Double-walled pipe

Steel casing pipes, short SMR, are used in the district heating to hot water or steam from the producer to transport to the customer. They differ in structure and in the technical design of much of the plastic sheath composite pipe. They are also used in industrial plants for transporting hot, rarely, cold, gaseous and liquid media, and because of its double hull construction also for the transport of gaseous and liquid hazardous materials.

Technical structure

Steel casing pipes consist of an outer protective and support tube, called the casing pipe, which is made of steel and depending on the application either by a corrosion protective coating for overhead lines or through a sheath of polyethylene or bitumen is protected for buried pipes.

The carrier pipe is generally centrally located within the jacket tube and is usually made of steel, rare from another, generally metallic material, and is mounted axially movably in the rule. Two or more media tubes are arranged Rare within a steel casing pipe. For storage of the medium pipe roller bearing or plain bearing designs are with Eternit rails ( in use for 40 years ), so-called non-locating bearing, used, care is taken in their construction in addition to the required static on the lowest possible heat loss. More rarely, such as in the range of fittings, branches or transitions to other piping systems, the axial displacement of the carrier pipe must be stopped. In these areas, carrier pipe and casing pipe will be connected to a fixed point rigidly together. Even with the fixed points is taken in addition to the required static to minimizing heat loss.

The tubes are usually supplied in rods in lengths of 12 up to 24 meters, 6 meters or less frequently depending on the customer in any length, the maximum length is limited by the necessary transport from the manufacturer to the site. Since it is a rigid pipe, are for bows etc. appropriate fittings which are constructed as a steel casing pipe supplied. Also fittings are available which are built like a steel casing pipe. Steel casing pipes are produced due to the relatively low demand only after the order, the degree of prefabrication is relatively high. Steel pipes, non-locating bearing and fixed points are usually ready prefabricated. Because of the partial safety-critical applications, the welds on the steel casing pipes are normally subjected to 100 % inspection.

The space between the carrier pipe and casing pipe is called the annulus. You can usually find a thermal insulation made of mineral fiber wool ( rock or glass wool), but does not fill the entire annular space on the outside of the carrier pipe. For further reduction of heat loss, but also for leak alarm, a rough vacuum is usually drawn in the annulus whose residual pressure is set so low that convection is suppressed reliably in the annulus, heat transfer thus only by gas conduction and thermal radiation in the annulus as well as by heat conduction through the solid points and floating bearing is done. Usual residual pressures are approximately 100 ... 500 Pa (1 ... 5 mbar). With careful production, this vacuum holds after the initial warm-up over the years and the annulus is usually nachevakuiert only every 2 to 4 years. In larger steel casing pipe networks, in which you can not, for example, by existing fittings leaks completely avoided, a stationary vacuum pump with fully automatic control is connected for maintaining the vacuum in the annulus after the evacuation.

Steel casing pipes shall be designed geometrically usually, this is a safe operation at the intended maximum continuous operating temperature possible in the annulus without vacuum. The maximum temperature on the jacket pipe is determined by the envelope and is usually at a coating with polyethylene at 50 ° C and a sheath with bitumen at 60 ° C., since these materials begin to flow at higher temperatures. As transmission line laid steel casing pipes without wrapping the jacket pipe must, if the casing pipe temperature can exceed 60 ° C, are surrounded by fences, people and larger animals from temperatures keep > 60 ° C.

The vacuum is used in the transportation of hot water or steam primarily the reduction of heat loss. The leak alarm plays a minor role, as the basis of sucked from a stationary vacuum pump in the event of a leak gases a distinction between leaks in the carrier pipe and the casing pipe is hardly possible. In both cases atmospheric gases (nitrogen, oxygen, argon ) and water vapor are sucked. If the line in very wet conditions, and can not even be made a distinction between leaks in the carrier pipe and the casing pipe by means of the water vapor concentration U..

The vacuum is used in the transportation of hazardous materials primarily for leakage monitoring. The reduction of heat loss plays a role only when the medium temperature is very different from the ambient temperature. For the leakage monitoring a gas detector is between annulus and stationary vacuum pump is installed which reacts to one or more gaseous components of the to -transporting hazardous material, the gas detector is selected so that it is responsive to gases that are not normally present in the environment in measurable concentrations.

When transporting hot or cold media, the axial displacement of the carrier pipe must be compensated. Meanwhile, U- or Z- bends are used but if possible, because the sooner often built Faltenbalgkompensatoren have proven to be weak points.

Limitations

Steel casing pipes are usually only used when the maximum operating temperature of 145 ° C, as for applications with lower operating temperatures plastic sheath composite pipes can be installed much lower cost. Exceptions are special structures such as culverts and pipe bridges, where structures with steel casing pipes can be much cheaper than other designs, or even represent the technically only possible construction. The maximum operating temperature is approximately 400 ° C. The minimum operating temperature for Refrigeration applications is approximately -50 ° C.

Manufacturer

• INPAL INDUSTRIES
• FW- DISTRICT -TECHNIK GmbH
• ISOBRUGG steel casing pipe GmbH