Nominal diameter – Nominal diameter or also DN (derived from French for ‘diamètre nominal’, English: Inside diameter) for ‘passage standard’. The nominal diameter indicates the inner diameter of a tube or the connection dimension of a mounting part. In connection with the nominal pressure level and the material class the nominal diameter determines all dimensions of a tubing. It is often the case that steel is not explicitly referred to as a material but is nevertheless required. It is also important to note that the actual inner diameter and the specified nominal diameter often differ by a few millimetres, for instance because the wall thickness of some steel tubes grows inwards with increasing pressure stage – as a consequence, the free cross-section is reduced. Therefore, tubes from different manufacturers can only be combined with one another without any difficulty if the nominal diameter is specified referring to the same DIN standard. The outer diameter on the other hand – except in case of very thick-walled tubing for very high pressure and for tubing made of glass fibre reinforced plastic – remains constant whereby the same thread can be cut for virtually all tubes of one nominal diameter. Thus it is possible to use the same pipe sleeves, also known as fittings, for all pressure stages. The nominal diameter is introduced in accordance with EN ISO 6708 with the abbreviation DN followed by a dimensionless number roughly corresponding to the inner diameter in millimetres. In accordance with ANSI the nominal diameter is specified in NPS (Nominal Pipe Size) in inches. For STRIKO specifications of the nominal diameter play an essential role when it comes to finding the right product for the customer’s demands. In the area of bursting discs and burst indicators STRIKO process technology usually provides products with nominal diameters between DN 20 and DN 600.