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
risk to the public because asbestos fibres are carcinogenic only when
inhaled but not ingested.
17. In road opening, it is sometimes noted that asbestos cement
pipes are broken up into pieces. Why? (P2)
Asbestos cement is mainly a mixture of cement and asbestos fibres with
density greater than 1000kg/m 3 . It contains about 10% asbestos fibres and
it is a light grey hard material. The fibres are tightly held in cement mixture
and they shall be discharged if asbestos cement undergoes significant
disturbance such as drilling and sawing.
However, one of the important characteristics of asbestos cement is the
relative brittleness. As such, asbestos cement pipes can be broken easily
when falling at height or driven over by heavy vehicles. Hence, it is not
uncommon that asbestos cement pipes are observed to break up into
pieces in inspection pits.
18. What is the difference between air chamber and surge tank in
pressurized pipelines? (DW8)
Air chambers and surge tanks are normally installed in watermain to ease
the stress on the system when valves or pumps suddenly start up and shut
down. A surge tank is a chamber containing fluid which is in direct contact
with the atmosphere. For positive surge, the tank can store excess water,
thus preventing the water pipes from expansion and the water from
compression. In case of downsurge, the surge tank can supply fluid to
prevent the formation of vapour column separation. However, if the relief of
surge pressure is significant, the height of surge tank has to be large and
sometimes it is not cost-effective to build such a large tank. On the contrary,
an air chamber can be adopted in this case because air chamber is an
enclosed chamber with pressurized gases inside. The pressure head of the
gas inside the air chamber can combat the hydraulic transient. The volume
of liquid inside the air chamber should be adequate to avoid the pressure in
the pipelines falling to vapour pressure. The air volume should be sufficient
to produce cushioning effect to positive surge pressures. In essence, air
chambers can usually be designed to be more compact than surge tanks.
Air chamber has the demerits that regular maintenance has to be carried
out to check the volume of air and proper design of pressure level of gas
has to be conducted.
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