| 摘要 |
To protect a manhole cover 2 against the effect of effluent gases escaping from the manhole shaft 7 in a sewerage system, a barrier plate 4 is seated on a base flange 10 of the manhole frame 3 below the cover. The barrier plate 4 is of a material which is resistant to corrosion such as a glass fibre reinforced synthetic plastics. A gasket 5 is bonded around the margin of plate 4 to seal against the base flange rebate 11. The gasket 5 is urged into sealing engagement with the rebate 11 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. The frame 3 may be formed by bolting together four L-section bars and sealing the joints with molten lead. <IMAGE> |
