Lecture 5 Draft tube and Cavitation Draft Tube The draft tube is a pipe of gradually increasing area, which connects the outlet of the runner to the tailrace. It is used for discharging water from the exit of the turbine to the tail race. This pipe of gradually increasing area is called a draft tube. One end of the draft tube is connected to the outlet of the runner while the other end is submerged below the level of water in the tail race. The draft tube, in addition to serve a passage for water discharge, has the following two purposes also: 1. It permits a negative head to be established at the outlet of the runner and thereby increase the net head on turbine. 2. It converts a large proportion of the kinetic energy rejected at the outlet of the turbine into useful pressure energy. Draft tube helps to convert the exit velocity head into pressure or potential head. The energy recovered in the draft tube is large enough in high speed turbines. Especially, for mixed flow turbines, this exit energy at rotor outlet varies from 4 to 25 % and for axial flow turbines from 20 to 50% of the total available energy. This unused energy can be extracted by the draft tube. The conical type draft tube has the shape of a frustum of a cone with an angle of flare not greater than 80 to avoid separation. The length of the conical draft tube should be long enough (2.5 to 3times the diameter of the runner) so that the exit velocity is about 1 m/s. This type is efficient and nearly 90% of the energy is converted into potential head. Model experiments also show that a flare of more than 60 results in loss of efficiency of the draft tube. HsHsHsConicaldrafttubeSimpleelbowdrafttubePrasildrafttubeMoodySpreadingdrafttubeTailrace TailraceTailraceFigure6.11.:Differentformsofdrafttubep = 9u%p = 8u -8S%p = 6u% To determine the limiting height of a draft tube by which a turbine can be set above the tailrace level, Bernoullis equation may be applied between the bottom of the runner which is also the top of the draft tube and the tailrace water level. Applying Bernoullis equation between points (2) and (4) as shown in the Figure above and neglecting the losses in the draft tube, we have, p2y + I222g + Z2 = p4y + I422g + Z4 Or, p2y = p4y -|Z2 - Z4] - jv22-v422g [ But, p4y = p3y - |Z3 - Z4] Here, p3= atmospheric pressure; therefore, p2y = p3y - |Z2 - Z3] - _I22 - I422g _ Or, p2y = p3y -[bs +v22-v422g Here, hs= static suction head =height of the runner outlet above the tail race level; Z2Z4Z3hs234Figure6.12:Drafttubev22-v422g=dynamic suction head. To reduce the exit loss as much as possible and to avoid tailrace erosion, modern designs limit the velocity at draft tube exit to 1.5 m/s to 1.8 m/s. The efficiency of the draft tube is given by pdt = Rcco:cry o tbc prcssurc bcoJIclocity bcoJ ot cxit pdt = I22 - I422g - b]dtI222g Where, V2 = velocity at the exit of the runner; V4= velocity at the tail race; hfdt= loss of head in the draft tube 0.05v222g. Cavitations The boiling temperature of a liquid depends directly upon the pressure and whenever, the pressure at any point inside the turbine falls below the evaporation pressure, the liquid water will boil and a large number of small bubbles of vapor and gases (are dissolved in the liquid) will be formed, which leads to a phenomenon called cavitation. Cavitation occurs when the static pressure of the liquid falls below its vapor pressure Cavitation is defined as the formation of voids within a body of moving liquid (or around a body moving in a liquid) when the local pressure is lower than vapour pressure and the particles of liquid fail to adhere to the boundaries of the passageway. The failure of the particles to adhere to boundaries occurs when there is insufficient internal pressure within the liquid to overcome the inertia of the moving particles and force them to take sufficiently curved paths along the boundary. The liquid enters the hydraulic turbines at high pressure; this pressure is a combination of static and dynamic components. Dynamic pressure of the liquid is by virtue of flow velocity and the other component, static pressure is the actual fluid pressure which the fluid applies and which is acted upon it. Static pressure governs the process of vapor bubble formation or boiling. The voids thus formed fill with vapour of the liquid and result in vapour bubbles. Because the inertia of a moving particle of a liquid varies with the square of the velocity, because the greater the inertia, the greater the pressure required to force the particles to take a curved path, it becomes obvious that cavitations is associated with three conditions: high-velocity flow, low pressures, and abrupt changes in the direction of the flow. The effect of cavitation is to cause pitting of the boundary surfaces. This pitting is the actual removal of the metal because of the violent collapse of the vapour bubbles formed by cavitation. Prof. Thoma (Chandramouli, et al., 2012) has suggested a critical value for the cavitation factor. That is, oc = (bu - b) - bsE Where, ha= average atmospheric pressure head; hv= vapor pressure head; hs= suction head; H= working head of the turbine in meters of water. Cavitation can be avoided in the turbines, if and only if, o > oc For Francis turbine: oc = u.62S[ Ns380.782 For Kaplan turbine:oc = u.Su8 +_ 16.82[ Ns380.782] Values of c for different values of specific speed Francis turbine Propeller turbine Specific speed 89 178 267 355 444 444 667 888 oc 0.025 0.10 0.23 0.40 0.64 0.43 0.73 0.15