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
If the downstream gate valve is not installed in position, then the pipe
section of branched-out watermain will be left dry during normal operation
and there is a high probability that damage to watermain and frequent
leakage would occur. With the downstream gate valve installed, the
segment of branched-out watermain contains water in normal operation. In
case there is any leakage, it can be readily detected by using the two gate
valves.
11. What is the purpose of embedding puddle flange inside the walls
of closed valve chambers? (DW4)
When valves are closed to stop water flow in a pipeline, a thrust is
generated along the direction of the pipeline. Hence, it is necessary to
restrain the valves during closure to prevent it from moving in the thrust
direction. If the closed valve is situated inside valve chambers, it is
connected to a puddle flange embedded inside a wall of the chamber. As
such, the closed valves can be effectively restrained from the thrust action
during the closure of valves.
12. What is the difference between “linear characteristic” and “equal
percentage characteristic” in controlling butterfly valves? (DW5)
For “linear characteristic”, the flow rate is directly proportional to the
amount of travel of butterfly disk. For example, at 25% open of butterfly
disk, the flow rate of 25% of maximum flow.
For “equal percentage characteristic”, equal increment of opening of
butterfly disk leads to equal percentage change in flow rate. For example,
when butterfly disc open from 30% to 40%, it generates a change in flow
rate of 50%. Therefore, when butterfly disc open from 40% to 50%, it also
generates a change in flow rate of 50%. If the flow rate at 30% is 200m 3 /s,
then the flow rate of 40% and 50% open are 300m 3 /s and 450m 3 /s
respectively.
The use of different characteristics depends on the amount of pressure
drop available to the butterfly value. Should more than 25% of system
pressure drop is available to the butterfly value, then the employment of
linear characteristic would provide the best results. On the contrary, if less
than 25% of system pressure drop is available to the butterfly value, then
the employment of equal percentage characteristic would be a better
choice.
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