Draft Tubes

DRAFT TUBES The draft tube is an integral part of reaction turbine, and its design criteria should be specified by the turbine manufacturer. The draft t ube used in hydraulic reaction turbine has gradual increase in cross sectional area from its inlet t o outlet. outlet. It is one of the important components of reaction turbine and connects runner exit to tail race. The main functions of  draft tube is to allow the installation of turbine above the tail race level without loss of head and to convert major part of kinetic energy coming out of runner into pressure energy. In mixed flow reaction turbines, kinetic energy from runner is up to 15% whereas in low head and high speed axial flow turbines, kinetic energy leaving the runner may go up to 50% of  total input energy. The recovery r ecovery of kinetic energy is achieved by increasing the crosssectional area of the draft tube in the flow direction. It has two important functions p

To enable the turbine to be se above a bove the tailwater level without losing any head thereby. A reduced pressure is produced at the upper end of the draft tube, which compensates for the height at which the turbine runner is set. Within limits the turbine can be set at different elevations without altering the net head. By its use there is an unbroken stream of liquid from headwater to tail water.

p

The second function of the draft tube is to reduce the head loss at submerged discharge to thereby increase the net head available to the turbine t urbine runner. runner. This is accomplished by using a gradually diverging tube whose cross-sectional area at discharge is considerably larger than the cross-sectional area at entrance to the tube.

Types of Draft Tubes As shown in Figure 5.23, the draft tubes are of the following three types: (1) Conical or divergent draft tube (2) Elbow type draft tube (3) Hydracone or Moody spreading draft tube (1) Conical or divergent draft tube: The shape of the tube resembles that of a frustrum of a cone. It is commonly used in the Francis turbine. The cone angle varies from 4° to 8°. The efficiency of the conical tube is about 8.5% to 90%. (2) Elbow type draft tube: It may be in the form of a simple elbow type or elbow tube with a circuit inlet and a rectangular outlet section. The latter type is used in the Kaplan turbine with an efficiency of about 70%. (3) Hydracone or Moody spreading draft tube: This is a modification of conical tube t ube and a solid conical cone is provided in the centre of the tube with a flare at the bottom end. Such an arrangement allows a large exit area without excessive length. length. The solid core cor e at the centre enables full flow and reduces the eddy losses. The efficiency of the tube is about 85%.

Draft Tube Theory: Consider

a turbine fitted with a draft tube (conical) as shown in Figure 5.24. Let y Let  y - distance of the bottom of draft tube from tail race, and  pa - atmospheric pressure at the surface of tail rac e. Applying Bernoulli's Bernoulli's equation to the section 2-2 2- 2 (representing the runner  exit or inlet of the draft tube) and the section 3-3 (representing the draft tube exit); assuming section 3"3 at the datum line, we have-

Where hf = loss of energy between sections 2-2 and 3-3. Rewriting the above expression (i) for  p for  p2/w 2/w , we obtain

The term (y2 - y) which represents the vertical distance between the runner runner exit and the tail water level is called the suction head of draft tube and is denoted by  H.

In equation it is l ess ess than atmospheric pressure . The efficiency of a draft tube (   ) is defined as the ratio of net gain in pressure head to the vel ocit  ocit  y head at entrance of draft tube . Thus

Where V2 = velocity of water at section 2-2 (inlet of draft tube), V 3 = velocity of water at section 3-3 (outlet of draft tube).