This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Pressure Conduites”.
1. The flow through the penstocks and pressure conduits is generally ____________________
c) both laminar and turbulent
Explanation: Penstocks are the huge diameter pipe which carries water under pressure and the structural design is similar to that of pressure pipes. Since there is a possibility of sudden load changes in penstocks which changes the pressure and flow velocity which characterizes the turbulent flow in fluid dynamics.
2. The head loss in the flow of water through a penstock pipe of given length ____________
a) increases with the increase in flow velocity
b) decreases with the increase in flow velocity
c) decreases with the increase in the roughness of the pipe surface
d) decreases with time
Explanation: The head loss (HL) by pipe friction is given by Darcy-Weisbach equation –
HL = f’. V2. L / 2gd where L is the length of pipe in meters, d is the diameter of the pipe, V is the velocity of the pipe and f’ is the friction factor which depends on Reynold’s number and the relative roughness of the pipe.
It is clear from the given equation that the head loss is directly proportional to the length of the pipe hence the head loss increases with the increase in the length of the pipe.
3. Hoop’s reinforcement is provided in cement concrete pressure pipes in order to counteract the _________
a) water hammer pressure
b) internal hammer pressure
c) stresses caused by the external backfills
d) both water hammer pressure and internal hammer pressure
Explanation: Hoop’s tension is the internal pressure exerted on the walls of the pipe by flowing water and the circumferential tensile stress (T1) is given by –
T1 = p1 d/2t where p1 is the internal static pressure, d is the diameter of the pipe and t is the thickness of the pipe shell.
Similarly, the circumferential tensile stress caused by water hammer pressure is given by –
T2 = p2 d/2t where p2 is the maximum water hammer pressure developed in pipelines.
4. The more rapid the closure of the valve, greater is the water hammer pressure developed.
Explanation: When a liquid is flowing in a pipeline is abruptly stopped by the closing of the valve, it destroys the momentum and retards the velocity of water column behind. This exerts a thrust on the valve, an additional pressure on the pipe shell and it may be so high as to cause bursting of the pipe shell. The more rapid is the closure of the valve, the more rapid is the change in momentum and hence, greater hammer pressure is developed.
5. Which of the following types of pressure conduits is preferably used for large heads?
a) PCC pipes
b) Cast iron pipes
c) Pre-stressed concrete pipes
d) Asbestos pipes
Explanation: Pre-stressed concrete pipes have more strength than RCC pipes and are more economical. The strength can be achieved by circumferential pre-stressing so as to increase the tensile stress. The main advantage is that they offer a cost advantage over other pipes for large diameter and higher pressures.
6. Which head loss formula is also applicable to turbulent flow in pressure conduits?
a) Darch-Weisbach equation
b) Manning’s formula
c) Hazen-Williams formula
d) Both Darch-Weisbach and Manning’s formula
Explanation: Manning’s formula is also applicable to the turbulent flow in pressure conduits. It yields good results provided the roughness coefficient is accurately determined. The head loss is expressed as –
HL = n2. V2. L / R4/3 where, n = manning’s rugosity coefficient, L is the length of pipe, V is the flow velocity through pipe and R is the hydraulic mean depth of the pipe.
7. __________________ joint is often used for connecting cast iron pipes.
a) Flexible joint
b) Expansion joint
c) Collar joints
d) Bell and spigot joint
Explanation: The pipes which are to be joined are made in such a way that one end is enlarged (also called socket or bell) and the other end is normal (i.e. spigot). The spigot end is inserted into the bell and the remaining space is filled with molten lead which gets solidified and thus making a water-tight joint.
8. Which of the following conduits is used when large diameter pipes of smaller wall thickness are required?
a) Cast iron conduits
b) Galvanized steel pipes
c) Hume steel conduits
d) Centrifugal type RCC pipe
Explanation: Galvanised steel pipes (with circumferential corrugations) are much stronger than ordinary steel pipes. These are usually manufactured in various sizes varying from 20 cm to 2 meters in diameter. They are lighter and can be more easily transported at distances and are widely used where large pipes of smaller wall thickness are required.
9. A special flexible joint called simplex joint is generally used to join _______________
a) vitrified clay conduits
b) galvanised iron pipes
c) asbestos conduits
d) hume steel conduits
Explanation: Asbestos conduits are highly flexible and may permit as much as 120 deflections in laying them around curves. Expansion joints are not required as the coefficient of expansion is low and the joints are also flexible i.e. simplex joint. Its assembly consists of pipe sleeve and two rubber rings which are compressed between the pipe and the interior of the sleeve.
10. Which of the following statement is incorrect?
a) Steel conduits are lighter than cast iron pipes
b) RCC pipes are generally made from 1:2:4 cement concrete with a maximum size of aggregates as 6 mm
c) RCC pipes and Hume steel conduits are heavy and difficult to handle
d) Riveted pipes are smoother and stronger than welded pipes
Explanation: Steel is strong in tension and even large size diameter pipes can be made of thin shells. They are therefore lighter than cast iron pipes. Welded pipes are smoother and stronger than riveted pipes. But steel pipes get rusted easily and are protected on the inside as well as outside by protective coatings.
11. Which of the following pipes is used for carrying hot water in the interior of the building?
a) Wrought iron pipes
b) Copper pipes
c) Galvanised iron pipes
d) Plastic pipes
Explanation: Copper pipes are very costly but they are highly resistant to acidic as well as alkaline waters. They can be bent easily and do not sag due to heat. Hence, they are very useful in carrying hot waters.
12. Wrought iron pipes are heavier than cast iron pipes and cannot be easily fabricated.
Explanation: Wrought iron pipes can be more easily cut threaded and worked but are more costly. They are neat in appearance but are less durable and corrode quickly. These are lighter than cast iron pipes and are generally protected by galvanizing with zinc coatings.
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