Guidelines for proper coreless furnace maintenance.
Providing for the most efficient operation of induction melting and holding equipment and maximizing its useful service life are worthy goals for a foundry maintenance program. The most important goal, however, is the safe operation of equipment and the protection of workers and visitors. Poor, improper or delayed maintenance is a major contributor to accidents involving induction equipment in foundries.
This article provides maintenance officials with some of the key considerations in maintaining a safe, efficient melt operation so that site-specific maintenance procedures can be drafted.
Basic Furnace Structure
In a coreless furnace, copper coils encircle a layer of refractory material surrounding the entire length of the furnace interior. Running a powerful electric current through the coils creates a magnetic field that penetrates the refractory and quickly melts the metal charge material inside the furnace. The copper coil is kept from overheating by cooling water flowing through it [ILLUSTRATION FOR FIGURE 1 OMITTED].
The coreless induction furnace operates at low, medium and high frequencies from 6010,000 Hz and offers foundries the greatest melting flexibility. It can be started cold and usually is poured empty, simplifying alloy changes and enabling the furnace to be shut down as desired.
Shell and Coil Maintenance
In induction furnaces, systematic scheduled attention to general inspection, cleaning and adjustments can prevent equipment failure and loss of production time.
General housekeeping around the furnace and melt deck is important to maintaining a safe and dependable induction melting operation. Check cleanliness of the area around the furnace daily. Do not allow slag or metal scrap to touch furnace power leads, as hot slag or charge material can cause power Dead failure.
Furnace shunt and tie rod bolts should be checked monthly to ensure they are sufficiently tight.
Coils [ILLUSTRATION FOR FIGURE 2 OMITTED] should be inspected monthly for signs of arcing, overheating or discoloring on the coil insulation. Inspect wood\composite coil supports and termination blocks for charring. Remove any slag or metal chips that have accumulated inside the furnace shell. Overheating the coil can cause coil insulation to deteriorate and develop coil-to-ground or turn-to-turn problems.
Inspect all water connections for signs of leaks and test furnace water conductivity to ensure that the water meets the manufacturer's specifications. Without continuous cooling, induction furnaces cannot operate, and any event that interferes with normal furnace cooling can quickly lead to equipment damage or personal injury. Therefore, induction furnaces should have a backup cooling system, such as a battery-powered or engine-powered water pump or city water connection that can be engaged if normal pump operation fails. The proper operation of backup cooling systems should be checked regularly to avoid possible damage to equipment that could lead to a water/metal explosion.
Daily visual inspections of the hydraulic system by maintenance personnel can prevent severe injury and property damage. Fire-resistant fluids should be used with induction furnaces to prevent this danger. Any hydraulic leaks should be corrected immediately and fluid cleaned up. A hazard exists whenever heat, molten metal or flame is near hydraulic equipment.
Proper and well-maintained refractory linings [ILLUSTRATION FOR FIGURE 3 OMITTED] are important to the safe operation of any furnace. In induction furnaces, they are absolutely critical.
Electrical induction physics demand that the refractory lining between the induction coils and the bath is as thin as possible. At the same time, it must be thick enough to fully protect the coils and prevent metal runout in the face of attacks by molten metal, chemical agents and mechanical shocks.
Assuring that the furnace lining remains safely within manufacturer-specified limits requires careful treatment of the lining during all furnace operations as well as comprehensive inspection and monitoring procedures. Without question, metal runout ranks among the most severe accidents that can occur during melting and holding operations.
Runouts occur when molten metal breaks through the furnace lining. If cooling, electrical, hydraulic or control lines become damaged, there may be an imminent danger of a fire or water/molten metal explosion. Proper furnace maintenance is the key to maintaining the integrity of the furnace lining and thus, preventing a runout.
The integrity of the furnace lining can be compromised by:
* inadequate or improper installation of refractory;
* failure to monitor normal lining wear and allowing the lining to become too thin;
* the sudden or cumulative effects of physical shocks or mechanical stress;
* the sudden or cumulative effects of excessive temperatures or thermal shocks;
* slag or dross buildup.
Any of these situations can cause a metal runout. Therefore, careful attention and proper maintenance of a furnace's lining is vital to safe melting and holding.
Proper Installation - Proper lining installation is as important to a safe operation as refractory material selected. If the refractory is inadequately compacted during installation, voids or areas of low density may form, creating a weak spot easily attacked by the molten metal. If the crucible is created with a form or ram that is improperly centered, or one that has somehow been distorted during storage or shipment, lining thickness will be uneven. As a result, the lining may fail before the end of its predicted service life.
It is especially critical that the refractory supplier's procedures for drying and sintering be precisely adhered to and never hurried. If sufficient time is not allowed for the refractories to bond, the lining is more prone to attack by molten metal and slag.
Monitoring Normal Lining Wear - Refractory linings and crucibles are subject to normal wear as a result of the scraping action of the metal on the furnace walls. This is due largely to the inductive stirring action caused by the induction furnace's electromagnetic field [ILLUSTRATION FOR FIGURE 4 OMITTED].
In theory, refractory wear should be uniform, but in practice, lining wear is irregular. The most intense wear occurs at three locations - the slag/metal interface, where sidewalls join the floor and at thin spots caused by poor lining procedures.
The entire furnace should be visually inspected whenever it is emptied. Special attention must be paid to the high-wear areas described above, and observations should be accurately logged.
Although useful, visual inspections are not always possible. Further, visual inspection alone can't reveal all potential wear problems. The presence of a low-density refractory area can escape notice during visual inspections, These limitations make lining-wear monitoring programs essential.
Directly measuring the interior diameter of the furnace provides excellent information about the lining's condition. Ideally, a base-line plot should be made after each feline. Subsequent measurements will show the precise rate of lining wear or slag buildup. Determining the rate at which the refractory material erodes makes it possible to schedule relining before the refractory material becomes dangerously worn.
In situations where visual inspections of coreless furnaces are impossible, (for example, when they are used for continuous holding and dispensing), operators should remain alert to the following vital warning signs of lining wear:
* attainment of maximum power at a lower-than-normal applied voltage;
* in a fixed frequency power supply, an increase in the number of capacitors needed to be switched into the circuit to maintain unity power factor;
* in a variable frequency power supply, running at a higher than normal frequency.
Useful though they may be, changes in electrical characteristics must never be used as a substitute for physical measurement of the lining itself. Regardless of the system used to monitor lining wear, it is essential to develop and adhere to a standard procedure. Accurate data recording and plotting helps assure maximum furnace utilization between relinings, while minimizing the risk of using a furnace with a dangerously thin lining.
Physical Shock and Mechanical Stress - The sudden or cumulative effects of physical shocks and mechanical stress also can lead to a refractory lining failure.
Most refractories tend to be relatively brittle and weak in tension. Bulky charge material dropped into an empty furnace can easily cause the lining to crack upon impact. If such a crack goes unnoticed, molten metal may penetrate, leading to a runout with the possibility of a water/molten metal explosion. One option are remotely controlled, automated charging systems that are engineered to place charge materials into the furnace without damaging its lining.
Mechanical stress caused by the different thermal expansion rates of the charge and refractory material can be avoided by assuring metal does not become jammed within the furnace. Except when done for safety reasons (such as dealing with a bridge), the melt must never be allowed to solidify in the furnace. In the event of a prolonged power failure, a loss of coolant or other prolonged furnace shutdown, the furnace should be emptied.
Excessive Temperature/Thermal Shock - Refractories must be used only in applications that match a product's specified temperature ranges. Should actual furnace conditions heat or cool the lining beyond its specified range, the resulting thermal shock cart damage the integrity of the lining. Cracking and spalling can be early warning signs of excessive thermal shock and a potentially serious metal runout.
The best way to avoid overheating is to monitor the bath and take a temperature reading when the charge liquefies. Temperatures exceeding the refractory's rating can soften its surface and cause rapid erosion, leading to catastrophic failure. The high heating rates of medium-frequency coreless furnaces enable them to quickly overheat. Kilowatt-hour counters, timing devices and computerized control systems can help prevent accidental overheating.
When working with a cold holding furnace, be sure it is properly preheated to the refractory manufacturer's specifications before filling it with molten metal. In the case of melting cold-charge material, slowing the rate of the initial heat-up until metal becomes molten minimizes the risk of thermal shock to a cold furnace. The gradual heating of the charge allows cracks in the refractory to seal over before they can be penetrated by molten metal. Practices for cooling a furnace following a melt campaign also should follow refractory supplier recommendations.
Managing Slag - Slag (which forms when rust, dirt and sand from the charge, and eroded refractory separate from the melt and rise to the top of the bath) is an unavoidable by-product of melting metal. Chemical reactions between the slag and the melt increase the rate at which the lining erodes.
A highly abrasive material, slag erodes away refractory near the level of the molten metal. In extreme circumstances, this erosion may expose the induction coils, creating the risk of a water/molten metal explosion. Refractory linings in this condition should be removed from service immediately.
Ground Leak Detector
The ground leak detector system is crucial to safe melting and holding. The system, which includes both a ground detector circuit associated with the power supply and a ground leak detector probe located in the furnace, provides important protection against electrical shock and warning of metal-to-coil penetration [ILLUSTRATION FOR FIGURE 5 OMITTED], a highly dangerous condition that could lead to a furnace eruption or explosion.
Key to this protection in furnaces with rammed linings or conductive crucibles is the ground leak detector probe in the bottom of the furnace (as shown in [ILLUSTRATION FOR FIGURE 1 OMITTED]). The probe is composed of an electrical ground connected to several wires that extend through the refractory and contact the molten bath or a conductive crucible. The system serves to electrically ground the molten metal bath.
In some small furnaces with nonconductive, nonremovable crucibles, the ground leak detector probe takes the form of a wire cage located between the crucible and coil. This wire cage serves to ground the bath if metal penetrates through the crucible.
Both probe configurations provide shock protection to melt deck workers by assuring that no voltage potential exists in the molten bath. If molten metal were to touch the coil, the ground leak detector probe conducts the current from the coil to the ground. A ground detector module detects such an event and shuts the power off to stop any coil arcing. This also prevents high voltage from being carried by the molten metal or furnace charge. Otherwise, such high voltage would cause serious or even fatal electrical shock if the operator were to come into conductive contact with the bath.
The electronic circuitry in the ground leak detector circuit continually monitors the electrical integrity of the system. This circuit turns off power to the furnace if any improper ground or metal penetration is detected in the induction system. This is crucial to furnace safety. If the furnace refractory lining or crucible cracks or otherwise fails and a portion of the metal bath touches the energized furnace coil, the coil could arc and rupture. This could allow water into the bath, causing a molten metal eruption or explosion.
To keep the ground leak detector probe working properly in a rammed lining furnace, care must be taken when installing the lining to ensure that the ground leak detector probe wires come into contact with the lining form. Also, when patching a furnace lining, it is essential that the ground leak detector probe wires remain exposed, permitting contact with the furnace charge.
Testing the integrity of the probe requires the foundryman to take measurements using a special instrument that verifies that the molten bath is grounded. In rammed lining furnaces and furnaces with conductive crucibles, the frequent checking of probe wires is critical. These wires, located at the bottom of the furnace, can easily be buried during relining, covered with slag, burned off or otherwise damaged. Failure to ensure that the ground leak detector probe wires provide a sound ground contact will result in the loss of protection for the operator and furnace provided by the ground leak detector system.
The melting system's ground detector circuit also should be checked at least daily. In a typical system, this can be done by briefly simulating an actual ground fault.
Because of the crucial safety functions ground leak detection systems have in coreless induction melting and holding, furnaces should not be operated without a fully functional ground leak detection system. (Removable crucible furnaces and certain vacuum melting systems may be operated safely without ground leak detector systems.)
Spill Pit Maintenance
The condition of the spill pit should be checked at the start of each shift. No induction furnace should be operated without adequate, carefully maintained and dry spill pits [ILLUSTRATION FOR FIGURE 6 OMITTED]. Located under and in front of the furnace, these pits safely contain any molten metal spilled from the furnace as a result of accident, runout or dumping of the furnace in an emergency.
Without adequate pits, spilled molten metal would flow across the foundry floor, endangering workers and damaging furnaces, other equipment and structures. This free-flowing spilled metal also could produce devastating fires and explosions.
Spill pits must be kept clear of debris and flammable materials. Pit covers must be kept clear of slag and other blocking materials that could interfere with the passage of molten metal. Metal from any minor spills should be regularly removed from the pits to ensure that adequate capacity is maintained. If your spill pit is properly sized, dry and clear of debris, you can operate your furnace with the confidence of knowing that if an emergency occurs, you can safely dump any molten metal from the furnace into the spill pit.
Safety Maintenance Checklist
Below is a sample, safety-oriented maintenance checklist. While it does not cover every situation, it should be used as a starting point in preparing a checklist for your own induction system(s). Specific maintenance procedures should be based on the specific maintenance recommendations of your equipment and refractory suppliers.
WARNING: Do not perform any maintenance on the system with the power on. Place the cabinet switch or circuit breaker off. Lock or secure input power (circuit breaker switch) to off to prevent accidental live power on to the system. Be sure capacitors are discharged and circuit breaker is off.
Always use two independent methods to support a tilted furnace whenever working on or near it. A structural brace strong enough to keep the furnace from dropping if hydraulic pressure is lost must be used when working on a tilted furnace.
DAILY MAINTENANCE SAFETY CHECKLIST
* Check for, and correct, any water leakage from the furnace and power supply cooling system(s).
* Check to ensure that the primary and emergency cooling systems for the furnace(s) are operating properly.
* Check for signs of condensation (wipe clean with lint-free rag).
* Check connections and general cleanliness at hydraulic connections.
* Check cleanliness around the furnace - do not allow slag or water to touch furnace leads. Hot slag or charge material can cause power lead failure.
* Check operation of ground leak detector. Make sure the metal bath is grounded. Failure to ensure that the ground leak detector probe wires provide a sound ground contact will result in the loss of crucial protection for the operator and furnace. Test the ground leak detector on the power supply.
* Check furnace refractory for mechanical or thermal damage and repair or replace per refractory supplier's specifications. (Check each time the furnace is emptied.)
* Check furnace lining for excessive erosion in the high-wear areas such as the slag/metal interface and where sidewalls join the floor. Also check for excessive slag or dross buildup. Repair or replace per refractory manufacturer's specifications. (Check each time the furnace is emptied.)
* Check spill pit for any signs of debris, flammable materials and moisture. Do not operate furnace with wet spill pits.
MONTHLY MAINTENANCE SAFETY CHECKLIST
After removing the inspection panel covers from the furnace:
* Remove any slag or metal chips that have accumulated inside the shell or case.
* Check coil for signs of overheating or discoloration. Overheating the coil can cause coil insulation to deteriorate and develop coil-to-ground or turn-to-turn problems.
* Inspect all water connections for signs of leaks. Water leaks of the furnace coil cause high ground readings and possible coil damage.
* Inspect all hoses and leads for loose connections. Tighten or repair as necessary.
* Wipe all hydraulic rams and check for and tighten any loose connections.
After replacing the furnace panel covers:
* Inspect the water and hydraulic filters. Remove and replace if needed.
* Inspect the furnace lining for signs of deterioration, cracks or metal penetration. Repair or replace per refractory manufacturer's specifications.
* Inspect furnace leads for signs of external water jacket cracks or deterioration. Clean, repair or replace furnace leads that show signs of excessive oxidation, distortion, cracks or leaks.
* Remove and replace hoses that leak or show signs of fatigue.
* Before returning the unit to operation, make sure that all cleaning materials and flammable solvents have been removed.
* Tighten shunt and tie rod bolts per furnace manufacturer's specification.
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|Title Annotation:||includes related article on safety maintenance checklist|
|Date:||Apr 1, 1999|
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