Preventing Magnetic Particle Tank
Freezing When Using Water Particle Baths

by Gerald Kulchytsky*

 
For companies that use magnetic particle testing during the winter months, a simple yet inexpensive way to heat water based particle baths can now be employed. This simple, cost effective solution will prevent tank freezing and minimizes the potential risk of flash freezing on the part surface if part temperature is 273 K (32 °F) or less. Equipment downtime associated with frozen magnetic particle tanks will be eliminated and premature equipment failure can be significantly reduced. Candidates for this technique include forging facilities, casting foundries and steel mills where magnetic particle equipment can be exposed to adverse conditions.

 

Lower Radiator Hose Heater
To accomplish this, a device known as a lower radiator hose heater can be purchased from a local automotive parts store. It is a heater that is intended to be used for automobiles, trucks and farm implements to heat the antifreeze in the engine block to promote starting in subzero temperatures. These heaters operate using 120 V and are available in sizes ranging from 25.4, 31.8, 38.1, 44.4 and 50.8 mm (1, 1.25, 1.5, 1.75 and 2 in.; Figure 1). They are easily installed by removing a section of the plumbing and replacing it with an assembly of heaters joined (in series) to the piping with rubber hose and clamps.

Figure 1 - Lower radiator hose heaters in a variety of sizes.

A typical wet horizontal bench system may require one to three heaters, depending on the size of the tank. As a general rule of thumb, conventional magnetic particle tanks will require at least one heater per 38 L (10 gal) of particle bath in the harshest of conditions. The number of heaters needed depends on equipment design, outside air temperature and whether the processing station is enclosed within the test booth or if it is exposed to the adverse conditions outside the booth. The diameter of the plumbing (which also dictates the size of the heater used) determines how many heaters are actually needed due to the fact that the various sized heaters have differing wattage ratings (that is, different current requirements).

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The heaters should be installed in series within the existing plumbing located below the tank.

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The heaters should be installed in series within the existing plumbing located below the tank. To be effective, it is recommended that the heaters are installed before the agitation tube and spigot, such that the bath is in constant motion relative to the heating elements. Although these heaters feature a straight through design, it is good practice to install such heaters after the particle bath has been filtered to avoid any risk of clogging (Figures 2 and 3).

Figure 2 - Schematic showing heater installation position following the filter and before the agitation tube and hose.

 

Figure 3 - Photo of the heating arrangement.

Observed bath temperatures at our forging facility, with the magnetic particle testing system idling, is usually between 294 and 305 K (70 and 90 °F). Again, this depends on equipment design, outside air temperature and whether the processing station is enclosed or not. When left idling during a cold night or over a weekend, clear plastic sheeting can also be draped over the processing station to trap in heat, such that any plumbing above the tank is also kept from freezing. The use of plastic sheeting will also minimize evaporation of the bath. Since heat is constantly being lost to the outside environment, "overheating" the particles (which can strip the fluorescent pigment) should not occur provided that the bath is always circulated.

Bath temperature when a magnetic particle testing system is in use during production is typically cooler, due to the fact that the bath is cooled from cold parts being magnetized and the cascading action of the particle bath as it drops back into the tank. Observed bath temperature has ranged between 289 and 300 K (60 and 80 °F).

 

Heater Guidelines
Below are some additional guidelines for using the heaters.

• Use heaters only when the circulation pump is running. Although these heaters are thermostatically controlled, localized heating without the constant cooling action of the bath can potentially cause the heaters to malfunction or can cause premature pigment breakdown of the particle itself. Heaters can continually operate while the pump is operating during cold winter nights and weekends without any issues.
• Use heaters during normal production to minimize the risk of flash freezing of particle bath on the part surface when part temperature is 273 K (32 °F) or less.
• Although multiple heaters can be installed, it can be left to the technician's discretion as to how many heaters should be plugged in during production. A single heater can be utilized or multiple heaters can be simultaneously plugged in to maintain bath temperature at the technician's discretion. Bath temperature can be periodically checked with a waterproof digital or bimetal dial thermometer.
• Do not plug tank heaters into a circuit that is used for ultraviolet lights or space heaters. The electrical current necessary to operate the heaters can exceed the rating of the circuit breaker, thereby overloading the breaker. If possible, try to use a dedicated breaker/circuit. Be sure to use a breaker that will not be shut off or tampered with during nights and weekends. Depending upon the heater size installed, individual heaters can range from 375 to 600 W each.
• Use lower radiator hose heaters that are of aluminum diecast design. This will allow a maintenance technician an inspector to simply "feel" the heater housing to ensure that it is indeed operating. The entire aluminum housing transfers heat, which is easily detected by carefully touching the heater. Lower radiator hose heaters of the aluminum diecast design also feature a straight through orifice, therefore reducing the risk of clogging.
• Lower radiator hose heaters that are made of the stamped sheet metal design can be used; however, they are not recommended since the heating element is in direct contact with the liquid. With this design, it is difficult to verify by simple means if the heaters are indeed operating. Also, the heating element extends into the circulation path, thereby increasing the risk of clogging the plumbing system with debris.
• If flash freezing on the part surface is a problem that can affect particle mobility and overall quality, the particle bath should be heated and bath temperature should be monitored/recorded regularly as part of the quality system. As mentioned above, the temperature of the bath can be measured with a traceable digital or bimetal dial thermometer to verify its effectiveness in harsh temperatures.
• Always use a three prong safety cord with these heaters in the event of malfunction.
• Prior to installing the heaters, verify that the magnetic particle tank/frame are also electrically grounded in the event of heater malfunction.

 

Conclusion
Overall, heating the particle bath in such adverse conditions will enhance the integrity of the magnetic particle process by reducing "flash freezing" and will reduce the risk of equipment downtime due to frozen plumbing or equipment failure. Another advantage is that heating the particle bath can help eliminate the need of ethylene glycol as a supplemental additive, since disposal of ethylene glycol is also an issue.

 

* Jernberg Industries, 328 W. 40th Place, Chicago, IL 60609; (773) 326-4824; fax (773) 326-4924; e-mail <gkulchytsky@jernberg.com>.

 

Copyright © 2006 by the American Society for Nondestructive Testing, Inc. All rights reserved.

 

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