Peripherals

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.

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

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.

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:

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 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

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.

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

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:

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

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.