Elevator Inspection Section for the State of Tennessee.
Former member of The National Board of Boiler and Pressure Vessel Inspectors
54th General Meeting in 1985
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (7 printed pages)
The following material represents the author only and should not be assumed to be the opinion or policy of The National Board of Boiler and Pressure Vessel Inspectors or The American Society of Mechanical Engineers administration, staff or membership, unless so acknowledged.
I would like to talk with you for a while on the subject of Quick Actuating Closures or, as more commonly called, Quick Opening Doors. Quick opening doors are provided on certain types of unfired pressure vessels to facilitate loading and unloading of materials, or products or goods which are being processed under pressure and at elevated temperatures within the vessel. Depending upon the process, these vessels may be called autoclaves, sterilizers, vulcanizers, reactors, retorts, treating cylinders, and I'm sure you may refer to them by many other names. During this discussion, let's just classify and refer to them as autoclaves. These vessels are used for countless processes such as: sterilizers in hospitals and laboratories, food processing, curing of tires and many rubber-type materials, production of chemicals and pharmaceuticals, curing of brick, concrete blocks and other concrete products, metal bonding, impregnating timber and wood products, processing of textiles, and many other applications. Some of these processes require a comparatively short time of less than one hour to complete while others require ten or 12 hours or longer to complete.
Quick opening doors are usually provided on only one end of horizontal cylindrical vessels in place of the fixed concave or convex dished head, however, occasionally they are provided on both ends to further expedite loading and unloading. Quick actuating closures are also used as the top head on vertical vessels. Although they are more commonly used on cylindrical vessels, the quick opening doors are sometimes applied to use on rectangular sterilizers and process autoclaves.
Quick actuating closures command a great deal of respect from boiler and pressure vessel jurisdictional authorities, insurance companies, and owners and users. No doubt many of you have a small two or three quart size pressure cooker which you use on the kitchen range at home. Many of these have lids which may be referred to as quick actuating closure. Perhaps in your home you have experienced an accident whereby the lid was not properly secured and blew off the cooker and through the ceiling. These incidents are not uncommon. Simple calculation shows that a small pressure cooker, ten inches in diameter, operating at 15 psig exerts a load or force of 1178 pounds on the lid. That's a load of well over one half ton. Let's consider an autoclave or tire curing vulcanizer ten feet in diameter and operating at a pressure of 100 psig. Again, simple calculation shows this vessel to have an end force of 1,130,976 pounds acting against the quick opening door. That's a 565 ton load carried by the door. That's why we must have respect for these quick actuating closures. Loads, such as this, in a conventional pressure vessel with dished and flanged heads properly welded to the shell would not worry us unduly. However, when the head senses the function of a door which gives quick and easy access for loading and unloading the vessel, and must therefore be held in position by some form of locking mechanism, we have to be more concerned.
Review of the past issues of National Board BULLETIN and jurisdictional records of accidents in the states of the United States and provinces of Canada will disclose some of the catastrophic results of failures of quick opening doors on autoclaves. We have learned through bitter experience just how devastating such accidents can be. Let's briefly review some of these.
We have a report of an autoclave failure in a brick manufacturing plant. An eight feet, six inches in diameter by 108 feet long brick curing cylinder had the door blown off just as the steam pressure reached its normal working pressure of 145 psig at the start of a curing cycle. The door, after deflecting off a wall at the loading pit, pierced another solid brick wall and continued through an adjoining section of the building. Other walls and a large portion of the plant roof collapsed from pressure of the explosion. The 45 ton autoclave moved 150 feet from its foundation destroying a delivery truck, curing racks and numerous stocks of inventory that blocked its path of travel. Miraculously no one was seriously injured. This incident was fully investigated and the cause of the door failure was attributed to reduced bearing area of worn edges and possible slippage of the locking ring.
Here is another report of a concrete block manufacturing plant that was leveled as a result of an autoclave door failure. This vessel was 12 feet in diameter by 80 feet long and the report indicates that the door was blown off during normal operation. The roof of the plant was lifted ten feet into the air, one wall was knocked down, and steel plates and concrete blocks were hurled 150 feet from the site. The autoclave tore free from its concrete supports and ripped across an empty lot into a children's club. The club, usually full of children, was empty because the children were on an outing. The autoclave, after passing through the children's club, continued for another 200 feet smashing three pick-up trucks and two cars before slamming into the wall of a shop. One operator was killed and three others were critically injured. This was tragic, but it could have been much worse. Just imagine the results had the children's club been occupied.
We have another report where an operator was killed when the door of an eight feet by 30 feet metal bonding autoclave was blown open by residual pressure as he was about to open it. The door slammed the operator against a concrete wall and he died instantly.
Here is another report involving a ten and one-half feet in diameter by 124 feet long block curing autoclave. The vessel was being vented at the end of a cycle and pressure was reported to be down to five psig on the gage when the 17 year old operator, impatient at the delay in waiting for the pressure to dissipate, decided to open the door despite shouted warnings from a senior operator. The door flew open, hurling the youth against a wall killing him instantly.
Here is yet another report of a recent accident involving a five feet in diameter by 30 feet rubber curing vulcanizer. The operator was in the process of securing the vessel after a curing cycle and apparently thinking that all pressure had been relieved through the open vent line, was opening the door with aid of a "come-a-long" winch when the door suddenly blew open. The operator was scalded by steam and hot water and the last report was that he remains in critical condition.
The list of reported incidents goes on and on. I would suspect there are many, many "near miss" type incidents involving quick opening doors that are not reported. Why do these quick actuating closures fail? In almost all cases, its a combination of improper operation and poor maintenance.
As you of course know, there are a number of different designs or types of locking mechanisms provided on these quick opening doors. The main difference in design is the manner in which the door is locked and the method of obtaining a pressure-tight joint. Some of the more commonly used types can be classified as:
- the interlocking lug type door;
- expanding or contracting ring type door;
- clamp type door;
- cam latch type door;
- bar locking type door;
- swing bolt type door.
At this time we won't discuss the design features of the various locking mechanisms. Most states and provinces require ASME Code construction and stamping with National Board registration and we strongly endorse this requirement.
Some of the general design features which should be considered for quick actuating doors are as follows:
- Design the locking mechanism so that the failure of one locking element will not result in the release or failure of the other elements.
Arrange the locking and holding elements so that a visual external examination can be made of their condition to determine that the elements are fully engaged in the closed position.
Where the locking mechanism or the closure is completely released by limited movement of the locking mechanism or closure, and is hydraulically operated (or operated by any means other than manual), the unit should be designed or provided with interlocking devices so that the vessel cannot be pressurized until the closure mechanism is fully engaged and the mechanism cannot be released until the vessel has been depressurized.
For manually operated locking mechanisms designed to release the vessel's pressure before the mechanism has been disengaged, an audible or visible warning device should be provided to alarm when an attempt is made to pressurize with an incompletely engaged mechanism, or to alarm when an attempt is made to disengage a mechanism while the vessel is pressurized.
Provide at least one safety device that would verify that the pressure in the vessel has been reduced to atmospheric before the door locking mechanism is released.
Provide a pressure indicating device on all units that is visible from the operating area.
As was stated earlier, investigation shows that in most cases the failure of quick actuating closures is caused by a combination of improper operation and poor maintenance. Training and supervision of operators is of utmost importance. Every operator should be adequately trained in the proper operation of an autoclave. He should be made fully aware of the potential for accidents involving these vessels and of the tremendous forces acting on the quick opening doors. He should be made aware of and understand the importance of insuring that the vessel is completely vented before attempting to open the door. He should understand the function of all operating controls and door interlocking devices and the danger of interfering with or bypassing any safety device. An operator who has not yet acquired sufficient knowledge and experience with autoclaves utilizing quick opening doors should be closely supervised by a trained and experienced person.
Qualified management or supervising personnel must establish safe and proper operating procedures. The operating instructions provided by the manufacturer should always be strictly adhered to. After definite operating procedures have been established, they should be prominently displayed and followed by all plant personnel. These procedures should insure:
- that special care be exercised in loading the autoclave to prevent damage to the vessel, the door gasket and the gasket bearing surfaces;
closing of the door must be done only by authorized personnel who are thoroughly acquainted with the locking mechanism and safety devices,
the gasket and contact surfaces must be clean and free of any foreign matter;
should any binding or poor fit-up be encountered during closing of the door, the trouble must be determined and corrected. Under no conditions should the door or locking mechanism be forced into position. The use of "cheaters" or "jimmy" bars to pry doors into position or to apply additional leverage to locking mechanism should be prohibited;
at the end of the cycle, no attempt must be made to open the door until the operator is certain that all pressure has been dissipated.
In any type of quick opening door, one of the chief hazards is the door being opened while there is still some pressure within the vessel. Various designs of safety devices have been developed to prevent this from happening. One device is a pressure actuated unit that prevents the admission of pressure into the vessel until the door is fully closed and locked, and prevents opening of the door until the pressure within the vessel has been reduced to atmospheric. Another simple safety device is a pin design which prevents closing of the vent valve until the door is in the completely closed position. The hole in the flange lines up with the pin, permitting its inward movement and the rotation of the valve stem to its closed position. Conversely, disengagement of the end closure is prevented by the pin unless the valve has been turned to the wide open position, thereby venting the vessel. Still another simple safety device that can be made in the plant if desired consists of a vent valve of the rising stem type with an arm attached to the stem of the valve. A pin on the end of the arm prevents closing of the vent valve unless the door is properly closed and locked so that the pin can enter holes in the door ring and door which are aligned only when the door is in the fully closed and locked position. Conversely, the door cannot be opened unless the vent valve is fully open.
There are a number of other types of mechanical interlocks available as well as electrical, hydraulic and pneumatic safety devices. Safety interlocks are occasionally interconnected into process control systems. In some instances the vessel should remain closed until flammable, toxic, or other hazardous material in gaseous or liquid forms have been removed and the vessel purged. These materials may be in the vessel under essentially atmospheric pressure conditions.
Regardless of the safety devices and interlocking systems provided on quick opening doors, the human element cannot be overlooked in prevention of accidents with these vessels. All too often, even key operating personnel have a tendency to become overconfident after a long term of trouble free operation.
A continuing preventive maintenance inspection program is required. Periodic inspections of quick opening door assemblies should be made by properly trained personnel. The general conditions of the moving parts of the door assemblies and safety devices should be checked at least on a monthly frequency. A more thorough inspection should be made of the entire vessel annually. Nondestructive testing of critical elements should be considered on units subject to severe service.
Of course, records should be kept of all tests, maintenance and repairs. Some of the features which should be inspected include:
In conclusion, quick actuating closures are in widespread use throughout industry; because of this, owners and operators are often unaware of, or complacent to, the deadly potential of these devices. It is up to the designer to insure that his autoclave door offers adequate protection against the maximum pressures carried by these closures and to further insure that interlocking devices are provided which will prevent the door from being opened while the vessel is still pressurized. It is up to vendors, inspectors, and all of us here to drive home the importance of planned maintenance and strict operational procedures to owners, management, and operating personnel.
- All bearing surfaces should be carefully checked for evidence of excessive wear.
Gaskets should be checked for wear, damage and leakage. Replacement gaskets should always be in accordance with the manufacturer's specifications with no deviations. Door hinge mechanisms should be checked for proper alignment and to insure that adjustment screws and locking nuts are properly secured.
Door and locking ring lugs should be examined carefully for evidence of undue stress and for cracks at the junction of the lug and door or locking ring. Locking ring and door wedges should be checked to verify full engagement when closed and for proper bearing surface contact, wear patterns, and condition. For replacement of missing wedges or securement of loose wedges, the door manufacturer should be consulted.
Doors using the contracting ring locking device should have the ring checked for loss of springiness, cracks at the points of attachment of the operating lugs, evidence of undue wear on the ring and shear on the pins in the lugs and the operating mechanism.
With clamp type doors, the surfaces of the clamps should be checked for wear and the clamps should be examined for distortion at the portions overlapping the shell ring and door ring. Hinge pins and locking device parts should be checked for wear and evidence of shear.
With bar type doors, all bearing surfaces should be inspected for undue wear and the various parts checked for indications of undue stress as well as for distortion. Arm pivot pins should be checked to be sure they are securely held in place and are not bent. Pivot pin mounting brackets should be checked for cracks at the point of attachment to the head and evidence of undue stress in line with the pin holes. Threads of the operating screw should be checked for wear and fit in the nut or handwheel hub.
Closures of the swing bolt type should be checked for missing bolts. If any are missing, they should be replaced at once. Bolts should be checked for soundness, particularly at the eye, and the threads should be checked for evidence of stripping or excessive wear. The bolt washers should be flat. Washers that are distorted to a dish shape tend to allow bolt movement out of the slot when nuts are improperly torqued. The closure should also be inspected when closed to be sure the nuts are fully engaged. The pins should be examined for distortion and for secure fit.
At each inspection of the vessel, the door safety locking appliances should be checked and tested to be sure that they are operating properly and are in good repair.
Any time a locking ring binds or catches at some point during its movement. The point becomes a fulcrum and the entire ring tries to rotate around it. This may result in shifting of the ring's position and causing unequal overlap on the lugs. Therefore, it is important that any safety device that determines the positioning of the ring, such as microswitches, manually operated pins with two-way valves connected to steam signals, or any other type of device, be located at four equal quadrants of the ring. One safety device at one point is not sufficient to properly indicate the position of the ring. The four devices should be tested.
The door and the locking mechanism should be checked both in the closed and open positions. The position of the locking ring, the amount of overlap, and any shift in the ring's position should be observed.
The opening to the vessel should be checked for out-of-roundness at the outer edge. This is the difference between the maximum and the minimum inside diameter at any cross section. Under no condition should it exceed one percent of the nominal diameter of the cross section under consideration and preferably it should be zero.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.