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
channels.
Fig. Short pipe
16. Nowadays, most flap valves are made of HDPE. What are the
advantages of using HDPE when compared with cast iron? (C8)
(i) It has no reaction with sewage and seawater and does not suffer from
the corrosion problem associated with cast iron.
(ii) No protective coating is required and it is almost maintenance-free.
(iii) HDPE flap valves require very low opening pressure in operation (like
5mm water level difference). For cast iron flap valves, due to its own
heavy self-weight, the required opening pressure of cast iron flap
valves is higher than that of HDPE flap valves. This criterion is
essential for dry weather flow conditions.
However, the pressure resistance of HDPE flap valves is not as good as
cast iron flap valves. For instance, a typical 450mm wide HDPE flap valve
can only withstand about 5m water column.
17. What is the difference between on-seating and off-seating head in
penstock? (C9)
A penstock is commonly used to control the flow and water level and for
isolation of fluid. It mainly consists of a sliding door which is controlled by
mechanical spindle moving through a hole in a frame built onto a structure.
Penstock is the term used in UK while sluice gate is more commonly
adopted outside UK. In the design of penstock, it is important to identify if it
would take on-seating head or off-seating head.
148