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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”.

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

bonded system

system consisting of a service pipe, insulating material and casing, which are bonded by the insulating material

cold installed preinsulated bonded pipes

district heating systems where the pipes are installed and taken into operation without prior pre-stressing by pre­heating

creep

slow progressive strain under the influence of stresses

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

design temperature

maximum temperature used for the design of a component or pipe section

displacement-controlled action

action called forth by enforced deformation or movement, e.g. thermal expansion or settling

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.

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 %

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.

fabricated tees

tees manufactured by welding a branch pipe directly onto a run pipe

fatigue strength

stress range of constant magnitude which, under given circumstances, just causes fatigue failure

force-controlled action

action which maintains its size irrespectively of the deformation of the structure, e.g. pressure and weight

house service connection

pipeline leading from main pipeline to one consumer installation, see Figure 2.

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. Pumpstation

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.

3.1.18

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

3.1.28

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

The unit system applied in this standard is the SI system (Systeme International d'Unites), cf. ISO 1000 and others.

The following units and their multiples are used:

Length m (metre)

Other units applied:

3.2.2 Symbols

Table 1 — Symbols

Table 1 (continued)

Table 2 — Indices

4 General considerations for system design

Design and installation of district heating pipe systems shall ensure that the system is given:

1. 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,

2. sufficient safety that unusual operating conditions or accidents do not jeopardise persons or the environment,

3. good energy economy,

4. good operating properties,

5. 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.

When a system designed according to this standard is subject to temperatures exceeding 120 °С, for periods such that the requirement for a service life of 30 years at continuous operation at 120 °С, calculated in accordance with Annex В 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 120 °С.

If the cumulative ageing requires a lifetime exceeding the equivalence of 30 years at 120 °С special documentation for the ageing properties are required.

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,

— 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,

2. operating conditions of the system, e.g. variations in pressure and temperature and requirements for safety of supply,

3. function mode of the system during operating and maintenance stage as well as resistance to relevant impacts such as:

1. loads due to installation and operation,

2. internal and external loads and deformations,

2. consequences of possible kinds of failure of the system,

3. authorities’ requirements, environment and third party aspects, f) methods of execution.

4.4 Determination of project class

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:

1. escape of hot water due to bursting or leakage, involving a risk of scalding, flooding, tunnelling, etc.,

2. damage to the installation, involving interruption of the heat supply,

3. 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.

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

Application rule:

Special or complex constructions can be crossings with railways, major roads and waterways, which normally must be designed in consultation with the owners and/or authorities. For crossings with dykes and flood defences extra measures may be required to prevent flooding of the hinterland.

System parts which are not directly pressurised, but in which a failure may involve fracture or leakage in a pressurised section are referred to the same project class as the pressurised section.

Pipelines which are accessible during operation shall, as a minimum, be classified in project class B.

Based on the expected effects the project classes A, В and C are determined by Figure 3.350 -

300 -

CM E E z 250- I- <

P

GT 200-

B

До = Re(7)

P "s' o 150- <

26,9/2,0

100 -

5 10 15

Figure 3 — Definition of project classes for a steel with a specified minimum yield strength, Re(23oC) = 235
N/mm²

Table 4 — Requirements for project classes

Application rule:

In relation to project class the following has to be considered:

• requirements for documentation,

• determination of Xat in Palmgren-Miners formula,

• requirements for welding,

• scope of inspection of weld seams,

• quality management and scope of inspection.

An installation can always be classified in a higher project class than stated in Figure 3.

Application rule:

Following conditions can result in the choice of a higher project class:

• system design and complexity,

• soil and groundwater conditions,

• traffic conditions,

• position in relation to other structures and utility networks,

• experience with corresponding installations,

• new methods,

• location of the pipeline and possibilities for maintenance and replacement.