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It is not uncommon that manhole explosion occurs nowadays. Manhole covers are dislodged from the frames which is associated with a release of energy. Manhole explosion occurs mostly owing to the ignition and combustion of flammable gas. Sources of flammable gas include the followings:
Sewer manholes should preferably be designed to be watertight owing to the following reasons:
(i) Water-tightness is important in sewer system. Otherwise, leakage of sewage from manholes would seriously contaminate the environment.
Wetwells are designed to store temporarily water/sewage before it is pumped out. They are usually provided for sewage and stormwater pumping stations and they serve the following functions:
(i) They assist in attenuating the fluctuations of flow owing to the diurnal variation of sewage discharge.
The presence of sediment in sewers has adverse effects on the hydraulic performance of sewers. For the case of sewage flow carrying sediment without deposition, the presence of sediment in the flow causes a small increase in energy loss.
In case the sewer invert already contains a bed of sediment deposit, it reduces the cross sectional area of sewers and consequently for a given discharge the velocity increases. As such, the head losses associated with this velocity increase. Moreover, the increase in bed resistance induced by the rough nature of sediment deposit reduces the pipe flow capacity of sewers.
For solids in large sewers, forces on solids position them at different flow heights depending on their specific gravity.
There are generally two modes of movement of solids in small sewers, namely, floating and sliding dam. The floating mechanism operates when the size of solid is small when compared with the diameter of sewer.
When excessive pipe deformation occurs, it may impair the joint performance and affect the strain in pipes. Based on the information by PIPA, there is a 5% reduction of flow capacity when the pipe is deflected by 15%. Hence, pipe deflection has impact on flow capacity but its effect is not significant.
Inverted siphons are designed at locations in which a sewer system is blocked by underground utilities or stormwater drains. They are sometimes called depressed sewers because it is claimed that there is no actual siphon action. They connect the upstream and downstream sewers with U-shaped vertical alignment such that they are always running full.
CCTV is a well-established and prevalent method for pipe inspection. However, there are several limitations of CCTV technology:
(i) It could only provide the view of pipe internal surface above waterline.
(ii) It does not provide the view of soil conditions surrounding the pipe.
There are mainly three major effects of sediment deposition in sewers:
(i) The deposited sediment may cause initiate small blockage of flow. Later when larger solid builds up it may result in total blockage of sewers.
(ii) The deposited sewer restricts flows leading to the drop in hydraulic capacity.
Vitrified clay pipe is capable of withstanding extreme temperatures. However, vitrified clay pipe is liable to damage by thermal shock – a swift change of temperature. Such rapid temperature variation induces thermal gradients inside the pipe wall which produces stresses which damage the pipe.
