The correct answer is A. 2 metres/sec.
The self-cleansing velocity of water flowing through pipe lines is the minimum velocity required to prevent the deposition of sediment on the pipe walls. This velocity is dependent on the pipe diameter, the type of sediment, and the roughness of the pipe walls.
For clean, smooth pipes, the self-cleansing velocity is typically around 2 metres/sec. However, for pipes with rough walls or carrying sediment-laden water, the self-cleansing velocity may be significantly higher.
It is important to maintain a velocity above the self-cleansing velocity in order to prevent the build-up of sediment in pipes. This can be done by increasing the flow rate, by using larger pipes, or by using a centrifugal pump to increase the velocity of the water.
If the self-cleansing velocity is not maintained, sediment can build up on the pipe walls, which can lead to a number of problems, including:
- Reduced flow rate
- Increased pressure drop
- Increased risk of blockages
- Increased corrosion
Therefore, it is important to ensure that the self-cleansing velocity is maintained in all pipe systems.