Assistant to the Chief Engineer The Boiler Inspection
and Insurance Company of Canada
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (9 printed pages)
Pressure vessels with quick-opening doors command a great deal of respect from
the Boiler Inspection and Insurance Company of Canada (B.I.& I.). We know
by simple calculation that a pressure vessel eight feet in diameter with
working pressure 150 psi, has an end force of just over one million pounds
acting on the heads. This does not worry us unduly in a conventional pressure
vessel with welded heads. However, when the head serves as a door and it needs
to be held in position by some form of locking mechanism, we have to be more
concerned. Brick curing autoclaves, metal bonding autoclaves, autoclaves used
in the textile industry, wood treating cylinders, tire vulcanizers, even
sterilizers can all be likened to a loaded rifle with the hammer cocked when in
operation. It doesn't take much to set it off. It's a lethal weapon and it
needs to be handled very carefully.
Our respect for pressure vessels with quick-opening doors does not stem from
theoretical calculations alone but from bitter experience.
Some 12 years ago, an autoclave failure occurred at an insured concrete plant
in Hamilton, Ontario. An 8'6" diameter by 108' long brick hardening cylinder,
one of six installed in the plant, had the door blown off just as the steam
pressure reached its normal maximum 145 psig at the start of a curing cycle.
After deflecting off the low wall of the loading bridge pit, the door of the
autoclave pierced another wall and continued piercing the plant research
laboratory located over the steam kilns. The walls and a large portion of the
plant's roof collapsed from the pressure of the explosion.
The 45 ton autoclave moved 150 feet away from its foundation and destroyed a
delivery truck, curing racks, and numerous cubes of inventory block.
Fortunately and miraculously no one was seriously injured.
The incident was fully investigated and the cause of the door failure was
attributed to reduced bearing area of worn wedges and possible slippage of the
In June 1969, another concrete block manufacturing plane in Hamilton was
leveled by an autoclave door failure. The B.I. & I. was not involved in
this accident but we had previously insured the location and were naturally
shaken by the disaster.
The newspaper report on the incident gives the size of the autoclave as 12 feet
in diameter by 80 feet long and indications are that the door blew off during
operation. The roof of the plane was lifted 10 feet into the air, one wall was
knocked down, and steel plates and concrete blocks were hurled 150 feet into
the air. The vessel tore free of its concrete supports and ripped across an
empty lot into a children's club (thankfully the club was empty because the
children were away swimming). The club was flattened and the autoclave
continued for another 200 feet squashing three pick-up trucks and two cars
before slamming into a wall of an autobody shop. The wall fell on top of at
least four parked cars and the autoclave finally butted a Volkswagen 80 feet
into the middle of a nearby street.
Unfortunately, one operator died instantly and three others were reported to be
We do not have any details as to the cause of the accident but photographs 3
and 4 attest to the devastation. Note in both cases the plants appear to have
been bombed. They probably were, if you accept the fact that the energy stored
in autoclaves of this size is approximately equivalent to about 80 pounds of
Following these disasters, a special Ad Hoc Committee on Autoclave Safety was
set up by the National Concrete Producer's Association. C. A. Williams, B. I.
& I.'s vice president of engineering, represented the insurance industry on
this committee that was responsible for the 1970 publication of the pamphlet
entitled "Safety Precautions for Autoclaves." The foreword of this booklet is
of special interest.
"In the past 20 years (1950-1970) about 200 concrete block manufacturers in the
United States and Canada have installed autoclaves to high-pressure steam cure
their products. Prior to 1950 there were about 20 autoclave plants in
operation. In these 220 plants employing this curing process there are probably
well over 500 autoclaves in use today.
In the history of the concrete block industry, there have been eight autoclave
failures; one each in New Mexico, Texas, North Carolina, Virginia, Illinois,
Quebec and two in Ontario. Seven of the eight were quick-opening door failures,
and six of these seven were wedge-lock type doors.
A recent tour of autoclave plants in Canada pointed out major deficiencies that
effected the safe operation in three of the twenty-one plants inspected. Most
common deficiencies found were related to the autoclave door safety locking
Due to wear and tear and to a degree of over-confidence often associated with
long term trouble-free operation, most of the autoclave doors should be
thoroughly inspected NOW!
Subsequent inspections by a representative of the door manufacturer or other
qualified person should take place every three years. Weekly, monthly, and
yearly inspections are the responsibility of plant personnel and insurance
A copy of this pamphlet was issued to each inspector in the B. I. & I. with
instruction to read and study the material and retain for reference. The need
for adequate overlap on wedges was stressed to all inspectors.
We do not know of any major disasters in Canada since 1970 but we have, from
time to time, heard of incidents involving autoclaves that serve as grim
reminders of the hazards involved in operating these vessels.
On December 3, 1974, an operator was killed at an aircraft plant in Brampton,
Ontario, when the door of a metal bonding autoclave (an 8' by 30' vessel) was
blown open by residual pressure as the operator was about to open it. The
operator was slammed against a concrete wall and died instantly.
Newspaper reports on the inquest into the death of the operator indicated that
the vessel was being operated without a safety device that had been recommended
by the insurance carrier.
In April 1976, a 17-year-old youth was killed in a cindercrete block
manufacturing plant in Regina. This incident involved a 10-1/2' diameter by
124' long autoclave with a Ring-Lok Quick Opening Door. The autoclave was being
vented at the end of a cycle and pressure was down to 5 psig on the gauge when
the young man, 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, striking the concrete wall and killing the youth.
On February 2, 1977, the door of a tire curing autoclave 12' 8" diameter by 9'
long blew open at a plant in Montmagny, Quebec, shortly after it was placed in
service. It completely demolished a 50' by 200' building in which it was
located. Fortunately no one was injured.
In May 1977, a vessel used for curing retreaded tires blew open at a location
in Toronto. This vessel bears little resemblance to the vessels mentioned
previously except for the fact that the cover is held in place by a concealed
locking device and is therefore analogous to the loaded rifle. No one was hurt
and the vessel, which was badly damaged, was replaced by the manufacturer at
their own cost. At about the same time a similar failure occurred in Alberta,
and again no one was injured but building damage was substantial.
In August 1977, at a plant in Edmonton, the chief engineer and production
superintendent entered an 8'6" diameter by 140' long brick hardening autoclave
to examine the rails which carry the loading cars. While they were inside, the
operator, unaware of their presence, proceeded to push the tram of loaded cars
into the autoclave. The two men managed to climb on top of the stacked blocks
and began crawling desperately along the 14 inch space above the blocks toward
the front, but were carried inside by the train so they were 80 feet from the
door when the train stopped. They got to within 20 feet before the operator
closed the door, and their shouts could not be heard above the din of the
forklift truck that was used to push the train into the vessel. As steam was
being admitted, the men managed to remove some blocks and pull free a steel
tray that they used to pound on the door.
A welder working nearby heard faint thumping sounds and mentioned it to the
operator. But after both men listened near the door they could hear nothing
unusual. However, they decided to shut down the unit and open the door anyway.
Both men were unconscious but quickly revived. They had only suffered numerous
cuts and bruises in their desperate scramble over the blocks.
Why do these vessels fail? In almost all cases it is a combination of improper
operation and poor maintenance.
How can we ensure safe operation of these vessels? The answer is fairly
The vessel must be designed, fabricated, and inspected in accordance with
Section VIII of the ASME code.
The vessel must be properly installed on adequate foundations with provisions
for thermal expansion.
The vessel must be properly operated. This is the responsibility of the owners
or managers. Their attitude is all important. They are the ones who should
ensure that operators are fully trained and re-trained for the job.
The vessel must be properly maintained;
The vessel should have the required complement of safety devices.
The human element cannot be overlooked in the prevention of accidents. All too
often after a long association with an operation, even key operating personnel
have a tendency to become careless or lax. Personnel without complete knowledge
of the hazards involved in the operation are even more likely to become
careless and fail to follow safe operating procedures. Thorough training of
operating personnel is most important and special attention must be given to
A complete explanation of the entire process and hazards involved.
A thorough understanding of routine duties and emergency procedures.
Thorough instruction and explanation of the functions and operation of control
An explanation of how safety devices function.
Proper procedures for depressurizing the autoclave before opening the door.
Proper procedures for operating the quick-opening door.
The dangers involved by forcing operating mechanisms.
Instructions to report at once any malfunctioning of equipment or control
Instructions should include where to look for wear and evidence of possible
failure of equipment.
It is fairly certain that the autoclave disaster in Hamilton stemmed from the
fitting of an oversized gasket which prevented proper overlap of the wedges and
engagement of the mechanical interlock. To overcome this problem the operator
took a hammer to the interlock, bent it, and got it into position with
Though seemingly unrelated to autoclaves, there is a film that deals in depth
with the causes of an air disaster in Paris, some years ago, involving a DC-10
aircraft. The crash was caused by a cargo door blowing open shortly after
takeoff. The point in common with autoclaves is that the door closing mechanism
carried a small flap which sealed a hole in the fuselage when the door was
locked. The aircraft could not be pressurized unless the hole was sealed and
the flap served the same function as an interlock on an autoclave. On that
fateful day in Paris, the door locking mechanism did not engage properly but
the operator managed to get the flap into position by applying a little extra
force which bent the mechanism sufficiently and sealed the hole. Some 350 lives
A lack of adequate knowledge and understanding is dangerous, especially when
working with vessels under high pressure. A very important lesson to be learned
from past explosions is that providing properly constructed equipment alone
will not prevent accidents. Instructions must be understood and employees must
be adequately trained if potentially serious accidents are to be prevented.
Management should also establish definite operating procedures and have them
prominently displayed and followed by all plant personnel. These procedures
that special care be exercised in loading to prevent damage to the vessel, door
gasket, and gasket bearing surfaces;
closing of the door must be done only by authorized personnel thoroughly
acquainted with the locking mechanism and safety devices;
that the gasket and contact surfaces must be clean and free of any debris.
Should any binding occur during closing, the trouble must be determined and
corrected. Under no condition must the door or locking mechanism be forced into
that safety devices, controls, and interlocks must be checked for proper
at the end of the opening cycle, no attempt must be made to open the door until
the operator is CERTAIN that all pressure has been dissipated.
Since most of the accidents on vessels with quick-opening closures are caused
by improper operation and poor maintenance, we must make certain that neither
Safety devices are dealt within the ASME code and we do not dispute the merits
of the code.
However, it should be noted that the code itself recognizes that it is
impractical to write detailed requirements to cover the multiplicity of devices
used for quick access, or to prevent negligent operation or the circumventing
of safety devices. We, as an insurance company, have a definite preference for
the inclusion among safety devices of a positive mechanical locking device that
when in place ensures that the door is properly positioned and cannot be opened
and when released provides visual and audible warning of any residual pressure
in the vessel.
Thus far, we have dealt with the subject of ensuring safe operation of vessels
with quick opening closures in terms of action required by owners and
operators. What part does the B. I. & I. play in this? What do we do when
we are requested to provide insurance coverage on such vessels?
Our inspection procedures call for a critical inspection of the vessel when
coverage is first provided. The company sees the object through the eyes of our
field inspector who must verify that the vessel is constructed to code, is free
of defects, is properly installed, etc. Special attention is given to the door
if it is of the quick-opening type.
The inspector describes and provides a sketch of the door and reports on the
condition of wedges, wedge overlap, (if a wedge type door), condition of
hinges, alignment, etc. He describes or provides a sketch showing all
mechanical safety locking devices that serve to prevent accidental opening of
the door under pressure and any audible or visual alarms which are fitted. He
determines if the operators are competent, and if they are aware of the hazards
involved. He looks at the attitude of the management and of the plant. Are they
aware of the hazards? Have they issued safety instructions? Have they properly
delegated responsibility for the safe operation of the vessel?
In most cases, approval of coverage hinges on the presence of a mechanical
safety locking device. Our philosophy is that if an operator can see, feel or
hear steam or air blowing from a vent pipe immediately after disengaging a
mechanical interlock, his own common sense should tell him or her to wait until
the pressure has dissipated. The device buys time. It gives them a chance to
Pressure activated devices and electrical devices do fail despite the many
guarantees issued by the designers. Operators all too often regard such devices
as being infallible and look to them or protection if they, the operators, make
We feel that safety must not be compromised and cannot be overdone.
The engineering department of the B. I. & I. will not approve insurance
coverage of autoclaves and similar vessels unless they are fitted with an
approved mechanical locking device.
A well-known manufacturer had this to say about safety devices:
"There is a need for quick-opening doors with safety devices which remain
effective even when personnel operating the door will not concern themselves
about safety. We believe that, to be effective, a safety device must:
be easily operated so that the operator has no reason to object to its use;
be difficult to circumvent or make ineffective;
be impossible to circumvent in such a manner that it can quickly and easily be
returned to its original condition, say, at the end of a shift;
be mounted so that supervisory personnel walking by the door can see at a
glance that the device is ensuring safe operation;
be simple so that the principle of operation is obvious and rugged so that
confidence in its effectiveness is maintained."
The manufacturer goes on to say:
"The patented safety devices on our doors fulfill these conditions. The
relative locations of the parts are fixed in our shop before shipment to ensure
a safe overlap of wedges and no means of adjustment is provided. All parts are
welded permanently in place. The bolt operates between lugs on the door and
breech ring so that any maladjustment of the hinge would not affect the overlap
ensured. The proportions of the safety device parts are adequate to resist even
the full force of the hydraulic blinders tending to unlock the breech ring. As
offered, the safety device should provide complete protection in that a two
inch gate valve can only be closed when the door is safely locked, and the door
can only be unlocked when the valve has been fully opened, and a hand wheel
located in the path of the discharge from the valve has been operated.
"Various other means of providing safe operation can be envisaged but we
believe that any involving electrical switches and circuits can be circumvented
without difficulty and, therefore, cannot be considered prime protection.
However, if required, we can supply at extra cost, in addition to our standard
safety device, any equipment which may be considered to offer useful secondary
protection. For example, a limit switch may be mounted on our safety device and
arranged to operate only when the bolt is fully engaged; this switch could be
used to control admission of steam or hot oil to the autoclave or to control
the blow down valve. Also, a pressure actuated switch could be mounted on the
autoclave and used to control operation of the hydraulic power unit, but the
value of such an arrangement is limited by the degree of sensitivity of the
Finally, a few quotes from an article copied from the Autumn 1974 issue of Vigilance
- The Journal of National Vulcan Engineering Insurance Group Ltd.
of the United Kingdom. It is quite amazing how their experience closely
"Pressure vessels with quick-opening doors such as autoclaves, sterilizers, and
vulcanizers have long been subject to the danger of explosion and from time to
time doors have been blown off for various reasons such as incorrect closure,
worn parts of locking devices, and improper operation."
"When the door is open at the end of the cycle, there may be residual pressure
in the vessel which is not detectable by the pressure gauge. For example, a
pressure of one pound f per in2 produces a thrust of two tons on a
door two meters in diameter. In these circumstances, the person opening the
door may be struck and seriously injured or killed."
Let us digress for a moment and refer to the instruction manual for the Bandag
type autoclaves used for curing retreaded tires. The operator is advised as
Open the manually controlled chamber exhaust valve (i.e. the mechanical
interlock) after the curing cycle has been completed. Never try to open the
door until the chamber pressure gauge reads "0." As a double check, place your
hands around the silencer of the exhaust valve to make sure all pressure had
been exhausted. Even one or two psi of air over the entire surface area of the
door will create enough thrust to cause damage to equipment or personnel.
The safe operation of autoclaves depends upon the following measures. Failure
to implement any of them may give rise to an explosion.
The provision of appropriate interlocking safety devices for the door.
Periodical thorough examination of the vessel and its fittings.
Arrangements for regular systematic inspection and maintenance.
Conformity with proper operating procedures.
Adequate training and supervision of operators.
INTERLOCKING DEVICES FOR DOORS
A test cock or other equivalent device should give an audible and visual
indication of internal pressure in the vessel. This cock should also be
interlocked with the door locking mechanism, in such a way that the testcock
has to be completely open before the door can start to unlock.
B. L. Whitley, Director
Boiler and Pressure Vessel Division
North Carolina Department of Labor
Apparently, many owners/operators and inspectors are not aware that after many
months or possibly after a few years of cyclic operation, the locking mechanism
on quick actuated closures and other similar devices can fail with catastrophic
results without proper maintenance and care.
A close inspection of the items making up these closures include pins,
bushings, bearings, bolts, nuts and wear rings. In many circumstances, these
items reveal excessive wear that under certain conditions can cause the vessel
to be blown open while it is under pressure. This results in injury to
personnel and an untold thousands of dollars in property damage.
This problem is not unique nor is it confined to any one specific design. It is
common in all types of closures having moving parts that are subject to wear
and that have not been properly maintained.
Each inspector should be cautioned to examine every quick actuating device for
excess wear. Surfaces of small critical parts, such as pins, bushings,
bearings, etc., should be scrutinized closely for any existing abnormal
conditions such as looseness, excess wear, or improper fit.
Should any of these conditions be observed, they should immediately be brought
to the attention of management and the proper authorities. The inspector should
also be reminded that the pressure sensing device designed to prevent the
vessel from being inadvertently opened should under no circumstances be
bypassed. If an inspector discovers the pressure sensing device has been
bypassed, the inspector should immediately report the condition to the proper
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.