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What is Induction Heating?
Induction heating has been used on production lines for many years particularly in the metals, food, automotive and pharmaceutical industries. Now modern advances in solid state technology make this a simple, safe and cost effective method of on-site heating of conductive materials without the need for direct contact. Mannings Thermal & Environmental Engineers Limited explain how it works and some of the applications they have put it to in the power generation industry.
Discovered in 1831 Michael Faraday's monumental discovery of electromechanical induction in 1831 and how this could be used to generate electricity has had a profound effect on our everyday lives. Faraday demonstrated that, when a moving magnet is passed through a closed loop of wire it will induce an electric current in the wire. In modern transformers an alternating current is applied to the primary windings of the transformer and in accordance with Faradays law an electric current is induced in the secondary windings.
Induction heating occurs in a similar way since, when a material capable of conducting electricity is placed in a varying electric field the temperature of the material will rise. To apply this technology, induction coils are manufactured to suit the component to be heated and operations such as brazing, annealing, hardening, melting and bonding can be carried out rapidly and without the use of furnaces or any other method of direct heating.
This technology has many applications in the power industry, particularly during maintenance work where access to components may be limited but where safety and quality are of paramount importance.
Bolt Heating
At Innogy's Tilbury Power Station Mannings used patented induction heating equipment to safely and speedily remove the massive steel bolts which secure the HP and IP steam turbine covers. The HP turbine has 40 bolts which are each 5 inches in diameter and the turbine had not been opened up for at least four years.
The induction heaters used for bolt removal have a water-cooled wand which is inserted inside the bolt itself. A variable frequency power supply is then applied to the wand to induce heat in the bolt and cause expansion between the threaded areas. Because intense heat is directed into a small space in a short amount of time, heat transfer to the bolt threads as well as to the turbine casing is minimal. The resulting elongation of the bolts allows the securing nuts to be removed by hand or by normal hand tools. A major benefit of the water-cooled wand is that, once the operation is complete the wand is cool, and can be removed safely. Gary Kirby who is Turbine Mechanical Engineer at the Tilbury Power Station said "the main advantage to us of using induction heating is the safety aspect. The alternative is to insert a heated carbon rod into the bolt and this rod has to be removed by hand while it is still glowing red hot. Because of the working environment and high temperatures involved we consider this to be extremely dangerous and we welcome this new development".
Mannings expected, in this case, that all the bolts on the turbine would be seized. However using one induction heater and a standard 63 amp 415 volt power supply, each bolt was safely released in less than ten minutes.
Induction Brazing
The use of induction heating is also of particular advantage when used on the blades of turbo alternators since the heat can be accurately controlled and can be directed to where it is needed. This prevents the blades from overheating and avoids local hot spots which can result in hardening and cracks in the blades.
This proved to be of particular advantage to Dowding & Mills plc who are one of the UK's major electricity, electronics, mechanical and instrumentation service groups. Dowding & Mills were developing a process for brazing stainless steel de-tuning weights to turbine blades for British Energy at the Heysham 2 Power Station. It was necessary to find a method of carrying out the brazing without removing the blades from the rotor of a turbo-alternator and the work had to be completed within a tight time schedule.
It was considered that the use of a flame would not be suitable for brazing in this case due to the lack of control and close proximity between blades. The turbine rotor has 192 blades per row with only a 12mm gap between each blade.
Mannings were invited to propose a suitable means of brazing by means of induction heating and experimentation began to design a coil which would provide adequate heat distribution and control. They produced an induction heating coil which was designed to suit the curvature of the blade and could evenly distribute heat over the small 1½ x 1½ inch area. A thermocouple was attached to the blade and the temperature was raised to 630 degrees centigrade in ten minutes and held at this temperature. The coil was then removed and a light flame was used until the braze had formed a secure join.
Following these tests Dowding & Mills developed brazing procedures and methods and carried out extensive NDT and X-Ray tests to carefully examine the results. The work was then carried out within 20 days and the turbine could be put back on-line.
Dr Lol Miles who is Project Leader of the Turbine Support Group at Heysham Power Station said "this was the first time induction heating had been used at Heysham 2 and it proved to be very effective. We have now also used it for bolt tightening on the HP turbine and in-situ heat treatment of welds on the LP turbine blade". The rotor has now been re-installed and the turbo-alternator was put back into service on schedule.
Erosion Shield Removal
The removal and replacement of erosion shields on turbine blades poses similar technical problems. An erosion shield is a strip of metal which is attached to the tip of the last row of blades in a steam turbine to protect the blade against erosion by water droplets. This is necessary since the speed of the blade tip is near to the speed of sound and any water droplets which form before the steam enters the condenser can actually cause erosion of the blade. For this reason erosion shields which are made of a harder metal than the turbine blade, such as tool steel, are fitted and these must be replaced from time to time.
Mannings were able to use induction heating for Innogy Operations and Engineering at their TSG Workshop in Ferrybridge where overhaul of a turbine rotor for the Didcot A Power Station in Oxfordshire involved removal of erosion shields. Jeff Streets, who is Technical Engineer for Innogy said, "induction heating has proved to be an ideal solution and the controllability and accuracy of the process make it of considerable interest for future work".
Mannings were able to remove a total of 180 shields on each rotor within 4 hours. Steve Smith who is Induction Manager at Mannings said "as soon as the coil is placed on the erosion shield and switched on it is possible to see the solder start to melt. The power is then turned down and the area is maintained at a temperature of exactly 630 deg centigrade. The shield is then simply taken off and the end of the blade is cleaned up".
The alternative method of erosion shield removal is to flame heat the area. However this produces heat in a much larger portion of the blade and this should be avoided.
Heat Treatment of Rotors
One of the most recent applications for induction heating has been to stress relieve turbine rotors once they have been weld repaired. The conventional method has been to construct a temporary furnace around the rotor which involves considerable time and expense. However during heat treatment of these large rotors they must be continuously rotated to avoid the possibility of any bending taking place in the rotor. The non contact characteristic of induction makes this ideal for this purpose since static coils can be placed around the rotor to induce heat within it.
Up to 30 thermocouples are usually connected to a single rotor during the process and the signals are transmitted to a remote temperature data acquisition system from a transmitters attached to the rotor. This ensures that the correct temperature profile can be maintained.
The use of induction heating also cuts down the time required for this job from three days to only one day.
Future Applications
New uses for induction heat treatment are being constantly found due to the safety, quality and time saving benefits. Sreve Smith said "we have been discussing many interesting applications for this technology in a variety of industries and we are sure that it will replace many direct heating methods in years to come". Mannings specialise in heating systems and heat treatment equipment and supply products and expertise worldwide. They believe that they can solve any heating problem and new methods are continually being developed to suit modern heating applications.
Faraday's scientific work formed the basis of modern electromagnetic technology but even he could not have foreseen that the same principles which led to the development of the turbo-generator would also one day be used to keep them running safely.
For more information please e-mail us at services@mannings.uk.com
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