Abstract:
A method for controllably releasing venting gases from a manhole space around the perimeter of a manhole cover by allowing the manhole cover to rise up in stages upon development of upward forces in an explosion to vent gases developed by the explosion while being restrained from being completely blown free except under extreme conditions. A lock body mounts a latch slide which has an outer end and which can be extended to prevent removal of the manhole cover. When a shear pin fails the lock body pivots down, to a limited extent, and a secondary shear pin can also be included allowing the cover to be blown completely free upon development of pressures of a great magnitude. A slotted skirt is used to direct venting gas flow so as to retard the inflow of free air and attenuate the explosion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a division of U.S. patent application Ser. No. 11/761,711 filed on Jun. 12, 2007, now U.S. Pat. No. 7,484,908, which claims the benefit of U.S. provisional patent applications No. 60/921,975 filed on No. Apr. 6, 2007; 60/889,553 filed on Feb. 13, 2007; and No. 60/812,757 filed on Jun. 12, 2006. 

   BACKGROUND OF THE INVENTION 
   This invention concerns mounting of manhole covers used to close off access to utility passages extending beneath city streets. For security purposes, it is desirable to limit access to such passages by locking the manhole covers onto their supporting seats. 
   However, manhole covers are sometimes subjected to very high pressures caused by explosions as when an accumulation of methane gas, etc. in the space below is ignited. 
   Manhole vault explosions usually blow the manhole covers out of their seats and into the air with great force. Since each manhole cover must be reinstalled as soon as possible after an explosion to cover up the hazardous open manhole, a significant maintenance cost is entailed. 
   Until a dislodged manhole cover can be replaced into its frame, the open manhole presents a serious hazard. 
   Pressure rises rapidly beneath a manhole cover in an explosion, and even a relatively small pressure rise will lift the manhole cover off its seat. For example, a momentary pressure rise of only one PSI beneath a 700 square inch manhole cover weighing 200 lbs. equates to a 500 lb. force available to dislodge the cover from its seat. 
   Although such explosive events are rare, when they do occur, manhole covers are often blown high into the air, can cause much damage, and even become deadly if a manhole cover strikes a passerby. 
   Typically, a metal manhole cover frame is cemented to the top of a manhole site chimney and set into the surrounding pavement. 
   A complicating factor is that the dislodging of the manhole cover acts to relieve gas pressure in the manhole during explosive events. Fixing manhole covers in place on their seats could cause damage to enclosing structures if there is no venting of the rapidly expanding gases. Pressure must somehow be relieved to avoid this potential structural damage. 
   It is an object of the present invention to provide a method which allows a controlled pressure relief while avoiding launching of the manhole cover out of its seat when an explosion occurs in the space beneath the cover. 
   SUMMARY OF THE INVENTION 
   The above recited object as well as other objects which will become apparent upon a reading of the following specification and claims are achieved by a method including mounting a lock body on the underside of a manhole cover at the perimeter thereof. A lug is mounted on the opposite side of the manhole cover so it can hook an inwardly sloping rim on the manhole seat defining structure. 
   The lock body is pivoted at one end between a pair of mounting plates by a swivel pin. Advance of the actuator bolt with a special wrench engages a leading end thereof with the cam surface on the latch slide to force the latch slide to move radially outward beyond the perimeter of the manhole cover. In that position, the latch slide will engage a sloping sidewall feature of the manhole cover enclosure when the cover is lifted up off its seat a short distance by the force of an explosion, creating a gap between the enclosure seat and the cover perimeter, allowing the venting of gas about the perimeter of the cover while preventing the manhole cover from being blown free. 
   The pivoted lock body may be restrained from pivoting down by a primary shear pin which will fail at a predetermined force level, allowing the lock body to pivot down a short distance where a stop engages a side of an enlarged opening in the lock body to prevent any further downward pivoting motion. This arrangement allows the cover to rise a predetermined short increment higher to create a greater venting area for the exit of explosive gases while still preventing the cover from being blown free. 
   The stop may be comprised of a secondary shear pin designed to also shear at a very high pressure level, allowing the lock body to swing down completely and let the manhole cover be blown free if very large pressures are experienced during the explosion which cannot be sufficiently relieved by the partial venting to prevent great structural damage. 
   According to another feature of the present invention, the manhole cover depending skirt extending around its perimeter is formed with scalloped slots shaped to redirect the exiting gases back toward the clearance gap around the cover, retarding the entrance of fresh air into the manhole and to attenuate the explosive combustion of the unburned gases which would otherwise occur. This reduces the magnitude of the peak pressure developed beneath the manhole cover from that which would otherwise develop. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a fragmentary sectional view through a manhole metal enclosure and surrounding paving section with a manhole cover resting on a seat defined by the enclosure, having a security locking arrangement according to the present invention installed thereon including a lock body assembly and a fixed lug. 
       FIG. 1A  is a view of the arrangement shown in  FIG. 1  but with the manhole cover in the process of being installed. 
       FIG. 2  shows the components shown in  FIG. 1  with the manhole cover lifted as by the force of an explosion to bring fixed lug and latch slide portions into abutment with a sloping feature on the inside of manhole enclosure. 
       FIG. 3  shows the components shown in  FIGS. 1 and 2  with the primary shear pin failed, resulting in a pivoting down of the lock body in turn allowing an additional incremental rise of the manhole cover off its seat to increase the area of the gap available for venting gases. 
       FIG. 4  is a fragmentary sectional enlarged view of the lock body and latch slide components, with adjacent portions of the manhole cover and enclosure. 
       FIG. 5  is a view of the components shown in  FIG. 4  with the lock body pivoted down a short distance. 
       FIG. 6  is a fragmentary view of the manhole cover showing an end view of the lock body assembly components. 
       FIG. 7  is a diagrammatic representation of the gas flow past the manhole cover in an explosion illustrating the redirection of gas flow induced by scalloped slots in the skirt on the inside of the manhole cover. 
       FIG. 8  is a plan view of the bottom of the manhole cover showing the slotted skirt and the lock arrangement components. 
       FIG. 8A  is a diagrammatic representation of the gas flow path induced by the slots. 
       FIG. 9  is an enlarged fragmentary view of a portion of the slotted skirt on the manhole cover. 
       FIG. 9A  is an enlarged fragmentary view of another portion of the slotted skirt showing a centering guide. 
       FIG. 10  is a sectional view of a modified form of the lock body assembly. 
       FIG. 11  is a partially sectional view of a portion of a manhole frame with a manhole cover having a modified form of the lock body assembly mounted thereto. 
       FIG. 12  is an inside view of the manhole cover having an explosion indicator rope hung from the inside of the manhole cover. 
       FIG. 13  is an end view of the components shown in  FIG. 11 . 
       FIG. 14  is a side view of the lock body assembly in the fully pivoted down position completely releasing the manhole cover. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
   Referring to the drawings,  FIG. 1  shows a manhole cover  10  resting on a seat  16  defined by a metal enclosure  12  recessed into street paving  14  and defining the manhole cavity itself. The enclosure  12  has an inwardly sloping annular feature  18  having the seat  16  defined on the top surface. 
   According to the present invention, a security locking arrangement is provided, comprised of a lock body assembly  20  fixed to the underside of the manhole cover  10  adjacent to the outer perimeter thereof in the space between two parallel extending ribs  11  extending across the underside of the cover  10 . On the diametrically opposite side, a fixed lug  22  is integrally cast into the underside of the manhole cover  10  having an outwardly projecting portion  24  located to engage the sloping feature  18  when the manhole cover  10  is elevated off the seat  16  to a predetermined height. 
   The lock body assembly  20  includes a latching slide  26  which has an end portion  28  which will also engage the sloping feature  18  when extended out to the position shown in  FIG. 1 . 
     FIG. 1A  shows the latching slide  26  retracted within a lock body  34  for installation of the manhole cover  10  by angling it into the manhole opening within the enclosure  12 . After seating the manhole cover  10 , an actuator bolt  30  is advanced, as will be described below, to shift the latching slide  26  radially to the extended position shown in  FIG. 1 . 
     FIG. 2  shows the initial upward movement of the manhole cover  10  resulting from an explosion. The vertical space between the sloping surface  18  of the enclosure  12  and the lug portion  24  and latching slide portion  28  as seen in  FIG. 1  allows the manhole cover  10  to lift up an inch or two before engagement of the portions  24 ,  28  with the enclosure feature  18 . The resulting gap around the perimeter of the cover  10  allows the venting of the hot gases generated by the explosion. 
   If the forces on the cover  10  created by the explosion exceed a predetermined level, a primary shear pin  32  holding the lock body  34  from pivoting about a pivot pin  36  will fail, allowing the lock body assembly  20  to pivot down to a shallowly angled position shown in  FIG. 3 . This creates another inch or so clearance about the perimeter of the cover  10  as seen in  FIG. 3  such that the cover  10  can tilt up to open a larger gap, creating a staged additional venting area for the gases generated by the explosion so as to avoid structural damage by the development of high pressures in the manhole cavity. 
     FIG. 4  shows internal details of the lock body assembly  20 . The lock body  34  is pivoted at one end on the pivot pin  36  received between vertical ribs  11  to allow limited rotation down from the cover  10  when the primary shear pin  32  also received in the ribs  11  is sheared off by the forces acting through the slide portion  28 . 
   The latch slide  26  is slidably received in a bore  38  formed in the lock body  34 . A keeper blade  40  is received in a slot  42  in the latch slide  26  to prevent rotation of the latch slide  26  within the bore  38 . 
   A spring  44  interposed between keeper blade  40  and an end wall  46  of the slot  42  urges the latch slide  26  to the left to tend to retract the portion  28  radially inwardly. 
   The actuator bolt  30  has a rounded end  48  which engages a sloping cam surface  50  on the top of the latch slide  26  which forces the latch slide  26  to the right when the bolt  30  is rotated to be advanced until the fully advanced position is reached as seen in  FIG. 4 . The bolt can be turned using an anti-tamper special wrench tool  52  mating with a correspondingly specially shaped bolt head  51  to prevent unauthorized removal of the manhole cover  10 . Such a tool and bolt head is described in U.S. Pat. No. 6,764,261. A plug  60  can enclose the bolt head  51  for protection and to keep debris from filling the recess within the cover  10  accommodating the bolt head  51 . 
   A retainer ring  54  is fixed at one of the bore  38  preventing escape of the latch slide  26  to the left when the actuator bolt  30  is removed. 
   A stop pin  58  is received in an elongated arcuate slot  56 . When the primary shear pin  32  releases, the latch body  34  pivots down a short distance until a bumper  62  contacts stop pin  58  in the position shown in  FIG. 5  preventing further pivoting. 
   The actuator bolt  30  has an annular curved shaped groove  64  near its end which is positioned in a hole in a flat at the end of the latch slide  26 . This allows the latch slide  26  to be moved slightly further to the left by the spring  44  when the bolt  30  is fully advanced. When the bolt  30  is withdrawn, a slight camming action by the curved side of the groove  64  breaks the slide  26  free if ice or corrosion has developed seizing the latch slide  26  in the bore  38  allowing the spring  44  to again act to retract the latch slide  26  with portion  28  to enable removal of the manhole cover  10 . 
   During a manhole explosion, a high velocity flow of gases are directed against the under side of the manhole cover  10 . The high velocity gases thus produced fill a cup shaped cavity defined by a skirt  66  usually cast as an integral part of the manhole cover  10  for strengthening purposes ( FIGS. 7 and 8 ). The cavity defined by the skirt  66  when filled with high velocity gases helps to propel the cover  10  out of enclosure  12  during a manhole explosion. According to another feature of the invention, the skirt  66  is formed with scalloped slots  68  comprising a plurality of semi-circular openings. The scalloped slot surfaces are angled down at between 30° and 45° and are also radially canted between 30° and 45° from alignment with the axis of the manhole cover  10 . The canting of the slots  68  are reversed from each of the adjacent slots  60  to maximize swirl in the vertical pressure wave outside skirt  66  ( FIG. 8A ). The skirt portions between the slots  68  disrupt and diffuse the radial pressure wave created when vertical pressure wave within the skirt  66  is forced to turn 90° and exit at high velocity radially. 
   The slots  68  direct high pressure gases radially into the advancing vertical flame front outside the skirt  66 . Consequently, the vertical flame front outside the slotted skirt  66  is disrupted and diffused. 
   Angular pressure waves are shaped and directed by the slots  68  into the vertical column of expanding gases outside the skirt  66 . These actions disrupt laminar gas flow axially and radially by generating diffusion in these respective flame fronts. Diffusion induces swirl and tumble in the respective air masses, lowers temperatures, and shortens radial flame travel on street surface. Shortened flame travel lessens injury potential to pedestrians near manhole explosions. 
   According to another aspect of this feature, a flow retarding action is created by the slotted skirt  66  extending below the underside of the cover  10  ( FIGS. 7-9 ). A portion of the expanding gases from an explosion in passing through the series of downwardly angled slots  68  are directed down into the gap  70  where the outflow of gas occurs. This creates turbulence and an increased static pressure which retards the inflow of fresh air. This in turn attenuates the continued burning of the flammable gases such as methane to reduce the peak force of the explosion by reducing the amount of available oxygen to combust the flammable gases. 
   A series of centering guides  92  ( FIG. 9A ) are affixed around the outer perimeter of the cover to insure that the cover  10  will drop back into the seat  16  after the pressure returns to normal. 
     FIGS. 10-14  shows some modifications in the lock body assembly  20 . A plastic liner sleeve  72 , as of Teflon, may enclose the slide bore  38  to prevent seizing and insure free movement of the latch slide  26  therein. A stop roll pin  74  may be used to limit travel of the latch block  26  to the left instead of the retainer ring  54 . 
   An enlarged bore  76  provides the stop for the secondary shear pin  58 , an easier feature to machine than the arcuate slot  56  described above. 
   The integrally cast reinforcement ribs  11 A can be reduced in height at the middle by a radiused contour as seen in  FIG. 11 . 
   An RFID “sparse pulse” transmitter  78 , solar battery/charger  80  can be included ( FIG. 11 ) for detecting an explosion event or unauthorized cover removal at a monitoring station. 
   The lock body assembly  20  can be mounted on detachable retainer plates  82  secured to the underside of the cover with bolts  84  received in threaded holes in the cover rather than directly to the ribs  11 . This allows the entire assembly to be manufactured and assembled separately from the cover  10 , and to be easily installed or removed. In that case, the opposite ends of the pivot pin  36  can be captured in respective blind holes formed in the two plates  82 . Also, the lug  22 A can be a separate piece attached to ribs  11  with screws as shown. 
   The stop  58  can be designed to act as a secondary shear pin, which when sheared will release the lock body assemble  20  to pivot down to a sharply angled position ( FIG. 14 ), allowing the cover  10  to blow free in the event of a very powerful explosion of a magnitude that could still create great damage despite being partially vented. 
     FIG. 12  shows an indicator rope or strip  86  hung on an eye  90  which strip  86  will be blown out through the gap  88  in an explosion with a tag on end of rope (danger call utility). This will enable maintenance crews to be alerted to the fact that an explosion has occurred at the site of a particular manhole after the cover  10  has dropped back into its normal position.