A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
environment.
(ii) In the event of high water table, water would infiltrate into sewer
manholes. Therefore, a higher volume of sewage would be
delivered to sewage treatment plant for treatment and this
essentially increases the cost for treating this additional volume of
water.
13. What are possible causes of manhole explosions? (S3)
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:
(i) Natural gas as a result of leakage of nearby gas line;
(ii) In sewer manholes, it is rich in methane which tends to accumulate
inside manhole;
(iii) Gas generated by degradation of cable insulation.
14. In the design of dry well pumping stations, which arrangement is
better, “turned-down” bellmouth or horizontal intake? (P1)
Pumps can be installed as dry well or wet well. The wet well is commonly
used because of its simplicity and low cost. However, this type of pump
arrangement has the potential problem of maintenance. For instance, it
requires the de-watering of sump and removal of pumps out of the sump,
which is suitable for stormwater pumping station which does not require
pumping for most of the time. For dry well, the pumps could be assessed
and maintained all of the time.
In general “turned-down” bellmouth of pump inlet is more popular because
of the following two reasons:
(i) It is less susceptible to vortex action with similar water height.
(ii) It accommodates a lower water cover.
15. What is the purpose of bellmouth entry to a circular pipe for
pumps? (P1)
If sharp edged inlet to pipes connecting to a pump is adopted, flow
separation will occur. Flow separates from sharp edges and a recirculation
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