BASIC EQUATION. Rotational speed = = ABC 60

Transcription

1 CENTRIFUGAL PUMP

2 BASIC EQUATION Rotational speed = =?@ = ABC 60 = = linear velocity in m/s? = radius in = angular velocity in rad/s B = diameter in m C = rotation per minute Power OPQR? = S U = O V U = WXh VU = WXh Z Work OPQR? = WXZ h [P?\ = Torque Angular velocity [P?\ = Force Distance

3 TURBOMACHINES Turbomachines are the commonly employed devices that either supply or extract energy from a flowing fluid by means of rotating propellers or vanes. PUMP: Pump adds energy to a system, with the result that the pressure is increased. It also causes flow to occur or it increases the rate of flow. TURBINE: A turbine extracts energy from a system and converts it to some other useful form, typically, to electric power. Hydroturbine: is a machine that generates power from high-pressure water; relatively large conduits or tunnels deliver fluid to closed turbines in order to generate power. Another example: steam turbine and air turbine.

4 PUMP CLASSIFICATION

5 CENTRIFUGAL PUMP A centrifugal pump consists of two principal parts: (1) Impeller: which imparts a rotary motion to the liquid. (2) Housing or casing: which directs the liquid into the impeller region and transports it away under a high pressure. The impeller is mounted on a shaft and is often driven by an electric motor. The casing includes the suction and discharge nozzles and houses the impeller assembly. The portion of the casing surrounding the impeller is termed the volute. Liquid enters through the suction nozzle to the impeller eye and travels along the shroud, developing a rotary motion due to the impeller vanes.

6 It leaves the volute casing peripherally at a higher pressure through the discharging nozzle. Some single-suction impellers are open, with the front shroud removed. Double-suction impellers have liquid entering from both sides.

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8 HEAD OF PUMP (Manometric head) This is defined by British Standards as the sum of the actual lift (H) + the friction losses in the pipes (hf)+ the discharge velocity head. r s = r + h t + U u v 2X = O v O x WX + U v v v U x 2X However, for special pumps allowance must also be made for the velocity of flow towards the suction intake and any pressure differences at the water surfaces in the supply and receiving tanks. Commonly the suction and delivery pipes are of equal diameter. In which case: r s = O v O x WX

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11 VELOCITY TRIANGLE

12 Legend: At inlet (1) = x =? = Tangetial velocity of impeller { x = Absolute velocity at x to tangent { }x = { x = x = Relataive velocity to impeller blade Component velocity for { x : { ~x = Whirl velocity { tx = Radial flow velocity x = Inlet blade angle At outlet (2) = v =? = Tangetial velocity of impeller { v = Absolute velocity at v to tangent { }v = { v = v = Relataive velocity to impeller blade Component velocity for { v : { ~v = Whirl velocity { tv = Radial flow velocity v = Inlet blade angle

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14 BLADE TYPE 1. Forward blade 2. Radial blade 3. Backward blade FORWARD BLADE

15 RADIAL BLADE

16 BACKWARD BLADE

17 THE EFFECT OF BLADE TYPE Centrifugal pumps do not always have backward curved vanes. But when they do, it is mostly for fluids in the incompressible regime of operation such as water. For compressible operation of fluids such as air, forward curve-vaned centrifugal pumps are used. The net ideal head developed by a centrifugal pump is given by: r ÇuÉÑÖ = V ÜZ Z = volume flow rate at the impeller outlet V, Ü = constant for a given impeller running at a given speed Additionally, Ü cot v.

18 Do note that the value of the actual head developed by the pump will be lower than this ideal value owing to shocks r àâäãå = ç x Z u Z v Z u = design volume flow rate Z = actual volume flow rate Friction can be calculated by: h t = ç v Z v which together constitute hydraulic losses.

19 The power required to drive the pump to provide a given flow-rate is given as: O = WXZ r ÇuÉÑÖ The representative curves are given below.

20 As is evident from the power-discharge characteristics of the radial and forward vaned centrifugal pump, the power requirement increases monotonically with an increase in discharge. Hence, if the pump motor is rated for maximum power, then it will remain under-utilized for most of the operating time, and result in an increased cost due to its higher rating. On the other hand, if a motor is rated at the design point, and due to some reason the flow-rate exceeds the design flow rate, then the power requirement will shoot up (in case of forward and radial vanes only), causing overloading and motor failure. However, for backward curve-vaned centrifugal pumps, if the flow-rate exceeds the design flow rate (occurs quite close to the maxima of the power-discharge curve), then contrary to the earlier case, the power requirement drops down as evident from the curves. This enables the motor which is rated at the design power to handle the entire range of flow-rates without any problems. The actual design point is located corresponding to the flowrate at which maximum efficiency occurs.

21 EULER HEAD Torque = kadar perubahan momentum sudut Momentum sudut = (jisim) (halaju tangen) (jejari) Momentum sudut masuk = è{ ~x? x Momentum sudut keluar = è{ ~v? v è = kadar jisim mengalir sesaat Kadar perubahan momentum sudut: ê = è{ ~v? v è{ ~x? x è = WVU = WZ ê = WZ { ~v? v { ~x? x Diketahui power ialah: O = ê@ O = WZ { ~v? v { ~x? Diketahui: = =?@ = x =? = v =? x = ë í ì and? v = ë î ì

22 Diketahui power ialah: O = WZ { ~v? v { ~x? = v = WZ { { = = WZ { ~v = v { ~x = x Power juga boleh ditulis sebagai: O = WXZ h Jika power adalah maksimum, nilai h ialah nilai maksimum, iaitu niai power dalam keadaan tiada kehilangan tenaga (losses, friction, etc). Nilai h boleh ditulis sebagai r ñ (Euler head) O = WXZ r ñ = WZ { ~v = v { ~x = x r ñ = 1 X { ~v= v { ~x = x Ia kenali sebagai Euler head (turus Euler). Unitnya dalam meter (m). Ia adalah turus ideal yang dihasilkan oleh impeller (pendesak) dalam system pam.

23 PUMP EFFICIENCY (kecekapan pam) Manometric efficiency ó sñòä = Kuasa air yang dihasilkan Kuasa impeller = WXZ r s WXZ r ñ = WXZ WXZ r s 1 X { ~v= v { ~x = x ó sñòä = Xr s { ~v = v { ~x = x Mechanical efficiency ó séãâ = Kuasa impeller Kuasa yang diberikan kepada syaf ó séãâ = 1 X { ~v= v { ~x = x O Çòôëö

24 Overall efficiency ó ä = Kuasa air yang dihasilkan Kuasa yang diberikan kepada syaf ó ä = WXZ r s O Çòôëö

25 CENTRIFUGAL PUMP TUTORIAL O1

26 QUESTION 1 A centrifugal pump is driven by an electric motor at 1450 rpm. Outlet diameter of blade, outlet blade width and outlet blade angle are 600 mm, 400 mm and 30 o, respectively. Inlet diameter of blade, inlet blade width and inlet blade angle are 300 mm, 80mm and 20ᵒ respectively. Pressure at suction pipe and delivery are positive 13.5 bar and negative 0.5 bar, respectively. Assume that the diameter for suction and delivery pipes is equal. The flowrate inside the pump is 0.3m 3 /s. Determine: i. The monometric head, Hm ii. The manometric efficiency, W X iii. Power required by electric motor if overall efficiency is 98%. QUESTION 2 A centrifugal pump has inlet and outlet diameter of 30 cm and 60 cm, respectively. Impeller width at outlet is 12 cm. Blade thickness occupied 10 percent of the circumference. Blade is backward with inlet and outlet blade angle is 30ᵒ and 40ᵒ, respectively. The flowrate is 0.5m 3 /s. Assume there are no whirl at inlet and velocity of flow is constant determine: i. The rotation of pump in rpm ii. The output power if manometric efficiency is 85% iii. The pressure difference across the impeller

27 QUESTION 3 Centrifugal pump supplies water at the rate of 400 liter/s and the pressure difference across the pump is 200 kn/m 3. Outlet diameter and outlet width are 40cm and 10 cm, respectively. Blade thickness occupied 10% of the circumference. Impeller inlet diameter is half of the outlet diameter. Assume losses in casing and impeller are negligible and zero whirl at inlet. The diameter of suction and delivery pipes is equal. If the blades are radial, determine: i. The pump power input in horsepower if overall efficiency is 80% ii. The impeller speed in rpm iii. The inlet blade angle if velocity of flow is constant QUESTION 4 The inlet and outlet impeller diameter of centrifugal pump are 200 mm and 400 m, respectively. Impeller width at inlet and outlet are 15 mm and 8 mm, respectively. Blades are backward with angle of 38ᵒ. Pump operates at 1500 rpm. The flowrate is 15 liter/s. Determine the pressure changes in the impeller. Assume no energy losses.

28 QUESTION 01

29 QUESTION 02 QUESTION 03 QUESTION 04