new Zealand Hydraulic turbine

1. Selection of water turbine

1.1. Data and parameters to be collected before selecting hydraulic turbine:

1.1.1. Water conservancy and hydropower planning and parameters related to the scale of the power station: understand the hydrology, geology, river development mode, reservoir regulation performance, power station type and hub layout of the basin where the hydropower station is located. Master the characteristic parameters of water head and flow, such as maximum water head, minimum water head, average water head, design water head, maximum flow, average flow, installed capacity, altitude at the power station, water level and flow relationship curve at the downstream of the power station, etc.

1.1.2. Data of the power system: such as the capacity, load, user nature of the power system, the role and status of the design power station in the system, and the requirements of the system for the design power station.

1.1.3. Technical data of hydraulic turbine products: such as turbine model spectrum data, parameters and performance, technical data of similar power stations, etc.

1.1.4. Local construction and transportation conditions.

1.2. Selection principle of hydraulic turbine

The selection principle of water turbine is to select the water turbine with good performance and small size under the condition of meeting the output requirements of the hydropower station and adapting to the parameters (head and flow) of the hydropower station.

The so-called good performance includes good energy performance and good cavitation resistance. Good energy performance requires high efficiency of hydraulic turbine. Not only the highest efficiency of hydraulic turbine is high, but also its average efficiency is high under the change of head and load. Therefore, the turbine with gentle curve change shall be selected within the variation range of water head of hydropower station as far as possible. Good cavitation resistance, that is, the cavitation coefficient of the selected turbine should be small, which can ensure the stable and reliable operation of the unit.

To make the size of the water turbine small, the water turbine with higher specific speed shall be selected as far as possible. The water turbine with higher specific speed has higher speed and smaller runner diameter. Therefore, in the selection and calculation of water turbine, it shall be equal to or slightly higher than the optimal unit speed, and the value shall be the maximum unit flow recommended in the spectrum table, so as to make full use of the water discharge capacity of the water turbine and reduce the size of the water turbine.

In addition to the above basic principles, the selection of water turbine shall also consider that the selected unit is easy to supply, small transportation difficulties and convenient construction and installation, so as to shorten the construction period of hydropower station as far as possible and strive for early power generation.

1.3. Selection of hydraulic turbine

The selection procedure of hydraulic turbine is basically to draw up several possible schemes to be selected, calculate and compare the kinetic energy economic indicators of each alternative scheme, and finally determine the best scheme through optimization. For each alternative, the main contents of turbine selection include:

1.3.1. Select the number of units and unit capacity;

1.3.2. Select the model and device mode of hydraulic turbine;

1.3.3. Determine the runner diameter and speed of the water turbine;

1.3.4. Determine the maximum allowable suction height and installation elevation of the turbine;

1.3.5. Draw the operation comprehensive characteristic curve of hydraulic turbine (generally provided by the manufacturer);

1.3.6. Determine the form and size of spiral case and draft tube;

1.3.7. Select governor and oil pressure device.

 

2. Turbine classification and structure

2.1. Turbine classification

2.1.1. According to different energy conversion modes, it can be divided into counterattack type and impact type. The counterattack type uses the pressure energy and kinetic energy of water flow, and the impact type uses the kinetic energy of water flow. The counterattack type is divided into mixed flow type, axial flow type, diagonal flow type and tubular flow type. Impact type is divided into bucket type, oblique impact type and double-click type.

2.1.2. Mixed flow type: water flows into the runner radially from all around and flows out approximately axially. Application head range: 30m ~ 700m. Features: simple structure, stable operation and high efficiency.

 

2.1.3. Axial flow type: the water flow changes from radial movement to axial flow between guide vane and runner. Applied head: 3 ~ 80m. Features: suitable for medium and low head and large flow hydropower stations. Classification: axial flow fixed propeller and axial flow propeller.

2.1.4. Impact type: the runner is always in the atmosphere, and the high-pressure water flow from the penstock has been transformed into high-speed jet before entering the turbine, impacting the runner blade to achieve work. Water head range: 300 ~ 1700m. It is suitable for units with high water head and small flow.

2.2. Classification of main shaft layout of hydraulic turbine

2.2.1. According to the arrangement form of main shaft, hydraulic turbine can be divided into horizontal and vertical (also known as horizontal shaft and vertical shaft). The hydro generators arranged vertically are divided into suspended type and umbrella type.

2.2.2. The thrust bearing of the suspended generator is located on or in the upper frame on the upper part of the generator rotor. The thrust bearing of the umbrella generator is located in the lower frame at the lower part of the rotor or supported on the top cover of the water turbine with a support. Umbrella generators are divided into ordinary umbrella type (the upper and lower guide bearings are located in the upper and lower frames respectively), semi umbrella type (only the upper guide bearing, which is arranged in the upper frame without lower guide bearing;) and full umbrella type (only the lower guide bearing, which is arranged in the lower frame without upper guide bearing).

 

3. Main basic parameters of hydraulic turbine

3.1. Working head h refers to the energy difference per unit weight of water body at the inlet and outlet sections of hydraulic turbine,

The unit is meter (m), and the typical working head is as follows:

3.1.1. Maximum head (Hmax): the maximum net head allowed within the operating range of the turbine.

3.1.2. Minimum head (Hmin): the minimum net head allowed within the operating range of the turbine.

3.1.3. Design head (H set): the minimum head when the water turbine generator unit sends out rated power.

3.2. Flow Q refers to the amount of water passing through a given flow section of the turbine in unit time, and the unit is m³ / s.

3.3. Output n refers to the work (power) done by the water flow in unit time, which is related to the head and flow of the turbine, and the unit is kW. Calculation formula: N = 9.81QHn

3.4. Efficiency refers to the total efficiency of the turbine, which is the ratio of the input power to the output power of the turbine, and its value is always less than 1, because the turbine will inevitably produce some energy losses during operation, mainly including:

3.4.1. Hydraulic loss: that is, the head loss of water flowing through volute, diversion mechanism, runner and draft tube.

3.4.2. Mechanical loss: that is, the friction loss of the rotating part of the turbine. Such as the friction loss between runner and water flow, between shaft and bearing, and between leakage stopping devices.

3.4.3. Volume loss: loss caused by water leakage of runner and fixed parts.

3.5. Speed refers to the number of rotation cycles of turbine runner in unit time, expressed in N, and the unit is rpm.

 

4. Turbine model

4.1. According to JB84-74 Regulations on the Preparation of Hydraulic Turbine model number, the hydraulic turbine model consists of three parts.

The first part represents the type of turbine and runner model. Common are: mixed flow type, represented by HL; Axial flow paddle type, represented by ZZ; Through flow propeller type, represented by GZ; Bucket type, represented by CJ; wait.

The second part represents the layout form of turbine main shaft and the characteristics of Headrace chamber. Common are: vertical axis, represented by L; Horizontal axis, represented by W; Metal spiral case, represented by J; Concrete spiral case, represented by H; etc.

The third part represents the nominal diameter D1 (cm) of the turbine runner.

4.2. Example of water turbine brand:

4.2. Example of water turbine brand:

HL220-LJ-550 refers to Francis turbine with runner model of 220, vertical shaft, metal spiral case and runner with nominal diameter of 550cm.

ZZ560-LH-800 refers to the axial flow propeller turbine with runner model 560. The nominal diameter of vertical shaft, concrete volute and runner is 800cm.

 

5. Four flow passage components and functions of Francis turbine

5.1. Water diversion components

Composition: spiral case and stay ring

Function: the water flow is evenly and symmetrically introduced into the guide vane with small hydraulic loss, and a certain circulation is formed before entering the guide vane.

5.2. Water guide parts

Composition: guide vane and its operating mechanism, top cover and bottom ring

Function: adjust the flow into the runner and the circulation required to form the runner

5.3. Working parts

Composition: runner

Function: directly convert water flow energy into rotating mechanical energy

5.4. Drain parts

Composition: discharge cone and draft tube

Function: guide the water flow into the downstream, and the draft tube also forms a vacuum behind the runner. Use the potential energy between the runner outlet and the downstream tail water to recover part of the kinetic energy loss at the runner outlet, so as to improve the efficiency.

 

6. Main structure of Francis turbine

Francis turbine is mainly composed of runner, main shaft, guide bearing, main shaft seal, stay ring, volute, top cover, bottom ring, discharge ring (foundation ring), leakage stop ring, wear plate, guide vane and its operating mechanism, turbine pit lining, annular crane in turbine pit, draft tube, etc.

6.1. Rotating part: the rotating part is the core component of the unit and the key to converting hydraulic energy into mechanical energy / electric energy. The rotating part and its related parts mainly include: runner, main shaft, main shaft seal, etc.

6.1.1 runner: the runner is the main component to realize water energy conversion. It converts most of the water energy into the rotating mechanical energy of the runner and transmits it to the generator through the main shaft of the turbine. The runner of a hydraulic turbine converts hydraulic energy into mechanical energy. Runner composition: Francis turbine runner is mainly composed of blades, upper crown, lower ring, discharge cone, pressure reducing device and leakage stopping device. Blade is the core of hydraulic energy conversion of hydraulic turbine runner. The smoothness, waviness, size and shape of blade will have different effects on the performance of hydraulic turbine. The upper crown is used to connect the main shaft at the upper part, support the blade at the lower part and form an overflow channel together with the lower ring. The lower ring connects the blades of the runner as a whole to increase the strength and stiffness of the runner, and forms an overflow channel with the upper ring. The function of discharge cone is to guide the water flow from the blade channel to discharge downward quickly and smoothly, so as to prevent the water flow from colliding with each other, so as to reduce the hydraulic loss and improve the efficiency of hydraulic turbine. The function of the pressure reducing device is to reduce the axial hydraulic thrust acting on the upper crown of the runner, so as to reduce the load of the thrust bearing.

6.1.2. Main shaft: the main shaft bears the weight of the rotating part of the hydraulic turbine and the tension generated by the axial hydraulic thrust, and transmits the torque generated by the runner at the same time.

6.1.3 main shaft seal: the main shaft seal is one of the key components of the hydraulic turbine. During the operation of the hydraulic turbine, the safe and reliable operation of the main shaft seal is directly related to the safe operation of the hydraulic turbine. It can effectively prevent the water in the draft tube from overflowing from the gap between the main shaft and the top cover, prevent the guide bearing and the top cover of the turbine from being flooded, and maintain the normal operation of the bearing and the unit.

6.2. Maintenance seal: maintenance seal is a seal used when the unit is repaired, inspected or damaged due to the main shaft seal, also known as air shroud; When put into operation, the compressed air enters the air shroud, so that the protruding part of the air shroud hugs the processing surface or large shaft flange matched with the water guide rotating oil basin, and cuts off the tail water to prevent flooding of the vehicle room.

6.3. Fixed parts: top cover, bottom ring, stay ring, support ring, etc.

6.4. Embedded part: spiral case, stay ring, etc.

6.5. Functions of guide bearing of hydraulic turbine: first, bear the radial force transmitted through the main shaft when the unit operates under various working conditions; Second, maintain the adjusted axis position.

6.6. Circulation mode of cylindrical water guide bearing: the oil circulation mode of cylindrical tile adopts self circulation. The working principle of self circulation of lubricating oil: when the unit is running, the water guide bearing installed on the main shaft rotates, and the oil basin rotates with the main shaft. During rotation, the oil in the oil basin also rotates. Due to the action of centrifugal force, the oil level in the oil basin forms a state of high edge and low center, That is to form a paraboloid. Under the action of differential pressure, the oil enters the lower annular oil groove on the bearing surface through the oil inlet hole outside the circumference of the fixed bearing body. Due to the rotation of the large shaft, the oil moves up along the inclined oil groove on the bearing surface and flows through the whole bearing surface, so that the lubrication between the large shaft and the bearing bush is good, and the heat is taken away. The hot oil flows to the upper annular oil groove and flows to the cooler through the oil drain pipe, After cooling, the hot oil enters the oil basin through the oil inlet pipe, and the path of the above lubricating oil is a working process. During the operation of the unit, the lubricating oil circulates back and forth to meet the operation needs of the bearing.

6.7. Composition of water guide mechanism: the water guide mechanism is composed of top cover, bottom ring, guide vane, connecting rod mechanism and servomotor.

6.8. The main function of the top cover is to form a flow channel and bear the corresponding fluid pressure; Fix and support the movable guide vane and its connecting rod mechanism; Support the water guide bearing; Support and form the seal of the unit, including main shaft seal, maintenance seal, upper labyrinth ring, etc.

6.9. Function of bottom ring: form an overflow channel with the top cover and install the lower bearing of guide vane.

6.10. Function of foundation ring: place the stay ring during unit installation to become the foundation of the stay ring. It is used to place the runner during turbine installation and maintenance.

6.11. Draft tube: the draft tube is located below the runner and is the main flow passage part, which is used to guide the water flow in and out of the runner.

 

7. Turbine protection device

The hydraulic turbine is generally equipped with emergency pressure distribution valve, quick brake, shear pin, vacuum breaking valve and other protective devices. Function of each protective device

7.1 emergency pressure distribution valve (also known as overspeed limiter) is an effective measure to prevent the turbine from running at runaway speed for a long time. During normal operation of the unit, the emergency pressure distribution valve is only used as a channel for pressure oil to connect the pipeline from the main pressure distribution valve of the governor to the servomotor; In case of load rejection of the unit and governor failure, the emergency pressure distribution valve acts to cut off the connection between the main pressure distribution valve and the servomotor, and directly connect the pressure oil from the pressure oil device to the servomotor, so that the servomotor can be quickly closed to realize the emergency shutdown of the unit.

7.2. The function of quick brake is to close the quick brake, cut off the water flow and shut down the unit when the unit overspeed reaches 140% of the rated speed, so as to shorten the operation time of the turbine under overspeed or runaway speed and protect the turbine.

7.3. The shear pin protection device is composed of the shear pin and its annunciator. The connecting plate and the guide vane arm in the guide vane transmission mechanism are connected by a shear pin. Under normal conditions, during the operation of the guide vane, the shear pin has sufficient strength to drive the guide vane to rotate. However, when there is foreign matter stuck between the guide vanes, the guide vane shaft and guide vane arm cannot move, and the connecting plate rotates driven by the fork head. Therefore, shear force is generated on the shear pin. When the shear force is greater than 1.5 times the normal operating stress, the shear pin is cut off and the guide vane is out of control, However, other guide vanes can still rotate normally to avoid accident expansion.

7.4. The function of vacuum breaking valve is that when the unit is in load rejection or emergency shutdown for other reasons, when the guide vane is quickly closed, the water flow continues to flow downward due to inertia, resulting in a great vacuum in the runner room. Under the action of differential pressure, the tail water flow in the runner room flows back to the runner room to impact the runner blade and top cover, which will produce a great impact force and lift the turbine. The vacuum breaking valve is used to replenish air to protect the hydraulic turbine.

 

8. Cavitation of water turbine

When the temperature rises to a certain value, the liquid under a certain pressure begins to boil; On the contrary, when the temperature remains unchanged and the pressure decreases to a certain value, it will vaporize. At a certain temperature, the critical pressure when water begins to vaporize is called vaporization pressure. During the movement of water flow in the turbine, pressure drop will occur in some areas.

When the pressure drops to the vaporization pressure, water produces bubbles due to vaporization. Due to the formation of low pressure zone and the movement of high-speed water flow, bubbles and bubbles are also moving continuously. In motion, bubbles and bubbles will suddenly compress or expand and even disappear. At this moment, water molecules will produce a huge impact force. If this impact force points to the metal surface, the metal surface will be continuously impacted and the metal surface will be damaged, which is the phenomenon of cavitation. Cavitation damage first makes the metal surface lose luster and darken, then rough, and then develop into pitting, forming sponge like honeycomb holes until it is completely damaged.

8.1. According to the location where cavitation occurs, turbine cavitation is divided into three types:

8.1.1 airfoil cavitation: airfoil cavitation of Francis turbine mainly occurs near the discharge edge near the lower ring on the back of blade; In severe cases, cavitation will also occur at other parts on the back of the blade; Cavitation can occur even on the front of the blade.

8.1.2 cavity cavitation: it refers to the cavitation generated by the large flow vortex zone in the central cavity of the draft tube. Cavity cavitation mainly occurs at the lower ring of blade outlet and draft tube inlet. During patrol inspection, operators can directly hear the impact sound caused by cavity cavitation at the straight cone section of draft tube.

8.1.3. Gap cavitation: refers to the cavitation formed by the local pressure reduction caused by the local increase of flow velocity when the water flows through the narrow gap or around the solid concave convex surface. It often occurs at some local positions of hydraulic turbine. Such as the clearance between the Francis runner and the upper and lower crown stop rings.

8.2. Cavitation prevention measures in operation, the main measures are as follows:

8.2.1. Reasonably formulate the operation mode of the power station and avoid the operation condition area that may produce serious cavitation.

8.2.2. Air replenishing device shall be used to send air to the draft tube to destroy the high vacuum water flow vortex zone in the draft tube.

8.2.3. Improve the maintenance process level and ensure the surface finish of the blade after maintenance to reduce cavitation.

8.2.4. Apply anti cavitation paint, such as epoxy resin, on the blade.

 

9. Causes of turbine vibration

Turbine vibration is caused by mechanical, hydraulic and electrical factors.

9.1. Machinery: vibration at low speed caused by bending of main shaft, poor adjustment of thrust bearing, excessive bearing clearance, loose connection of main shaft flange, etc; Vibration and sound caused by the collision between rotating parts such as runner and stationary parts; The vibration caused by the weight imbalance of the rotating part increases with the increase of rotating speed.

9.2. Hydraulic: vibration caused by pressure pulsation caused by flow vortex in draft tube; Vortex train

Vibration caused by; Vibration caused by uneven clearance of runner stop ring.

9.3. Electrical aspect: vibration caused by uneven air gap and short circuit.