Hydropower Water Conveyance System (Part - 5) | Additional Documents & Tests for Civil Engineering (CE) PDF Download

Penstocks  

As mentioned before, a penstock is usually steel or reinforced concrete lined conduit that supplied water from the reservoir, forebay or surge tank at the end of a head race tunnel to the turbines. A penstock is subjected to very high pressure and its design is similar to that for pressure vessels and tanks. However, sudden pressure rise due to value closure of turbines during sudden load rejection in the electric grid necessitates that penstocks be designed for such water hammer pressures as well. Penstocks, at their lowermost end meets a controlling value, from where the water is led to the spiral casing of the turbine, details of which would be discussed in the next lesson. 

Since penstocks convey water to the turbines and form a part of the hydropower water conveyance system, it is necessary that they provide the least possible loss of energy head to the flowing water. According to the Bureau of Indian Standards code IS: 11625-1986 “Criteria for hydraulic design of penstocks”, the following losses may be expected for a penstock: 

• Head loss at trash rock
• Head loss at intake entrance
• Friction losses, and
• Other losses as at bends, bifurcations, transitions, values, etc. 

Based on the above losses, the diameter of the penstock pipes have to be fixed, such that it results in an overall economy. This is because if the diameter of a penstock is increased, for example, the friction losses reduce resulting in a higher head at turbine and consequent generations of more power. But this, at the same time, increases the cost of the penstock. This leads to the concept of Economic Diameter of Penstock which is one such that the annual cost, including cost of power lost due to friction and charges of  amortization of construction cost, maintenance, operation, etc. is the minimum.

A penstock made of steel may be constructed as a seamless pipe, rolled or drawn from mild steel if the diameter is within 0.5m. Larger diameter pipes are usually manufactured from steel plates welded together. The joints have to be carefully tested by ultrasonic or radiographic methods which ensures that high pressure may be tolerated by the pipes. 

Penstocks may also be classified according to their location with respect to the ground surface. If they are buried within ground or laid inside a tunnel drilled (see Figure 18) within the mass of a hill, then they have to be designed to take the load of the surrounding soil or rock. Such buried or embedded penstocks may be differentiated from those that are laid above the ground surface, termed as the surface penstocks, which are subjected to variation in temperature of the surroundings especially due to the sum’s direct radiation. Such and other advantages and disadvantages of embedded and surface penstocks may be listed as under: 

The following Bureau of Indian Standards codes may be referred for the design of embedded and surface penstocks respectively. 

IS: 11639-1986 “Criteria for structural design of penstocks”    Part1: Surface penstocks    Part2: Buried / embedded penstocks 

A penstock is not only a single straight piece of pipeline. It has to certain additional pieces, called specials, to allow it to be located over undulating terrain or within curved or contracted tunnels, provide access for inspection, etc. Design of these special attachments to a penstock is provided by the Bureau of Indian Standards code IS: 11639(Part3)-1996 “Structural design of penstocks-criteria (Specials for penstocks)”. The following paragraphs briefly described these specials and the purpose they serve. 

Bends  

Depending on topography, the alignment of the penstock is often required to be changed, in direction, to obtain the most economical profile so as to avoid excess excavation of foundation strata and also to give it an aesthetic look with the surroundings. These changes in direction are accomplished by curved sections, commonly called penstock bends. For ease of fabrication, the bends are made up of short segments of pipes with mitered ends. 

Bends may be only in one plane, in which case it is known as a simple bend. If the curvature or change in alignment is in two planes- horizontal as well as vertical- then it is called a compound bend.

Reducer piece

In the case of very long penstocks, it is often necessary to reduce the diameter of the pipe as the head on the pipe increases. This reduction from one diameter to another should be effected gradually by introducing a special pipe piece called reducer piece. The reducer piece is a frustum of a cone. Normally the angle of convergence should be kept between 5 degrees ton 10 degrees so as to minimize the hydraulic loss at the juncture where the diameter is reduced.

Branch pipe

Depending upon the number of units a single penstock feeds, the penstock branching is defined as bifurcation when feeding two units, trifurcation when feeding three units and manifold when feeding a greater number of units by successive bifurcations. Branch pipes of bifurcating type are generally known as “wye” pieces which may be symmetrical or asymmetrical.  

Generally the bifurcating pipe has two symmetric pipes, after the bifurcating joints, and the deflection angle of the branching pipes ranges between 30degrees to 75 degrees. In order to reduce the head loss, a smaller deflection angle is advantageous. However, the lesser the bifurcating angle, greater the reinforcement required at the bifurcating part. The wye branches should be given special care in design to ensure safety of the assembly under internal pressure of water. The introduction of a bifurcation considerably alters the structural behavior of the penstock in the vicinity of the branching. 

Expansion joints 

Expansion joints are installed in exposed penstocks between fixed point or anchors to permit longitudinal expansion, or contraction when changes in temperature occur and to permit slight rotation when conduits pass through two structures where differential settlement or deflection is anticipated. The expansion joints are located in between two anchor blocks generally downstream of uphill anchor block. This facilitates easy erection of pipes on steep slopes.

Expansion joints should have sufficient strength and water tightness and should be constructed so as to satisfactorily perform their function against longitudinal expansion and contraction. The range of variations to be used for calculation of expanded or contracted length of penstocks should be determined keeping in consideration the maximum and minimum temperature of the erection sites. 

Manholes

Manholes are provided in the course of the penstock length to provide access to the pipe interior for inspection, maintenance and repair.

The normal diameter of manholes is 500 mm. Manholes are generally located at intervals of 120-150 metres. For convenient entrance, exit manholes on the penstock may be located on the top surface or lower left or right surface along the circumference of the penstock.

The manhole, in general, consists of a circular nozzle head, or wall, at the opening of the pipe, with a cover plate fitted to it by bolts. Sealing gaskets are provided between nozzle head and cover plate to prevent leakage. The nozzle head, cover plates and bolts should be designed to withstand the internal water pressure head in the penstock at the position of the manhole. 

Bulk heads

Bulkheads are required for the purpose of hydrostatic pressure testing of individual bends, after fabrication, and sections or whole of steel penstock and expansion joints, before commissioning. Bulkheads are also provided whenever the penstocks are to be closed for temporary periods, as in phased construction.

Air vents and valves 

These are provided on the immediate downstream side of the control gate or valve to facilitate connection with the atmosphere. Air inlets serve the purpose of admitting air into the pipes when the control gate or valve is closed and the penstock is drained, thus avoiding collapse of the pipe due to vacuum excessive negative pressure. Similarly, when the penstock is being filled up, these vents allow proper escape of air from the pipes. 

The factors governing the size of the vents are length, diameter, thickness, head of water, and discharge in the penstock and strength of the penstock under external pressure. 

Manifold 

The portion beyond the main penstock which feeds the branches for the individual units, when two or more units are fed from one penstock. Apart from the above, the following are required for aligning and holding a penstock in place. 

Anchorage/ Anchor Block/Anchor pier  

This is a structure built to hold down penstocks in position at the points where the direction or inclination of the axis changes and also at some regular intervals. In the closed type of anchor, the penstock is embedded in concrete. In the open-type, the penstock is anchored to concrete by rings. Intermediate supports are also provided for penstocks between two anchor blocks, over which the pipe can slide while expanding or contracting. Sometimes thrust blocks are provided on either side of branch connections to resist unbalanced forces at the penstock connection and thus maintain alignment of outlet headers.

Concrete saddle supports 

These are a type of intermediate supports with concrete base shaped to suit the bottom of the pipe. A well lubricated steel plate, rolled to suit the shape of the pipe shell in contact, is provided in between the concrete surface and the pipe to facilitate smooth movement of the pipe over saddles.