EN 13941:2009

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Auslegung und Installation von werkmäßig gedämmten Verbundmantelrohren für die FernwärmeConception et installation des systèmes bloqués de tuyaux pré-isolés pour les réseaux enterrés d''eau chaudeDesign and installation of preinsulated bonded pipe systems for district heating91.140.10Sistemi centralnega ogrevanjaCentral heating systemsICS:Ta slovenski standard je istoveten z:EN 13941:2009SIST EN 13941:2009en,fr,de01-september-2009SIST EN 13941:2009SLOVENSKI
STANDARDSIST EN 13941:20041DGRPHãþD

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 13941July 2009ICS 91.140.10Supersedes EN 13941:2003
English VersionDesign and installation of preinsulated bonded pipe systems fordistrict heatingConception et installation des systèmes bloqués de tuyauxpréisolés pour les réseaux enterrés d'eau chaudeAuslegung und Installation von werkmäßig gedämmtenVerbundmantelrohren für die FernwärmeThis European Standard was approved by CEN on 23 May 2009.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the CEN Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as theofficial versions.CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre:
Avenue Marnix 17,
B-1000 Brussels© 2009 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 13941:2009: ESIST EN 13941:2009

2 Contents Page Foreword . 5Introduction . 61Scope . 82Normative references . 83Terms and definitions, units and symbols . 103.1Terms and definitions . 103.2Units and symbols . 144General considerations for system design . 174.1General requirements . 174.2Service life. 174.3Preliminary investigations . 174.4Determination of project class . 184.5Design documentation . 205Components and materials . 235.1Basic requirements . 235.2Steel pipe components . 245.3Polyurethane foam insulation . 275.4PE casing . 275.5Expansion cushions . 275.6Valves and accessories . 276Actions and limit states . 286.1General . 286.2Simplified analysis procedure . 306.3Actions . 306.4Limit states. 327Installation. 427.1General . 427.2Transportation and storage . 427.3Excavation of pipe trench . 437.4Installation of pipes and components . 437.5Welding of the steel pipe and testing of the steel welds . 447.6Strength pressure test and leak tightness test . 527.7Assembly of casing pipes, joint installation and site insulation . 537.8Backfilling of trench . 547.9Pipe bends and other components . 557.10Setting into operation . 567.11Special constructions . 567.12Construction work during the operation stage . 57Annex A (normative)
Design of piping components under internal pressure . 58A.1General . 58A.2Symbols . 58A.3Straight pipe and bends . 59A.3.1Straight pipes . 59A.3.2Bends . 59A.4Tees and branch connections. 59A.4.1General aspects and limitations . 59A.4.2Reinforcement . 60SIST EN 13941:2009

3A.5Reducers and extensions . 62A.5.1General . 62A.5.2Minimum wall thickness of the cone . 62A.5.3Offset reducers . 63A.6Dished ends . 63A.6.1General . 63A.6.2Minimum required wall thickness for internal pressure . 64A.6.3Straight cylindrical shells . 64Annex B (informative)
Geotechnics and pipe-soil interaction . 65B.1Scope . 65B.1.1General . 65B.1.2General requirements . 65B.2Symbols and units . 65B.3Soil parameters for global analysis (pipe-soil interaction) . 66B.3.1Modelling pipe-soil interaction . 66B.3.2Pipe to soil friction (axial) . 67B.3.3Coefficient of horizontal soil reaction (lateral) . 69B.3.4Combined stiffness of PUR foam, expansion cushions and soil . 73B.4Characteristic values for soil loads and soil parameters . 75B.4.1General . 75B.4.2Soil mechanics study . 76B.5Specific requirements for stability . 77B.5.1General . 77B.5.2Vertical stability . 77B.5.3Horizontal stability . 79B.6Specific requirements for parallel excavations . 79B.6.1General . 79B.6.2Reduced friction . 79B.7Requirements for soft soils and settlement areas . 79B.7.1General . 79B.7.2Differential settlements . 80B.8Ovalization and circumferential stresses from top load . 80B.8.1Traffic load . 80B.8.2Stresses and ovalization from top load . 82B.8.3Deflection . 85Annex C (informative)
Global- and cross sectional analysis . 86C.1General . 86C.2Symbols . 86C.3Survey of limit states for steel . 88C.4Locations to be assessed . 89C.4.1Components to be considered . 89C.4.2Areas to be considered . 91C.5Actions . 92C.5.1General . 92C.5.2Action cycles . 92C.6Global analysis . 93C.6.1General . 93C.6.2Flexibility . 94C.6.3Boundary conditions . 95C.6.4Boundary conditions for
pipe systems with single use compensators (SUC’s) . 98C.7Calculation of stresses . 101C.7.1General . 101C.7.2Simplified procedure . 101C.7.3Cross section analyses, steel . 101C.7.4Straight pipes . 103C.7.5Bends . 105C.7.6Tees . 107C.7.7Other components . 112SIST EN 13941:2009

4 C.7.8PUR foam and PE casing . 115C.8Fatigue analysis . 116C.8.1Fatigue strength data . 116C.8.2Fatigue strength data, detailed design . 117C.8.3Design fatigue lives . 117C.9Further actions . 117Annex D (informative)
Calculation of heat losses . 118D.1General . 118D.2Heat loss per pipe pair . 118D.3Insulance of the soil . 119D.4Insulance of the insulation material . 119D.5Insulance of the heat exchange between flow and return pipe . 119Bibliography . 120 SIST EN 13941:2009

5Foreword This document (EN 13941:2009) has been prepared by Technical Committee CEN/TC 107 “Prefabricated district heating pipe systems”, the secretariat of which is held by DS. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by January 2010, and conflicting national standards shall be withdrawn at the latest by January 2010. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document supersedes EN 13941:2003. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom. SIST EN 13941:2009

6 Introduction The standard has been prepared by JWG1, a joint working group with CEN/TC 267 "Industrial piping and pipelines". According to the scope from CEN/TC 107:  The task of CEN/TC 107/TC267/JWG1 is to specify rules for design, calculation and installation for preinsulated bonded pipe systems for underground hot water networks with pipe assemblies co-ordinated with EN 253, EN 448, EN 488 and EN 489.  CEN/TC 107/TC267/JWG1 may also specify rules for functional tests for preinsulated bonded pipe systems for underground hot water networks.  The basic rules for design, calculation and installation should be based on functional requirements.  The purpose of the work is to provide uniform basis for the design, construction and operation of district heating systems, to ensure that the system is reliable and efficient and safe for the surrounding area, the environment and public health.  Joint assemblies for pipe systems dealt with should be co-ordinated with EN 489. This standard takes account of experience acquired, of new knowledge available, of the behaviour of material and of distribution of stresses and allowable deformations and also evolution in installation techniques. When use is made of the standard, the different sections of which it is made up must be interpreted as being interdependent and, because of this, cannot be dissociated. The standard consists of a main part and four annexes. Depending on the character of the individual clauses, distinction is made in this standard between Principles and Application Rules. The principles comprise:  general statements, definitions and requirements, for which there is no alternative, as well as  requirements and analytical models for which no alternative is permitted unless specifically stated. The principles are printed in normal typeface (10 point font). The application rules are generally recognised rules, which follow the principles and satisfy their requirements. Application rule: The application rules and comments to principles and application rules are printed in a 8 point font. This is an application rule. It is permissible to use alternative design rules from the application rules given in this standard, provided that it is shown that the alternative rule accords with the relevant principles and it is at least equivalent with regard to the resistance, serviceability and durability achieved by the system. Annex A is part of the standard (principles). Annexes B, C and D have status as application rules. SIST EN 13941:2009

7This standard contains a number of requirements aimed at ensuring the sound execution of distribution networks for district heating. To the extent possible, the requirements specified in this standard are functional requirements. The requirements and regulations contained in this standard should be assessed and applied in compliance with the intentions of the standard and in due consideration of the development taking place in the field it concerns. It is therefore assumed that the user of the standard has the requisite technical insight and that the user of the standard has adequate knowledge of legal and other external regulations that are of consequence to the practical application of the standard. Special cases may occur within the scope of this standard in which its contents do not cover. An evaluation whether the contents cover shall be made in any specific case where the standard is used. Presently CEN/TC 107 "Pre-fabricated district heating pipe systems" is preparing standards for preinsulated flexible pipes and surveillance systems. SIST EN 13941:2009

8 1 Scope This European Standard specifies rules for design, calculation and installation for preinsulated bonded pipe systems for buried hot water distribution and transmission networks (cf. figure 2) with pipe assemblies in accordance with EN 253, for continuous operation with hot water at various temperatures up to 120ºC and occasionally with peak temperatures up to 140ºC and maximum internal pressure 25 bar (overpressure). Application rule: For larger pipe dimensions and pressures below 25 bar wall thickness bigger than specified in EN 253 can be required for straight pipes, bends and tees. The principles of the standard can be applied to preinsulated pipe systems with pressures higher than 25 bar, provided that special attention is paid to the effects of pressure. Adjacent pipes belonging to the network (e.g. pipes in ducts, valve chambers, road crossings above ground etc.) can be designed and installed according to this standard. The standard assumes use of treated water, which by softening, demineralisation, deaeration, adding of chemicals, or otherwise has been treated to prevent internal corrosion and deposits in the pipes. This standard is not applicable for such units as: a) pumps, b) exchangers, c) boiler installations, tank installations, d) consumer installations. However, the full functional ability and durability of such units should be ensured in consideration of the impacts from the district heating system and other impacts occurring from the conditions under which they have been installed. Guidelines for product quality inspection and in situ tests of joints are given in Annex A of EN 448:2009, Annex D of EN 253:2009, Annex A of EN 488:2009 and Annex B of EN 489:2009. Guidelines for welding of polyethylene casing are given in Annex B of EN 448:2009. The estimation of expected life with continuous operation at various temperatures is outlined in Annex B of EN 253:2009. 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. EN 253:2009, District heating pipes – Preinsulated bonded pipe systems for directly buried hot water networks – Pipe assembly of steel service pipe, polyurethane thermal insulation and outer casing of polyethylene EN 287-1, Qualification test of welders – Fusion welding – Part 1: Steels EN 444, Non-destructive testing – General principles for radiographic examination of metallic materials by X- and gamma-rays SIST EN 13941:2009

9EN 448:2009, District heating pipes – Preinsulated bonded pipe systems for directly buried hot water networks – Fitting assemblies of steel service pipes, polyurethane thermal insulation and outer casing of polyethylene EN 473, Non-destructive testing – Qualification and certification of NDT personnel – General principles EN 488, District heating pipes – Preinsulated bonded pipe systems for directly buried hot water networks – Steel valve assembly for steel service pipes, polyurethane thermal insulation and outer casing of polyethylene EN 489:2009, District heating pipes – Preinsulated bonded pipe systems for directly buried hot water networks – Joint assembly for steel service pipes, polyurethane thermal insulation and outer casing of polyethylene EN 571-1, Non destructive testing – Penetrant testing – Part 1: General principles EN 583-1, Non-destructive testing – Ultrasonic examination – Part 1: General principles EN 970, Non-destructive examination of fusion welds – Visual examination EN 1289, Non-destructive examination of welds – Penetrant testing of welds – Acceptance levels EN 1290, Non-destructive examination of welds – Magnetic particle examination of welds EN 1291, Non-destructive examination of welds – Magnetic particle testing of welds – Acceptance levels EN 1418, Welding personnel – Approval testing of welding operators for fusion welding and resistance weld setters for fully mechanized and automatic welding of metallic materials EN 1435, Non-destructive examination of welds – Radiographic examination of welded joints EN 1712, Non-destructive examination of welds – Ultrasonic examination of welded joints – Acceptance levels EN 1714, Non-destructive examination of welds – Ultrasonic examination of welded joints EN 10204, Metallic products – Types of inspection documents EN 10216-2, Seamless steel tubes for pressure purposes – Technical delivery conditions – Part 2: Non-alloy and alloy steel tubes with specified elevated temperature properties EN 10217-1, Welded steel tubes for pressure purposes – Technical delivery conditions – Part 1: Non-alloy steel tubes with specified room temperature properties EN 10217-2, Welded steel tubes for pressure purposes – Technical delivery conditions – Part 2: Electric welded non-alloy and alloy steel tubes with specified elevated temperature properties EN 10217-5, Welded steel tubes for pressure purposes – Technical delivery conditions – Part 5: Submerged arc welded non-alloy and alloy steel tubes with specified elevated temperature properties EN 13018, Non-destructive testing – Visual testing – General principles EN 13480-3:2002, Metallic industrial piping – Part 3: Design and calculation EN 25817:1992, Arc-welded joints in steel – Guidance on quality levels for imperfections (ISO 5817:1992) EN ISO 3834-1, Quality requirements for fusion welding of metallic materials – Part 1: Criteria for the selection of the appropriate level of quality requirements (ISO 3834-1:2005) EN ISO 3834-2, Quality requirements for fusion welding of metallic materials – Part 2: Comprehensive quality requirements (ISO 3834-2:2005) SIST EN 13941:2009

10EN ISO 3834-3, Quality requirements for fusion welding of metallic materials – Part 3: Standard quality requirements (ISO 3834-3:2005) EN ISO 3834-4, Quality requirements for fusion welding of metallic materials – Part 4: Elementary quality requirements (ISO 3834-4:2005) EN ISO 9692-2, Welding and allied processes – Joint preparation – Part 2: Submerged arc welding of steels (ISO 9692-2:1998) EN ISO 14731:2006, Welding coordination – Tasks and responsibilities (ISO 14731:2006) EN ISO 15607:2003, Specification and qualification of welding procedures for metallic materials – General rules (ISO 15607:2003) EN ISO 15609-1, Specification and qualification of welding procedures for metallic materials – Welding procedure specification – Part 1: Arc welding (ISO 15609-1:2004) EN ISO 15614-1, Specification and qualification of welding procedures for metallic materials –– Welding procedure test – Part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys (ISO 15614-1:2004) ISO 1000, SI units and recommendations for the use of their multiples and of certain other units ISO 3419, Non-alloy and alloy steel butt-welding fittings ISO/TR 15608:2000, Welding – Guidelines for a metallic materials grouping system 3 Terms and definitions, units and symbols 3.1 Terms and definitions For the purpose of this European Standard, the terms and definitions given in EN 253:2009 and the following apply. 3.1.1 action set of concentrated or distributed forces acting on the pipe system (force-controlled action), or cause of imposed or constrained deformations in the system (displacement-controlled action). Actions are often referred to as “loads”. 3.1.2 action cycle impact with a given stress range. An action cycle comprises one full action course (which is twice the action amplitude calculated from an average value).
Key 1
One action cycle 2
Temperature or stress range Figure 1 — Action cycle SIST EN 13941:2009

113.1.3 bonded system system consisting of a service pipe, insulating material and casing, which are bonded by the insulating material 3.1.4 cold installed preinsulated bonded pipes district heating systems where the pipes are installed and taken into operation without prior pre-stressing by pre-heating 3.1.5 creep slow progressive strain under the influence of stresses 3.1.6 design pressure
internal pressure equal to or greater than the maximum operating pressure at any point of the pipeline acting in a component or pipe section multiplied by a partial safety factor 3.1.7 design temperature maximum temperature used for the design of a component or pipe section 3.1.8 displacement-controlled action action called forth by enforced deformation or movement, e.g. thermal expansion or settling 3.1.9 distribution pipeline pipeline leading from place of production or transmission line to heating installations. Distribution mains are primarily main pipelines or house service connections, see figure 2. 3.1.10 ductile materials materials which with good approximation are linearly elastic up to the yield stress or to the 0,2% proof stress, and which have a minimum elongation at rupture of 14 % 3.1.11 extruded tees tees manufactured by drawing a collar on which the branch pipe is welded. The collar is welded onto a transitional piece with increased wall thickness, so that the local stress intensification for the tee is reduced before the straight pipe with normal wall thickness. 3.1.12 fabricated tees tees manufactured by welding a branch pipe directly onto a run pipe 3.1.13 fatigue strength stress range of constant magnitude which, under given circumstances, just causes fatigue failure 3.1.14 force-controlled action action which maintains its size irrespectively of the deformation of the structure, e.g. pressure and weight 3.1.15 house service connection pipeline leading from main pipeline to one consumer installation, see figure 2. SIST EN 13941:2009

12 Key 1
Transmission system 2
Distribution system 3
Transmission pipe 4
Main pipe 5
House service connection 6
Supply pipe 7
Consumer 8
Return pipe 9
Valve chamber 10
Heat production 11
Heat exchanger station 12
Pump station Figure 2 — Distribution and transmission systems 3.1.16 installation temperature temperature arising from the ambient conditions during laying or installation, prevalent at the time when action is taken 3.1.17 main pipeline pipeline supplying several heating installations. See figure 2. SIST EN 13941:2009

133.1.18 number of equivalent full action cycles number of action cycles with constant full action range calculated from a known or presupposed temperature history using Palmgren-Miner’s formula and the respective SN-curve 3.1.19 operating pressure
maximum internal pressure acting against the pipe wall at any point or in any section of the pipeline at a given operating temperature Application rule: This is generally the internal pressure needed to take account of the static head, friction losses and required outlet pressure. 3.1.20 operating temperature water temperature in a component or pipe section during specified operating conditions 3.1.21 pre-heated system system which, after being assembled, but before backfilling, is heated to a pre-heating temperature allowing the system to expand without introducing additional stresses
3.1.22 preinsulated systems systems assembled at site consisting of prefabricated pipe elements and components with integrated protective casing, insulation and service pipe 3.1.23 pressure over-pressure or sub-pressure as compared to normal atmospheric pressure. Unless otherwise indicated, pressure refers to gauge pressure. 3.1.24 pre-stressing temperature temperature applied during pre-stressing of a pre-heated system Application rule: The pre-heating temperature is chosen such as approximately average axial stress is obtained, compared to the axial stress levels at ambient temperature and maximum operating temperature. 3.1.25 reference stress stress calculated (with sign) from the membrane or resulting stresses by Tresca or by von Mises’ formula Application rule: The formulae are presented in 6.4.3. 3.1.26 resulting stresses all stresses occurring in one point, i.e. membrane stresses plus stresses varying over the wall thickness 3.1.27 service life span of time during which the network is expected to function without major replacements, given normal maintenance and operation conditions as described in the project SIST EN 13941:2009

143.1.28 service pipe steel pipe that contains the water 3.1.29 single action compensator compensator functioning during pre-heating. After pre-heating the compensator is locked. 3.1.30 strain unit deformation, e.g. elongation or reduction per unit of length 3.1.31 stress range difference between maximum stress and minimum stress for one single load cycle, the stress being computed with preceding sign, see figure 1. 3.1.32 surge pressure (water hammer) variation of pressure for relatively short period, resulting from a change in velocity of the circulating water. Such a change may be a consequence of valve closing, pump failure, boil over, impacts from non-return valves, blockage, fractures in the pipeline, etc. 3.1.33 system complete pipeline installation including joints, branches, accessories, etc., and adjacent pipelines 3.1.34 temperature range absolute value of the difference between the two extremes of temperature occurring during a cycle, taking account of operational and environmental influences, see figure 1. 3.1.35 test pressure internal pressure occurring within the pipeline or a part of the pipeline during strength testing (strength test pressure) or leak tightness testing (leak tightness test pressure) 3.1.36 transmission pipelines major pipelines leading from place of production to distribution pipelines, see figure 2. 3.1.37 valves and accessories surveillance, operating and safety equipment directly fitted to a district heating pipeline 3.1.38 weld-in tees tees e.g. made by forging and usually seamless 3.2 Units and symbols 3.2.1 Units The unit system applied in this standard is the SI system (Système International d'Unités), cf. ISO 1000 and others. The following units and their multiples are used: SIST EN 13941:2009

15Length
m (metre)
mm (millimetre) Mass
kg (kilogram) Force
N (Newton) Stress
N/mm² (Newton per square millimetre) Pressare
Pa (Pascal = Newton per square metre) Other units applied: Temperature
°C (degree centigrade) Pressure
bar (1 bar = 105 Pa = 0,1 N/mm2) 3.2.2 Symbols Table 1 — Symbols Symbol Name A Area c Cohension of the soil, fabrication tolerance D Diameter of casing d Diameter of service pipe E Modulus of elasticity F Friction force f Design stress, friction force per area unit, deflection G Selfweight I Momentum of inertial i Stress intensification factor L Friction length l Length M Bending moment N Normal force, number of full action cydes n Number p Internal pressure Re Specified minimum upper yield strength Rm Tensile strength R Bend radius r Pipe radius T Temperature SIST EN 13941:2009

16Table 1 (continued) Symbol Name t Pipe wall thickness W Section modulus Z Depth of burial (measured to centreline of pipe) α Coefficient of thermal expansion γ Specific gravity, partial safety coefficient δ Friction angle between pipe and soil, displacement from thermal expansion ε Strain θ Angle λ Coefficient of thermal conductivity µ Coefficient of friction between pipe and soil ρ Density σ Normal stress τ Shear stress ν Poisson’s ratio ϕ Internal friction angle of soil
Table 2 — Indices a : Action min : Minimum b : Branch pipe (at tee connections) n : Nominal, number (of fatigue cycles) c : Casing o : Outer, outside d : Design r : Run pipe (at tees) fat : Fatigue res : Resulting i : Inner, inside u : Fracture j : Reference v : Vertical m : Mean, membrane, material
NOTE Separate symbol lists are found in Annexes A, B and C. SIST EN 13941:2009

174 General considerations for system design 4.1 General requirements Design and installation of district heating pipe systems shall ensure that the system is given: a) sufficient durability, robustness and reliability in relation to the internal and external loads and impacts, to which it is likely to be subjected in normal operation, b) sufficient safety that unusual operating conditions or accidents do not jeopardise persons or the environment, c) good energy economy, d) good operating properties, e) safety of supply. Application rule:
Installation expenses, maintenance expenses and operating expenses arising throughout the service life of the system should be included in the assessment of the system. The assessment of operating properties should pay regard to the possibilities of inspection and maintenance. 4.2 Service life When a system designed according to this standard is subject to temperatures exceeding 120 oC, for periods such that the requirement for a service life of 30 years at continuous operation at 120 oC, calculated in accordance with Annex B of EN 253:2009, is exceeded, the service life of components subject to ageing must be assessed. Application rule: The minimum requirements for the type test of EN 253 (based on the shear strength between PUR foam and steel pipe) is a service life of 30 years for continuous operation at 120oC. If the cumulative ageing requires a lifetime exceeding the equivalence of 30 years at 120 oC special documentation for the ageing properties are required. 4.3 Preliminary investigations Preliminary investigations comprising an assessment of all conditions of importance to a district heating project shall be carried out. These preliminary investigations shall elucidate matters in the planning, design, execution and operating stages as well as consequences of any kind of failure of the system. The principal basis of the preliminary investigation is the main data for the current system, e.g.:
 function,
 pressure and temperature,
 dimensions,
 depth of burial,
18 materials,  distances and heat transfer to other utility networks, buildings and trees,
 geotechnical and groundwater parameters, etc. Application rule: The preliminary investigations may include elucidation of the following matters: a) pipeline route, b) operating conditions of the system, e.g. variations in pressure and temperature and requirements for safety of supply, c) function mode of the system during operating and maintenance stage as well as resistance to relevant impacts such as: loads due to installation and operation, internal and external loads and deformations, d) consequences of possible kinds of failure of the system, e) authorities’ requirements, environment and third party aspects, f) methods of execution. 4.4 Determination of project class 4.4.1 Risk assessment Possible coincidences involving a risk of personal damage or consequences to the society or environment shall be assessed. Application rule: When evaluating possible risk, both the probability of a failure and the effects of a failure should be taken into account. The effect of failure of a district heating pipeline system to its environment is related to temperature, pressure and diameter of the pipeline. The probability of a failure is based on internal and external factors and the quality of design, installation and operation. Possible risks are: a) escape of hot water due to bursting or leakage, involving a risk of scalding, flooding, tunnelling, etc., b) damage to the installation, involving interruption of the heat supply, c) damage to the installation, involving a risk of further spread of the damage in the installation, d) loss of safety of supply. The consequences of failure may be related to the entire system or to a section only. The project class determines the level for design and installation of the pipeline system. SIST EN 13941:2009

194.4.2 Project classes The choice of project class is related to the level of safety and complexity of execution expressed as requirements with respect to design procedures and construction. Based on preliminary investigations and risk assessment the pipeline system shall be classified in one of the following classes: Table 3 — Project classes Project class A 
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