Fluid Mechanics & Hydraulic Machinery – 8th UNIT
Assignment - 8
1. (a) What is priming? Why is it necessary? Explain how it is done.
(b) Find the number of pumps required to take water from a deep well under a total head of 156 m. Also, the pumps are identical and are running at 1000 r.p.m. The specific speed of each pump is given as 20 while the rated capacity of each pump is 150 lit/s. [8+8]
2. (a) Define a centrifugal pump. Explain the working of a single-stage centrifugal pump with sketches.
(b) The internal and external diameters of the impeller of a centrifugal pump are 30 cm and 60 cm respectively. The pump is running at 1000 r.p.m. The vane angles at inlet and outlet are 200 and 300 respectively. The water enters the impeller radially and velocity of flow is constant. Determine the work done by the impeller per unit weight of water. [8+8]
3. (a) How will you determine the possibility of the cavitation to occur in the installation of a turbine or a pump? [8+8]
(b) A single-acting reciprocating pump running at 30 r.p.m., delivers 0.012 m3/s of water. The diameter of the piston is 25 cm and stroke length 50 cm. Determine:
i. The theoretical discharge ii. Co-efficient of discharge iii. Slip and percentage slip
5. (a) What is cavitation and what are its causes? How will you prevent the cavitation in hydraulic machines?
(b) A single-acting reciprocating pump has a cylinder of a diameter 15 cm and of stroke length 30 cm. The centre of the pump is 4 m above the water surface in the sump. The atmospheric pressure head is 10.3 m of water and pump is running at 40 r.p.m. If the length and diameter of the suction pipe are 5 m and 10 cm respectively, determine the pressure head due to acceleration in the cylinder:
i. At the beginning of the suction stroke ii. In the middle of suction stroke. [8+8]
6. (a) Find an expression for the head lost due to friction in suction and delivery pipes.
(b) A centrifugal pump (diffusion type) has a suction lift of 1.5 m and the delivery tank is 13.5 m above the pump. The velocity of water in the delivery pipe is 1.5 m/s. The radial velocity of flow through the wheel is 3 m/s. and the tangent to the vane at exit from the wheel makes an angle of 1200 with the direction of motion. Assuming that the water enters radially and neglecting friction and other losses, determine:
i. Velocity of wheel at exit ii. Velocity and pressure head at exit from the wheel
iii. Direction of fixed guide vanes. [8+8]
7. (a) Define the terms: suction head, delivery head, static head and manometric head.
(b) A Centrifugal pump is running at 1000 r.p.m. The outlet vane angle of the impeller is 300 and velocity of flow at outlet is 3 m/s. The pump is working against a total head of 30 m and the discharge through the pump is 0.3 m3/s. If the manometric efficiency of the pump is 75%, determine:
i. The diameter of the impeller ii. The width of the impeller at outlet. [8+8]
8. (a) Show from first principle that the work saved, against friction in the delivery pipe of a single-acting reciprocating pump, by fitting an air vessel is 84.8% while for a double-acting reciprocating pump the work saved is only 39.20%.
(b) What is negative slip in a reciprocating pump? Explain with neat sketches the function of air vessels in a reciprocating pump. [8+8]
1. A three stage centrifugal pump has impellers 40cm in diameter and 2cm wide at outlet. The vanes are curved back at the outlet at 450 and reduce the circumferential area by 10%. The manometric efficiency is 90% and the overall efficiency is 80%. Find the head generated by the pump when running at 1000 r.p.m delivering 50lps. What should be the shaft horse power?
2. A four-stage centrifugal pump has four identical impellers keyed to the same shaft. The shaft is running at 400 r.p.m and the total manometric head developed by the multistage pump is 40m. The discharge through the pump is 0.2 m3/ s .The vanes of each impeller are having outlet angle as 450. If the width and diameter of each impeller at outlet is 5cm and 60cm respectively find the manometric efficiency.
3. Explain the performance characteristic curves of a centrifugal pump.
4. A single-stage centrifugal pump with impeller diameter of 30cm rotates at 2000r.p.m and lifts 3m3 of water per second to a height of 30m with an efficiency of 75%. Find the number of stages and diameter of each impeller of a similar multistage pump to lift 5m3 of water per second to a height of 200 meters when rotating at 1500 r.p.m.
5. (a) How will you obtain an expression for the minimum speed for starting a centrifugal pump?
(b) Find the rise in pressure in the impeller of a centrifugal pump through which water is flowing at the rate of 15 lit/s. The internal and external diameters of the impeller are 20 cm and 40 cm respectively. The widths of impeller at inlet and outlet are 1.6 cm and 0.8 cm. The pump is running at 1200 r.p.m. The water enters the impeller radially at inlet and impeller vane angle at outlet is 300. Neglect losses through the impeller.