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(civl351)[2007](f)mid~PPSpider^_10076.pdf
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Course: CIVL351 Municipal Hydraulic Systems
Examination: Midterm
Date: November 2, 2007
Full Mark: 100 pts.
Time: 2 hours
Type: Open note examine
Semester: Fall 07/08
Instructor: Dr. Chii SHANG

A. Briefly answer the following questions (40 pts, 4 pts each)



1) Explain why the head losses in fittings and valves are called minor losses. Give one situation when these minor losses cannot be neglected.



Normally, the energy loss due to fittings and valves in the distribution system is negligible as compared to energy loss introduced by friction in pipe lines. In pumping stations these minor losses cannot be neglected.




2) Define the equivalent length that is used to describe minor losses from fittings and valves. What is the purpose to introduce this term, in addition to the loss coefficient k?



The equivalent length is defined as the length of an imaginary straight pipe (with the same diameter of the fittings or valves) which results in the same head loss as caused by certain kinds of fittings and valves. The concept of equivalent length can fit in existing empirical equations for calculating headloss, and therefore facilitate hydraulic calculations.





3) Among different maximum and average demands, what is the design flow rate we shall choose if there is no distribution storage? Why the pumping cost in Hong Kong is still high, with the use of many distribution storage reservoirs?



When there is no distribution storage, the maximum hourly demand should be chosen for design of flow rate. In HK, most distribution storages were installed underground due to space limitations. Therefore the water head will be totally lost when water is stored in such underground storage which will result in extra pumping cost when the water is distributed from the storage.




4) Explain why there is a minimum sewage velocity requirement? Explain why the requirement is higher for storm water than for sewage lines.



The minimum sewage velocity is employed to prevent grit and solids content in the gravity sewer from settling in the sewage collection system. Since the particle in storm water is heavier than sewer, the minimum sewage velocity requirement for storm water is higher.





5) What are the pros and cons of grid distribution systems? Explain the ideal location for installing grid distribution systems.



In a grid system, water is completely circulated; if repair is needed, the area getting cut-off is limited. On the other hand, a grid system needs more pipes. Grid distribution system is ideal for cities.






6) Why the treatment schemes are different in treating water from lakes vs. rivers?



The reason for such difference lies in the different quality of raw water from lakes and rivers. Normally, raw water from lakes contains high concentration of taste and odor causing compounds which deserve pre-chlorination. Raw