Hydraulic Turbines for small Hydro(Selection, Economic Viability and Model Testing)
...ively at heads down to about 20 m. Pelton turbines are not used at lower heads because their rotational speeds becomes very slow and the runner required is very large and unwieldy. If runner size and low speed do not pose a problem for a particular installation, then a Pelton turbine can be used efficiently with fairly low heads. If a higher running speed and smaller runner are required then there are two further options: - increasing the number of jets. Having two or more jets enables a smaller runner to be used for a given flow and increases the rotational speed. The required power can still be attained and the part-flow efficiency is especially good because the wheel can be run on a reduced number of jets with each jet in use still receiving the optimum flow. - twin runners. Two runners can be placed on the same shaft either side by side or on opposite sides of the generator. This configuration is unusual and would only be used if the number of jets per runner had already been maximized, but it allow the use of smaller diameter and hence faster rotating runners. Turgo turbine The Turgo turbine is an impulse machine similar to a Pelton turbine but which was designed to have a higher specific speed. In this case the jets aimed to strike the plane of the runner on one side and exists on the other. Therefore the flow rate is not limited by the discharged fluid interfering with the incoming jet (as is the case with Pelton turbines). As a consequence, a Turgo turbine can have a smaller diameter runner than a Pelton for an equivalent power. With smaller faster spinning runners, it is more likely to be possible to connect Turgo turbines directly to the generator rather than having to go via a costly speed-increasing transmission. Turgo runner blades and water jet Like the Pelton, the Turgo is efficient over a wide range of speeds and shares the general characteristics of impulse turbines listed for the Pelton, including the fact that it can be mounted either horizontally or vertically. A Turgo runner is more difficult to make than a Pelton and the vanes of the runner are more fragile than Pelton buckets. At one time they were exclusively made by Gilbert, Gilkes and Gordon a UK manufacturer who owned the patent rights, but they are now manufactured in several other countries. The Ghatta and the Multi-Purpose Power Unit The Ghatta is a traditional Nepalese waterwheel with a vertical axis. The water enters the waterwheel from above. The turbine is made out of wood to enable simple building and repair techniques to be used. A consequent of this design are low efficiency and power output (maximum 12 kW). Out of this traditional Ghatta the improved Ghatta was developed. The wooden waterwheel was improved and replaced later with a steel one with round buckets. This improved the momentum transfer of the water and doubled power output. Traditional wooden runner and Improved Metallic Runner metallic runner Multi-Purpose Power Unit The Multi-Purpose Power Unit (MPPU) is chronological situated in between the Ghatta and the improved Ghatta. The name multi-purpose refers to the construction of the MPPU which enables the connection of various machinery to it. The concept of the MPPU is basically the same as that of the improved Ghatta: a vertical axis with a fixed and a rotating grinding stone. Technical complexity, power output and price are in between those of the improved Ghatta and crossflow turbines. All components are of steel instead of wood, water supply is improved and friction losses are reduced compared to the improved Ghatta. Design philosophy was to produce a device as cheap and simple as possible. Special attention was given to transportability. Crossflow turbine Also called a Michell-Banki turbine a crossflow turbine has a drum-shaped runner consisting of two parallel discs connected together near their rims by a series of curved blades. A crossflow turbine always has its runner shaft horizontal (unlike Pelton and Turgo turbines which can have either horizontal or vertical shaft orientation). Operation: In operation a rectangular nozzle directs the jet onto the full length of the runner. The water strikes the blades and imparts most of its kinetic energy. It then passes through the runner and strikes the blades again on exit, impacting a smaller amount of energy before leaving the turbine. Although strictly classed as an impulse turbine, hydrodynamic pressure forces are also involved and a mixed flow definition would be more accurate. Crossflow turbine Part flow efficiency: A high part-flow efficiency can be maintained at less than a quarter of full flow by the arrangement for flow portioning illustrated in the figure. At low flows, the water can be channeled through either two-thirds or one third of the runner, thereby sustaining a relatively high turbine efficiency. Part-flow efficiency of a partitioned crossflow turbine Reaction turbines The reaction turbines considered here are the Francis turbine and the propeller turbine. A special case of the propeller turbine is the Kaplan. In all these cases, specific speed is high, i.e reaction turbines rotate faster than impulse turbines given the same head and flow conditions. This has the very important consequences that a reaction turbine can often be compiled directly to an alternator without requiring a speed-increasing drive system. Some manufacturers make combined turbine-generator sets of this sort. Significant cost savings are made in eliminating the drive and the maintenance of the hydro unit is very much simpler. The Francis turbine is suitable for medium heads, while the propeller is more suitable for low heads. On the whole reaction turbines require more sophisticated fabrication than impulse turbines because they involve the use of larger and more intricately profiled blades together with carefully profiled casings. The extra expenses involved is offset by high efficiency and the advantages of high running speeds at low heads from relatively compact machines. Fabrication constraints make these turbines less attractive for use in micro-hydro in developing countries. Nevertheless because of the importance of low head micro-hydro, work is being undertaken to develop propeller machines, which are simpler to construct. Most reaction turbines tend to have poor part-flow efficiency characteristics. Francis turbine Francis turbines can either be volute-cased or open-flume machines. The spiral casing is tapered to distribute water uniformly around the entire perimeter of the runner and the guide vanes feed the water into the runner at the correct angle. The runner blades are profiled in a complex manner and direct the water so that it exits axially from centre of the runner. In doing so the water imparts most of its pressure energy to the runner before leaving the turbine via a draft tube. The Francis turbine is generally fitted with adjustable guide vanes. These regulate the water flow as it enters the runner and are usually linked to a governing system, which matches flow to turbine loading in the same way as a spear valve or deflector plate in a Pelton turbine. When the flow is reduced the efficiency of the turbine falls away. Francis turbines Propeller turbine The basic propeller turbine consists of a propeller, similar to a ship's propeller, fitted inside a continuation of the penstock tube. The turbine shaft passes out of the tube at the point where the tube changes direction. The propeller usually has three to six blades, three in the case of very low head units and the water flow is regulated by static blades or swivel gates ("wicket gates") just upstream of the propeller. This kind of propeller turbine is known as a fixed blade axial flow turbine because the pitch angle of the rotor blades cannot be changed. The part-flow efficiency of fixed-blade propeller turbines tends to be very poor. Kaplan Large-scale hydro sites make use of more sophisticated versions of the propeller turbines. Varying the pitch of the propeller blades together with wicket gate adjustment enables reasonable efficiency to be maintained under part flow conditions. Such turbines are known as variable pitch or Kaplan turbines. Reversible pump turbines Centrifugal pumps can be used as turbines by passing water through them in reverse direction. Research is currently being done to enable the performance of pumps as turbines to be predicated more accurately. The potential advantages are the low cost due to mass production (and in many cases also local production), the availability of spare parts and the wider dealer/support networks. The disadvantages are the as yet poorly understood performance characteristics and very poor part-flow/load efficiency. Various companies have used pumps as turbines at various times, but the technology remains unproven and relatively poor in performance. Economical Viability of Small Hydro Schemes: As the cost per KW produced of small schemes is high and to make it economical feasible the following factors may be considered for effective cost reduction: a) Standardization of Equipment: Various manufacturers are adopting standardization in design, rating, runner diameter and size of generating units. The use of such standard equipment should be freely resorted to, even if there is little adjustment in rating or efficiency. This will reduce the cost to a great extent. b) Reduction in Manufacturing Cost: There is great scope for reducing the manufacturing cost. Some of the suggestions are as follows: i) Utilization of most economical, indigenous and easily available material for runner blades, guide vanes, other hydraulic passage etc. There is general tendency to ask for stainless steel but not always required for small sets. ii) Using fixed blade propeller runners where ever possible and elimination of guide vanes. iii) Adoption of self lubricated or water lubricated brushes and bearings. iv) Using induction generator in place of synchronous generator as it is about 25% cheaper. It does not require separate exciter and elaborate governing system but proximity of a large grid is, however, necessary. If large grid is not available then adoption of synchronous generator with brush less exciter is recommended. v) Simplified governing and control system. vi) Selection of the most economical terminal voltage. The number of step-up and step-down transformers can also be kept minimum. Sub-station should be made as simple as possible. vii) Various controls, protection devices, cabinet of governor and other instruments etc. may be clubbed in the minimum number of panels and placed suitably so as to occupy minimum working space. viii) The powerhouse structure can be made as simple as possible without any auxiliary bays for housing various power station auxiliary, ancillary system and controls. The supper structure of the powerhouse can be made in brick masonry instead of RCC thus reducing the civil work cost. ix) Electrical overhead crane may be substituted by the chain pulley block or hand operated traveling beam. x) Inlet valves may be dispensed with and substituted by gates. xi) Specifications for smaller turbines should not be based upon the specifications of larger conventional turbines. Table 1: Various types of turbines suitable for small hydro schemes Head in meters of water Discharge in Cumecs Capacity in KW Type of Turbine 0.5 to 10 - - Simple wood and metal wheel 0.5 to 12 0.05 to 8 - Scehneider Hydro Engine 2 to 502 to 151.25 to 25 3 to 201.5 to 403 to 25 -50 to 5000150 to 3500 Axial Flowa) Straflob) Tubular(Fig. 5&6)c) Bulb (Fig. 2, 3 & 4) 1 to 70 3 to 40 - Kaplan 8 to 300 0.3 to 20 500 to 5000 Francis (Fig. 1.) 45 to 300 1 to 8 - Turgo 1 to 200 0.03 to 9 50 to 1000 Cross flow, Banki, Mitchel or obserger 45 to 1000 0.06 to 3 100 to 5000 Pelton TESTING METHODOLOGIES AND INSTRUMENTATION To test a hydraulic turbine for acceptance, investigations can be carried out on prototype itself or on a scale model manufactured as per International standards. The investigations of water turbine are normally undertaken to establish the efficiency characteristics, performance against cavitation and runaway features. The technique of model investigation is based of attempting to simulate the flow conditions on a small scale replica in laboratory to assess the performance of full scale prototype. The model investigations are preferred over the prototype investigations due to the following reasons:- a) In model investigations it is possible to obtain the full performance data of the turbine over the entire operating range where as prototype investigations are difficult to carry out due to head and speed limitations and loading constraints. b) The model investigations are more accurate. They have established an accuracy of the order of ㊣ 0.25% as against 2% in case of prototype. More over it is very difficult to measure accurately the discharge through prototype machine. c) The model investigations are carried out independently, where as prototype investigations interrupt the normal operation of the plant and results in considerable loss of revenue. d) The observation of flow after runner and cavitation phenomenon is only possible in model as transparent draft tube cone, runner chamber etc. can be used. It is now internationally accepted that the prototype performance can be contractually relied upon the model investigations. Though advanced theoretical methods have come into existence for designing actual turbines but they have their own limitations being based on certain simplifying assumptions such as flow is steady, axi-symetric, incompressible, in-viscid or ir-rotational. Experimental support therefore becomes essential to evolve optimum design. Prototype performance is predicted by applying similarity laws Similarity Laws: In general three types of similarities are observed in model and prototype. i) Geometric similarity ii) Kinematic similarity iii) Dynamic similarity Geometric similarity: For this similarity the flow passages of model and prototype are to be similar. This is possible when the ratio's between the corresponding lengths, depths, width, areas, volumes etc. of model and prototype are same. But it is very difficult to keep same the relative surface roughness, clearances and blade thickness especially at exit edge etc. Keeping in view the practical difficulties and manufacturing tolerances. I.E.C. test code 193 recommends permitting certain deviations of model from prototype. Kinematic similarity: If the ratios of the corresponding velocities and accelerations are the same for model and prototype then the kinematic similarity is fulfilled. Dynamic similarity: Dynamic similarity demands that the proportionality of forces acting on the elements of turbine space is same. Similarity relations in Hydraulic Turbines: Specific speed of turbine: Where ns = specific speed of turbine, n is the rotational speed of turbine shaft, N is the power produced and H is the head acting on the turbine. The Kinematic similarity is satisfied if the geometrical similar turbines have the same value of specific speed (specific speed is defined as the speed of a geometrical similar turbine producing 1 HP when working under 1 m head). Unit discharge: Where QI' is the unit discharge, Q is discharge through turbine, D is the diameter of runner, H is the head acting on the turbine. The unit discharge is derived based on the Euler's criterion of dynamic similarity (unit discharge is defined as the discharge through a geometrical similar turbine whose runner diameter is 1m and working under 1m head) thus the equality of Q'1 in model and prototype satisfy the dynamic similarity. Unit Speed: Where n'I is the unit speed, n is the rotational speed of turbine shaft, D is the diameter of runner, H is the head acting on the turbine. The unit speed is derived on the basis of Strouhal's criteria of periodicity. Equality of unit speed (which is defined as the rotational speed of geometrical similar turbine whose runner diameter is 1 m and working under 1m head) of model and prototype satisfies the Strouhal similitude. Cavitation similarity: The Reynolds numbers (defined as the ratio of inertia and viscous forces) are proportional to ﹟H.D and the Froude's numbers (defined as the ration of inertia and gravitational forces) are proportional to H/D. Thus it is not possible to simultaneously satisfy Froude and Reynolds similitude conditions. Reynolds number effects can be made insignificant if the model tests are carried out at higher value of Reynolds number. This limit is specified by I.E.C. code 193 as Re ≡ 2xl06. Mini - Micro machines are low head machines and Froude number is considered as an essential parameter. By adhering to Froude similitude cavitation performance can be faithfully reproduced based the Thoma's cavitation co-efficient 考 defined as: (where Ha = Atmospheric head, Hs = submergence or height of runner above tail water level, Hv = Vapour pressure of water, H = Turbine net head), which is maintained same in model and prototype. In short, the turbine similitude laws which should be satisfied are as follows: (ns)m = (ns)p (Q'I)m = (Q'I)p (n'I)m = (n'I)p (考)m = (考)p where suffix m is for model and p is for prototype. Contractual acceptance tests: Contractual acceptance testing work is directly related to the specific terms of reference agreed by the manufacturer/supplier and purchaser/client. It involves, to check the guaranteed values of efficiency, cavitation, maximum output and runaway speed. Efficiency Performance: Good efficiency of turbine enables maximum utilisation of available water energy to produce higher power generation and increased revenue. Normally the manufacturer indicates values of turbine efficiency at different operating conditions of head and output. In view of widely varying values of turbine efficiency at these operating conditions usually value of weighted average efficiency is guaranteed which can be computed from the formula indicated by purchaser based on study of normal operating conditions. Depending upon the duration of operation at different head and output conditions and also depending upon the importance of the operation at specific operating conditions, varying weightage is given for efficiencies at different head and output conditions to arrive at weighted average efficiency formula during project stage itself. 灰av = a.灰100 + b.灰90 + c.灰75 + d.灰50 + ＃＃. Where 灰av is the weighted average efficiency, a is the weightage to efficiency at 100% load, b is the weightage to efficiency at 90% load, c is the weightage to efficiency at 75%...