in these
...