Steel jacket pipe
Steel jacket pipes , SMR for short , are used in district heating to transport hot water or steam . They differ in structure and technical design from plastic jacket composite pipes and flexible composite pipes . In industrial plants, they are used for the transport of hot and, more rarely, cold, gaseous and liquid media and, due to the double-hull construction, also for the transport of gaseous and liquid hazardous substances .
technical structure
Steel jacket pipes consist of an outer protective and support pipe, the so-called jacket pipe, which is made of steel and, depending on the application, is protected either by a corrosion protection coating for overhead lines or by a coating made of polyethylene or bitumen for underground lines.
The medium pipe is mostly arranged in the middle within the casing pipe and consists mostly of steel, more rarely of another, mostly metallic material. It is usually mounted so that it can move axially. Two or more media pipes are rarely arranged within a single steel jacket pipe. To support the media pipe, roller bearings or plain bearing constructions with Eternit rails (in use for 40 years) are used as so-called floating bearings. In addition to the required statics, attention is also paid to the lowest possible heat losses when they are designed. Less often, e.g. B. in the area of fittings, branches or transitions to other pipe systems, the axial displacement of the media pipe must be prevented. In these areas, the media pipe and jacket pipe are rigidly connected to one another with a fixed point. In addition to the required resistance, the fixed points also ensure the lowest possible heat losses.
The pipes are usually delivered in lengths of 12 to 24 meters (rarely 6 meters) or, depending on the customer's requirements, in any length, the maximum length being limited by the necessary transport from the manufacturer to the construction site. Since it is a rigid pipe, suitable fittings are used for bends , which are constructed in the same way as the steel jacket pipe. Fittings that are constructed like a steel jacket pipe are also available. Steel jacket pipes are only made to order due to their relatively low demand, and the degree of prefabrication is relatively high. Steel pipes, floating bearings and fixed points are usually ready to use. Because of the sometimes safety-critical applications, the weld seams of the steel jacket pipes are normally subjected to a 100% test.
The space between the carrier pipe and the jacket pipe is called the annulus. On the outside of the medium pipe there is usually thermal insulation made of mineral fiber wool (rock or glass wool), but this does not fill the entire annulus. To further reduce the heat losses, but also to report leakages, a rough vacuum is usually created in the annulus, the residual pressure of which is selected to be low enough to reliably prevent convection in the annulus. A heat transfer thus only takes place through gas heat conduction and thermal radiation in the annulus and through heat conduction via the fixed points and floating bearings. Usual residual pressures are around 100 to 500 Pa (1 to 5 mbar). With careful production, this vacuum will last for years after the initial warm-up and the annulus only needs to be re-evacuated every 2 to 4 years. In larger steel jacket pipe networks, in which z. B. can not completely avoid leaks in the fittings, a stationary vacuum pump with automatic control is connected to the annulus after evacuation to maintain the vacuum.
Design of the pipes
Steel jacket pipes are usually geometrically designed in such a way that safe operation is possible at the intended maximum continuous operating temperature without a vacuum in the annulus. The maximum temperature on the jacket pipe is determined by its casing and is usually 50 ° C for a casing with polyethylene and 60 ° C for a casing with bitumen, since these substances begin to flow at higher temperatures. Steel casing pipes laid as overhead lines without covering the casing pipe must be provided with barriers to prevent contact by people and larger animals if the casing pipe temperature can exceed 60 ° C.
When transporting hot water or steam, the vacuum primarily serves to reduce heat losses. The leak detection plays a subordinate role, as the gases sucked in by a stationary vacuum pump in the event of a leak can hardly be used to distinguish between leaks in the medium pipe and in the jacket pipe. In both cases, gases (nitrogen, oxygen, argon) and water vapor contained in the air are sucked in. If the line is in a damp environment, it may U. even the water vapor concentration does not provide information about whether the leak is in the carrier pipe or in the jacket pipe.
When transporting hazardous substances, the vacuum is primarily used to monitor leaks. The reduction in heat losses only plays a role if the medium temperature deviates significantly from the ambient temperature. For leakage monitoring, a gas detector is installed between the annulus and the stationary vacuum pump, which reacts to one or more gaseous components of the hazardous substance to be transported. The gas detector is chosen so that it responds to gases that are normally not present in measurable concentrations in the environment.
When transporting warm or cold media, the axial displacement of the medium pipe must be compensated. If possible, U- or Z-bends are used for this, as the bellows expansion joints that were often installed in the past represent weak points.
Application limits
Steel jacket pipes are mostly only used if the maximum operating temperature is above 145 ° C, since plastic jacket composite pipes can be laid much more cheaply for applications with lower operating temperatures . Exceptions are special structures such as culverts and pipe bridges, where constructions with steel jacket pipes can be cheaper than other constructions or represent the only technical option. The maximum operating temperature is approx. 400 ° C. The minimum operating temperature for cooling applications is approx. −50 ° C.
Manufacturer
- INPAL Industries
- FW-Fernwärme-Technik GmbH
- ISOBRUGG Stahlmantelrohr GmbH
Individual evidence
- ↑ inpal.com Homepage of INPAL Industries, as of October 10, 2009
- ↑ fw-gmbh.de Homepage of the company FW-Fernwärmetechnik, as of October 10, 2009
- ↑ isobrugg.de as of October 10, 2009