High efficiency, low emissions and cost-optimised construction processes – these are the demands placed on power plants and their construction, and in meeting them the structure and design of pipelines has some significance, for two main reasons: pipes made of wear-resistant and resilient material that exhibit small bending radii improve the efficiency of power plants; and lean tube bending processes and automated production lines can themselves provide a decisive contribution to cost efficiency in power plant construction.

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Temperature resistance

More efficient power plants tend to be run at higher temperatures. For a pipeline to resist temperature loads of 600°C or more, steels such as T91 are increasingly being used in their manufacture. In comparison to normal steels, these have the greater creep strength that is required to resist the higher loading through a range of pressure and flow values. When forming these tubes it is necessary, however, to take various factors into consideration. The steel is stronger, so the bending machines used must have the necessary bending forces. Yet despite their strength, these materials react more sensitively, and wall-thickness tapering and out-of-roundness can occur in the tube bend. It is therefore essential that the bending machines meet the corresponding tolerance specifications. These include, for example, the European standard EN 12952 and that of the American Society of Mechanical Engineers ASME B31.1. Meeting these standards plays a significant part, in particular in view of the fact that power plant construction makes increasing use of smaller boilers with higher capacity. Heavier wall thicknesses and smaller bending radii are required to manufacture them, and not infrequently, the minimal bending radius must be 1 x D, depending on tube diameter (in part also less than 1 x D). The bending challenge behind this is correspondingly significant.

Two bend heads

Lean production processes provide an essential contribution towards cost efficiency. That is the reason why machines with two bend heads have established themselves for the production of pipelines, because they permit time-optimised production of complex tube serpentine shapes. The CNC 100 DB Twin by Schwarze-Robitec has been conceptualised specifically for power plant construction, by taking into consideration the special properties of tubes made of temperature- resistant steels. It has two bend heads which can be moved vertically and horizontally (1x left bending, 1x right bending), which bend the tube alternately clockwise and anticlockwise. In this manner, the machine offers high flexibility, for example when manufacturing superheater coils. The tube serpentine does not need to be turned after each bend, which guarantees quick and economic production processes and the manufacture of long side lengths. As an option, both bend heads can also bend in the same bending direction. By using different sized bend formers, different sized bending radii can be produced.

Fully automatic manufacturing

Fully automatic manufacturing chains make a further contribution towards cost efficiency in power plant construction. When storage, materials-handling and manufacturing technology interact optimally, they reduce cycle times and therefore affect the competitive edge of a company. Schwarze- Robitec has developed an automation concept by which all machines can be fitted with optional extras, depending on bending order, and expanded to a fully-automatic manufacturing system. This rationalisation extends across the complete production chain, from material supply to handling and quality testing all the way to the dispatch of the ready bent pipelines.
First the tube goes through quality control of the weld seams by x-ray, then flaw-free tubes are moved to the bending machine and flip-over table. The flip-over table is specially designed for handling long pipelines. Its arms turn the tube serpentines quickly and securely between the individual bending procedures. The next stage is the terminal-end-bender with a gripper unit. This is a further bending machine which undertakes bends in both directions at the rear end of the tube serpentine. With the help of such automated manufacturing chains higher manufacturing speeds and low reject rates can be realised.

Membrane tube walls

Also offered is a specialised bending technology for the complete membrane tube walls usual in modern boilers. The FL machine series processes tube walls, up to a width of 3600 mm and a length of 25000 mm, more precisely and quickly by means of a compression bending method. It ensures that the tube wall is clamped securely at all times. In addition, there is the option of bending the tube walls diagonally. This enables simple manufacture of the "gradient" of a boiler wall and saves considerable levelling and assembly costs when setting up the boiler.

The company has also developed a heavy duty series of machines tailored to power plant construction. Equipped with special tools these machines bend thin and thick- walled tubes up to a maximum diameter of 426 x 22 mm.

 


(Originally published in MPS January 2016)