Patent Publication Number: US-3876385-A

Title: Fail-safe system for operating an autoclave door

Description:
United States Patent Markus et al. Apr. 8, 1975 1 FAIL-SAFE SYSTEM FOR OPERATING AN 3.780.904 12/1973 Stanford .0 220/553 AUTOCLAVE DOOR FOREIGN PATENTS OR APPLICATIONS [75] In entors: rt ur Russell Markus. ng ood; 686.071 1/1953 United Kingdom 23/290 John Ostapovich, Denver, both of C010 Primary Examiner-M0rris O. Wolk [73] Assignee: Johns-Manville Corporation, New Am&#39;flam &#34;&#39;B York NY. Attorney, Agent, or F1rmRobert M. Krone; John D.  
  Lister [22] Filed: July 31, 1972 [2i] Appl. No.: 276,803 [57] ABSTRACT A fail-safe key interlock system prevents an autoclave [52 us. (:1. 23/290; 49/31; 49/35; door from being Opened when Steam Pressure Within 220/553 the autoclave is above a predetermined level, and pre- 511 Int. CL B01]- 3 0o; 5 45, 0 os 65/00 vents a steam supply valve 0f the autoclave from being 58 Field of Search 23/290; Opened auwclave door is closedlatched 2|/94 98; 49/31 35 220/553 and locked. The system is fail safe in that if there is a loss of electrical power or control air pressure, the [56] References Cited door cannot be opened and the steam supply valve is UNITED STATES PATENTS closed or remains closed. 168L008 14/1972 Black 0. 23/290 x 12 Claims 3 Drawing Figures FAIL-SAFE SYSTEM FOR OPERATING AN AUTOCLAVE DOOR BACKGROUND OF THE INVENTION The present invention is directed to a system for safely operating an autoclave door and, in particular, to a fail-safe system which assures that the autoclave door cannot be opened unless the pressure within the autoclave is below a predetermined level and a steam supply valve is shut off.  
  Autoclaves are usually thick-walled steel vessels utilized to carry out chemical reactions and the like under relatively high pressure and temperatures. These vessels, which typically employ steam under pressures of ll50 psi, have massive steel doors which range from -9 feet in diameter and can have up to about 1.700.000 pounds of force exerted on them when the autoclave is pressurized. When one of these doors is ac cidentally opened while an autoclave is still pressur ized, the results are disastrous. The hugh door is blown off its hinges and sometimes through walls while the main portion of the oven is generally moved several feet off its foundation in the opposite direction. Obviously. anyone standing in front of one of these massive doors is generally killed and even those who are not hit by the door can be severely burned by the steam re leased from the autoclave. Not only have these accidents resulted in the loss of human life but, in addition. the damages caused are sometimes extensive, amounting to hundreds of thousands of dollars. Consequently, there is a need to provide a fail-safe system to prevent the occurrence of such accidents.  
 SUMMARY OF THE INVENTION Accordingly. it is an object of the present invention to provide a fail-safe system for preventing an autoclave door from being opened when steam pressure within the autoclave is above a certain level and for assuring the steam supply valve is closed when the autoclave door is being opened, is open or is being closed.  
  The present invention provides a fail-safe key interlock system for an autoclave door. The system includes a control means for opening and closing the steam supply valve for the autoclave; a means. including a key interlock means, for preventing the door from being unlocked or opened when the pressure within the autoclave is above a predetermined level and for preventing the steam supply valve from being accidentally turned on when the door is not closed, latched and locked; a door-locking means; and hydraulic mechanisms for latching or unlatching and opening and closing the door.  
  The system is fail-safe in that if there is a power failure or a loss of control air pressure, the steam supply valve is closed or remains closed and the door can not be unlatched.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view of the fail-safe autoclave door-opening system of the present invention as applied to a rotating door which is shown closed. latched and locked;  
  FIG. 2 is a front elevational view of a typical rotating door utilizing the control system of FIG. 1;  
  FIG. 3 is a side elevational view of the rotating door of FIG. 2 with portions broken away for clarity and to illustrate a latching mechanism of the door;  
  FIG. 4 is a diagrammatic representation of a solenoid key-release unit of the key interlock of the system;  
  FIG. 5 is an elevational view with portions broken away of a locking mechanism for the door of FIGS. 2 and 3 with a second component of the key interlock illustrated;  
 FIG. 6 is a plan view of FIG. 5;  
  FIG. 7 is a side elevational view of FIG. 5 taken generally along lines 77; and  
  FIG. 8 is a schematic representation of a second embodiment of the invention. modified somewhat for use with an autoclave door which is latched by a rotating ring with the door shown closed, unlocked and unlatched.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS The fail-safe door-opening system of the present invention is primarily intended for use with autoclaves. While the detailed description will primarily deal with an autoclave having a rotating door, as illustrated in FIGS. 1-7 of the specification, the system with slight modifications can be used equally as well with a rotating ring autoclave door assembly such as that illustrated in FIG. 8 plus other types of autoclave doors.  
  As best shown in FIGS. 1, 2 and 3, an autoclave 20 comprises a thick-walled steel vessel 22 with a door 24 mounted thereon. The door 24 is rotatably mounted on a hub 26 for rotation about its central axis. The hub 26 is located at the outer end of a support arm 28 which is pivotally secured to the vessel 22 by a hinge assembly 30. The periphery of the door 24 is provided with circumferentially spaced-apart lugs 32. These lugs cooperate with lugs 34 on a retaining ring 36, which is welded or otherwise affixed to the vessel 22 to retain the door in a closed position when the autoclave is pressurized. The length of each of the lugs 32 corresponds to the spacing between the lugs 34 and vice versa. Consequently, when the lugs 32 on the door 24 are properly aligned with the spaces between the lugs 34 on the retaining ring 36, the lugs on the door can pass between the lugs 34 on the retaining ring and the door can be opened or closed.  
  The autoclave door 24 is rotated about its central axis by a pair of double acting hydraulic cylinders 38 and 40. These cylinders are each pivotally secured at one end to the door 24 and at the other end to support arm 28. Since the hydraulic cylinders 38 and 40 are both double acting piston-type cylinders, they can be used in unison to rotate the door 24 both clockwise and counterclockwise about its central axis to latch or unlatch the door.  
  In addition to the hydraulic cylinders 38 and 40, the autoclave door 24 is provided with a double acting piston-type hydraulic cylinder 42 for swinging the door about the hinge assembly 30 to open and close the door. The cylinder 42 is pivotally secured at one end to the support arm 28 and at its other end to the vessel 22.  
  The door 24 is also provided with a locking mechanism 44 which is illustrated in detail in FIGS. 5, 6 and 7. The locking mechanism 44 is provided with a locking bar 46 that is pivotally mounted on a steam vent valve 48 which is secured to the vessel 22. The locking bar 46 cooperates with and is received within a slot of a yoke 50 mounted on the door 24 to prevent rotation of the door about its central axis. The locking bar 46 also cooperates with and is received within a slot of a yoke 5l which is affixed to the retaining ring 36. The yoke 51 holds the locking lever bar in position when the door is locked and the locking bar 46 cooperates with yoke 50 on the door to keep the door from rotating.  
  The door 24 can also be provided with a second locking mechanism to prevent rotation of the door about its central axis. This second locking mechanism comprises a lever 52 that is pivotally secured to the door and provided with notches 54. The notches 54 cooperate with a protrusion 55 on an extension of the support arm 28 to secure the door in the correct angular position for opening and closing the door and the correct angular position for maintaining the door in a latched position.  
  The overall fail-safe door-opening system of the present invention is best illustrated in FIG. 1. To facilitate the understanding of the Figure, electrical connections between components are indicated by lines having Crossbars thereon; air control lines are indicated by dashed lines, and hydraulic lines and all other lines are indicated by solid lines. The sources of electrical power, control air and hydraulic fluid are not shown.  
  As shown in FIG. I, the autoclave is provided with a steam inlet valve 56 which connects the autoclave with a source of pressurized steam (e.g., pressurized to 100-150 psi 3. The valve 56 is a two-way pneumatically actuated spring return valve. Consequently, the valve is closed when the actuating mechanism 58 of the valve is vented or not pressurized and the valve is open when the actuating mechanism 58 is pressurized The actuating mechanism 58 is connected to a supply of pressurized air through an air control line which is provided with a three-way solenoid air valve 60. a three-way pneumatically actuated spring return air valve 62. and a conventional controller 64.  
  When the solenoid valve 60 is energized, the three way solenoid air valve 60 permits the passage of pressurized air through the air control line to the actuating mechanism 58 of the steam valve 56. When the solenoid valve 60 is de-energized the valve 60 is shifted to interrupt the passage of air from the controller 64 through the valve and to vent that portion of air control line leading from the valve 60 to the actuating mechanism 58. Thus, when the solenoid valve 60 is de energized, the valve 60 functions to depressurize mechanism 58 and thereby shut off the steam supply valve 56 for the autoclave.  
  The three-way pressure actuated spring return air valve 62 permits the passage of pressurized air through the air control line leading to the actuating mechanism 58 when the valve 62 is pressurized. If the valve 62 is depressurized, the spring shifts the valve to a position where the flow of pressurized air through the valve to the actuating mechanism 58 is cut off and that portion of the air control line leading from the valve 62 to the actuating mechanism 58 is vented. Thus, when the valve 62 is depressurized, the actuating mechanism 58 of steam supply valve 56 is depressurized to shut off the steam supply valve 56.  
  The controller 64 is connected to a supply of pressurized air and functions to regulate the degree of pressurization of the actuating mechanism 58. When the pressure within the autoclave is low, the controller 64 pressurizes the actuating mechanism 58 to fully open the valve 56. As the pressure within the autoclave in- 6 creases, the controller 64 gradually reduces the pressurization of the actuating mechanism 58 to gradually close the steam supply valve 56. When the correct op crating pressure is reached within the autoclave, the controller 64 reduces the pressure in the actuating mechanism 58 until the steam supply valve 56 is shut off.  
  A control panel of the system is provided with an on-of switch having an on&#34; button 66 and an of button 68. This switch energizes and de-energizes the solenoid valve 60 to turn the steam supply valve 56 on or off.  
  The system also includes a pressure gauge 70 and a pressure switch 72. The pressure gauge 70 is provided to give a visual confirmation of the actual pressure within the autoclave. When the pressure within the autoclave drops to eight inches of water, the pressure switch 72 is actuated to turn on green signal lights 74 and 76 which are located on the control panel and a solenoid key release unit 78 respectively. At the same time, the switch turns off amber signal lights 80 and 82 of the control panel and the solenoid key release unit. Thus, the signal lights give visual indication as to whether the pressure within the autoclave is above or below eight inches of water. In addition to the above, pressure switch 72 energizes a solenoid 84 of the solenoid key release unit 78 when the pressure in the autoclave drops below eight inches of water.  
  As shown in FIG. 4, the solenoid key release unit 78 includes the solenoid 84, a limit switch 86, an actuating bar 88, a lock and a key 92. The lock 90 has two positions. In a first position, a slot in the actuating bar 88 is engaged by a rod of the solenoid 84 and the key 92 cannot be removed from the lock. In a second position, where the limit switch is opened by the actuating bar 88, the key can be removed from the lock. When the solenoid 84 is energized by the pressure switch 72, it retracts the rod to free the actuating bar 88 for movement. The key 92 can now be turned in lock 90 from its first position to its second position to move the actuating bar 88. When the actuating bar 88 has been moved, a protruding portion on the upper side of the actuating bar 88 depresses the limit switch 86 to open that switch and shutoff all power to the on-off&#34; switch of the control panel. Thus, the solenoid valve 60 cannot be energized to open the steam supply valve 56. With the key 92 in its new position, the key can now be removed from the lock 90 and utilized in the other component of the interlock system to complete the unlocking of the door.  
  The door-unlocking mechanism 44 which comprises the other component of the interlock unit, is provided with a lock 94 which is also compatible with the key 92. By inserting the key 92 in the lock 94 and turning the key, a bolt 96 is retracted from an aperture within the locking lever bar 46. With the bolt 96 removed, the locking lever bar 46 can now be pivoted about the end which is secured to valve 48 to open that valve and unlock the door 24. The locking lever bar 46 is provided with a plate 98 which remains beneath the bolt 96 when the lever 46 is swung open. Thus, the bolt 96 cannot be re-extended unless it is aligned with the aperture in the locking lever bar 46. Since the aperture in the locking lever bar 46 is only aligned with the bolt 96 when the bar is engaged with yoke 50 and since the key cannot be removed unless the bolt 96 is extended, the key 92 cannot be removed from the lock 94 unless the locking lever bar is in the locked position.  
  Since the locking lever bar 46 is secured to a shaft 100 of the steam vent valve 48 which is connected to the interior of the autoclave. When the lever 46 is swung from its locked position, where it engages yokes 50 and S1, to its opened position, the lever 46 turns the shaft 100 of the steam vent valve 48 to open this valve. Consequently, the turning of the lever 46 vents any steam which may still be left in the autoclave.  
  Locking lever bar 46 also carries a four-way cam operated spring return air valve 102 which is connected directly to a supply of pressurized air. The valve 102 is provided with a cam follower 104 which cooperates with a cam 106 secured to the door of the autoclave. When the cam follower 104 is depressed by the cam 106, the valve 102 connects the air actuated valve 62. a conventional pressure actuated indicator light 108, and a pressure switch 110 with an air control pressure line. The pneumatic indicator light 108 is on when the air control pressure is normal. However, when the air control pressure drops below its normal level, the indicator light is automatically turned off to give a visual indication of the pressure loss. The pressure switch 110 is connected in series with the limit switch 86 and the on-off switch of the control panel. Thus, the valve 102 normally maintains the valve 62 open, the light 108 on, and the pressure switch 110 closed so that switch 110 does not interrupt the circuit for the control panel. At the same time, the valve 102 vents a two-way air actuated spring return valve 112 which is in a hydraulic line connecting a control valve 114 with the hydraulic cylinders 38 and 40. With the valve 112 vented, the spring return of the valve 112 holds the valve in a closed position thereby preventing the actuation of the cylinders 38 and 40 by means of the control valve 114.  
  When the lever 46 is swung to the unlocked position, the cam follower 104 is no longer depressed by the cam 106 and the valve 102 is shifted to vent the valve 62, to vent the pneumatic indicator light 108, to vent the pressure switch 110, and to pressurize the valve 112. With the valve 62 vented, the valve 62 interrupts the air control line leading to the actuating mechanism 58 of the steam supply valve 56 and vents the actuating mechanism 58 to assure that the steam supply valve 56 cannot be opened while the door is unlocked. With the pressure switch 110 vented, the switch is opened to interrupt the control panel circuit for the on-off switch. With the valve 112 activated, the hydraulic circuit between the control valve 114 and the hydraulic cylinders 38 and 40 is complete and thus the actual unlatching and opening of the door can now be commenced.  
  The valve 114 is a four-way lever operated valve. The valve is a three-position valve and is spring centered so that when an actuating lever 116 is in its neutral position, the ports leading to the piston-type double-acting hydraulic cylinders 38 and 40 are blocked and the pressure line for the hydraulic fluid is open back to the res ervoir. When the lever 1 16 is shifted to the left, as viewed in FIG. 1, the hydraulic cylinders 38 and 40 are pressurized to rotate the door in a clockwise direction. When the lever 116 is shifted to the right from its neutral position, as viewed in FIG. 1, the hydraulic cylinders 38 and 40 are pressurized to rotate the door in a counterclockwise direction.  
  The hydraulic circuit for the cylinders 38 and 40 which unlatch and latch the door by rotating the door about its central axis is provided with a two-way valve 118. When the door is in its closed position, the valve 118 is open. When the door is not closed, the valve 118 is closed and interrupts the hydraulic circuit for the cylinders 38 and 40. The valve 118 is mounted on the retaining ring 36 and cooperates with a plate on the door hinge 30 which depresses the valve roller when the door 24 is in the closed position. As soon as the door 24 starts to open, the valve 118 is spring biased to a closed position to thereby prevent any rotation of the door 24 by means of the hydraulic cylinders 38 and 40 when the door is open.  
  The final component of the system is the hydraulic circuit for actually swinging the door opened and closed. This circuit comprises a control valve 120 having an actuating lever 122. This valve is identical in construction to valve 114 and is connected to the source of pressurized hydraulic fluid and the reservoir in exactly the same manner as the valve 114. The hydraulic circuit connecting the piston-type doubleacting hydraulic cylinder 42 with control valve 120 is also provided with a two-way valve 124. This valve is mounted on the support arm 28 and is spring biased to a closed position. However, the valve is provided with a cam follower 126 which cooperates with a cam 128 on the door 24 to open the valve and complete the hydraulic circuit for the cylinder 42. The cam on the door and the cam follower on valve 124 are so oriented that when the door 24 is in its unlatched position, the hydraulic circuit from control valve 120 to cylinder 42 is complete. However, when the door is latched or only partially unlatched, the circuit is interrupted. in this manner, the door cannot be swung open or closed unless the lugs 32 on the door are properly oriented with the lugs 34 on the retaining ring.  
  With this circuit, when the actuating lever 122 is swung to the left from a neutral position, the hydraulic cylinder 42 is pressurized and vented to the reservoir to swing the door open. When the lever is swung to the right from a neutral position, cylinder 42 is pressurized and vented to the reservoir to swing the door closed. The hydraulic circuit for the cylinder 42 is provided with a pair of valves 130 which can be adjusted to control the rate of flow of the hydraulic fluid to and from &#39;the cylinder 42 to regulate the rate at which the door is opened or closed.  
  The sequence of operation for opening and closing the door with the system disclosed in FIG. 1 will now be set forth. To open the door. the operator first presses of button 68. This de-energizes the solenoid valve 60 thereby cutting off control air to the actuating mechanism 58 of the steam supply valve 56 and venting the actuating mechanism 58 of the steam supply valve 56 to the atmosphere. Thus, the steam supply valve 56 is closed so that no more pressurized steam is admitted to the autoclave. The operator then opens the blowdown valve (not shown) for the autoclave to depressurize the autoclave and exhaust the steam from the autoclave. When the tank pressure drops to 8 inches of wa ter, the pressure switch 72 turns off the amber lights and 82 and turns on the green lights 74 and 76. In addition, the pressure switch energizes the solenoid 84 of the key release unit 78 of the interlock system to permit the key 92 to be turned in the lock to open the limit switch 86. With the limit switch 86 open, all power to the on-off switch of the control panel is cut off. Consequently, if the on button 66 is accidentally pushed, the valve 60 will remain closed and so will the steam supply valve 56.  
  With the key 92 turned to its new position. the key can now be removed from the lock 90 and inserted into the lock 94 of the doonanlocking mechanism 44 which is the second component of the interlock system. The key 92 is turned in the lock 94 to retract the bolt 96 from the locking lever bar 46. With this bolt retracted. the locking lever bar 46 can now be swung from its normal position within the yoke 50 to its open position. The rotation of the bar 46 from its normal operating position to the open position opens the steam vent valve 48 to the atmosphere to thereby assure that the autoclave is free of any pressurized steam. At the same time the valve 48 is being opened, the valve 102 vents and closes the valve 62 to further assure that the steam supply valve 56 cannot be turned on; vents and opens pressure switch 110 to open the circuit for the on-off&#34; switch of the control panel and pressurizes and opens valve 112 to permit the unlatching of the door. it should be noted that since the cam follower 104 of valve 102 cooperates with the cam 106 of the door. once the door 24 is rotated to the unlatched position or opened. the steam supply valve cannot be turned on. Even if the lever 46 is swung back to its original position. the key removed from the lock 94 and inserted into interlock unit 78 in an attempt to energize the con trol panel. the steam supply valve still cannot be turned on for two reasons. The line from the air supply to the actuating mechanism 58 of the steam supply valve 56 is still cut off by means of the valve 62. The pressure switch 110 is open and interrupts the circuit for the control panel so that even if limit switch 86 is closed. the circuit will still be interrupted Once valve I12 is open. the door 24 can be rotated from its latched position clockwise to its unlatched position by moving lever [16 to the left from its neutral position. Since the door is closed at this time. the valve I18 is open and permits the flow of fluid through the hydraulic circuit to rotate the door by means of the cylinders 38 and 40. Once the door has been rotated to its unlatched position, the valve 124 is opened and the hydraulic circuit for the cylinder 42 is complete. The lever [22 of the valve 120 can now be moved to the left and the door will be swung open. With the door swung open. the valve 118 is now closed and the door cannot be rotated by means of hydraulic cylinders 38 and 40.  
  To close the door. the lever 122 is .moved to the right and as long as the door is in the proper position. the valve 124 is open and the door can be closed by means of the hydraulic cylinder 42. Once the door is closed. the valve 118 is opened and the hydraulic circuit for cylinders 38 and 40 is complete whereby the door can be rotated back to its latched position by the hydraulic cylinders 38 and 40. Once the door is rotated back to its latched position. the lever 46 can be swung back to its normal position to close the valve 48 to prevent steam from escaping through the valve once the autoclave is repressurized; to close the valve 112 so that the hydraulic circuit for the cylinders 38 and 40 is interrupted; to open the valve 62 so that it no longer interrupts the control air line leading to the actuating mechanism of the steam supply valve; and to close the pressure switch 110 so that it no longer interrupts the circuit for the on-off&#34; switch of the control panel.  
  With the lever 46 in its closed position, the key 92 can now be turned in lock 94 to reengage the locking bolt 96 within the lever 46. With this done, the key 92 can be removed from the lock 94 and inserted in the lock of the solenoid key release unit 78. The key is then turned in the lock 90 to move the bar 88 to the right to close the limit switch 86. With the switch 86 closed, it no longer interrupts the circuit for the &#34;onoff switch and the solenoid valve 60 can now be opened to pressurize the actuating mechanism of steam supply valve 56 by pushing on button 66. Once the valve 56 has been opened, pressure begins to build within the autoclave and as the pressure reaches and exceeds 8 inches of water, pressure switch 72 is actuated to de-energize the solenoid 84 to thereby lock the bar 88 and, consequently. key 92 within the lock 90 so that the door can no longer be opened. In addition, the green lights go out and the amber lights go on indicating that the autoclave is now under pressure.  
  The system is fail-safe. in the event of an electrical failure. the solenoid valve 60 will be deenergized and steam valve 56 will be closed. Furthermore, since the solenoid 84 will remain de-energized. the key 92 can not be removed from this interlock component to permit the unlocking of the lever 46. In addition, if there is a failure in the control air. the steam supply valve 56 will be closed by valve 62; the pressure switch will be opened to interrupt the circuit for the control buttons 62, 64 and the valve 112 will remain closed to prevent the unlatching of the door,  
  The embodiment of FIG. 8 is identical in many respects to the embodiment of FIGS. l7. Consequently. to avoid unnecessary repetition. like components of the embodiment of FIG. 8 will not be discussed in detail.  
  The autoclave 200 comprises a thick-walled steel vessel 202 with a door 204 mounted thereon. The door 204 is mounted on a support arm 206 which is pivotally secured to the vessel 202 by a hinge assembly 208. A retaining ring 210 is rotatably mounted on the vessel 202 to retain the door in a closed position when the autoclave is pressurized. As with the first embodiment. the door 204 and the ring 210 are provided with lugs which cooperate to maintain the door in a closed position when they are aligned, and which permit the door to be opened or closed when the lugs on the door and the ring are aligned with the spaces between the lugs of the other member.  
  The retaining ring 210 is rotated about its central axis by a pair of double-acting piston-type hydraulic cylinders 212 and 214. These cylinders are each pivotally secured at one end to the ring 210 and at the other end to the vessel 202. Since hydraulic cylinders 212 and 214 are both double-acting piston-type cylinders, they can be used in unison to rotate the ring both clockwise and counterclockwise about its central axis to latch or unlatch the door 204.  
  The door 204 is provided with a double-acting piston-type cylinder 216 for swinging the door about the hinge assembly 208 to open and close the door. The cylinder 216 is pivotally secured at one end to support arm 206 and at its other end to the vessel 202.  
  The ring 210 is provided with a ring-locking mechanism 218 which is substantially identical in construction to the door-locking mechanism 44 of the first embodiment. The locking mechanism 218 is provided with a locking bar 220 which is pivotally mounted on a steam vent valve 252 of the vessel 202. The locking bar 220 cooperates with a plate 222 mounted on the ring 210 to prevent the rotation of the ring 210 about its central axis when the door is locked.  
  The autoclave is provided with a pneumatically actu ated spring return steam inlet valve 224 which is identical in construction to the valve 56 of the first embodiment. An actuating mechanism 226 of the valve is connected to a supply of pressurized air through an air control line which is provided with a three-way solenoid air valve 228, a three-way air valve 230 and a conventional controller 232.  
  The three-way solenoid valve 228 is identical in construction and function to the three-way solenoid air valve 60 of the first embodiment. Thus. when the solenoid valve 228 is energized. it opens the steam supply valve 224 and when the valve is de-energized. the valve functions to close the steam supply valve 224 for the autoclave.  
  The three-way cam-actuated spring return air valve 230 permits the passage of pressurized air through the control air line leading to the actuating mechanism 226 when a cam follower 232 of the valve is depressed by a cam 234 mounted on the retaining ring 210. When the cam follower is no longer engaged by the cam 234, a spring in the valve shifts the valve to a position where the flow of pressurized air through the valve to the ac tuating mechanism 226 is cut off and that portion of the air control line leading from the valve to the actuating mechanism 226 is vented to shut off the steam supply valve 224.  
  The controller 235 is identical in construction and function to the controller 64 of the first embodiment and is connected to a supply of pressurized air.  
  A control panel of the system is provided with an &#34;on-off&#34; switch having an on&#34; button 236 and an &#34;off button 238. These buttons energize and deenergize the solenoid valve 228 to turn the steam supply valve 224 on or off.  
  The system also includes a pressure gauge 240 and a pressure switch 242. The pressure gauge 240 is pro vided to give a visual confirmation of the actual pressure within the autoclave. When the pressure within the autoclave drops to 8 inches of water. the pressure switch 242 is actuated to turn on a pair of green signal lights located on the control panel and a solenoid key release unit 244. At the same time. the switch turns off a pair of amber signal lights on the control panel in the solenoid key release unit. 244. Thus, the signal lights give a visual indication that the pressure in the autoclave is above or below 8 inches of water.  
  The solenoid key release unit 244 is identical in construction to the solenoid key release unit 78. Consequently. when a solenoid of the unit is energized by the pressure switch 242, a key 246 can be turned in a lock 248 of the unit 244 to open a limit switch of the unit and thereby cut off all power to the on-off switch. As with the other solenoid key release unit, the key 246 can only be removed from this unit when the limit switch of the unit has been opened.  
  The locking mechanism 218, which comprises the other component of the interlock unit. is substantially identical in construction to the locking mechanism 44 and is provided with a lock 250 which is compatible with the key 246. As in the first embodiment, the locking lever bar 220 is provided with a plate to prevent the removal ofthe key 246 when the lever is not in its lock ing position. The bar 22 is secured to a valve stem of the steam vent valve 252 of the autoclave. Thus, when the lever 220 is swung from its locking position where it engages plate 222 to an unlocked position. the lever turns the valve stem of the steam vent valve 252 to open this valve and vent any remaining steam within the autoclave.  
  As in the previous embodiment, the locking lever bar 220 also carries a four-way cam operated spring return air valve 254 which is connected directly to a supply of pressurized air. The valve is provided with a cam follower 256 which cooperates with a cam 258 on the retaining ring of the autoclave. When the cam follower 256 is depressed by the cam 258, the valve 254 con nects a pressure switch 260 and a pneumatic visual indicator 262 with an air control line. The pneumatic visual indicator 262 is identical in construction and function to indicator 108. The pressure switch 260 is connected in series with the limit switch of the solenoid key release unit 244 and the on-of switch of the control panel. Thus. the valve 254 keeps the pressure switch 260 closed when the door is locked so that the switch 260 does not interrupt the circuit for the control panel. At the same time. the valve 254 vents a two-way airactuated spring return valve 264 which is in a hydraulic circuit connecting a control valve 266 with the hydraulic cylinders 212 and 214. With the valve 264 vented. the spring return of the valve 264 holds that valve in a closed position thereby preventing the actuation of cylinders 212 and 214 by means of the control valve 266.  
  When the lever 220 is swung to the unlocked position, the valve 254 is shifted to vent the pressure switch 260, to vent the pneumatic visual indicator 262 and to pressurize valve 264. With the pressure switch 260 vented. the switch is opened to interrupt the control panel circuit for the on-of switch. With the valve 264 activated. the hydraulic circuit from the control valve 266 to hydraulic cylinders 212 and 214 is complete and thus the actual unlatching and opening of the door can be commenced.  
  The valve 266 is a four-way lever-operated valve. The valve is a three-position valve and is spring centered so that when an actuating lever 268 of the valve is in its neutral position, the ports leading to the pistontype double-acting hydraulic cylinders 212 and 214 are blocked and the pressure line is open back to the reservoir. When the lever 268 is shifted to the left, the hydraulic cylinders 212 and 214 are pressurized to rotate the ring in a counterclockwise direction. When the lever 268 is shifted in the opposite direction. the hydraulic cylinders 212 and 214 are pressurized to rotate the ring in a clockwise direction.  
  The final component of the system is the hydraulic circuit for swinging the door opened and closed. This circuit comprises a control valve 270 having an actuating lever 272. This valve is identical in construction to valve 266 and is connected to the source of pressurized hydraulic fluid in the reservoir in exactly the same manner as 266. The hydraulic circuit connecting the pistontype double-acting hydraulic cylinder 216 with control valve 270 is also provided with a two-way valve 274. This valve is mounted on the autoclave and is spring biased to a closed position. However. the valve is provided with a cam follower 276 which cooperates with a cam 278 on the rotating ring to open the valve and complete the hydraulic circuit for cylinder 216. The cam on the ring and the cam follower on the valve 274 are so oriented that when the ring is in the unlatched position. the hydraulic circuit from the control valve 270 to the cylinder 216 is complete. However. when the ring is rotated to latch the door or when the door is only partially unlatched, the circuit is interrupted. In this manner, the door cannot be swung open or closed unless the retaining ring is properly oriented with the lugs on the door.  
  With this circuit. when the actuating lever 272 is swung to the left the hydraulic cylinder 216 is pressurized and vented to the reservoir to swing the door open. When the lever is swung to the right from a neutral position, the cylinder 216 is pressurized and vented to the reservoir to swing the door closed. The hydraulic circuit for the cylinder 216 is provided with a pair of valves 280 which can be adjusted to control the rate of flow of hydraulic fluid to and from the cylinder 216 to regulate the rate at which the door is opened or closed.  
  The sequence of operation for opening and closing the door is essentially the same as the sequence for the embodiment of H6. 1. The only major distinctions are valve 230 of the second embodiment which corresponds with valve 60 of the first embodiment and valve 274 of the second embodiment which corresponds with valve 124 of the first embodiment. Valve 230, instead of being pressure-actuated as is valve 60, is actuated by means of cam 234 on the retaining ring. Thus, this valve acts to cut off the supply of pressurized air to the actuating mechanism of steam valve 224 when the ring 210 is rotated to the unlatched position, rather than cutting off the supply when the lever is swung out as in the first embodiment.  
  Valve 274 which completes or interrupts the circuit for the hydraulic cylinder 216 used to open the door takes the place of valves 124 of the first embodiment. When the retaining ring is in the proper position for swinging the door open and closed. the circuit for hydraulic cylinder 216 is complete. If hydraulic cylinders 212 and 214 are actuated at any time while the door is opened to change the position of the retaining ring. the circuit is interrupted, thereby preventing the closing of the door. Consequently, while the ring can be rotated when the door is open. the valve 274 assures that the door is properly oriented before it is swung either open or closed.  
  The system is fail-safe. In the event of an electrical failure. the solenoid valve 288 will be de-energized and the steam valve 224 will be closed. Furthermore, since the solenoid of the solenoid key release unit 244 will remain de-energized, the key 246 cannot be removed from this interlock component to permit the unlocking of lever 220. A loss in the air control pressure will cause the steam supply valve 224 to close. the pressure switch 260 to open to interrupt the circuit for the on off&#34; switch of the control panel and the valve 264 to re main closed to prevent the unlatching of the door.  
 What we claim is:  
  1. A system for safely operating a door of an auto clave which is normally pressurized when in operation comprising:  
 a. means for sensing pressure within the autoclave and for preventing the door from being opened when the pressure within the autoclave is above a predetermined level, the means for sensing pressure within the autoclave and for preventing the door from being opened including a first key inter lock unit said first unit having a first lock and a key for said lock. and said first unit having a means for preventing removal of the key from said lock until the pressure within the autoclave is below the predetermined level;  
 b. locking means for locking the door in a closed position, said locking means including a second key interlock unit, said second unit having a second lock which is fit by said key so that the door can be unlocked only after said key has been removed from said first unit; and  
 c. means for closing a steam supply valve of the autoclave and for maintaining the steam supply valve closed when the door is being opened, is open or is being closed.  
 2. The system of claim 1 wherein:  
 a. the means for maintaining the steam supply valve closed includes means for preventing the steam supply valve from being opened in said first key interlock unit which is actuated by moving the key in the first lock to a position where the key can be removed from the first lock.  
 3. The system of claim 2 wherein:  
 a the means for maintaining the steam supply valve closed includes means for preventing the steam supply valve from being opened which is actuated when the door is unlocked.  
 4. The system of claim 1 wherein:  
 a. the means for maintaining the steam supply valve closed includes means for preventing the steam supply valve from being opened which is actuated when the door is unlocked.  
 5. The system of claim 1 wherein:  
 a. vent means is provided for venting the autoclave;  
 and  
 b. the locking means includes means for opening the vent means as the door is being unlocked.  
  6. A system for safely operating a door of an autoclave oven which is pressurized when in operation comprising:  
 a. a door having lugs spaced about the perimeter of the door;  
 b. a retaining ring on the autoclave, the retaining ring having lugs spaced about the ring for cooperating with the door lugs to retain the door in a closed position;  
 c. rotating means for rotating the door and the ring relative to each other to latch or unlatch the door by engaging or disengaging the lugs;  
 d. means for opening and closing the door;  
 e. a steam supply valve for supplying the autoclaves with pressurized steam;  
 f. means for sensing pressure within the autoclave and for preventing the door from being opened when the pressure within the autoclave is above a predetermined level, the means for sensing pressure within the autoclave and for preventing the door from being opened including a first key interlock unit, said first unit having a first lock and a key for said lock, and said first unit having a means for preventing removal of the key from said lock until the pressure within the autoclave is below the predetermined level;  
 g. locking means for locking the door in a closed position, said locking means including a second key interlock unit, said second unit having a second lock which is fit by said key so that the door can be unlocked only after said key has been removed from said first unit; and  
 h. means for closing a steam supply valve of the autoclave and for maintaining the steam supply valve closed when the door is being opened, is open or and is being closed. c. the rotating means rotates the door to effect the 7. The system of claim 6 wherein: latching or unlatching of the door. a. vent means is provided for venting the autoclave; 11. The system of claim 9 wherein:  
  and a. the retaining ring is rotatably mounted on the autob. the locking means includes means for opening the clave; and  
  vent means as the door is being unlocked. b. the rotating means rotates the retaining ring to ef- 8. The system of claim 7 wherein: feet the latching or unlatching of the door. a. the locking means includes means for preventing 12. The system of claim 9 wherein:  
  actuation of the rotating means when the door is 10 the means for sensing pressure within the autoclave locked. and for preventing the door from being opened and 9. The system of claim 8 wherein: the means for closing the steam supply valve and a. means is provided to prevent actuation of the for maintaining the steam supply valve closed are means for opening and closing the door unless the electrically and pneumatically operated in such a door is unlatched. manner that an electrical or pneumatic failure pre- 10. The system of claim 9 wherein: vents the door from being opened and closes the a. the retaining ring is stationary; steam supply valve. b. the door is rotatably mounted on the autoclave;