Systems and methods for venting gas in the event of an explosion in a space covered by a manhole cover

1. A closure for an underground space, the closure being adapted to limit damage caused by combustion of one or more explosive gases that might accumulate in the underground space, the closure comprising:

a substantially cylindrical sleeve having a plurality of vents extending radially through the sleeve, the sleeve defining a manhole extending axially through the sleeve for providing access to the underground space so a person can enter the underground space through the manhole, and

a manhole cover moveable relative to the sleeve and adapted to be moved to a closed position in which the manhole cover is supported by an upper end of the sleeve and covers the manhole to limit access to the underground space through the manhole while the manhole cover is in the closed position,

wherein the sleeve has a recessed shoulder at an upper end of the sleeve and the manhole cover is seated on the recessed shoulder in the closed position.

2. A closure as set forth in claim 1 wherein the manhole cover and sleeve are formed separately from one another.

3. A closure as set forth in claim 1 wherein the vents comprise a plurality of notches extending up from a bottom end of the sleeve.

4. A closure as set forth in claim 1 wherein the vents include a plurality of vents spaced circumferentially around the sleeve.

5. A closure as set forth in claim 1 further comprising a low-friction corrosion resistant liner on the outside surface of the sleeve.

6. A closure as set forth in claim 1 further comprising one or more locks adapted to releasably secure the manhole cover to the sleeve, said one or more locks being adapted so they can be unlocked from a position above the manhole cover while the manhole cover is in the closed position, the manhole cover and sleeve being configured so the manhole cover can be lifted from the closed position by moving the manhole cover straight upwardly when the one or more locks are unlocked.

7. A closure as set forth in claim 1 further comprising one or more locks operable to releasably secure the manhole cover to the sleeve, said one or more locks being adapted so they can be unlocked from a position above the manhole cover while the manhole cover is in the closed position, the manhole cover and sleeve being configured so when the locks are unlocked any portion of the perimeter of the manhole cover can be lifted off the sleeve while an opposite portion of the perimeter of the manhole cover remains supported by the sleeve to facilitate moving the manhole cover away from the closed position in any radial direction.

8. A closure as set forth in claim 1 wherein the manhole cover has a bottom that is substantially flat.

9. A closure as set forth in claim 1 further comprising a frame for securing the closure to a riser connecting an underground space to an above-ground environment, the frame having a frame opening, wherein the sleeve is configured to be slideably received in the frame opening for telescoping movement of the sleeve relative to the frame.

10. A closure as set forth in claim 9 further comprising a retainer adapted to limit movement of the sleeve from the retracted position beyond the extended position.

11. A closure as set forth in claim 1 further comprising:

(i) a frame for securing the closure to a riser connecting an underground space to an above-ground environment, the frame having a frame opening, wherein the sleeve is configured to be slideably received in the frame opening for telescoping movement of the sleeve relative to the frame; and

(ii) one or more locks operable to releasably secure the manhole cover to the sleeve, said one or more locks being adapted so they can be unlocked from a position above the manhole cover while the manhole cover is in the closed position, the one or more locks having sufficient collective strength to withstand a force that is sufficient to move the sleeve in an upward direction relative to the frame against the force of gravity to an elevation at which the vents extend above a top of the frame for venting gas through the vents.

12. A closure as set forth in claim 1 further comprising:

(i) a frame for securing the closure to a riser connecting an underground space to an above-ground environment, the frame having a frame opening, wherein the sleeve is configured to be slideably received in the frame opening for telescoping movement of the sleeve relative to the frame between a retracted position in which the vents do not extend above a top of the frame and an extended position in which the vents extend above the top of the frame for venting gas through the vents; and

(ii) a biasing member biasing the sleeve toward the retracted position.

13. A closure as set forth in claim 1 wherein the sleeve has a flange at its upper end extending radially outward relative to the manhole.

14. A closure as set forth claim 13 further comprising one or more retaining lugs, wherein the sleeve and retaining lugs are adapted for attachment of the retaining lugs to the sleeve at an end of the sleeve opposite the flange.

15. A closure as set forth in claim 1 wherein the manhole cover is substantially free of explosion mitigation features.

16. A closure as set forth in claim 1 wherein the manhole cover consists essentially of a unitary structure formed as one piece in a metal casting process.

17. A closure as set forth in claim 16 wherein the manhole cover has a substantially flat bottom.

18. A closure

as set forth in claim 1 further comprising one or more security bolts adapted to releasably lock the manhole cover to the sleeve, said one or more security bolts being adapted so they can be unlocked from a position above the manhole cover while the manhole cover is in the closed position, the manhole cover and sleeve being configured so the manhole cover can be lifted from the closed position and separated from the sleeve by moving the manhole cover straight upwardly when the one or more security bolts are unlocked.

19. A closure as set forth in claim 18 wherein the manhole cover and sleeve are configured so when the one or more security bolts are unlocked any portion of the perimeter of the manhole cover can be lifted off the sleeve while an opposite portion of the perimeter of the manhole cover remains supported by the sleeve to facilitate moving the manhole cover away from the closed position in any radial direction.

20. A closure as set forth in claim 18 wherein the manhole cover has bottom that is substantially flat.

21. A closure for an underground space, the closure comprising:

a substantially cylindrical sleeve having a plurality of vents extending radially through the sleeve, the sleeve defining a manhole extending axially through the sleeve for providing access to the underground space so a person can enter the underground space through the manhole, and

a manhole cover moveable relative to the sleeve and adapted to be moved to a closed position in which the manhole cover is supported by an upper end of the sleeve and covers the manhole to limit access to the underground space through the manhole while the manhole cover is in the closed position,

wherein the manhole cover has a substantially flat bottom and the closure is adapted to limit damage caused by combustion of one or more explosive gases that might accumulate in the underground space.

22. A closure as set forth in claim 21 wherein the manhole cover does not have any skirt on its bottom.