EN 62086-2:2005

SLOVENSKI SIST EN 62086-2:2005

STANDARD
december 2005
Električne naprave za eksplozivne plinske atmosfere - Električni uporovni
grelni trakovi - 2. del: Vodila za zasnovo, vgraditev in vzdrževanje (IEC 62086-
2:2001)
Electrical apparatus for explosive gas atmospheres - Electrical resistance trace
heating - Part 2: Application guide for design, installation and maintenance (IEC
62086-2:2001)
ICS 29.260.20 Referenčna številka
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

EUROPEAN STANDARD EN 62086-2
NORME EUROPÉENNE
EUROPÄISCHE NORM October 2005

ICS 29.260.20
English version
Electrical apparatus for explosive gas atmospheres –
Electrical resistance trace heating
Part 2: Application guide for design,
installation and maintenance
(IEC 62086-2:2001)
Matériel électrique pour atmosphères Elektrische Betriebsmittel für
explosives gazeuses – gasexplosionsgefährdete Bereiche –
Traçage par résistance électrique Elektrische Widerstands-Begleitheizungen
Partie 2: Guide d'application Teil 2: Anwendungsleitfaden
pour la conception, l'installation für Entwurf, Installation und
et la maintenance Instandhaltung
(CEI 62086-2:2001) (IEC 62086-2:2001)

This European Standard was approved by CENELEC on 2005-02-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and
notified to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden,
Switzerland and United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2005 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 62086-2:2005 E
Foreword
The text of the International Standard IEC 62086-2:2001, prepared by IEC TC 31, Electrical apparatus
for explosive atmospheres, was submitted to the CENELEC Unique Acceptance Procedure and was
approved by CENELEC as EN 62086-2 on 2005-02-01 without any modification.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2006-05-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2008-02-01
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 62086-2:2001 was approved by CENELEC as a European
Standard without any modification.
__________
- 3 - EN 62086-2:2005
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE Where an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
1)
IEC 60079-0 1998 Electrical apparatus for explosive gas -
-
atmospheres
Part 0: General requirements
IEC 60079-10 Part 10: Classification of hazardous areas EN 60079-10 2)
1995 1996
IEC 60079-14 Part 14: Electrical installations in hazardous EN 60079-14 3)
1996 1997
areas (other than mines)
IEC 60079-17 Part 17: Inspection and maintenance of EN 60079-17 4)
1996 1997
electrical installations in hazardous areas
(other than mines)
IEC 62086-1 Electrical apparatus for explosive gas EN 62086-1 2005
atmospheres - Electrical resistance trace
heating
Part 1: General and testing requirements

1)
IEC 60079-0:1998 is superseded by IEC 60079-0:2004, which is harmonized as EN 60079-0:2005 (mod).
2)
EN 60079-10:1996 is superseded by EN 60079-10:2003, which is based on IEC 60079-10:2002.
3)
EN 60079-14:1997 is superseded by EN 60079-14:2003, which is based on IEC 60079-14:2002.
4)
EN 60079-17:1997 is superseded by EN 60079-17:2003, which is based on IEC 60079-17:2002.

NORME CEI
INTERNATIONALE IEC
62086-2
INTERNATIONAL
Première édition
STANDARD
First edition
2001-03
Matériel électrique pour atmosphères
explosives gazeuses –
Traçage par résistance électrique –
Partie 2:
Guide d'application pour la conception,
l'installation et la maintenance
Electrical apparatus for explosive
gas atmospheres –
Electrical resistance trace heating –
Part 2:
Application guide for design,
installation and maintenance
 IEC 2001 Droits de reproduction réservés  Copyright - all rights reserved
Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in
utilisée sous quelque forme que ce soit et par aucun procédé, any form or by any means, electronic or mechanical,
électronique ou mécanique, y compris la photocopie et les including photocopying and microfilm, without permission in
microfilms, sans l'accord écrit de l'éditeur. writing from the publisher.
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62086-2 © IEC:2001 – 3 –
CONTENTS
Page
FOREWORD.7
Clause
1 Scope.9
2 Normative references.9
3 Definitions .9
4 Application considerations .11
4.1 General .11
4.2 Corrosive areas .11
4.3 Installation considerations.11
4.4 Process temperature accuracy.13
5 Thermal insulation .13
5.1 General .13
5.2 Selection of insulating material .15
5.3 Selection of weather barrier (cladding) .15
5.4 Selection of economical thickness.19
5.5 Double insulation .19
6 System design .23
6.1 Introduction .23
6.2 Purpose of, and major requirement for, trace heating .23
6.3 Heat loss calculations .23
6.4 Heat-up considerations .25
6.5 Heat-loss design safety factor.29
6.6 Selection of trace heater.29
6.7 Maximum temperature determination .31
6.8 Design information.35
6.9 Power system .37
6.10 Start-up at low ambient temperatures.39
6.11 Long cable runs .39
6.12 Flow pattern analysis .39
6.13 Dead-leg control technique .43
6.14 Chimney effect .43
7 Control and monitoring equipment.43
7.1 General .43
7.2 Mechanical controllers .43
7.3 Electronic controllers .45
7.4 Application suitability .45
7.5 Location of controllers .45
7.6 Location of sensors .45
7.7 Alarm considerations .47

62086-2 © IEC:2001 – 5 –
Clause Page
8 Recommendations for installation, testing and maintenance .49
8.1 Introduction .49
8.2 Application.49
8.3 Preparatory work .49
8.4 Installation of trace-heating systems .55
8.5 Installation of trace heaters.59
8.6 Installation of control and monitoring equipment.63
8.7 Installation of thermal insulation system (see also clause 5) .65
8.8 Commissioning.67
8.9 Maintenance.67
8.10 Repairs.69
Figure 1 – Thermal insulation – Weather-barrier installation.17
Figure 2 – Typical temperature profile .21
Figure 3 – Heated tank example.41
Figure 4 – Bypass example .41
Table 1 – Process types .11
Table 2 – Pre-installation checks.53
Table 3 – Pre-commissioning checks and heater installation record .73
Table 4 – Heater commissioning record.75
Table 5 – Maintenance schedule and log record .77

62086-2 © IEC:2001 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES –
ELECTRICAL RESISTANCE TRACE HEATING –
Part 2: Application guide for design,
installation and maintenance
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62086-2 has been prepared by IEC technical committee 31:
Electrical apparatus for explosive atmospheres.
The text of this standard is based on the following documents:
FDIS Report on voting
31/347/FDIS 31/359/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 3.
The committee has decided that the contents of this publication will remain unchanged until
2003. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
62086-2 © IEC:2001 – 9 –
ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES –
ELECTRICAL RESISTANCE TRACE HEATING –
Part 2: Application guide for design,
installation and maintenance
1 Scope
This part of IEC 62086 provides guidance for the application of electrical resistance trace-
heating systems in areas where explosive gas atmospheres may be present.
It provides recommendations for the design, installation and maintenance of trace-heating
equipment and associated control and monitoring equipment.
This part supplements the requirements specified in IEC 62086-1.
2 Normative references
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 62086. For dated references, subsequent amendments
to, or revisions of, any of these publications do not apply. However, parties to agreements
based on this part of IEC 62086 are encouraged to investigate the possibility of applying the
most recent editions of the normative documents indicated below. For undated references, the
latest edition of the normative document referred to applies. Members of IEC and ISO maintain
registers of currently valid International Standards.
IEC 60079-0:1998, Electrical apparatus for explosive gas atmospheres – Part 0: General
requirements
IEC 60079-10:1995, Electrical apparatus for explosive gas atmospheres – Part 10:
Classification of hazardous areas
IEC 60079-14:1996, Electrical apparatus for explosive gas atmospheres – Part 14: Electrical
installations in hazardous areas (other than mines)
IEC 60079-17:1996, Electrical apparatus for explosive gas atmospheres – Part 17: Inspection
and maintenance of electrical installations in hazardous areas (other than mines)
IEC 62086-1, Electrical apparatus for explosive gas atmospheres – Electrical resistance trace
1)
heating – Part 1: General and testing requirements
3 Definitions
For the purposes of this part of IEC 62086, the definitions in IEC 62086-1 apply.
___________
1)
To be published.
62086-2 © IEC:2001 – 11 –
4 Application considerations
4.1 General
This standard supplements the requirements of IEC 60079-14 and IEC 60079-17.
Where trace-heating systems are to be installed in explosive gas atmospheres, full details of
the hazardous area classification(s) (IEC 60079-10) should be specified. The specification
should state the zone of risk (1 or 2), gas group (IIA, IIB or IIC) and temperature classification
in accordance with IEC 60079-0. Where special considerations apply or where site condi-
tions may be especially onerous, these conditions should be detailed in the trace-heating
specification.
Where trace-heating systems are to be installed on mobile equipment or interchangeable skid
units, the specification for these trace-heating systems should be designed to accommodate
the worst conditions in which the trace-heating system may be used.
Where any parts of the trace-heating system are likely to be exposed to ultraviolet radiation,
those parts should be suitable for use in such conditions.
4.2 Corrosive areas
All components of electric trace-heating systems should be examined to verify that they are
compatible with any corrosive materials that may be encountered during the lifetime of the
system. Trace-heating systems operating in corrosive environments may have a higher
potential for failure than in non-corrosive environments. Deterioration of the thermal insulation
system is made worse by corrosion of the weather barrier and the possibility of pipeline and
vessel leaks soaking the thermal insulation. Particular attention should be given to the
materials of piping systems, as well as the electric trace-heating systems, as related to the
effective earth-leakage/ground-fault return path. The use of non-metallic or hybrid piping
systems may further complicate the earth-leakage/ground-fault return path and special
consideration should be given to these piping systems. Earth-leakage/ground-fault return paths
that are established at the time of installation may become degraded due to corrosion during
the operation of the plant.
4.3 Installation considerations
For convenience, various process types, according to the degree of application criticality and
process temperature accuracy required, may be specified (see table 1). However, it should be
recognized that each specific application may involve a combination of considerations.
Table 1 – Process types
Process temperature accuracy required
Within a Within a narrow
Above a
Application criticality
minimum point moderate band band type III
type I type II
a
Critical (C-) C – I C – II C – III
b
Non-critical (NC-) NC – I NC – II NC – III
a
Critical applications
b
Non-critical applications
62086-2 © IEC:2001 – 13 –
When the trace heating is critical to the process, consideration should be given to circuit
monitoring (heating circuits should be monitored for correct operation and alarms provided to
indicate damage or fault) and to back up (redundant) heating systems. Spare or back-up trace
heating controls may be specified to be automatically activated in the event of a fault being
indicated by the monitoring/alarm system. This is sometimes known as “redundance”. Back-up
trace heating allows maintenance and repairs to be performed without a process shutdown.
4.4 Process temperature accuracy
4.4.1 Type I
A type I process is one for which the temperature should be maintained above a minimum
point. Ambient sensing control may be acceptable. Large blocks of power may be controlled by
means of a single control device and an electrical distribution panel board. Heat input may be
provided unnecessarily at times and wide temperature excursions should be tolerable. Energy
efficiency may be improved through the use of dead-leg control techniques (see 6.13).
4.4.2 Type II
A type II process is one for which the temperature should be maintained within a moderate
band. Pipeline sensing mechanical thermostats may be typical.
4.4.3 Type III
A type III process is one for which the temperature should be controlled within a narrow band.
Pipe-sensing controllers using thermocouple or resistance-temperature detector (RTD) units
facilitate field (work site) calibration and provide maximum flexibility in the selection of
temperature alarm and monitoring functions. Heat input capability may be provided to heat up
or raise the fluid temperature, or both, within a specified range and time interval. Type III
considerations require strict adherence to flow patterns and thermal insulation systems.
5 Thermal insulation
5.1 General
The selection, installation and maintenance of thermal insulation should be considered a key
component in the performance of an electrical trace-heating system. The thermal insulation
system is normally designed to prevent the majority of heat loss with the trace-heating system
compensating for the remainder. Therefore, problems with thermal insulation will have a direct
impact on the overall system performance.
The primary function of thermal insulation is to reduce the rate of heat transfer from a surface
that is operating at a temperature other than ambient. This reduction of energy loss may
– reduce operating expenses;
– improve system performance;
– increase system output capability.

62086-2 © IEC:2001 – 15 –
Prior to any heat loss analysis for an electrically traced pipeline, vessel or other mechanical
equipment, a review of the selection of the insulation system is recommended. The principal
areas for consideration are as follows:
– selection of an insulation material;
– selection of a weather barrier (cladding);
– selection of the economic insulation thickness;
– selection of the proper insulation size.
5.2 Selection of insulating material
The following are important aspects to be considered when selecting an insulation material:
– thermal characteristics;
– mechanical properties;
– chemical compatibility;
– moisture resistance;
– personnel safety characteristics;
– fire resistance;
– smoke, toxicity;
–cost.
Insulation materials commonly available include:
– expanded silica;
– mineral fibre;
– cellular glass;
– urethane;
– fibreglass;
– calcium silicate;
– isocyanurate;
– perlite silicate.
For soft insulants (mineral fibre, fibre-glass, etc.), actual pipe size insulation may be used in
many cases by banding the insulation tightly. Care should be taken to prevent the heater from
being buried within the insulation, which may cause damage to the heater or may restrict
proper heat transfer. As an alternative, the next largest pipe size insulation that can easily
enclose pipe and electric trace heater is also acceptable. Rigid insulation (calcium silicate,
expanded silica, cellular glass, etc.), may be pipe-size insulation if board sections are cut to fit
the longitudinal joint. This type of installation technique is commonly referred to as an extended
leg installation. Alternatively, the next largest insulation size may be selected to accommodate
the trace heater. In all cases, the insulation size and thickness should be clearly specified.
5.3 Selection of weather barrier (cladding)
Proper operation of an electrically trace heated system depends upon the insulation being dry.
Electric tracing normally has insufficient heat output to dry wet thermal insulation. Some
insulation materials, even though removed from the piping and force dried, never regain their
initial characteristics after once being wet.

62086-2 © IEC:2001 – 17 –
Straight piping may be weather-protected with metal jacketing, polymeric, or a mastic system.
When metal jacketing is used, it should be smooth with formed, modified “S” longitudinal joints.
The circumferential end joints should be sealed with closure bands and supplied with sealant
on the outer edge or where they overlap (see figure 1).
IEC  156/01
Key
1 – Metal jacket 5 – Closure band
2 – Insulation 6 – Insulated strap
3 – Metal jacket insulated pipe 7 – Movement
4 – Mastic sealer 8 – Pipe
Figure 1 – Thermal insulation – Weather-barrier installation

62086-2 © IEC:2001 – 19 –
Jacketing that is overlapped or otherwise closed without sealant is not effective as a barrier to
moisture. A single, unsealed joint can allow a considerable amount of water to leak into the
insulation during a rainstorm.
The type of weather barrier used should, as a minimum, be based on a consideration of the
following:
– effectiveness in excluding moisture;
– corrosive nature of chemicals in the area;
– fire protection requirements;
– durability to mechanical abuse;
–cost.
5.4 Selection of economical thickness
At a minimum, an economic consideration of the insulation will weigh the initial costs of the
materials and installation against the energy saved over the life of the insulation. It should be
noted that the actual insulation thicknesses do not always correspond exactly to the nominal
insulation thickness. When choosing the insulation size, considerations should be made as to
whether or not the actual pipe-size insulation is suitable for accommodating both pipe and trace
heater.
5.5 Double insulation
The double insulation technique may be employed when the pipe temperature exceeds the
maximum allowable temperature of the trace heater. Prevention of the freezing of condensate
in high-temperature steam lines when these lines are not in use is a typical application. It
consists of locating the trace heater between two layers of insulation surrounding the pipe. The
essence of the double-insulation technique is to determine the correct combination of inner and
outer insulation type and thickness that will result in an acceptable interface temperature for
the trace heater. This relationship is illustrated in figure 2. Note that maximum ambient
temperature conditions should be considered in this determination.

62086-2 © IEC:2001 – 21 –
IEC  157/01
Key
1 – Pipe 6 – Maximum temperature pipe
2 – Inner insulation layer 7 – Interface temperature
3 – Heat tracer 8 – Outer insulation surface temperature
4 – Outer insulation layer 9 – Ambient temperature
5 – Metal foil (aluminium) 10 – Radius
Figure 2 – Typical temperature profile

62086-2 © IEC:2001 – 23 –
6 System design
6.1 Introduction
When designing trace-heating systems for use in explosive gas atmospheres, additional
constraints are imposed due to the requirements and classification of the area under
consideration. The design of any trace-heating system should conform to all IEC requirements
for the use of electrical equipment in these areas and with the requirements of this standard.
Each process imposes unique demands on the designer in order to achieve the desired
temperature and maintain it within the specified conditions. Trace-heating systems necessarily
interface with other specified items of equipment such as thermal insulation and the electrical
supply available to power the system. The final system will be an integration of all these
component parts so the values of these interface items have to be known and controlled in
order to design systems that will perform as required. Local ambient temperature data, both
maxima and minima, have to be known and incorporated into the design.
Consideration should be given as to the maintenance of the systems and process equipment,
to energy efficiency, and to testing the installed systems for operational satisfaction and safety.
6.2 Purpose of, and major requirement for, trace heating
Trace heaters should be selected and installed so as to provide sufficient power for
a) compensation of heat loss when maintaining a specified temperature of a workpiece (no
heat exchange is assumed with the workpiece or its contents); see calculation method in
6.3; and/or
b) raising the temperature of a workpiece, and its content when specified, with a specified
value to be established within a specified period (see calculation method in 6.4);
c) a combination of a) and b).
It should be observed that heat losses established in accordance with 6.3 and 6.4 should be
multiplied by a safety factor determined on the basis of 6.5. The excess of installed power over
the power as required, and the way in which trace heaters are applied, installed and operated
shall not be the cause, not even after evaluation of worst-case conditions, of any unacceptable
risk in an explosive gas atmosphere.
6.3 Heat loss calculations
To determine heat loss for a given set of conditions, a complete insulation specification is
required, including details of the thermal conductivity of the insulation at several mean
temperatures, the type of weather barrier specified, insulation size and thickness, desired
workpiece maintenance temperature, and the minimum ambient temperature and wind
conditions.
Given these parameters, the heat loss may be determined by calculation. For example, the
heat loss for pipes and tubes may be evaluated using the following equation:
(T − T )
p a
q = (1)
 D   D 
2 3
In   In  

  
D D
1 1 1
 1   2 
+ + + +
πD h 2πK 2πK πD h πD h
1 i 1 2 3 co 3 o
62086-2 © IEC:2001 – 25 –
where
q is the heat loss per unit length of pipe in watts per metre (W/m);
T is the desired maintenance temperature in degrees Celsius (°C);
p
T is the minimum design ambient temperature in degrees Celsius (°C);
a
D is the inside diameter of the inner insulation layer in metres (m);
D is the outside diameter of the inner insulation layer in metres (m) (inside diameter of the
outer insulation layer when present);
D is the outside diameter of the outer insulation layer when present in metres (m);
K is the thermal conductivity of the inner layer of insulation evaluated at its mean
temperature (W/mK);
K is the thermal conductivity of the outer layer of insulation, when present, evaluated at its
mean temperature (W/mK);
h is the inside air contact coefficient from the pipe to the inner insulation surface when
i
present (W/m K);
h is the inside air contact coefficient from the outer insulation surface to the weather barrier
co
when present (W/m K);
h is the outside air film coefficient from the weather barrier to ambient (W/m K) (typical
o
values for this term range from 3 W/mK to 284 W/mK for low-temperature applications
below 50 °C).
Vessel heat losses are more complex due to the heat sinks that penetrate the insulation
surface. They often require a more complex analysis to determine total heat loss and the trace
heating supplier should be consulted.
For ease of product selection, most trace-heating suppliers will furnish simple charts and
graphs of heat losses for variously maintained temperatures and insulations, which usually
include a safety factor.
A simplified version of equation (1), expressing heat loss per unit length, is as follows:
2πK (T − T )
p a
q = (2)
 
D
ln  
 
D
 1 
6.4 Heat-up considerations
In certain plant operations, it may be necessary to specify that the trace-heating system is
capable of raising the temperature of a static product within a certain time period. For example,
the heat delivery requirement of a trace-heated system on piping may be evaluated by use of
the following equation:
q U()T T P V h
 − − 
c i a 1 c1 f
t = H ln + (3)
 
q − U()T − T q − U()T − T
 c f a  c sc a
where
U is the heat loss per unit length of pipe per degree of temperature difference.

62086-2 © IEC:2001 – 27 –
(T − T )
p a
q = (see equation (1))
   D 
D
2 3
In   In  
   
D D
1 1 1
 1   2 
+ + + +
πD h 2πK 2πK πD h πD h
1 i 1 2 3 co 3 o
where
H is the thermal time constant, which is the total energy stored in the mass of pipe, fluid, and
insulation per degree of temperature divided by the heat loss per unit length per degree
temperature differential.
P C V + P C V + 0 ,5P C V
1 p1 c1 2 p 2 c 2 3 p 3 c 3
H = (4)
U
and
P is the density of the product in the pipe (kg/m );
C is the specific heat of the product (J/kgK);
p1
V is the internal volume of the pipe (m /m);
c1
P is the density of the pipe (kg/m );
C is the specific heat of the pipe (J/kgK);
p2
V is the pipe wall volume (m /m);
c2
P is the density of insulation (kg/m );
C is the specific heat of the insulation (J/kgK);
p3
V is the insulation wall volume (m /m);
c3
T is the initial temperature of the pipe in degrees Celsius (°C);
i
T is the final temperature of the fluid and the pipe in degrees Celsius (°C);
f
T is the ambient temperature in degrees Celsius (°C);
a
T is the desired maintenance temperature in degrees Celsius (°C);
p
t is the desired heat-up time in seconds (s);
U is the heat loss per unit length of pipe per degree of temperature (W/mK);
H is the thermal time constant in seconds (s);
K is the thermal conductivity of the inner insulation evaluated at its mean temperature
(W/mK);
K is the thermal conductivity of the outer insulation evaluated at its mean temperature
(W/mK);
D is the outside diameter of outer insulation layer in metres (m);
D is the outside diameter of the inner insulation layer in metres (m);
D is the inside diameter of the insulation layer in metres (m);
h is the inside air contact coefficient of the weather barrier (W/m K);
co
h is the outside air contact coefficient of the weather barrier to the ambient (W/m K);
o
h is the inside air contact coefficient from the pipe to the inside insulation surface (W/m K);
i
T is the temperature at which phase change occurs in degrees Celsius (°C);
sc
h is the latent heat of fusion for the product (J/kg);
f
q is the output of the trace heater(s) (W/m).
c
62086-2 © IEC:2001 – 29 –
The preceding relationships also assume that system densities, volumes, thermal conduc-
tivities and heat losses are constant over the temperature range of interest. Note that some
products do not undergo a phase change during heat-up. Although the model is representative
of a straight pipeline, it does not have provisions for equipment such as pumps and valves.
Insulation for valves, flanges, pumps, instruments, and other irregularly shaped equipment may
be constructed for the particular configuration. This can be fabricated from block, insulation
segments or flexible removable covers.
Non-insulated or partially insulated pipe supports or equipment require additional heat input to
compensate for the higher heat loss. Insulating cements or fibrous materials should be used to
fill cracks and joints. Where insulating cements are used for total insulation of an irregular
surface, a proportionally thicker layer of cement may be applied to achieve the desired
insulating capability.
6.5 Heat-loss design safety factor
Since heat-loss calculations result in theoretical values and do not account for imperfections
associated with actual work site installations, a safety factor should be applied to the calculated
values. The calculated safety factor should be based upon past experience and typically will
range from 10 % to 25 %. The addition of a safety factor is used to compensate for tolerances
in the trace-heating system. These tolerances are generally connected with the efficiency of the
thermal insulation system, the voltage supply and the characteristics of the heaters.
6.6 Selection of trace heater
For a particular application there are some basic design requirements which may determine the
choice of trace heater(s). These are as follows.
a) Trace heaters should be certified for use in explosive gas atmospheres.
b) The maximum withstand temperature of the trace heaters should be greater than the
maximum possible workpiece temperature (which may be greater than the normal operating
temperature).
c) Trace heaters should be suitable for operation in the environmental conditions specified, for
example, a corrosive atmosphere or a low ambient temperature.
NOTE Any change to certified apparatus may invalidate the certificate.
For any application, there is a maximum allowable power density at which a trace heater can
be used without damaging either the workpiece or its contents. Sometimes this value is
particularly critical, such as with lined pipes, vessels containing caustic soda or with heat-
sensitive materials. This limiting value shall be recorded in the system documentation. Multiple
tracing or spiralling of a single trace heater may be required. The choice of trace heater may be
further limited by whether fabrication on site is possible. Site-fabricated trace heaters may be
permissible provided that
a) installation personnel are competent in the special techniques required;
b) trace heater(s) pass the field (site work) tests specified in 8.5.2;
c) trace heater(s) is/are marked in accordance with 6.3 of IEC 62086-1.

62086-2 © IEC:2001 – 31 –
Trace heater(s) not excluded by the above considerations are technically suitable for the
application, but it is still necessary to determine the maximum allowable power density of each.
This is a function of the construction, the maximum withstand temperature, the required
temperature classification of the trace heater, the maximum operating temperature and
maximum permissible temperature of the workpiece and thermal insulation.
For each particular trace heater the maximum allowable power density should be determined
from the manufacturer’s data which are based on tests specified in clause 5 of IEC 62086-1.
The value used should be chosen so that neither the maximum withstand temperature nor the
required temperature classification is exceeded. The limiting value of maximum allowable
power density for each trace heater is either the value chosen from the manufacturer’s data or
that specified for the process, whichever is the lower. However, the power density may be
further limited by the need for multiple tracing.
The designer may then select the type, length or size and loading of the trace heater. The
actual installed load should be not less than the design loading and the actual power density
not greater than that obtained above. The type of trace heater and the values of installed load
and power density should be recorded in the system documentation.
6.7 Maximum temperature determination
6.7.1 Maximum workpiece temperature – Stabilized design
For a stabilized design, the maximum possible pipe temperature is calculated at maximum
ambient temperature with the trace heater continuously energized. The formula for calculating
the maximum potential pipe temperature is a rearrangement of the terms of the heat loss
formula which is as follows:
 
 D 
ln  
 
 
D
W 1 1 1
 1 
 
T = + + + + T (5)
pr a
 
π 2
D h k D h D h
1 i 2 co 2 o
 
 
 
where
T is the maximum calculated pipe temperature in degrees Celsius (°C);
pr
NOTE The maximum process pipe temperature may exceed the calculated value.
W is the trace heater output (W/m) corrected for 110 % rated voltage and maximum
manufacturer’s output tolerance;
k is the thermal conductivity of the insulation at its mean temperature (W/mK).
Other terms are defined in equation (1). Iterative techniques may need to be applied to the
calculation of equation (5) in order to arrive at T , since the thermal conductivity of the
pr
insulation and the heating cable output may be a function of pipe temperature.
6.7.2 Sheath temperature – Metallic applications
For metallic pipe or vessel applications, the sheath temperature of a trace heater should be
considered to the extent that product ratings are not to be exceeded in the application. This
includes not only the electrical insulation and sheath materials, but also the maximum
temperature limitations of the thermal insulation and pipe wall material or process material.

62086-2 © IEC:2001 – 33 –
The maximum sheath temperature of a trace heater may be determined by product testing or
using the system approach as described in 5.1.11 of IEC 62086-1. The sheath temperature of a
trace heater is as follows:
W
m
T = + T (6)
sh pm
UC
where
T is the trace heater sheath temperature in degrees Celsius (°C);
sh
C is the trace heater circumference in metres (m);
U is the overall heat transfer coefficient (W/m K);
W is the trace heater output (W/m) at the maximum pipe temperature as defined below at
m
110 % rated voltage and maximum output tolerance (declared by the manufacturer);
T is the maximum pipe temperature (°C).
pm
The overall heat transfer coefficient is different for each trace-heater type, installation method
and system configuration. It is a combination of conductive, convective and radiation heat
transfer modes. The value of U can vary from 2,2 for cylindrical heating cable in air (primarily
convective), to 30 or more for a trace heater applied using heat transfer aids (primarily
conductive). Upon request, the trace-heating supplier should provide the U-factor for given
applications, or furnish calculated or experimentally determined sheath temperatures.
The power output in W of the trace heater selected should provide the stabilized design and
m
should not exceed the temperature classification or any other maximum temperature limitations
listed above.
6.7.3 Sheath temperature – Non-metallic pipe applications
For non-metallic pipe applications, the pipe wall thermal resistance should be considered, as
the non-metallic pipe is a poor heat transfer mediu
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