Pipework

When installing pipework systems, the terms nominal diameter (DN) and nominal pressure (PN) are used to identify characteristics of pipework for the purpose of defining compatible parts, e.g. flange connections. Nominal diameter and nominal pressure are standardised according to the geometrical increment.

When sizing pipework, i.e. defining the nominal diameter and nominal pressure for pipework and valves, a balance must always be struck between the technical requirements, such as keeping the pressure loss or heat loss as low as possible, and the associated investment and operating costs. The optimum overall cost balance between investment and operating costs that emerges is different for every pipe and system. Owing to the curve characteristics in the minimum range of the total costs, two nominal diameters often lie within the optimum range.

Example of schematic cost trend for pipework dimensioning

Example of schematic cost trend for pipework dimensioning

Total costs

Operating costs

Investment costs

The following steps must be observed when designing pipework:

  • Define nominal diameter
  • Define nominal pressure
  • Select material
  • Define spans
  • Consider thermal expansion
  • Take special characteristics of the medium into account during installation

As many system-specific, technical and commercial individual parameters would need to be considered for a detailed analysis, pipework is normally designed for the permissible flow speed according to economically sound principles and technical necessities based on experience. Depending on the medium and use, the recommended values have proven to be practise-compliant in many systems.

Medium

Area of application

Recommended speed

Steam

0 – 1 bar

20 – 25 m/s

1 – 40 bar

30 – 40 m/s

Water

Suction line

0.4 (0.25 – 0.6) m/s

Pressure line

2 (1.5 – 3) m/s

Condensate

Steam fraction

15 m/s

Water fraction

2 m/s

Flue gas

16.5 m/s

Oil

Light fuel oil intake side

0.5 m/s

Light fuel oil discharge side

1 m/s

Heavy fuel oil intake side

0.3 m/s

Heavy fuel oil discharge side

0.5 m/s

Natural gas

No specifications (design via pressure loss)

Standard design speeds (recommended speeds) for pipework sizing

Definition of nominal diameter DN

The nominal diameters in the following table are specified without units. They correspond roughly to the inner diameter of the pipework in mm. This is for production reasons as the tools used in the manufacturing of pipes are defined via the external diameter and the clear inner diameter therefore varies depending on the wall thickness. The nominal diameter will normally suffice as calculation variable for rough sizing of the inner diameter.

Nominal diameter
DN

External diameter d1
[mm]

 

Nominal diameter
DN

External diameter d1
[mm]

 

Nominal diameter
DN

External diameter d1
[mm]

6

10.2

 

80

88.9

 

500

508.0

8

13.5

 

100

114.3

 

600

610.0

10

17.2

 

125

139.7

 

700

711.0

15

21.3

 

150

168.3

 

800

813.0

20

26.9

 

200

219.1

 

900

914.0

25

33.7

 

250

273.0

 

1,000

1,016.0

32

42.4

 

300

323.9

 

1,200

1,219.0

40

48.3

 

350

355.6

 

1,400

1,422.0

50

60.3

 

400

406.4

 

1,600

1,626.0

65

76.1

 

450

457.0

     

Pipe diameter (EN 10255:2004+A1:2007, EN 1092-1:2013-04, Table A.1)

The necessary nominal diameter can then be calculated as follows:

 
Berechnung

Equation for calculation of required nominal diameter

Example calculation for determining the required nominal diameter

kgh 3,600sh ⋅ 4 π ⋅ kg m s
⋅ 1,000 mm m = 44 mm ≤ DN 50
 

DN

Nominal pipe diameter [mm]

Flow rate [m³/s]

Mass flow rate [kg/h]

ρ

Density [kg/m³]

u

Recommended speed according to table [m/s]

To optimise nominal diameters which have been designed according to a permissible recommended speed, it may be advisable in individual cases, e.g. if the pipework is very long, to recalculate and optimise the nominal pipe diameter using special design programs.

Defining the nominal pressure PN

The nominal pressure is a standardised pressure stage for pipework and valves. It represents a parameter for the mechanical and dimensional characteristics of a component. Components with the same nominal diameter and same nominal pressure are compatible. The nominal pressure corresponds the maximum permissible positive pressure [bar] at a reference temperature of 20°C.

However, in addition to the material, the maximum permissible positive pressure of a component depends first and foremost on the temperature. At higher temperatures, the maximum permissible operating pressure falls below the nominal pressure. Pipework or valves can then not be operated at the nominal pressure.

The pressure-temperature assignment of flanges is based on the material groups. The following materials and groups are customary in the area of steam boilers:

Material group

Material type

Material number

Material

3E0

Unalloyed steels with guaranteed strength characteristics at higher temperatures

1.0352

P245GH

3E1

Unalloyed steels with defined characteristics ≤ 400°C, upper yield point > 265 N/mm²

1.0460

P250GH

4E0

Low alloy steels with 0.3% molybdenum

1.0426

P280GH

12E0

Standard carbon content, stabilised with Ti or Nb

1.4541
1.4550
1.4941

X6CrNiTi18-10
X6CrNiNb18-10
X6CrNiTiB18-10

15E0

Standard carbon content, alloyed with molybdenum, stabilised with Ti or Nb

1.4571
1.4580

X6CrNiMoTi17-12-2
X6CrNiMoNb17-12-2

Material groups according to EN 1092-1:2013-04 Table 9, G.2.2, G.3.2, Table D.1

The following diagram shows the pressure-temperature curves for different nominal pressure stages. In this case also observe the information in the chapter Tools – Pressure-temperature assignment, which contains the tables for the diagram.

Info on Pressure-temperature assignment

Pressure-temperature assignment for flanges according to EN 1092-1

Pressure-temperature assignment for flanges according to EN 1092-1

3E0

3E1

4E0

12E0

15E0

Defining the material

The following table states only the minimum requirement for material selection. Other materials can be also used where special installation conditions, customer requirements or national or local regulations apply.

Information

Materials containing copper must not be used for any pipework to and from the steam boiler, in the condensate and make-up water area.

Application area

Pipework material

Steam pipes

Steel or stainless steel with inspection certificate

Feed water lines

Steel

Safety valve blow-off pipes

Steel

Ventilation and drain lines

Steel

Seat drainage (safety valve)

Copper or stainless steel

Softened water

Plastic (cold) or stainless steel (following heating)

Osmosis water

Stainless steel

Minimum requirement for material selection

Definition of spans

It must be ensured through a sufficient number and correct construction of holders that pipework does not deform beyond acceptable limits due to weight forces (own weight, content, valves and insulation) and other forces acting on it (e.g. at deflections).

Requirements for pipework are explained in EN 13480-3.

Pipework and flanges for water and steam


DN


Ø valves

PN 40
S

Max. span
L11)

10

17.2

2.0

15

21.3

2.0

20

26.9

2.3

25

33.7

2.6

2.9

32

42.4

2.6

3.2

40

48.3

2.6

3.5

50

60.3

2.9

3.9

65

76.1

2.9

4.7

80

88.9

3.2

5.4

100

114.3

3.6

6.2

125

139.7

4.0

6.9

150

168.3

4.5

7.5

200

219.1

6.3

8.6

250

273

7.1

9.7

300

323.9

8.0

10.6

350

355.6

8.8

11.1

400

406.4

11.0

11.8

500

508

14.2

12.5

600

610

16.0

13.2

Pipework spans (holder-to-holder distance)
1) Requirements for span L1:
– 
According to EN13480-3:2014 – filled with water, thickness of insulation 80mm

– With additions through interpolation
– L1 limitation of deflection, up to DN 50 = 3mm deflection, from DN 65 = 5mm deflection
– For details see EN13480-3

Thermal expansion

Substances expand when they are heated and contract when they cool down again.

This effect must be taken into account at many points in a boiler system, especially in locations where high temperatures can occur during operation.

The following points must for example be taken into account during planning and installation:

Location

Use of  –  to absorb the elongation

Piping

  • Steam

  • Flue gas

  • Surface/bottom blowdown

  •  ...

Pipework compensators

  • Expansion legs (L-legs)

  • Expansion bends

  • U-bends (with long straight pipework)

  • Friction bearing

Boiler and container

Friction bearings on feet and base frame

Expansion joints and expansion legs on incoming and outgoing pipework

Location and types of measures used to absorb thermal expansion

The following equation can be used to calculate the linear thermal expansion:

 
Berechnung

Equation for calculating linear thermal expansion

 

Δl

Linear thermal expansion [mm]

l

Length [mm]

α

Expansion coefficient [mm/m]

ΔT

Temperature difference [K]

Information

Expansion coefficients of different steels

Low alloyed steel (ferritic):
α ≈ 1 – 1.3 [mm/m ∙ 100K] = 10 – 13 ∙ 10-6 [1/K]

Stainless steels (austenitic):
α ≈ 1 – 1.8 [mm/m ∙ 100K] = 10 – 18 ∙ 10-6 [1/K]

The leg length required to absorb thermal expansion must be determined according to the general codes of practise.

Minimum distance to structure and adjacent pipework

A clearance of at least 50 – 100mm should be maintained in order to install the pipework and insulation and also carry out repairs. The frequently used technical standard for insulation work DIN 4140 recommends a minimum clearance of 100m.

To minimise the clearances, flange connections should have an offset arrangement on pipe bridges.

Functional clearances of pipework on pipe bridges and offset arrangement of flange connections Functional clearances of pipework on pipe bridges and offset arrangement of flange connections