by Jeffery Cowan
The increasing desirability of laminated glass products has caused autoclave usage to grow significantly in recent years. New building codes that require glass with improved breakage resistance, architectural and artistic demands and new and improved interlayer products mean the need for autoclaved glass will continue to increase. As autoclave usage becomes more common, it will become even more important to ensure safety issues are addressed.
Some autoclaves have been in use for many years. In fact, because of their generally reliable operation they often outlive their operators and sometimes their owners. It is common for new operators to be less familiar with the autoclaves than the previous operator, and for them to rely even more heavily upon whatever inherent safety devices the autoclave possesses.
New autoclaves often are designed with advanced features requiring less operator interaction than older autoclaves. This means operators become less familiar with, and more reliant on, the equipment.
A conscientious and thorough program of operator training and testing should be adopted to ensure each operator knows not only basic operation procedures, but also more importantly, what to do in an emergency. "How do you override a non-responsive control system? What do you do if the autoclave will not release pressure?" are typical scenarios to be addressed.
Semi-annual reviews of the operators knowledge should be encouraged and recorded in a log. The autoclaves operation manual, along with posted instructions and warnings, should be available at the autoclave at all times.
A tremendous amount of energy is contained inside the operating autoclave and it is vitally important to its proper control. One of the most obvious equipment related safeties is the door safety interlock mechanism. In the United States, the accepted design and construction code for pressure vessels is detailed in the American Society of Mechanical Engineers, (ASME), Section VIII, Division 1. This code mandates the use of a mechanism, with redundant features, to prevent the accidental opening of an autoclave door while pressurized.
Typically, internal autoclave pressure holds the mechanism in a locked position, and releases the mechanism after autoclave pressure has been vented. This mechanism also includes mechanical interference, electrical control system inputs and hydraulic system blocking capabilities. An independent pressure switch is used frequently to prevent the doors hydraulic system from operating. Monthly inspection of this device will assure proper and safe operation.
Autoclaves large enough to permit operator entry are subject to the OSHA Confined Spaces Act. They must allow safe operator entry by either mechanical or procedural means. After the door safety interlock mechanism, the most common operator safety is the man-inside alarm. This is a safety cable that runs along the length of the internal workspace that can be pulled by the operator if the autoclave door closes accidentally while the operator remains inside. The man-inside alarm shuts down all autoclave functions, heating, pressurizing and air circulation and initiates audible and visual alarms.
Internal autoclave energy emanates mainly from two sources: pressure and thermal energy. Safe control of both of these sources are essential. The control system should not allow operation that results in temperature or pressure higher than the equipments maximum design points. Independent control devices must be incorporated to override the control systems attempt, intentional or otherwise, to exceed safe values. Temperature high-limit controls, over pressure sensors or switches can actively handle the appropriate conditions.
Passive mechanical safeties for over-pressure control should be part of all autoclaves. The ASME code requires a safety relief valve be installed with a set pressure of no more than the design point of the pressure vessel. It is to open automatically to vent off excess pressure. If autoclave pressure continues to rise, a second pressure relief device can be called upon to provide additional venting capacity. The rupture disc is a simple device that will burst open, or rupture, to provide large venting capabilities and reverse the over pressure condition.
Periodic inspection of the pressure vessel itself should be conducted. The frequency of inspection will vary depending on the age and condition of the autoclave. Some insurance carriers or state or local jurisdictional agencies may require annual inspection. Unfortunately, there is no national standard for inspection requirements. Requirements may be limited to visual inspection of the vessel, the door safety interlock and a properly rated safety relief valve. The autoclave owner should treat this as a minimum and should supplement this with a more thorough inspection at reasonable intervals.
Autoclave vessels generally are constructed of carbon steel which is susceptible to corrosion. Areas subject to the most corrosion should be checked for sufficient material thickness using an ultrasonic thickness tester. This simple test can be performed by an authorized boiler and pressure vessel manufacturer.
Areas of concern usually are related to the internal water cooling system. The water piping itself can corrode enough to leak into the autoclave workspace, or more commonly, the internal cooling coil, heat exchanger and connecting piping, will leak. The water will then find its way down through the insulation to the pressure vessel wall where it will be unseen or ignored. The vessel wall can continue to corrode until internal operating pressure causes a failure. Repair of a corroded pressure vessel component can be done for a fraction of the replacement cost of a complete autoclave.
Autoclaves are also subject to cyclic fatigue from the pressure and temperature variations during the laminating process. Door locking mechanisms are subject to complex stresses. Most doors are of the breech-lock type design, employing a rotating locking ring to engage a number of lugs around the perimeter of the door. Testing for fatigue-induced cracking is best accomplished using magnetic particle methods. Again, the local ASME fabricator is a good source for this service. Depending on the location, size and quantity of cracks discovered it may be possible to repair the cracks.
Safety features common to all industrial equipment should be inspected and verified regularly. These include lock-out tag-out compatible components and procedures, equipment guards, safety railing and protection, and adequate warning signs and reminders.
Many safety features of autoclaves are designed by the original manufacturer, however, all safety features should be incorporated or retrofitted into existing autoclaves and their proper operation should be verified periodically.
Jeffery Cowan, PE, is chief engineer at Melco Steel Inc., located in Azusa, CA.
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