Preventive Maintenance Keeps Induction Systems In Peak Condition: Part 1
Maintenance of induction equipment can easily be ignored by assuming that everything is fine as long as the equipment is operating. Ignoring original equipment manufacturers' (OEM) recommended maintenance schedules can create problems that are not apparent from just looking at the system. It has long been understood that as much as 90% of induction-system problems are water related. High-conductivity water usually is the culprit that causes cooling system erosion due to electrolysis. This reaction causes the erosion of vital copper components resulting in the collection of contaminating material, which reduces water flow. Lower water flow allows the system devices that are being cooled to overheat and fail prematurely. This is most common in water-cooling systems where high electrical potentials are present, such as silicon-carbide rectifier (SCR) and diode heat sinks and chokes (reactors). Use of lake, well and city tap water to cool an induction power supply can reduce the service life of an induction system by 30 to 50%.
The following information is not intended to replace OEM equipment manuals, which usually cover induction heating equipment maintenance, but to provide easy-to-use checklists for regular system maintenance.
These usually are closed-loop systems that cool the power supply, capacitor (or heat station), water-cooled leads and bus, and the induction coils. It is important to remember that about 90% of the problems with induction systems are water related. This also is the most neglected item in induction, causing the most down time and damage. The following preventative maintenance will minimize water-related problems in an induction system.
Daily. Check the level of the water in the cooling system and top it off if necessary using only approved high-quality water. Never use well water, city tap water and high-conductivity water in the system as these will damage the system in a matter of weeks due to erosion or corrosion. OEMs recommend different types and qualities of water and additives (ethylene glycol, for example). Distilled, deionized (DI) and reverse osmosis (RO) are approved types of water, but usually require a glycol addition.
The glycol addition prevents the water from freezing and serves as a buffer to corrosion. A 30 to 40% ethylene glycol (uninhibited type) addition should prevent freezing in severe cold weather conditions, even during power outages. Glycol also prevents the water from becoming too aggressive to the materials in the system components. DI water by itself usually is too low in electrical conductivity (< 1 Kmho/cm) and is called "hungry." Always add glycol to plain DI water.
Every three months. Check the conductivity of the recirculating water using a hand-held conductivity meter. When the conductivity reaches or exceeds the OEM-recommended threshold, drain, flush and refill, then drain and refill the system again. Purge the system to remove trapped air. (See OEM manual for recommended cleaning and flushing practices.) Damage caused by poor water quality and system charging can result in tens of thousands of dollars in repair costs. Some power supplies have replaceable targets (anodes), which should be checked and replaced if necessary.
An example of the cooling water specification for recirculating water used to cool induction power supplies that do not use targets is 15 ppm total water hardness (CaCO3), 25 ppm total dissolved solids, 20 - 70 Kmho/cm conductivity, 10 ppm maximum suspended solids and 7.0 - 7.5 pH. An example of the cooling water specification for recirculating water used in an induction power supply that has replaceable targets is 100 ppm total water hardness (CaCO3), 200 ppm total dissolved solids, 50 - 300 Kmho/cm conductivity, 10 ppm maximum suspended solids and 7.0 - 7.5 pH.
Every spring. For maximum cooling efficiency, all cooling water systems should be drained and flushed with fresh clean water for a few hours, drained again, then recharged. Also, clean the "Y" strainers in the system at this time. Remove a few hoses during draining and cleaning, especially where hoses are looped, and inspect for erosion and deposits on the copper fittings and inside the hoses at both ends. Clean hoses indicate good system maintenance. The presence of corrosion and deposits indicates the need for more frequent conductivity testing, flushing and cleaning. Corrosion usually is due to water solutions that have degraded, allowing the conductivity to rise.
Spring and fall. If necessary, adjust the temperature control sensor for the recirculating water-cooling system to prevent condensation from forming. The water-cooling controls usually have an adjustable set point connected to a solenoid valve that controls the temperature of the cooling water. In winter conditions, cooling water generally is controlled to operate at a temperature of about 75F (24C). Summer conditions often require a higher temperature (about 85F, or 30C) to operate above the higher dew point (higher summer humidity) and prevent damage caused by condensation. The presence of condensation in the power system can create destructive paths around high-voltage components.
Every other year in the spring. Remove the water to water heat exchangers for both the water-cooling system and the quench system. Use a solution of humidifier cleaner and a low-pressure pump to circulate the fluid through both paths of the heat exchanger to remove calcium scale build-up, which can restore full cooling efficiency of the heat exchanger. This requires using a small valve to regulate flow and some hoses for connection. Pumping for two hours usually is sufficient. However, the heat exchanger and the humidifier cleaning fluid may need to be replaced if there is a large amount of calcium inside. This cleaning method is preferred because the humidifier cleaner is user friendly and nontoxic.
Cooling towers are designed to remove heat from water and dissipate the heat into the atmosphere. Two common types are dry towers and evaporative towers.
Dry tower systems consist of a motor and pump with an expansion tank and tower, which is made up of cooling tubes, fins and fans. They are low maintenance systems compared with evaporative systems, but lack the cooling capabilities of the latter. Yearly maintenance of a dry tower requires the following:
- Drain the water in the spring, flush with clean water and recharge with a 30 - 40% ethylene glycol-water solution (depending on geographic location) to prevent damage due to corrosion and freezing
- Remove dirt from the surface area of the cooling fins using a high-pressure power washer to restore the tower to maximum cooling efficiency
- Lubricate the pump and motor as required and check seals and gaskets for leaks
- Lubricate the fan bearings and bushings as required
Evaporative tower systems, consisting of a recirculating pump, heater, sump pump, fans and cooling fins, require more maintenance than dry types. Yearly maintenance is the same as for dry tower systems plus the following:
- Lubricate the pump(s), motor, bearing and bushings
- Check seals and gaskets for leaks.
- Remove dirt that has accumulated in the sump pump and reservoir.
- Check to ensure that the sump pump heater is functioning properly. This is very important because the heater prevents freezing of the sump pump water during winter operation.
Suggested water quality requirements for both dry and evaporative cooling towers are 100 ppm total water hardness (CaCO3), 200 ppm total dissolved solids, 20 - 300 Kmho/cm, 10 ppm suspended solids and 7.0 - 7.5 pH.
Power supplies and heat stations
The power supply and heat station, which are the most expensive parts of a production induction heating system, are the most likely to be ignored. Maintenance requirements and frequency schedules are given below.
- Inspect inside the cabinet for water leaks, drips and condensation. This must be done with the cooling pump on, at the beginning of the shift after the unit has been idle all night, and again at the end of the shift while the system is warm. If tightening a hose clamp cannot stop a leak, replace the hose. Never reduce the length of a hose; some hose connections have a specified length and may have loops to prevent electrical conduction and subsequent electrolysis of the copper tube at the end of high-voltage dc potentials.
- Test the ground leak detector; see the OEM manual for details.
- Check for loose connections of all ribbon connectors and wires on terminals.
- Tighten loose connection using only a screwdriver; never overtighten.
- Visually inspect for overheating (discoloration) of bus connections, SCRs, diodes and other components including transformer and capacitor connections, and tighten using appropriate tools. Do not over- torque SCRs. If the system has mechanical contactors, inspect the contact pads and springs and adjust or replace worn components.
- Test all door interlocks. Check the OEM manual as each manufacture has its own requirements for this very important safety test.
- Inspect the power supply and heat station using infrared thermal imaging, which identifies hot spots and provides information that can be used to identify potential problems during operation.
- Check power-supply battery backups used for logic memory; some batteries require replacement annually, while others have a five-year life. Check the OEM manual.
- Check water-cooling hoses. Hoses become brittle and are susceptible to leaks and complete failure. Depend-ing on many factors, such as ambient heat and humidity, water-cooling hoses may need replacement every five to ten years. Use only the same type of nonconductive hose as originally supplied, and never shorten the length of any hose when replacing it.
- Wipe the inside of the power supply and heat station interior using plain hot water and detergent to remove any loose dirt. Allow the unit to dry before starting it. Rust spots on the floor of the cabinet should be removed by sanding and painted using white enamel, which makes it easier to detect leaks later. Never operate the system with the doors open and never use cooling fans to cool the inside (to prevent the unit from overheating during extreme hot weather). This practice allows a large amount of dirt to get into the unit, which is not easily removed.
Every five years: Replace all door gasket material to keep a good seal and to keep dirt out.
AcknowledgmentsThe authors thank all Ajax Magnethermic Engineering, R&D, Sales, Customer Service Department, Park Precision and Ajax Induction Services personnel and customers who contributed to this article.
For more information: George Welch is product manager, Heat Treating, Ajax Magnethermic Corp., 1745 Overland Ave., Box 991, Warren, OH 44482; tel: 330-372-8511; fax: 330-372-8608; e-mail: firstname.lastname@example.org.
Part two of this article covering maitenance of other induction system components will appear in the August 2001 issue.