| 摘要 |
A manhole cover unit 1 comprises a cover 2 which is seated in a frame 3 to cover over a pit opening 7 the mouth of which is bounded by the frame. To protect the cover 2 against the effect of effluent gases escaping from the pit opening 7 when the unit 1 is used in a sewerage system, a barrier plate 4 is seated on a base flange 10 of the frame 3 so as to extend across the mouth of the pit opening below the cover. The barrier plate 4 is of a material which is resistant to corrosion such as a glass fibre reinforced synthetic plastics. To ensure that effluent gases do not escape past the barrier plate 4, a gasket 5 is bonded around its margin to seal against the base flange 10 on a ledge 12 thereof formed by a rebate 11. The gasket 5 is urged into sealing engagement with the ledge 12 by the weight of the cover 2 when resting on the base flange 10. The cover may have a solid top braced by a honeycomb ribbed structure on its underside. The cover 2 and frame 3 may be of cast iron. |
