LDPE plants using the tubular or autoclave process are built, to a large extent, from high pressure tubes. It is the single most important item to focus on when you invest in new equipment. For many years, Uhde has manufactured high pressure tubes by drilling forged bars. But only recently did this technology become so successful that 9 out of 10 tubes are now manufactured in this way. We at Uhde are fully utilising the benefits of this new technology in order to provide equipment suited for the highest demands of the new plants with a world-scale capacity.

We melt the crude steel in a modern electric-arc furnace, followed by vacuum degassing. The steel purity achieved by this modern equipment is already very high but can be further improved by electroslag remelting or, for the highest demands, by vacuum-arc remelting. The ingots produced in this way then go through the largest radial forging or rolling machines in the world where the raw bars are produced. The capabilities of the steel plant cover all the requirements in terms of metallurgical capabilities which are necessary for the production of materials for the LDPE process licensors. A team of experienced specialists is ready to always further improve the steel quality and to adapt the alloys and the production technology to the market needs. This is how our standard alloy, A-723 Grade 3 mod. (DIN 1.6957 mod.), came into being: An alloy design focused on the required function in LDPE high pressure duty.

Precise machining is used in order to obtain very narrow tolerances on the bore diameter and the outer diameter. This helps the process designers to precisely determine the flow rate and pressure drop in the tube and in the jacket pipe. The final surface quality on the bore which we achieve is Ra = 0.1 µm. This quality assures the longest service life of the tubes. The tubes are then finish-machined including a sealing face, a thread and a shrink-ring seat for jacketing.

High pressure tubes can be effectively protected against corrosion attack from the cooling medium at the outside by cladding with a copper tube. The thickness of the copper is appr. 1.6 mm and protects the tube for at least 20 years. The heat transfer remains unaffected, due to an excellent contact between the copper and the high pressure tubes and high conductivity.

All high pressure tubes are subjected to an autofrettage treatment. This operation helps to extend the service life of the tubes. It is carried out by exposing the tubes to extremely high pressures close to their burst pressure. The tube is deformed in such a way that we achieve a high level of compressive stress at the tube bore. This protects the tube against cracks developing from the inside.

The complete tube manufacturing line has been developed in order to guarantee 100 % defect-free production. Every process step is carefully monitored so that the specifications are met. Therefore, the tube orders can be made to the precise number without any need to worry about an allowance for overproduction.