TPi March 2020

for tyres, industrial rubber products, plastics, electrostatic discharge (ESD) compounds, coatings or other products is required, high temperature corrosion will undoubtedly be a challenge. Suitable tube grades should be sought for air pre- heaters (APH), oil pre-heaters, scrubbers, gas cleaning unites and continuous rotary dryers. The combustion of heavy petroleum, whether in a single or double shell preheater at 800°C to 900°C in a reactor’s preheating can be highly corrosive and lead to adverse deposits. However, the ability to recover process gas heat is an excellent way of reducing fuel consumption and increasing feedstock flow, while at the same time boosting the production yield. Careful material selection can also go a long way to extending the lifetime of muffle tubes, flares and pigtails, as exposure to shielding or flue gases at high temperature can cause problems. High temperature muffle tubes for use with furnaces come in a number of types from wire and Bundy tube to razor blades and tube annealing. In these applications some of the protective ‘shielding’ gases used in the tubes can be highly aggressive and when combined with high temperatures, can shorten the service life of the tubes due to oxidation, nitridation or even loss of mechanical strength. Using the best available grade will prevent problems and go a long way to extending the lifetime of the tube. In particularly aggressive environments with cracked ammonia, Sandvik Sanicro 31HT or similar will protect against rapid nitriding of the material and prevent loss of mechanical strength. In industries where flare off excess gas to reduce over- pressure is used, there are materials suitable for elevated vertical or ground level flares. Such applications tend to be in petrochemical and chemical plants, LNG facilities, offshore operations or downstream refineries. Corrosion resistant tubing will increase the lifetime of the flares’ tip and reduces the need for unscheduled maintenance and downtime. Meeting the demands for production precision, reliability and safety are also key when looking to select material. One such application is that of mineral insulated cables (MIC). Manufacturers of MIC seek tighter tolerances for the sheath, higher temperatures resistance and the best precision control, and this is driven by safety requirements for temperature, control and measurement engineering. The choice of tube material for the protective tube sheath is a critical consideration, particularly for working with corrosive environments and at elevated temperatures. Whether for thermocouples, measurement or heating, the tubing used for the sheath of MIC is exposed to corrosive gases at high temperature without any cooling process. A metal sheath is therefore essential to protect the thermocouple wires from corrosion. Replacement of the tubing is both time consuming and dangerous due to the fact they are usually located in high temperature zones or difficult to access areas. Corrosion and creep prevention and securing structural stability can be provided by using a sheath material that will complement the insulation material.

production plant, no energy should be going to waste and using surplus heat to preheat process gases is an obvious way to save energy and keep costs down. With energy intensive industries such as glass, steel, iron and pig iron, recuperators can go a long way to make savings on fuel or boost the process efficiency by working at higher temperature and/or recovering waste gas heat at higher temperature. In these applications such gases are quite dirty and full of impurities from the process and so they can build up on tubes and lead to their degradation and in turn create an increase in maintenance time and costs. To avoid this, specifying a material with excellent HT corrosion resistance such at Sandvik 253 MA tube will make a significant difference, particularly for the direct reduction of iron ore (DRI). Again, payback on investment can be as short as one year. An example would be when changing from an air preheat of 650°C using common ferritic or austenitic materials to air preheat of 800°C with a superior material. For higher flue gas temperatures in the 1,200°C range, such as in glass manufacture, materials with resistance to sulfidation environments can make a big difference. With lime kiln applications, material failure can occur especially in tip close to the burning zone of the lance. Internal and external corrosion challenges can be presented by using high temperature tubing in all kinds of kilns involving the need to tackle oxidation and carburisation. When kiln temperature reaches 900 – 1,000°C, high temperature corrosion can occur with oxidation scale forming on the surface of the tube as well as sulfidation and carburisation, so it is important to seek a material that will extend the service life and require less maintenance. It is also vital to note that different fuels affect the corrosion mechanism differently, so care needs to be taken at the material selection stage. Carbon black related products also need consideration. If the production of carbon black pellets or powder as a filler

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March 2020 TUBE PRODUCTS INTERNATIONAL