Draft tube (refer Fig 6 1 and Plate 17)-The water after doing work on the runner passes on to the tail race through a draft tube which is a welded steel plate pipe or a concrete tunnel, its cross-section gradual increasing towards the outlet. The draft tube is a conduit which connec the runner exit to the tail race. The tube should be drowned-approxmately one metre below the lowest tail race level. The functions of draft tube are as follows:
(i) If the water is discharged freely from the runner, turbine work under a head equal to the height of the head race water level above the runner outlet. If an airtight draft tube connects the runner to the tailrace, workable head is increased by an amount cqual to the height of the runner outlet above tail race.
The draft tube will, thus, permit a negative (suction) head to be established at the runner outlet thus making it possible to install the turbine above the tail race without loss of head. This can be explained as follows:
The pressure in the draft tube at the tail race level is atmospheric. If the cross-section of draft tube is kept uniform, the pressure at the runner outlet is equal to the atmospheric pressure minus the height of runner outlet above the tail race level. The available head, measured from head race level to the discharge side of the turbine, is thus the same as if the turbine were erected at tail race level and discharged under atmospheric pressure.
Thus the reaction turbines may be installed in three ways, (a) at the tail race level, (b) above the tail race level and (c) below tail race,
Example-Let the difference in level of head race and tail race be 100 m, and for a turbine installed at the tail race level and discharging at atmospheric pressure, the head available for the turbine is 100 m (refer -Fig 6.6a).
Now in a case when the turbine is installed 5 m above the tail race level and no draft tube is installed, the head available for the turbine is 95 m (refer Fig 6.68).
With the use of draft tube the water is not discharging at atmospheric pressure but at a negative pressure that is -5 m. Therefore, the available head for the turbine will be 95-(-5)=100 m.
Thus it is possible to install the turbine above tail race level without any - loss of available head.
(61 +5m . (c) TURBINE In certain cases the turbine is installed below the tail race level withg 6,6c). Let the turbine be installed 5 m below the tail race level, 1 Pressure at the runner exit will be 5 m gauge. The turbine runner 09. m below the head race level. There the available head for the
Then the pressure at the runner exit will There the available a exit is 105. m below the head turbine will be 105-5=100 m. Thus the turbine can be installed below or above the tail race level with the help of draft tube and the available head remains the same.
It may be noted that reaction turbines are seldom installed discharging in atmosphere at the tail race level because the tail race level is variable during the draught and flood periods of the year.
Further the turbine installed below the tail race level will reduce the possibility of cavitation because of the absence of negative head.
(ii) The water leaving the runner still possesses a high velocity and this kinetic energy would be lost if it is discharged freely as in a Pelton turbine. By employing a draft tube of increasing cross-section, the enclosed conduit is extended up to the outlet end of the tube and discharge takes place at a much reduced velocity thus resulting in a gain of pressure head. This increases the negative pressure head at turbine runner exit with which the net working head on the turbine increases. With the increase in act working head on the turbine, output will also increase, thus raising the efficiency of the turbine.
6.8 Different Types of Draft Tubes-The draft tube is an integral part of a reaction turbine. The velocity energy of water at rummer
2
Fig 6.7 (6) Moody's Bell
Mouthed Draft Tube
Fig 6.7 (a) Straight Divergent Tube
2 exit is 3 to 15% of the net working head in case of Francis turbines depend. ing upon the specific speed. As the specific speed increases, the value of velocity energy at the runner rises and it will be nearly 45% in the case of a Kaplan turbine. Hence or high specific speed Francis turbines as well as for Kaplan turbines, the second function of draft tube, described in Art 6.7(ii),
.e., recovery of kinetic energy at discharge is more important. Therefore great attention is paid to the shape of draft tubes
The following are some types of draft tubes, employed in the field
(a) Straight Divergent tube (refer Fig 6.7 (a)].—The shape of this tube is that of a frustum of cone. It is employed for low specific speed
Fig 6.7 Different Types of Draft Tube vertical shaft Francis turbine. The maximum cone angle is 8 degree (or half cone angle = 4°). Experiments have shown that if this angle is greater than 8°, the water detaches away from the inner wall of the tube while slowing downwards forming vortices and causing loss of head. The tube must discharge sufficiently low under tail water level. The maximum efficiency which this type of draft tube can yield is 85%, because the kinetic head to be recover. ed is less. This type of draft tube improves speed regulation on falling load.
(6) Moody's Spreading Tube or "Hyrlracono" (refer Fig 6.7 h and c)
—
Moody suggested a bell rnouthed drast tube (refer Fig 6.7 ) having a solid conical core in the entire central portion of the tube, thus allowing a large cxit area without excessive length When the turbine works at part load or due to high velocity of water at runner exit, the discharging water velocity has a whirl component, it is likely to cause eddy losses. The central cone arrangement is made to reduce the whirl action of discharging water. The efficiency of such a draft tube is about 85%
(c) Simple Elbow Tube (refer Fig 6.7 d)-In order to keep down the cost of excavation, particularly in rock, the vertical length of the drafi tube should be minimum. Since the draft tube exit diameter should be as large as possible to recover the kinetic head and at the same time the maximum value of the cone angle is fixed, the draft tube must be bent to keep its definite length. Simple elbow type draft tube will serve such a purpose. Its efficiency is, however, low, about 60%.. .
(2) Elbow Tube with a Circular Inlet and a Rectangular Outlet Section (refer Fig 6.7 e) - This type of draft tube has been designed to turn the water from the vertical to the horizontal direction with a minimum depth of excavation and at the same time having a high efficiency. The transition from a circular section in the vertical leg to a rectangular secho
e horizontal leg takes place in the bend. The horizontal portion of the draft tube is generally inclined upwards to lead the water gradually to the the exit end. level of the tail race and to prevent entry of air from exit end of the tube must be totally immersed in water.
In order to avoid any whirl component of velocity of water at runner exit, one or two piers are constructed in the bend of the draft tubes. Such piers behave similar to the central core of Moody's spreading tube, describing the above.
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(i) If the water is discharged freely from the runner, turbine work under a head equal to the height of the head race water level above the runner outlet. If an airtight draft tube connects the runner to the tailrace, workable head is increased by an amount cqual to the height of the runner outlet above tail race.
The draft tube will, thus, permit a negative (suction) head to be established at the runner outlet thus making it possible to install the turbine above the tail race without loss of head. This can be explained as follows:
The pressure in the draft tube at the tail race level is atmospheric. If the cross-section of draft tube is kept uniform, the pressure at the runner outlet is equal to the atmospheric pressure minus the height of runner outlet above the tail race level. The available head, measured from head race level to the discharge side of the turbine, is thus the same as if the turbine were erected at tail race level and discharged under atmospheric pressure.
Thus the reaction turbines may be installed in three ways, (a) at the tail race level, (b) above the tail race level and (c) below tail race,
Example-Let the difference in level of head race and tail race be 100 m, and for a turbine installed at the tail race level and discharging at atmospheric pressure, the head available for the turbine is 100 m (refer -Fig 6.6a).
Now in a case when the turbine is installed 5 m above the tail race level and no draft tube is installed, the head available for the turbine is 95 m (refer Fig 6.68).
With the use of draft tube the water is not discharging at atmospheric pressure but at a negative pressure that is -5 m. Therefore, the available head for the turbine will be 95-(-5)=100 m.
Thus it is possible to install the turbine above tail race level without any - loss of available head.
(61 +5m . (c) TURBINE In certain cases the turbine is installed below the tail race level withg 6,6c). Let the turbine be installed 5 m below the tail race level, 1 Pressure at the runner exit will be 5 m gauge. The turbine runner 09. m below the head race level. There the available head for the
Then the pressure at the runner exit will There the available a exit is 105. m below the head turbine will be 105-5=100 m. Thus the turbine can be installed below or above the tail race level with the help of draft tube and the available head remains the same.
It may be noted that reaction turbines are seldom installed discharging in atmosphere at the tail race level because the tail race level is variable during the draught and flood periods of the year.
Further the turbine installed below the tail race level will reduce the possibility of cavitation because of the absence of negative head.
(ii) The water leaving the runner still possesses a high velocity and this kinetic energy would be lost if it is discharged freely as in a Pelton turbine. By employing a draft tube of increasing cross-section, the enclosed conduit is extended up to the outlet end of the tube and discharge takes place at a much reduced velocity thus resulting in a gain of pressure head. This increases the negative pressure head at turbine runner exit with which the net working head on the turbine increases. With the increase in act working head on the turbine, output will also increase, thus raising the efficiency of the turbine.
6.8 Different Types of Draft Tubes-The draft tube is an integral part of a reaction turbine. The velocity energy of water at rummer
2
Fig 6.7 (6) Moody's Bell
Mouthed Draft Tube
Fig 6.7 (a) Straight Divergent Tube
2 exit is 3 to 15% of the net working head in case of Francis turbines depend. ing upon the specific speed. As the specific speed increases, the value of velocity energy at the runner rises and it will be nearly 45% in the case of a Kaplan turbine. Hence or high specific speed Francis turbines as well as for Kaplan turbines, the second function of draft tube, described in Art 6.7(ii),
.e., recovery of kinetic energy at discharge is more important. Therefore great attention is paid to the shape of draft tubes
The following are some types of draft tubes, employed in the field
(a) Straight Divergent tube (refer Fig 6.7 (a)].—The shape of this tube is that of a frustum of cone. It is employed for low specific speed
Fig 6.7 Different Types of Draft Tube vertical shaft Francis turbine. The maximum cone angle is 8 degree (or half cone angle = 4°). Experiments have shown that if this angle is greater than 8°, the water detaches away from the inner wall of the tube while slowing downwards forming vortices and causing loss of head. The tube must discharge sufficiently low under tail water level. The maximum efficiency which this type of draft tube can yield is 85%, because the kinetic head to be recover. ed is less. This type of draft tube improves speed regulation on falling load.
(6) Moody's Spreading Tube or "Hyrlracono" (refer Fig 6.7 h and c)
—Moody suggested a bell rnouthed drast tube (refer Fig 6.7 ) having a solid conical core in the entire central portion of the tube, thus allowing a large cxit area without excessive length When the turbine works at part load or due to high velocity of water at runner exit, the discharging water velocity has a whirl component, it is likely to cause eddy losses. The central cone arrangement is made to reduce the whirl action of discharging water. The efficiency of such a draft tube is about 85%
(c) Simple Elbow Tube (refer Fig 6.7 d)-In order to keep down the cost of excavation, particularly in rock, the vertical length of the drafi tube should be minimum. Since the draft tube exit diameter should be as large as possible to recover the kinetic head and at the same time the maximum value of the cone angle is fixed, the draft tube must be bent to keep its definite length. Simple elbow type draft tube will serve such a purpose. Its efficiency is, however, low, about 60%.. .
(2) Elbow Tube with a Circular Inlet and a Rectangular Outlet Section (refer Fig 6.7 e) - This type of draft tube has been designed to turn the water from the vertical to the horizontal direction with a minimum depth of excavation and at the same time having a high efficiency. The transition from a circular section in the vertical leg to a rectangular secho
e horizontal leg takes place in the bend. The horizontal portion of the draft tube is generally inclined upwards to lead the water gradually to the the exit end. level of the tail race and to prevent entry of air from exit end of the tube must be totally immersed in water.
In order to avoid any whirl component of velocity of water at runner exit, one or two piers are constructed in the bend of the draft tubes. Such piers behave similar to the central core of Moody's spreading tube, describing the above.
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