# Waste Water Engineering Questions and Answers – Minimum and Maximum Velocity of Flow

This set of Waste Water Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Minimum and Maximum Velocity of Flow”.

1. The maximum velocity in a channel section often occurs _______ the water surface.
a) Above
b) Below
c) Between
d) Top

nswer: b
Explanation: A closed or an open conduit always has different velocities of flow within the section. The bottom of the channel is provided with the certain slope for the water carrying foreign particles to flow easily. Hence the velocity is maximum at the bottom of the water surface.

2. What is the maximum velocity of tap water such that the flow causes low noise?
a) 0.2-0.3 m/s
b) 0.5-0.7 m/s
c) 1.0-2.5 m/s
d) 1.5-2.5 m/s

Explanation: The high rate of flow of water can lead to increasing sound levels of flow which leads to noise. Hence minimum flow of water should be considered. 0.5-0.7 m/s is the maximum velocity of tap water that produces less noise levels. 1.0-2.5 m/s of water flow leads to high noise levels.

3. Which of these equations is used to calculate the water velocity inside of a pipe?
a) A=Q/I
b) V=0.408 Q/ D2
c) L=C*Q
d) Q=A*V

Explanation: The first formula is used to calculate the infiltration area. The third formula is used to calculate the load of wastewater. The fourth formula is used to calculate the rate of flow. The second formula is used to calculate the velocity of flow inside of a pipe (V) where Q stands for flow rate of water inside pipe and D stands for pipe diameter (inside).

4. What is the minimum velocity required in sewers to keep the foreign particles from settling?
a) 3 ft/sec
b) 5 ft/sec
c) 10 ft/sec
d) 12 ft/sec

Explanation: If the velocity is too less then the solid or foreign particles which are very minute in size may settle down. Hence a minimum velocity is required to keep these particles moving along the flow is likely to be 3 ft/sec.

5. Exceeding 10 ft/sec may increase the likelihood of hydraulic surges.
a) True
b) False

Explanation: 8 ft/sec is the maximum allowable velocity in public water mains. Increasing velocities beyond this limit may lead to hydraulic surges which lead to the excessive pressure in the water systems.

6. What is the maximum allowable velocity in public water mains?
a) 3 ft/sec
b) 8 ft/sec
c) 10 ft/sec
d) 12 ft/sec

Explanation: The velocity of 3 ft/sec is considered to be minimum to keep the foreign particles moving in sewers. The Washington State Department of Health (DOH) recommends a maximum velocity of 8 ft/sec but permits greater velocities during fire flow conditions.

7. The increasing flow velocity in water systems not only creates noise but also leads to the wear and tear of the pipes.
a) True
b) False

Explanation: The minimum flow velocity is 3 ft/sec to keep the foreign particles moving. In certain mains higher velocity is required such as fire hydrants. If the velocity is increased higher than the limits, the foreign particles moving with higher speeds may cause abrasion against the inner walls of the sewer which leads to wear and tear of the sewers.

8. Storm water is collected from streets into the __________ which in turn discharged into main drains of open type.
b) Lateral drains
c) Sub mains
d) Interseptal mains

Explanation: Lateral drains are small diameters of pipe which is aligned on one side of the curb line that collects wastewater from different sources like residences, schools, etc. Link drains collect storm water from streets and conveys it to the min drains.

9. The depth in open drain should preferably be kept _______ the man height.
a) Less than
b) Greater than
c) Equal to
d) Sufficient to

Explanation: Open drains often carry storm water. In rainy seasons the water level increases in the drains. If it contains clogging or any operations to be done, it becomes difficult if the depth is more.

10. Which of the following is considered if sewers are placed on the softer grounds?
a) The trench is excavated to confirm the shape of the pipe
b) Sewers are laid embedded in the concrete
c) The width of the trench is kept to 15 cm
d) The desired slope is considered

Explanation: The bottom of the trench is kept at a width of 15 cm and top as 60 cm. This is to facilitate the joining of pipes. When the ground is not hard enough, it becomes necessary to insert pipes on harder surfaces. Hence sewers are embedded in concrete surfaces.

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