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Induction coil life varies from coreless furnace to the next, but furnaces with properly maintained coils generally achieve longer lining life, too.

Maximizing Lining Life and Coil Life

May 10, 2018
In a coreless induction melting furnace, promoting longer lining life can also improve coil life, resulting in less downtime, lower production cost-per-ton, and reduced maintenance costs.

Many people don’t realize that maintenance is interrelated for induction-furnace linings and coils: Short lining life and coil life result in increased downtime, costs, and frequent coil changes. To understand this correlation better, let’s focus on a coreless furnace melting ferrous materials, with silica refractory as the working lining. The observations would be similar with nonferrous materials and an alumina-based refractory. 

Over the years, melt shops have reported coil life as short as one year or less while others report coils lasting three to five years, or even longer. Generally, melt shops reporting longer coil life cycles experience good lining life campaigns too. 

Let’s review reasons for a shorter lining life, and its effect on the coil life. 
1. Selecting refractory material. The refractory material must be suitable for the material to be melted and its pouring temperature. Refractory manufacturers customize the amount of binding agent or binders according to the desired pouring temperature. The melt shop should consult with the manufacturer for proper binder content in the refractory. Too much binder can cause the lining to sinter all the way to the coil; too little binder will not give the refractory a sufficiently thick hot face. Short lining life due to finning or severe erosion may result from improper refractory material. 

2. Installation procedure. The manufacturer’s instructions for installing the refractory should be followed precisely. Silica linings are installed using vibrating tools to compact the refractory between the coil and melt-out lining form. The wall thickness of the melt-out form must be thick enough to ensure that the form does not buckle or deform during vibration—or prematurely during the sinter. The thickness and shape of the form should conform with the furnace OEM’s recommendation. Charge material or weights should be placed inside the form to prevent it from lifting during vibration, which otherwise can cause a void under the form. Not following the proper installation procedure can
result in low silica density, causing metal penetration and excessive erosion. Improper installation will shorten lining life. 

3. Drying coil grout before lining. If the coil grout cement is replaced, this new grout material must be dried properly before installing the lining. Consult the grout refractory manufacturer to obtain the proper dry-out schedule. Improper dry-out may cause the grout to lose strength and allow premature cracking. Failure to dry out the grout can leave excess moisture at the coil, which can lead to coil arcing or grounding during sintering.

4. Sintering procedure. The sintering procedure for the lining must be followed precisely. Rushing the sinter and dry-out procedure will not allow sufficient time for moisture in the refractory to escape, causing excessive moisture next to the coil, which leads to coil arcing or the coil going to ground. A proper procedure ensures the binder in the refractory creates a proper working face. Not following the sintering schedule will result in short lining life. 

5. Taking temperature of the bath. The pouring temperature of the molten metal in the furnace must always be measured with a calibrated immersion dip thermocouple rather than by eye, or by guessing. Overshooting the pouring temperature by 100°F beyond a target pour temperature of 2,800°F, for example, will cause a much shorter lining life than pouring at the correct temperature. The higher temperature will cause higher refractory wear, resulting in a short refractory life. 

6. Ground leak detection. The furnace ground leak detection system must be functioning and tested daily for proper operation. Under no circumstances should a coreless furnace be operated without a fully functioning ground and leak detection system. Furnaces without proper ground leak detection risk metal penetrating to the induction coil which can cause coil arcing and resultant water leakage, and possible catastrophic explosion. A handheld furnace ground leak detector tester is available for
Inductotherm coreless furnaces to verify that the molten metal bath is properly grounded. This test must be performed at every heat to ensure safe operation. 

7. Improper charging techniques. Dropping extra-large pieces into the furnace can crack the lining, creating a run-out during melting. Bridging also can occur due to improper charging, causing molten metal under the bridge to superheat beyond the refractory’s maximum temperature capability. 

Superheating under a bridge condition can result in a run-out and cause a molten metal-water explosion. Many melt shops use automated charging with conveyors that keep up with the melt rate of the furnace. Not keeping up the charge procedure with the melt rate can cause excessive temperature in the molten bath, eroding the refractory and shortening life. An improper charging technique can damage the refractory, resulting in a short lining life. 

8. Improper cold-start procedures. Improper cold-start procedures can lead to short lining life due to molten iron getting into lining cracks. Melt shop operators should consult the refractory manufacturer for the proper cold-start procedure for their particular furnace size. 

9. Lining removal systems. Many melt shops use a mechanical device to push out the refractory lining. Others with older furnaces still use chisels to break up and remove the lining manually. If a worker is not careful when using the chisel hammer while removing the lining, the induction coil tubing can be damaged to the point of a water leak. Improper lining removal will cause short coil life. 

10. Furnace coil sweating. Modern melt shops use closed water-cooling and recirculating systems to conserve water and reduce costs. Some older cooling systems don’t have controls that keep the cooling water to the furnace and power supply above the dewpoint. When the cooling water in the system gets below the dewpoint, sweating can occur inside the power supply and on the induction coil. Moisture on electric conductors in the power supply can cause arcing across the components, such as SCRs, diodes, transformers and capacitors, and may cause component failure. 

The sweating that occurs on the furnace coil also may allow electrical arcing across the coil turns and to ground. Arcing can be severe enough to cause a turn-to-turn short with a resultant water leak. 

Newer, closed-loop cooling systems incorporate a temperature diversion valve placed in the recirculating system so that the cooling water bypasses the heat extraction device (cooling tower, dry air cooler, etc.) to allow the water to maintain a temperature above 75°F which is typically above dewpoint. Coil sweating and resultant coil arcing obviously affect overall coil life span. 

11. Preventive maintenance (PM). The furnace manufacturer should be contacted to obtain the recommended PM schedule and procedure for the power supply, furnace, and associated equipment in the melt department. 

If the furnace assembly is not properly maintained, short coil life will result. If magnetic shunts are used in the furnace design, they need to be torqued periodically to ensure they stay in the proper location supporting the coil. The torque procedure should comply with the furnace manufacturer’s recommendations. Loose shunts can move during operation and come in contact with water connection pipes and hose barbs, causing arcing and coil grounding. 

Power lead and hose connections need to be checked for leaks during PM. Water leaks can cause coil arcing, resulting in short coil life. In addition to power leads and hoses, other furnace components such as top cast blocks, Faraday rings, tie rods, furnace covers and cylinders should be checked during PM. 

12. Monitoring lining wear. Most melt shops check the refractory-lining wear regularly. Many take physical measurements of the lining after the lining has cooled off, or between shifts. 

Some melt shops also have computer controls that monitor and record furnace lining wear over time. This information along with physical dimensions is useful in predicting when the lining needs to be changed. 

Melt shops that don’t monitor lining life risk having a run-out that would no doubt cause coil damage. 

13. Lining life vs. Coil life cost-per-ton. The above charts illustrate the approximate refractory and coil change-out repair cost per ton of melted metal based on a 2500 kW, 200 Hz batch melting system melting iron on a two-shift, five-day basis, 50 weeks per year. It is assumed that the iron pouring rate is about four tons of iron per hour for the comparison. 

As shown in the charts, melt shops experiencing both short lining life and short coil life can have a cost per ton more than three times those that experience long lining and long coil life campaigns. Those experiencing “life on the low side” should consider all of the above factors that contribute to poor life. Reviewing current procedures may uncover areas for immediate improvement in both lining life and coil life. 
Charles Fink is Vice President Customer Relations and Training at Inductotherm Corp. Contact him at [email protected] or visit www.Inductotherm.com