US 3396375 A
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68 D. D. LOW 3,396,375
FAIL- SAFE ALARM SYSTEM Filed May 20, 1965 PRESSURE SWITCH INVENTOR DONALD D. Low
3,396,375 FAIL-SAFE ALARM SYSTEM Donald D. Low, Oakland, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed May 20, 1965, Ser. No. 457,535 10 Claims. (Cl. 340-236) The present invention relates generally to apparatus for monitoring fluid pressure and more particularly to a failsafe malfunction detecting means for use with an air exhaust system or the like.
The invention was developed for use with an air exhaust system for removing either radioactive or chemically polluted air from enclosed gloveboxes of the type wherein radioactive materials or toxic chemicals are handled. In gloveboxes of this type an exhaust blower continually operates to maintain a slightly negative pressure within the enclosed region. This prevents an excessive accumulation of contamination within the box and insures containment inasmuch as the air flow through any leaks in the box will be inward rather than outward. It is very important for the safety of personnel that the air exhaust system operate continuously and reliably, and that an alarm be activated in the event of failure.
The present invention is a monitor for such air exhaust systems and follows the Scully-Rowell theory of fail-safety as described, for example, in the article F ail-Safe Gets a New Meaning in the publication Control Engineering, March 1956, McGraw-Hill. The basic principle in this approach to fail-safety is to periodically simulate an unsafe condition and to provide for actuation of the alarm signal whenever the alternation between safe and unsafe indications ceases or changes frequency. Thus the alarm is actuated not only by a continuous unsafe signal but is also actuated by a continuous safe signal. In effect, the alarm system is continually monitoring its own operation as well as that of the apparatus to which it is connected. The apparatus is fail-safe in that failure of the monitoring apparatus itself is also detected.
The present invention is an adaptation of the basic monitoring principle discussed above, and is particularly suitable for use with air exhaust equipment, although the invention may be used with other fluid systems.
In the invention, a pressure responsive switch is periodically operated by being alternately communicated with the partial vacuum in the system from which air is being exhausted and with the atmosphere, motor driven means being provided to cycle the switch input in the described manner. The switch is connected between a power supply and the heating elements of a pair of thermostatic delay relays, in such a manner that the relay heating elements are alternately energized by the cyclical operation of the pressure switch. Such energization of the heating elements causes the normally closed relay contacts to open after a predetermined time period of energization of the heating elements. However, as utilized in the present invention the period between cycles of the pressure responsive switch, and thus the period of energization of the relays, is normally less than the delay period of the relays so that the relays remain unoperated under normal closed conditions. However, in the event of failure in the exhaust system or in the monitoring apparatus itself, the pressure switch is no longer able to periodically energize the relay. Thus the delay time of the relay is exceeded, allowing the relay contacts to open and actuate an alarm or otherwise initiate corrective steps. The invention may,
for example, be adapted to automatically energize emergency stand-by apparatus in the event of failure in the primary system. Previous fail-safe systems of this general type are much more complex, with correspondingly United States Patent 3,396,375 Patented Aug. 6, 1968 greater maintenance costs and difiiculties. The present invention uses readily available, low cost components and provides significantly more trouble-free operation than previous alarm systems since many fewer components are required.
Therefore it is an object of the present invention to provide a simplified fail-safe system for detecting a change in the pressure of air or other gases within an enclosure.
It is another object of the present invention to provide a fail-safe system for protecting apparatus and personnel associated with gloveboxes in which radioactive or toxic chemicals are handled.
It is another object of the present invention to provide a fail-sate pressure change which is relatively free from malfunctions and relatively maintenance free.
It is another object of the present invention to provide an improved fail-safe system particularly adapted for monitoring the operation of an air exhaust system used with a glovebox.
The invention will be best understood by reference to the accompanying drawing which is a circuit diagram with mechanical elements shown schematically. The relays in the drawing are shown in the de-energized condition.
Considering first the exhaust system, there is shown an integral main blower and motor assembly 11 which is coupled by suitable piping 12 to a standard glovebox 13. The apparatus serviced by the exhaust system is not in itself a portion of the invention and may be any apparatus in which a negative or positive pressure is required. A standby blower and motor assembly 14 is ordinarily connected in parallel with the main blower 11 and is activated by means of the present invention in the event a rise of pressure is detected in glovebox 13.
Mechanical elements of the invention include a disc 16 which is rotated slowly by an electric motor 18 through a speed reducing gear train 17. A single-pole double-throw pressure sensitive switch 19 is periodically operated by being temporarily communicated through piping 21 with the glovebox 13 and alternately being communicated with the atmosphere by rotation of the disc 16. To effect such an operation, communication between the pressure switch 19 and the glovebox 13 is interrupted by the disc 16 which is interposed transversely across a gap 20 in the pipe 21, between facing first and second pipe ends 24 and 26. When the disc 16 is rotated, a plurality of holes 22 and a slit 23 provided in the disc 16 effect a periodic variation in the pressure applied to the pressure sensitive switch 19. The holes 22 penetrate completely through the disc, and when aligned with pipe ends 24 and 26, bridge across gap 20 to connect pressure switch 19 and glovebox 13. Each hole 22 has a diameter approximately the same size as the inner bore of the piping 21. The holes 22 are regularly spaced around a semicircle on the disc while the slot 23 occupies the remainder of the circle. The slot 23, which does not penetrate through the disc 16, faces the end 24 of the pipe 21 leading to the pressure switch 19. Thus during half of each turn of the disc 16, the pressure switch 19 is coupled through the holes 22 to the glovebox 13 exhaust system while during the other half turn of the disc, the pressure switch is coupled to the atmosphere via the slot 23. It will be noted that the pressure switch 19 is not continuously coupled to the glovebox 13 during the half revolution of disc 16 when the holes 22 successively bridge the gap 20, however, the end 24 is sealed off by the surface of disc 16 between the times the holes bridge gap 20 so that the pressure at the switch is maintained at the pressure of the exhaust system. A hermetic seal 27 is dispose-d around the end 24 to prevent passage of air into the pipe section 21 except via the holes 22 and slot 23. A similar seal is provided at the facing end 26.
Considering the electrical circuitry of the invention, operating power, usually volts at 60 cycles per second,
is provided through a standard double power plug 31, one side 32 of the power line being considered as neutral or ground and the other side 33 being considered as the high voltage line. The movable arm 34 in a first relay 36 connects the high voltage line 33 to the motor of the standby blower 14 when the relay coil is de-energized but connects the high voltage line to the main blower 11 when the relay 36 is energized. The other side of the motors of blowers 11 and 14 are connected to the neutral line, thus one of the blowers is always in operation. Normally, the first relay 36 is energized so that the main blower 11 is in operation. One side of the first relay 36 coil is connected directly to the neutral line 32 while the other side is connected to the high voltage line 33 through a second relay arm 37 of relay 38 when the coil of the relay 38 is energized. A first side of the coil of the second relay 38 is connected to the neutral line 32, while the second side is connected to a switch arm 41 of the pressure switch 19. Normally closed relay arms of a first and a second thermostatic delay relay 42 and 43 are connected in series from the high voltage line 33 to a first movable arm 39 of relay 38. Arm 39 is a holding arm, that is, power to energize the coil of the second relay 38 is provided through the arm 39. If power to the second relay 38 is momentarily interrupted, the circuit between the relay coil 40 and the high voltage line 33 is broken at the first arm 39 and the relay 38 is not automatically reenergized when power is restored.
The thermostatic relays 42 and 43 each include electrical heating elements 44 and 46 respectively. When rated current is passed through the heating elements, after a short delay time which typically may be fifteen seconds, bimetallic relay arms 45 open. One end of each of the heating elements 44 and 46 is connected to the neutral line 32 while the other ends are connected to opposite terminals of the single-pole double-throw pressure switch 19. When the pressure switch 19 is operated by the means previously described, the heating elements 44 and 46 are alternately energized by current through the movable arm 41. If the pressure switch should stay in one position for a time period exceeding the delay time of one of the delay relays 42 and 43, indicating a malfunction in the system, the delay relay contacts will open causing the second relay 38 to be de-energized. Power for operating the motor 18 is obtained through the movable relay contact 39 of relay 38 so that if the second relay 38 is deenergized, the motor 18 is also de-energized.
With de-energization of second relay 38, an alarm bell 47 or other form of warning indicator is actuated by relay arm 37. Simultaneously, first relay 36 is inactivated owing to the switch of relay arm 37 thereby causing auxiliary blower 14 to operate. The bell 47 is connected between the neutral line 32 and the arm of a single-pole double-throw alarm switch 48. The terminals of the alarm switch 48 are each connected to one of two contact terminals for the arm 37 of second relay 38. Such switch is used to test operation of the bell 47 and to silence the bell after an alarm has sounded.
Operation of the fail-safe circuit is initiated by operation of a spring-loaded start switch 49 which is normally open. The stationary contacts of the switch 49 are connected across the stationary contacts of the presssure switch 19 while a spring-loaded movable contact 51 is connected to the high voltage line 33. When operation of the fail-safe circuit is initiated, the movable arm 51 is momentarily moved to one of the switch positions so that the second relay 38 is energized through the movable arm 41 of pressure switch 19. The switch 49 is also useful for test purposes to check the operation of the various relays.
Considering the operation of the invention, the fail-safe circuit is set or turned on by depressing the start switch 49 momentarily so that the second relay 38 is activated, the relay 38 thereafter being maintained in an activated condition by current passing through the holding relay arm 39. Simultaneously with the energization of the second relay 38, the first relay 36 is activated, causing main blower 11 to operate. Also, motor 18 is activated, causing the the presssure switch 19 to be operated periodically owing to the rotation of the disc 16. If, for example, the delay time for the delay relays 42 and 43 is fifteen seconds then the disc 16 should complete a single complete rota.- tion in less than thirty seconds to avoid exceeding the delay time of the delay relays.
In the event of malfunction of the main blower 11, then a, constant gas pressure is applied to the pressure switch and one of the delay relays 42 and 43 will open, causing the alarm bell 47 to ring. While in the particular embodiment of the invention described an auxiliary blower 14 is activated automatically in the event of a vacuum loss, such feature is optional and in many instances merely sounding an alarm may be suflicient.
It will be noted that not only malfunction of the blower system will cause the alarm to be sounded, but failure of the motor 18-disc 16 pressure cycling means, or failure of the pressure switch 19 to respond to the pressure changes, or any other condition causing inactivation of the second relay 38 will all result in sounding of the alarm. Thus the proper functioning of much of the pressure monitoring apparatus is itself monitored and the apparatus is, to a large degree, fail-safe.
Obviously, the invention is not restricted to air-exhaust systems. The invention can be utilized with any fluid system wherein two different fluid pressures must be maintained.
While the invention has been disclosed with respect to a specific embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and it is not intended to limit the invention except as defined in the following claims.
What is claimed is:
1. In apparatus detecting a change in a first fluid pressure in a system in which said first pressure differs from a second reference fluid pressure, the combination comprising a switch of the class having a first and a second position determined by the fluid pressure at an input, coupling means cyclically communicating said pressure switch to said first pressure and alternately to said reference fluid pressure within a predetermined time period, a first and a second delay relay means alternately energized by said pressure switch at said first and second positions thereof respectively, said relay means having a delay time exceeding one-half of said time period and a pressure change indication means actuated by operation of any one of said relay means when said delay time thereof is exceeded.
2. In apparatus monitoring the presence of a first degree of fluid pressure within a system wherein said first fluid pressure normally differs from a second fluid pressure in a region outside said system, the combination comprising a pressure switch having a first switch position when subjected to said first fluid pressure and having a second switch position when subjected to said second fluid pressure, means alternately and cyclically communicating said pressure switch with said first and second pressures, a power source, a first and a second relay of the class which operates after energization for a delay time period, said delay time period exceeding one-half of the time period in which said pressure switch completes one cycle of operation between said first and second positions thereof, said first relay being connected through said pressure switch in said first switch position to said power source, said second delay relay being connected through said pressure switch in said second switch position to said power source, and a malfunction sensing means connected to said relays and responsive to operation of either thereof.
3. A monitoring apparatus as described in claim 2 wherein said means cyclically communicating said pressure switch with said first and second pressures comprises a pipe extending between said pressure switch and said system and having a transverse gap, a rotatable disc interposed between said pressure switch and said first pressure system at said gap of said pipe, said disc being perforated by at least one first opening for communicating said pressure switch to said system at a first angular position of said disc, said disc further having a groove therein communicating said pressure switch to said second fluid pressure region at a differing angular position of said disc, and rotary drive means coupled to said disc.
4. A monitoring apparatus as described in claim 2, wherein said first and said second relays each have a heater element connectable to said power source through said pressure switch, said first and said second relays each being of the type having relay contacts operable when heated, said malfunction sensing means being connected to said relay contacts for activation upon operation thereof.
5. In apparatus for detecting malfunction of an air exhaust system, the combination comprising a double-throw switch having a first and a second contact and a switch arm movable therebetween in response to a change in pressure at an input, means alternately communicating said exhaust system and the atmosphere to said switch input, a first and a second thermostatic delay relay each having a heating element which is connected to said first and said second contact respectively of said switch and each having a normally closed relay contact, a power source connectable across said heating element through said switch arm, and means detecting the opening of one of said relay contacts as a result of said switch having been in one switch position for a time longer than the delay time of said relays.
6. In a fail-safe apparatus for detecting malfunction of an exhaust system for an enclosure in which air is maintained at a differing level than the air pressure of the atmosphere, the combination comprising a pressure sensitive switch having a first position at atmospheric pressure and having a second position when at said differing level of air pressure, an air conduit connecting said enclosure to said switch, said conduit having a gap therein, a rotatable disc transecting said conduit at said gap and being transpierced by at least one aperture for communicating said enclosure with said switch through said conduit at a first angular position of said disc, said disc further having a groove on the side adjacent said switch at a second angular position on said disc for alternately communicating said pressure switch to the atmosphere,
means rotating said disc whereby said switch is alternately opened and closed, an electrical power source, a first delay relay means connected to said power source through said pressure switch at one position thereof, a second delay relay means connected to said power source through said pressure switch at the other position thereof, the delay time of said first and said second delay relay means exceeding one-half the time it takes for said disc to cycle said pressure switch whereby said relays do not normally operate, and an indicating circuit connected to said power source through said first and second delay relay means and adapted to be activated if one of said first and second delay relays operates.
7. In a fail-safe alarm as described in claim 6, the further combination of a normally unenergized stand-by air exhaust system coupled to said enclosure and said indicating circuit, said stand-by means being energized when said indicating circuit is activated.
8. A fail-safe apparatus for detecting malfunction of an exhaust system as described in claim 6 wherein a plurality of said apertures are provided on a first half of said disc, said apertures being equidistantly spaced from the axis thereof, and wherein said groove is semi-circular and situated on the second half of said disc.
9. A fail-safe apparatus for detecting malfunction of an exhaust system as described in claim 6, wherein said indication circuit is provided with an available alarm energized upon operation of one of said relays and providing an audible indication thereof.
10. A fail-safe alarm as described in claim 6, wherein said power source has a first side and a second side and said first and second delay relays each have normally closed switch contacts, said switch contacts being connected in series, and wherein said indicating circuit comprises a third relay having a first and second movable arm each with an energized position and an unenergized position, said third relay further having a relay coil with one side electrically connected to said first side of said power supply and the other side connectable to said first movable arm in said energized position, said first movable arm being connected in series through said switch contacts to said second side of said power supply, and wherein said indicating circuit comprises an alarm means connectable to said power supply through said second movable arm in said unenergized position.
No references cited.
JOHN W. CALDWELL, Primary Examiner.
D. K. MYER, Assistant Examiner.