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
The following steps must be observed when designing pipework:
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.
Area of application
0 – 1 bar
20 – 25 m/s
1 – 40 bar
30 – 40 m/s
0.4 (0.25 – 0.6) m/s
2 (1.5 – 3) m/s
Light fuel oil intake side
Light fuel oil discharge side
Heavy fuel oil intake side
Heavy fuel oil discharge side
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.
External diameter d1
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:
Equation for calculation of required nominal diameter
Example calculation for determining the required nominal diameter
Nominal pipe diameter [mm]
Flow rate [m³/s]
Mass flow rate [kg/h]
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:
Unalloyed steels with guaranteed strength characteristics at higher temperatures
Unalloyed steels with defined characteristics ≤ 400°C, upper yield point > 265 N/mm²
Low alloy steels with 0.3% molybdenum
Standard carbon content, stabilised with Ti or Nb
Standard carbon content, alloyed with molybdenum, stabilised with Ti or Nb
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
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.
Materials containing copper must not be used for any pipework to and from the steam boiler, in the condensate and make-up water area.
Steel or stainless steel with inspection certificate
Feed water lines
Safety valve blow-off pipes
Ventilation and drain lines
Seat drainage (safety valve)
Copper or stainless steel
Plastic (cold) or stainless steel (following heating)
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
– With additions through interpolation
– L1 limitation of deflection, up to DN 50 = 3mm deflection, from DN 65 = 5mm deflection
– For details see EN13480-3
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:
Use of – to absorb the elongation
Expansion legs (L-legs)
U-bends (with long straight pipework)
Boiler and container
Friction bearings on feet and base frame
Expansion joints and expansion legs on incoming and outgoing pipework
The following equation can be used to calculate the linear thermal expansion:
Equation for calculating linear thermal expansion
Linear thermal expansion [mm]
Expansion coefficient [mm/m]
Temperature difference [K]
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.