Diversion Headworks in ROR Type Hydropower Plant

Fig: Schematic Diagram of Headworks Components in ROR Type Hydropower Plant

Hydropower plants may be of different types. Based on the type of hydropower plants, the general arrangement and components in headworks may vary. For example, a run of river hydropower plant generally consists of a weir and undersluice structures separated by a divide wall where as in a storage type hydropower plant, it consists of a dam and reservoir system with an spillway sitting on the dam crest. Here, in this article, the major components of diversion headworks in typical ROR type hydropower plant are described. The principal function of each components of headworks are as follows:

Components & Function of Diversion Headwork in Typical ROR Type Hydropower Plant

1. Weir

• To raise the water level upstream of the weir that builds the sufficient driving head to divert water into the water way.
• To allow safe passage of flood through the weir crest.

2. Underlsuice / Scouring Sluice

• To create a still pocket of water in front of the intake so that sediment free water can be withdrawn thorough it.
• To flush sediments deposited in front of the intake by opening undersluice gates.
• To allow a portion of flood discharge to pass safely through it.

3. Divide Wall

• To separate the weir and under sluice portions from each other
• Helps to keep the comparatively less turbulent pockets of water by preventing cross currents near the intake area

4. Intake

• To regulate supply of water into the waterway
• To control entry of sediment and trashes into the waterway
• To prevent flood entering into the waterway

5. Fish Ladder

• To allow migratory fishes to travel between upstream and downstream of weir

6. River Training works such as Flood Walls, Boulder Riprap, etc.

• To prevent flooding of surroundings by safe passage of flood discharge
• To prevent river eroding the river banks
• To prevent river from changing its course

7. Gravel Trap

• To settle coarser sediments entering into the intake
• To avoid small trashes that has passed through intake
• To escape excess water through side spillway if any.

8. Approach Canal / Approach Culvert

• To convey water from intake and gravel trap to settling basin
• To maintain straight approach and controlled velocity before settling basin.

9. Settling Basin

• To settle fine sediments and silts and to flush them out
• To escape excess water through side spillway if any.

10. Head Pond / Forebay

• To maintain submergence for pressure conduit
• To escape excess water through side spillway if any.
• It also serve the function of head regulation.

Forebay & Its Functions in Hydropower Plant

What is Forebay in Hydroelectric Power Plant ?

It is a structure located at the beginning of penstock pressure shaft that satisfy the function of head regulation and elimination of water hammer effect. It is a large water body that fulfills similar functions as the surge tank. A forebay is sometimes also referred as head pond. A forebay may consists of trash rack at the beginning of penstock pipe. The functions of a forebay can be pointed as below:

• It allows transition of watercourse from open channel flow to pressurize flow maintaining the necessary submergence.
• It protects the penstock pipe form water hammer effect caused due to sudden acceptance and rejection of load in turbines.
• Temporary storage of water when load on turbine is suddenly decreased.
• Temporary supply of water when load on turbine is suddenly increase.
• It serves the function of head regulation.
• Spilling of excess water through side spillway if any.

What is Difference Between a Surge Tank & Forebay ?

A surge tank or a fore bay is constructed in a hydroelectric power plant with the same purpose of eliminating the effect of water hammer. But a surge tank is constructed when there is a pressurized flow in headrace that feeds to surge tank where as if the headrace flow is of open channel type, then construction of forebay is necessary.

Functions & Types of Penstock in Hydropower Plant

What is Penstock in a Hydropower Project?

A penstock is a steeply sloping pipe that carries water at high pressure from surge tank or a forebay to the turbine. It is designed to operate under very high head of water with least possible head loss. A penstock pipe experiences heavy stress and vibration due to the effect of water hammer during sudden closure of valve that control flow to the turbine. Anchor blocks and support piers are provided along the span of the penstock pipe in order to held it firmly in its position. Expansion joints are provided in case of long penstock pipes so as to avoid the forces due to thermal expansion and contraction. These joints are usually placed immediately downstream of an anchor block. A penstock pipe may have vertical and horizontal bends as per the project needs. It may have bifurcation or trifurcation before it enters into the powerhouse so as to feed the different turbine units. The major and minor head loss in penstock pipe can be determined as explained in pipe flow literature. A penstock pipe may be of following types:

• Exposed Penstock
• Embedded Penstock
• Buried Penstock

A penstock pipe in hydropower plant serves following different functions:

• To convey water from surge tank to the turbines with minimum loss of head
• To withstand the effect of water hammer during sudden closure of valve

What is Economical Diameter of Penstock?

The economic diameter of penstock is a diameter which minimizes the sum of annual cost of penstock pipe and annual value of power loss due to loss of head in the penstock pipe. The diameter of penstock pipe can be optimized either by analytical method or by graphical method so as to obtain the economical diameter of pipe.

Water Hammer & Surge Tank in Hydropower Plant

Concept of Water Hammer

In a long pipe, when velocity of liquid is abruptly decreased as a response to the operation of flow control device (eg: valve closure), the momentum of liquid gets destroyed. As a result, the kinetic energy of liquid is converted into internal pressure energy due to which a wave of high pressure is set up which is transmitted upstream through the pipe with a velocity equal to that of sound and produces noise and vibration in pipeline. This phenomena of sudden rise in pressure in pipe is called water hammer. It is termed as 'water hammer' because it produces a loud noise that resembles to the sound of a hammer striking a metal surface.

What is Surge Tank in Hydropower Plant?

It is a chamber, provided in between low pressure tunnels and steeply sloping high pressure penstock pipes, that permits liquid to surge in it so as to minimize the effects of water hammer caused due to sudden acceptance and rejection of load in the turbine. During sudden closure of valve that feeds to the turbine, a wave of very high pressure is established in the pipeline system which may cause bursting of penstock pipe if proper safety measures are not provided. In order to counter this, either the thickness of penstock pipe should be increased or an open place should be provided to release the surge. The high pressure set up in the penstock pipe is released through the oscillation of water surface in surge chamber and is dampened by frictional resistance.

Functions of Surge Tank

• Protection of penstock pipe and low pressure tunnel from the effect of water hammer.
• Temporary storage of water when load on turbine is suddenly decreased (load rejection case)
• Temporary supply of water when load on turbine is suddenly increase (load acceptance case)
• It performs the function of head regulation.

Working Principle of Surge Tank

When load on turbine is suddenly decreased, water mass in penstock is rapidly decelerated and also the flow in tunnel is retarded. One part of continuous supply from tunnel then tend to fill the surge chamber and water level in surge chamber rises above static level in reservoir. Due to the counter pressure created by this over travel, flow in the tunnel is decelerated to the extent that the supply becomes smaller than the turbine demand. As a result, water level in the chamber starts to recede and will drop below steady state level. In an attempt to establish steady state condition, the water surface will again start to rises from low level and due to inertia of moving water, it will again over travel the steady state level. This cycle is repeated until the oscillation is dampened by friction.

When load on turbine is suddenly increased, the water level in surge chamber will fall abruptly below its initial position at reservoir level creating a sufficient head to accelerate the water mass in tunnel. This head difference becomes greater than that necessary to maintain steady flow at normal losses. Consequently, the discharge in tunnel increases and exceeds that required by turbine. As a result, surplus water will rise in the chamber above the initial static level. Now, similar to load rejection case, the water level in chamber will fall below steady state level due to counter pressure and will again over travel the steady state level while rising up. This cycle is repeated until the oscillation is dampened by friction.

Head Loss in Open Channel Flow | Major & Minor Losses

Major Loss / Friction Loss in Open Channel Flow

As described in pipe flow, the head loss in open channel flow is also categorized into two parts as Major Loss (or friction loss) & Minor Loss. The friction loss in open channel is calculated from Manning's equation as below:

$$v=\frac{1}{n}R^{2/3}S^{1/2}$$

$$or, v=\frac{1}{n}R^{2/3}(\frac{h_f}{L})^{1/2}$$

$$\therefore h_f=\frac{n^2v^2}{R^{4/3}}L \tag{1}$$

where,

$n$ = Manning's Constant

$R=\frac{A}{P}$

     = Hydraulic Radius

$L$ = Length of Channel

$v$ = Velocity of flow

Equation (1) gives the head loss in open channel caused by friction.

Minor Loss / Local Loss in Open Channel Flow

Minor head loss occurs due to change in velocity of flow either in magnitude or in direction such as at channel contraction and enlargements, at bends, etc. Minor losses in open channel flow can be formulated as below:

1. Head loss due to channel enlargement

$$h_m=k_e\left(\frac{v_1^2}{2g}-\frac{v_2^2}{2g}\right)$$

2. Head loss due to channel contraction

$$h_m=k_c\left(\frac{v_2^2}{2g}-\frac{v_1^2}{2g}\right)$$

where, 

$k_e$= Enlargement coefficient

$k_c$= Contraction coefficient

The value of these coefficients can be adopted as below:

3. Head loss at channel bends

Tests on large canals showed that losses due to bends could be estimated from following equation:

$$h_m=0.001(\Sigma \Delta^{\circ})\frac{v^2}{2g}$$

where, $\Sigma \Delta^{\circ}$ is summation of deflection angles in the reach.

Sprinkler & Drip Irrigation I Applicability, Advantages & Disadvantages

Sprinkler Irrigation

It is a method of irrigation in which water is spread uniformly over the land in the form of natural rainfall that comes from the nozzle of a sprinkler. Water is applied at the rate less than infiltration rate of soil so as to avoid any surface ponding and runoff. Also, the rate of application of water must match the rate of usage of water by the plants else the water will percolate deep below the root zones and will be lost to the plants. Surface ditches and prior land preparation is not necessary for sprinkler irritation system. It consists of a pipe network having main pipe, distribution pipe and laterals with the sprinklers placed at regular intervals along the lateral line. A sprinkler irrigation system can be of following different types:

1. Permanent type
2. Semi permanent type
3. Portable type

If the distribution pipe and laterals are fixed in the land i.e. they are often buried then it is called permanent type sprinkler irrigation system. If the distribution pipes are fixed but laterals are movable from place to place then it is termed as semi permanent system. If the entire network can be moved from place to place then it is called portable type sprinkler irrigation system.

Advantages of Sprinkler Irrigation System

• Suitable to all types of soil except heavy clay.
• Suitable for both row and scattered cropping patters.
• Possibility of using soluble fertilizers and chemicals by mixing in water.
• Accurate and easy to measure amount of water distributed.
• No soil erosion as surface runoff can be completely eliminated.

Disadvantages of Sprinkler Irrigation System

• Sediment free water is necessary otherwise frequent clogging occurs.
• Efficiency of water distribution decreases under heavy winds.
• It is not suitable for paddy crops.
• Due to circular spraying of water, there is chance of over irrigation and no irrigation at some area.

Drip Irrigation

It is a method of applying water directly to the plants through a number of low flow rate outlets called emitters or drippers, generally placed at short interval along a small tubing. One of the main characteristics of this method is point irrigation as compared to area irrigation in sprinkler irrigation system. This method of irrigation is suitable only for row crops such as tomatoes.

Advantages of Drip Irrigation System

• Deep percolation and evaporation losses can be minimized because only small portion of ground (especially root zone of plants) is supplied with water.
• Weed growth can be controlled effectively.
• Surface runoff and soil erosion can be minimized.
• Fertilizers can be used with high efficiency.
• Field levelling is not necessary.

Disadvantages of Drip Irrigation System

• Sediment free water is necessary otherwise frequent clogging occurs.
• It is not suitable for scattered cropping patterns.
• Sun heat decreases the useful life of tubes used for drip irrigation.
• Waste of water, time, and harvest if not installed properly.

Suitability of Sprinkler and Drip Irrigation System for Hilly Areas

The sprinkler and drip irrigation methods are suitable for remote hilly area of Nepal because of following reasons:

• The necessary head for the operation of sprinkler and drip irrigation is easily available in hilly area due to its steep topography.
• Sediment free water is necessary for effective operation of sprinkler and drip irrigation system which can be obtained from spring sources in hilly areas.
• These methods can be adopted for both row and scattered cropping patterns. Drip irrigation method is applicable only for row crops where as sprinkler is suitable for both row and scatter crops.
• Drip and sprinkler irrigation system can be established with a simple pipe network in hilly areas where construction of headworks and canal structures is not feasible. 
• Components of sprinkler and drip irrigation system can be transported easily to remote hilly areas.

Layout & Components of Typical ROR Type Hydropower Plant

Fig: Schematic diagram of typical ROR type hydropower plant

Components of typical ROR type hydropower plant

The general layout and components of hydropower plants may differ based on the types of hydroelectric project. A general layout of typical ROR type hydropower plants constructed in Nepal is shown in figure above in schematic  form. The name of each components is listed below:

1. Headworks Components

The diversion headwork is constructed in a hydro power plant so as to divert necessary amount of sediment free water into a waterway. A diversion headwork may consists of following components:

• Weir / Spillway

• Undersluice / Scouring sluice

• Divide wall

• Flood wall

• Side Intake

• Bed Sluice

• Gravel Trap

• Gravel Trap Side Spillway

• Approach Culvert / Approach Canal

• Settling Basin

• Head Pond

2. Headrace Waterway

It is a long watercourse or passage that conveys water from headworks component to the a surge chamber or a forebay. The flow in the waterway may be either open channel or pipe flow. The structure in waterway my be one of the following types:

• Headrace Tunnel
• Headrace Canal
• Headrace Pipe

3. Surge Tank / Surge Chamber

The chamber provided in between headrace pressure conduit and steeply sloping penstock pipes with a purpose to minimize the effect of water hammer is called surge tank. If the the waterway in headrace is of open channel type instead of pressurized system, forebay is installed in place of surge tank.

4. Penstock

It is a steel pipe that supplies water from surge chamber to the turbines placed inside powerhouse. Penstock pipe operates under very high water pressure.

5. Anchor Block and Saddle Support

These are the structures that provide supports to the penstock pipe. Anchor blocks are provided at the bends where as saddle supports are provided in between the anchor blocks at regular spacing.

6. Powerhouse

It is a building which consists of different hydromechanical and electromechanical equipment in which hydraulic energy is converted into mechanical energy by turbines and the generators convert mechanical energy into electrical energy.

7. Tailrace Culvert / Tailrace Canal

Tailrace is a water channel constructed in the downstream of hydroelectric powerhouse in order to discharge water back to the river or other water bodies safely.

Types of Irrigation | System of Irrigation

Types of irrigation can be broadly divided into following categories:
a) Surface Irrigation

b) Subsurface Irrigation

c) Micro Irrigation

a) Surface Irrigation

It is further divided into two parts:

i) Flow Irrigation

ii) Lift Irrigation

I) Flow Irrigation

When irrigation water is supplied to the agricultural land by flow of water solely under gravity is called flow irrigation. Flow irrigation is further divided into two parts as:

1) Inundation Irrigation

2) Perennial Irrigation

1) Inundation Irrigation

When water is diverted by excavating a canal at the bank of inundation river (i.e. the river which overflows in rainy season and nearly dries up in summer and winter) without regulator at the head of canal is called inundation irrigation. The bed level of canal is fixed in such a way that water flows through the canal when water level rises above the canal bed and stops automatically when it falls below the canal bed level. As there is no head regulator, over irrigation may occur resulting in damage of crops.

2) Perennial Irrigation

If water is diverted into the canal by constructing headworks at a perennial river (i.e. the river which flows throughout the years in its full capacity) is called perennial irrigation. It consists of regulator at the head of canal to control the flow into the canal. It is of following types:

• Direct Irrigation

In this system, headworks consist of a weir or a barrage across the river in order to raise water level upstream of the weir or barrage so that water can be diverted into the canal.

• Storage Irrigation

In this method, a dam is constructed across the river so as to form a storage reservoir at upstream of the dam and flow of water into the canal is controlled by a head regulator. The storage reservoir may be utilized for other purposes as well in addition to irrigation such as water supply, hydroelectricity, fishery, etc.

II) Lift Irrigation

If water is lifted from surface or subsurface sources by means of manpower, mechanical power or electrical power and directly supplied to the agricultural land then it is called lift irrigation. For example, water from a well may be lifted manually in bucket and supplied to the field via same bucket, a water boring may be installed inside the field that operates by electrical power, etc. It is mostly suitable for irrigating small isolated area. The method of lift irrigation has following advantages:

• Water is supplied to the field as per requirements and there is no possibility of over irrigation.
• Water is supplied directly to the field and there is no conveyance loss as in canals.
• There is no loss of valuable land because construction of distribution canals inside agricultural land is not necessary.
• Installation and maintenance cost is low.

Lift irrigation has following disadvantages:

• It may not be feasible for irrigating large areas.
• Pumping from deep wells may not feasible in the areas where water tables decreases below the suction head.

b) Subsurface Irrigation

In this method, water is supplied to the plants by maintaining an artificial water table at some depth below the ground surface. Moisture then moves upward till the root zones by capillary action.

c) Micro Irrigation

The sprinkler and drip irrigation system falls under this category. In sprinkler irrigation, water is sprayed from the nozzle of sprinkler uniformly over the field. In case of drip irrigation, water is supplied to row crops through a small tube and released from the drippers provided at regular spacing along the tube.

Head Loss in Pipe Flow I Major and Minor losses in Pipe

Concept of Head Loss in Pipe Flow

When a fluid under pressure flows through a pipe, the pipe offers resistance to the flow due to which total head (or energy) of flowing fluid gets reduced by certain amount which is called head loss $(h_L)$ in pipe flow. The head loss in fluid flow may occur due to several reasons which are mainly divided into two parts as:

1. Major Head Loss
2. Minor Head Loss

The head loss in fluid flow due to friction is called major head loss. It is also called friction loss $(h_f)$. The head loss in fluid flow caused by various factors such as bend in pipe, change in pipe diameter, presence of valves and fittings, etc are termed as minor head loss $(h_m)$. It is called so because its value is small in relation to friction loss for a large length of pipe. However, in case of short pipe, minor loss may become greater than the friction loss. The total head loss in the pipe is sum of major loss and minor losses.

Major Loss / Friction Loss in Pipe Flow

The friction loss in a pipe of length $(l)$ and diameter (d) is generally calculated using Darcy-Weibach Equation as below:

$$h_f=\frac{flv^2}{2gd}$$

where,

f = friction factor

l = length of pipe

v = velocity of flow in the pipe

g = acceleration due to gravity

   = $9.81 m/s^2$

d = diameter of pipe

The friction factor is evaluated depending upon the regime of flow (laminar or turbulent) and the type of surface boundary (smooth or rough). The friction factor for pipe flow is determined using Moody diagram for which Reynold's Number, pipe roughness, and diameter of pipe is required to be known. Moody diagram can be used for laminar as well as turbulent flow regime. Alternatively,  for laminar flow, friction factor can be formulated as below:

$$f=\frac{64}{Re}$$

Here, the Renyold's Number (Re) can be calculated as below:

$$Re=\frac{v\rho d}{\mu}$$

$$Re=\frac{vd}{\nu}$$

where, kinematic viscosity $(\nu)$ for water is approximately equal to $10^{-6}m^2/s$ and is formulated as below:

$$\nu=\frac{\mu}{\rho}$$

In case of turbulent flow, friction factor is determined based on whether the pipe is hydrodynamically smooth or rough. For turbulent flow in hydrodynamically smooth pipe, friction factor is given as:

$$f=\frac{0.3164}{Re^{1/4}}\space for \space 4*10^3<Re<4*10^5$$

The friction factor for turbulent flow in commercial pipes can be calculated using Colebrook-White Equation as below:

$$\frac{1}{\sqrt{f}}=-2log\left(\frac{e}{3.70d}+\frac{2.51}{Re\sqrt{f}}\right)$$

Here, the roughness height (e) for different pipe material is given below:

Minor Loss / Local Loss in Pipe Flow

When the velocity of a flowing liquid changes suddenly either in magnitude or in direction, there is a large scale turbulence generated due to the formation of eddies in which the energy possessed by the flowing liquid is utilized which is ultimately dissipated as a heat and hence loss of energy occurs. This loss of energy of flowing fluid due to sudden change in velocity is known as minor loss. Minor losses in pipe flow can be formulated in different situations as below:

1. Head loss due to sudden enlargement

$$h_m=\frac{(v_1-v_2)^2}{2g}$$

$$=k\frac{v_1^2}{2g}$$

where,

$$k=\left(1-\frac{A_1}{A_2}\right)^2$$

2. Head loss due to sudden contraction

$$h_m=k\frac{v_2^2}{2g}$$

where,

$$k=\left(\frac{1}{C_c}-1\right)^2$$

$C_c$ is coefficient of contraction. The value of $C_c$ or k is not constant but depends on the ratio $(\frac{A_2}{A_1})$. The value of 'k' is generally taken as 0.50.

3. Head loss at the entrance to a pipe

$$h_m=0.5\frac{v^2}{2g}$$

4. Head loss at exit from a pipe

$$h_m=\frac{v^2}{2g}$$

5. Head loss due to bend

$$h_m=k\frac{v^2}{2g}$$

The value of 'k' depends on total angle of bend and relative radius of curvature R/d, where, R is radius of curvature of pipe axis and d is diameter of pipe.

6. Head loss in various pipe fittings

$$h_m=k\frac{v^2}{2g}$$

The value of coefficient 'k' actually depends on the type of pipe fittings.