The materials and process technology for manufacturing high quality steels is complex, with finished steel being produced from iron ore in several steps. In addition to coke, iron ore, alloy metals and various additives such as limestone, gravel and dolomite, large amounts of auxiliary operating materials are required. Most importantly, water and steam for heating, cooling, cleaning and rinsing. Chemicals and oils are also required for pickling and other refining processes.
Due to the amount of water consumed, steel works are often built on rivers or the coast to guarantee the water supply as well as for logistical reasons. Long pipeline systems are required to supply and dispose of the different materials mean that many different valve types must be provided to support the systems. The processes in the coking plant, the blast furnace, the pickling plant and the electroplating shop are outlined below and the valves used there described in more detail.
The coking plant
Before the blast furnace can be supplied with iron ore, coke and additives, the raw coal first has to be turned into coke. The volatile components are removed in the coking plant. This takes place in a process which lasts approximately 20 hours. The coal is heated indirectly in a furnace and then extinguished or quenched with water before removal. Up to 2 cubic metres of extinguishing water can be required per ton of coke. Butterfly valves of the Desponia type, butterfly valve body cast iron (GG25) or S.G. iron (GGG 40.3) epoxy coated with EPDM liner are used for supplying the necessary extinguishing water. The GEMÜ 520 globe valve with mounted GEMÜ 1435 positioner is used for regulation.
Contamination by dust and hydrogen sulphide makes cleaning of the waste water and steam problematic. The butterfly valve bodies in the flue gas and waste water lines are therefore made of GGG 40.3 coated with Rilsan, the packing is NBR. Pneumatically actuated GEMÜ 620 diaphragm valves and GEMÜ 628 with motorised control actuator and PP-lined S.G. iron bodies and PTFE diaphragms are used in parallel. The diaphragm valves are less sensitive to solids and flue gas particles. The nominal sizes of the butterfly valves are between DN 350–1400, diaphragm valves between DN 50–100 and the globe valves DN 50–150. In the fresh and waste water areas, the operating pressures are between 6 and 10 bar, the temperatures are up to 80°C. The pneumatically actuated butterfly valves are usually equipped with position indicators or positioners. In some places, the valves and electrical components must meet the requirements for explosion protection.
The blast furnace
Raw iron is melted from ore in a continuous process in the blast furnace. Medium sized blast furnaces have an output of approximately 6000 tons, large blast furnaces up to 13 000 tons of raw iron per day. This process therefore places very high demands on the reliability of plant components. The blast furnace is fed with the ‘charge’ (eg, limestone, gravel and dolomite) as well as coke, iron ore and additives. The charge reduces the melting point of the ores and serves for better liquefaction of the mixture.
At the base of the furnace, the carbon monoxide formed from coke and oxygen in the air oxidises to carbon dioxide. The necessary oxygen is drawn from the iron oxide which is reduced to iron as a result. For continuous operation, hot air is blown in at a constant temperature of 1200°C through the tapping opening. It is heated to about 1300°C in the air heaters and then mixed with fresh air to keep a constant 1200°C.
Temperatures up to 2000°C occur in the blast furnace in the air feeder section. The blown air is fed and regulated by temperature resistant, metallic sealing butterfly valves and the fresh air by butterfly valves of the Desponia or Saturnia series, equipped with GEMÜ 1435 positioners.
The outside wall of the blast furnace is cooled constantly. Modern furnaces are equipped with several closed cooling circuits to minimise the amount of water and energy lost. DN 100 GEMÜ 512 and GEMÜ 520 globe valves with GEMÜ 1435 and GEMÜ 1436 positioners and process controllers are used for cooling water control. Butterfly valves of the Desponia type are used for bigger sizes. Plastic diaphragm valves of the GEMÜ 690 and GEMÜ 677 types with PTFE diaphragms as well as PVC-U or PP valve bodies of DN 25–50 are mainly used for water treatment.
The pickling plant
The raw iron is refined into steel in further process steps. To do this, it is mixed with steel scrap and alloy components and melted again in special furnaces of different types. The liquid steel is drawn from these furnaces in a continuous casting process. From there it reaches the hot rolling mill directly where it is rolled into steel section or steel band depending on the plant setting. An oxide layer forms on the surface of the steel during rolling.
The oxides are partly broken and dissolved by the mechanical stress in the rolling mill. This simplifies the downstream chemical pickling. There, the oxides are removed by hydrochloric or sulphuric acid as well as caustic soda. The processes take place at operating pressures up to 6 bar and temperatures between 20 and 90°C. Pickling is followed by rinsing at a station in which the chemicals and the dissolved scale coating are washed off by desalinated water. Pickling is followed – depending on the desired end product – by further processing steps such as rolling, cutting and surface coating by galvanic or other application processes.
For distribution of the clean acids, centric butterfly valves of the Bianca type are used. The valve body is made of GGG 40.3, the liner of PTFE and the shaft seal of PFA. The acid contaminated by hard scale particles is usually disposed of via diaphragm valves of the GEMÜ 675 and GEMÜ 620 series. The valve bodies are also made of GGG 40.3 lined with PFA, the shut-off diaphragm is PTFE.
The diaphragm valves are relatively insensitive to the hard oxide particles. Clean rinsing water and concentrated caustic soda for neutralisation are distributed via butterfly valves with EPDM seal and Halar coating. The rinsing water is disposed of via diaphragm valves because of the particle contamination. At temperatures up to 80°C, plastic diaphragm valves GEMÜ 690 and GEMÜ 677 with valve bodies made of polypropylene (PP) and shut off diaphragms made of PTFE are also used for caustic soda. The advantage of the plastic lined S.G. iron body is the higher pressure and temperature resistance to solid plastic valves. At GEMÜ, the metal valve bodies are lined in injection moulding machines. This produces a very high quality, long lasting lining with a defined and constant material strength. Glass lined bodies are rarely used in steelworks today because of their sensitivity to impacts.
A part of the hydrochloric acid reacts with the iron oxide during pickling. The chemical reaction is as follows:
Fe2O3 + Fe + 6 HCl → 3 FeCl2 + 3 H2O
Fe3O4 + Fe + 8 HCl → 4 FeCl2 + 4 H2O
FeO + 6 HCl → FeCl2 + H2O
Pickling is followed by regeneration of the acid. In the process, FeCl2 is oxidised to Fe2O3 and HCl in a spray roaster. The hydrochloric acid is fed back into the pickling process, the acid is agglomerated, pelletised and fed back into the blast furnace. The chemical reaction is as follows:
4 FeCl2 + 4 H2O + O2 → 2 Fe2O3 + 8 HCl
Wherever the pneumatic valves have to perform control tasks, they are fitted with GEMÜ 1435 and GEMÜ 1436 positioners and process controllers. Many customers prefer the GEMÜ 1435 for the rugged environment of a steelworks because of its robust metal housing. The steam required for heating in the processes is controlled by GEMÜ 512 and GEMÜ 520 globe valves. The valves are also equipped with the above positioners.
Surface coating
Part of the surface refining also takes place in a today’s modern steelworks. Galvanic processes are mainly used for this. Finished rolled bands run through the respective baths and process steps in a continuous process. They are then delivered as coated coils for further processing. The popularity of chemically resistant plastic valves began in galvanising industries of Europe with the invention of the first plastic solenoid valve by Fritz Müller in 1964. Solenoid valves, diaphragm valves, butterfly valves and ball valves made of PP, PVC and PVDF are used depending on the process media. Depending on the type of valves, the nominal sizes are between DN 15 and DN 100. Solenoid valves from DN 2 are also available for smaller volumes and dosing tasks. In continuous processes, they are integrated into plant automation via electric position indicators, switchboxes, positioners and fieldbus interfaces.
Steelworks usually incorporate a large amount of automation due to their round the clock operation. Therefore, pneumatic and electric actuators are mainly used with the appropriate automation components and controllers. All GEMÜ valves and automation components have a very high quality standard. They are ideally adapted to each other and therefore guarantee a long life and high plant availability. Another advantage for the plant manufacturer and owner is the fact that GEMÜ’s wide range of products allows a large number of the valves used in the steelworks to be ordered from one source.
For more information contact Keith Hedges, Gemü Valves Africa, +27 (0)11 462 7795, [email protected], www.gemue.de
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