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
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
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
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
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.
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
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
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
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:
All bearing surfaces should be carefully checked for evidence of
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.
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
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
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
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 and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.