Distributor pipe: pelton turbine

Soon after main inlet valve,the water is conveyed to the nozzle of Pelton turbine via the distributor.Distributor pipe is a mild steel pipe with diameter decreasing along the direction of the flow.It is designed in such a way that the velocity of the water remains constant at each outlet.

Procurement Process in Nepal

Procurement process of the goods or work by the public companies (Ministries, Departments, NEA, NTC, NAC , CAAN etc) is governed by the procurement act 2007A.D/ 2063 B.S. The process by which procurement should be done based on the following factors:
1)  value of procurement,
2) source of fund for the procurement,
3)  degree of emergency of requirements of goods/ work
4) requirement  of community involvement and employment empowerment

Based on the above factors, the procurement can be done by public entity in the following way

1) National Competitive bidding:

• Value of the procurement: above 10 lacs for the goods and above 20 lacs for the work.
• The tender notice should be published in national newspaper with period of  at least 30 days.
• The tender documents must be available from two or more than two public entities.

2) International Competitive bidding

• Value of procurement: Same as NCB
• The tender notice should be published in English language with at least period of 45 days

ICB is done in the following cases:

• If the goods/work cannot be available at competitive price inside the Nepal
• If the agreement with donor agency has been done in such a way that the goods/work must be buyed from the donor's conuntry
• If none of the responsive bid has been obtained previously while doing NCB in Nepal
• If the work/goods required is of complex nature

3) Sealed Quotation

• Value of the procurement:below 10 lacs for the goods and below 20 lacs for the work.
• The tender notice should be published in national/local level newspaper with period of  at least 15 days.
• The lowest sealed quotation fulfilling the technical requirement will be selected .

4)  Direct procurement

1. Procurement value: procured directly without any competition between the vendor
2. Allowed upto 5 lacs only

Direct procurement is done in the following cases

• If there is only a single competent vendor for providing the particular goods/works
• If there is an emergency requirement 
• if any single vendor has exclusive rights to provide goods/works and ther e is no other options available

5) Work through user committee

The main objective of this type of procurement is to achieve the involvement of local community and provide more and more employment opportunity to them. The maximum value of procurement through this type is sixty lacs only

6) Work may be done by Force Account

In this method the public company hire few  worker on wage basis and do the construction activity through them. The maximum procurement value allowed is 1 Lacs

Governing of Hydro-generator

If we have hydro power plant connected to grid, the generator needs to be synchronized to the grid for power evacuation.To synchronize generator with the grid, the  frequency of the grid must match with the frequency of the current generated by the generator.For this, generator must be always rotated at same speed which is called synchronous speed. However, the speed of generator varies as the load varies.The speed of generator decreases as load increases and increase as the load decreases. The load fluctuation is inevitable and therefore a mechanism must be developed to run the generator at constant synchronous speed which is called the governing mechanism.

The governing of water turbine generator is similar to the diesel generator.In diesel generator we vary the mass flow rate of the  fuel (diesel) going to the engine. However, in hydro generator we vary the flow rate of the water.Whenever the speed of generator decreases we increase the flow rate of water  and thus higher flow of water helps to maintain its synchronous speed.The vice-versa will occurs when the speed of generator decreases.

The flow rate is varied by varying the area of the flow.Different type of  mechanism has been developed for different types of turbine to vary the area of the flow.Following are the mechanism for the different turbines.

1) Francis Turbine and Kaplan Turbine : Guide Vane Mechanism

Figure: Guide Vane Mechanism

As shown in the left side of the figure, the guide vane (also called wicket gates) is in completely closed position which will block the flow of water to the the turbine.In the right side the guide vane is fully opened allowing all the water to flow to turbine. The guide vane can also be partially opened.

Since the energy of water  reaching the turbine,is very high, and since the guide vanes are heavier in weight, large amount of power needs to be applied to move the wicket gates.This is made possible by the use of hydraulic servo-motor and linkage bar assembly.The servo motor is supplied with the pressurized hydraulic fluid provided by the Oil Pumping Unit (OPU)  of the Power House.

 Pelton Turbine: Spear Valve mechanism/ Deflector plate mechanism

Spear Valve Mechanism:  Spear Valve is a conical type valve having  it's pointed edge facing toward the runner. The linear motion of the spear valve varies the area of the water flow inside the nozzle.. Whenever the spear valve comes near toward the runner the area of the flow inside nozzle decrease and  vice versa will occurs when the moves backside.This type of mechanism is used for slow response to the load fluctuation (5-10 sec)

Deflector Plate Mechanism:

This is the mechanism which is for very fast response to the load fluctuation. It brings the generator back to its original speed within 2-3 second. In this mechanism instead of varying the flow of water , the water is diverted to the tailrace instead of hitting the turbine .This will cause the loss of water

Control Mechanism: The function of control mechanism is to sense the rotational speed of the generator and provide signal to the mechanical device (Guide Vane/Spear Valve/Deflector Plate assembly) for opening or closing.Previously, the control mechanism was mechanical using the principle of centrifugal governor. However, regarding the issue of accuracy and the response to the fluctuation of load, it has become obsolete. Digital Governor is widely used recently which uses the principle of PID controller.

For detail understanding, I have also posted the video of working of governing of Francis turbine and Pelton turbine. The video is courtesy of Voith Hydro GmBH Germany.

Please feel free to comment and have further discussion

Cooling system architecture in Hydropower

The main function of cooling system is to cool Generating lubricating oil and turbine shaft seal. So, basically, two main consumer of cooling system in Power house are

a)     Generating lubricating oil cooler:   Generator bearing is cooled by Lubrication oil and Lubrication oil is cooled by the water, circulated by cooling system..

      Fig: Heat Flow Diagram

b) Turbine shaft seal: Water also used to cool/seal turbine shaft seal.

Water from draft tube/tailrace is used as coolant .Cooling system can be open loop or closed loop. In open loop cooling water system water is finally discharged to tail race level or MIV pit. However, in closed loop cooling the same water is recirculate back.

The major components of cooling system are as follows:

1)      Pump: An appropriate pump is used to circulate the water. Normally, there are two centrifugal pumps in parallel per unit. One will be the main pump and other will be stand-by pump. Standby pump will operate main pump fails thereby increasing reliability of the system.

2)      Filter/Strainer: Water is filtered by duplex/simplex filter to remove silt and abrasive material from it before reaching to consumer. Two or more filters of different capacity can be used in series connection based on the size of the particles to be filtered.

3)      Water Tank: Pumped water is stored in a tank. The Tank will acts as buffer

4)      Fitting and Arrangement: It consists of valves, Pressure switch, piping and pressure gauge.

5)      Heat exchanger: An appropriate heat exchanger is used to extract heat to water from the consumer.

                                  

Heat Treatment

There are different heat treatment method.Heat treatment change the crystalline structure and hence change the property of material.Hardness, Strength and ductility can be controlled by heat treatment process.

1. Annealing: It is the process of heating the material above re crystallization temperature followed by very slow cooling at the furnace. Annealing is done to increase the toughness.However, hardness is reduced in the process.Following are the significance of annealing
   It increases the ductility of the material
   It improves the grain size of crystal structure
   It helps to relieves internal stress
   
2. Quenching: It is the process of heating the material above re crystallization temperature (to red hot) followed by immediate cooling at brine solution. Quenching increase hardness of the material.However, the material becomes brittle after quenching
   
3. Tempering: Tempering is usually done after quenching(hardening) to reduce the brittleness.In tempering, the metal is heated below the critical point for some time and then allowing it to cool in still air.
   
4. Normalizing: Normalization is the process of heating the metal to upper critical temperature and then cooling in still air.Tempering provides a way to carefully decrease the hardness of steel, thereby increasing the toughness to more desirable point. Normalizing increases the ductility and help to relieve the internal stress.
   
   
   If you find difficult to visualize these processes the following video might be useful
   
   https://www.youtube.com/watch?v=fLvZkZxiXnE&list=LLaXo5zVPTkRjLHl4CHv9Vbw&index=2