FW Steel-Cased Pipe-in-Pipe

"The pipe with the red cap.”

“According to unconfirmed tradition, in 1906 a Frenchman should have inserted two steel pipes into one another – the steel-cased pipe system was born.”

Quote from the FW company founder Volkwart Harders

FW steel-cased pipes (SIS) are a pipe system that has been established for over 50 years for the transport of thermally demanding media. They cover a temperature range from -196 °C to +400 °C and are therefore suitable for both cryogenic and high-temperature applications.

SIS (STEEL-IN-STEEL) is primarily used in district heating systems, particularly where high operating temperatures and increased demands on mechanical stability and service life are required. Furthermore, they are used wherever media need to be conveyed safely and in a controlled manner.

An essential scope of application is their use as safety pipes for environmentally hazardous substances. In the event of a leak, the double-walled design will enable controlled drainage and monitoring to be carried out. SIS systems are also utilized in the LNG sector, for example in cryogenic piping systems on ships.

Typical areas of application:
  • District heating and high-temperature networks
  • Transport of cryogenic media (e.g. LNG)
  • Safety pipe systems for critical media
  • Industrial process pipelines with extended temperature requirements

FW steel-cased pipes combine high thermal resistance with mechanical robustness and enable safe operation even under demanding conditions.

Steel-cased pipelines can be installed underground or above-ground, even in very demanding conditions. When installed as an above-ground line, they offer high protection against damage and vandalism and provide the advantage of long spans, with the casing pipe acting as the load-bearing element for self-supporting pipe bridges.

When laid underground, they are particularly suitable for laying long sections using the horizontal directional drilling (HDD) method, e.g. for river crossings.

The temperature-induced linear expansion of the medium pipe is ideally compensated naturally in the bends of the route. Where necessary, thermal and mechanical pre-stressing, as well as inner pipe compensators or compensation chambers, are also used.

  • The project-specific dimensioning of the FW steel-cased pipes allows to be tailored to customer requirements:
    • Flexible application for many media thanks to the choice of inner pipe material
    • Can be installed underground or above ground
    • Casing pipe coating with PE, PP or paint coating
    • Can be supplied with cathodic corrosion protection (CP)
  • Static calculations and detailed design are executed by FW.
  • The high degree of pre-fabrication enables simple installation on site. All fittings and special components are already assembled at the factory into the 12 to 18 m long construction units.

The FW steel-cased pipe-in-pipe consists of a medium pipe that is guided concentrically within a casing pipe and is mounted to allow axial movement. The concentric alignment is ensured due to the use of various bearing systems designed according to the specific project requirements.


As a rule, the medium pipe is fitted with thermal insulation consisting of rock wool shells bonded with synthetic resin; in special applications, aerogel insulation is also used.


In the ring space between the medium pipe and the casing pipe, there is a vacuum. This reduces heat loss and, due to the removal of oxygen, provides additional protection against corrosion.