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Amelioration of technical characteristics
of Volkhov Hydro Power Station (Russia)
after prolonged working period.
I.P.Ivanchenko, I.A.Kovalev (St-Petersburg Central Boiler and Turbine Institute), A.Triandafillidis (Entegro Ltd)
The prototype of soviet power generating facilities, the Volkhov HPS, was built according to state plane of Electrification of Russia more than 70 years ego. For those times this HPS was the most powerful object of the country.
Eight hydro- agregates have been installed there rating 8,6 MW each, under the Head HP = 10,5 m. The turbines were equipped with radial-axial (Francis) runners of 4,5 m diameter manufactured by the Swedish Company "Verkstaden Kristinenhamn". The runner has 12 steel blades cast in hub and rim made of cast iron.
The runner blades are of constant profile with 30 mm thickness with the exception for the inlet and outlet edges having thickness of 16 mm.
The main peculiarity of this turbines is the positioning of guide vanes too close to the runner, which is not typical for modern turbines. The diameter of guide vanes location is DO = 1,11D1 while modern turbines have DO = 1,2D1. As a result the egdes of the guide vanes (24 pieces) are ovehanging the runner blades in case of great opening angles.
The Volkhov HPS turbines have become obsolete long time ago. In the period of 1994-1996 three turbines were replaced by new ones with higher rated capacity having by 6% move efficiency and giving by 20% more generated power at the same Head (HP = 10,5 m). So the necessity to replace all remainig turbines for new ones is not disputable, but all reconstruction works have been suspended due to heavy economical situation in present Russia.
Under these conditions there remained the only target - to increase the generated power of old turbines by upgrading the existing blading system.
Practically this task comprised the increasing of maximum turbine rated capacity (by 5-10% depending on the Head) as well as agregate working efficiency, especially while functioning at high opening angles of guide vanes ( the most frequent regime).
On the first step in fulfillment of these objectives the relevant calculations were carried out to select the optimum variant of modernization. The applied theoretical methods with special program packets allowed to determine the values of efficiency, flow, capacity and other cinematic, loading and cavitation characteristics of the turbines.
Fourteen modifications were investigated theoretically. The block-diagram of modification options is shown at Fig 1. The modifications M1-M5 comprise different variations of partial cutting of the running blades outlet edges (Fig 2).
The runner according to M1 was obtained by the cutting the initial structure all along the outlet profile with the constant width of cut portion equal to bn = 100 mm.
The M2 was received by the cutting of the initial runner along the periphery of blade from point C up to point D of outlet edge on the length lCD =900 mm with changing width of cut portion: from O at point C up to hn =100 mm at point D.
M3, M4 and M5 were obtained by cutting of initial runner along the hub region of the blades from point A up to point C of outlet edge along the length lAC = 1270 mm (for M3) and from point A up to point B along the length lAB =635 mm (for M4 and M5). The width of cut portion varies according to the linear law from zero at point C (runner M3) and at point B (M4 and M5) up to bn =180 mm at point A for M5.
As a result of mentioned cutting, the thickness of outlet edge increases in the zone of cutting up to the value tk =30 mm (in case of cutting width bn more than 80 mm). So all other runner modifications were achieved on the base of M1-M5 variations by means of decreasing the outlet edges changing the working surface profile without touching the blades back surfaces (Fig 3).
All modifications titled as M1.1, M1.2, M3.1 etc differ in different ways for decreasing the outlet profile.
For example the modifications M4.1 and M5.1 were organized by decreasing the outlet edge thickness in order to receive the constant thickness equal to 16 mm all along the outlet profile.
The calculations for all modifications have been carried out for Hp =10,5 m for different opening of the guide vanes (ao =260
400 mm).
The relevant basic data (geometry of flow path, spiral chamber drawings, running blades parameters, etc) were delivered by administration of Volkhov HPS and partially obtained by site measurements and analysis of archive documents.
The results of efficiency and capacity calculations for initial runner as well as for modified one are shown in Table 1.
As it is seen the cutting of outlet edge along all the runner profile (M1) decreases the turbine efficiency at every working values of guide vanes opening but slightly increases capacity at high openings due to increased flow-rate through the runner.
For the Volkhov HPS runners the most effective variant of modification is the outlet cutting in the vicinity of the hub (M3-M5).
In this case the efficiency maximum is shifted to higher values of flow and capacity and even the value of efficiency becomes higher as compared with initial run.
So for high guide vanes opening the higher values of efficiency and capacity are achieved. Thus the cutting of outlet edge along the length AC (Fig 2, M3) ensures the increase of efficiency and capacity at ao > 315 mm.
The positive effect is even more for M4 and M5 (Fig 2). It should be noted here that cutting of inlet edge influences power characteristics only negligibly (M1.4).
Taking into consideration all the aspects the best runner modification is assumed to be M5.1.
According to calculations the cavitational characteristics of M5.1 runner does not differ practically from initial design. Also it was considered and recognised that M5.1 requires minimum scope of modernization works.
The additional calculations were carried out for H=8 m and H=12 m. The a results of calculations are shown in comparison to initial runner on Fig 4 as function of efficiency =f(NT) under H = Const.
With the increase of the Head the efficiency and capacity effect is growing.
Now the nominal maximum guide vane opening is ao =350 mm, but in case of utilization of accessible opening margin (the limited opening is ao =400 mm according to the project) it is possible to achieve essential increase of turbine capacity with the modified runner.
The proposed option of modernization was realized in February 1999 on one of the turbines (Unit 4) without disassambling of the agregate.
In order to develop the machining methods for edges cutting the investigations of mechanical properties of metal were carried out with prediction of metal behavior while cutting. The air welding arc cutting was selected as most preferable technique.
With the help of this method the cutting and elimination of metal from working surface were effectuated. Afterwards the final dimensions were achieved by grinding. The dimensions were controlled with the tamplets.
The site tests made before and after the modifications have approved the correctness of calculated values.
For example under the head Hnp =9,22 m for a certain opening range (controlled by the movement of servo-motor rod) the increase of capacity was noted as it is shown on Fig 5.
Alongside with power tests the vibration investigations were carried out with the aim to determine the change of hydrodynamic loading on turbine after modification. The best way to control this change was the analysis of amplitude-frequency spectrum of supporting elements vibration.
In the process of the tests the vertical vibrations of supporting cross-like construction, the radial vibrations of turbine and generator bearings and relative displacements of the shaft were measured.
The most interesting data were obtained by measurements of supporting cross-beams and bearings vibration (Table 2,3).
The separated spectral lines of vibration components may be explained in the following way.
The oscillations on frequencies less than 1.0Hz has distinctly hydrodynamic origin. They are caused by the revolving vortex under the runner on regimes laying far from optimal efficiency working conditions. Under partial load (less that optimal) the vortex rotation coincides with that of the turbine, but under the loading higher than optimal the vortex is revolving against turbine rotation thus influencing the runner dynamics exciting in running blades the dynamic stresses with the frequency equal to the difference between turbine and vortex speeds.
The phenomenon is most pronounced in the vibrations of turbine bearing housing (see Table 3).
Before blades modernization the vortex phenomena were noted under both partial and maximum loads, the minimum vortex effect being on Na =6,44 MW.
After upgrading the vortex forces substantially lessened, the minimum vortex effect shifting to the maximum load Na =7,53 MW.
The oscillations on frequency 1,25Hz comprise the displacements on rotational frequency. These vibrations of supporting elements are brought about by the forces rigidly bound to the turbine rotor and generally may have mechanical, hydrodynamic or electrical origin.
In our case these forces are assumed to be of hydrodynamic origin. The amplitude of the oscillations caused by the said forces became much less after upgrading, mostly due to more accurate new geometry of the runner blading.
The oscillations on frequency 15Hz are caused by cyclic forces acting on blades frequencies (fblade = frot x zblade = 1,25 x 12). These oscillations are influenced by the difference in individual geomentical dimensions of the blades.
The vibrations of this nature remained the same and even slightly diminished.
The oscillations on frequency 30Hz are caused by the nonuniformity of the flow and are bound with the number of guide vanes (fbl = frot x zgv = 1,25 x 24). The forces acting on 30Hz frequencies playing the leading role in accumulating fatigue phenomena in the runner elements. The upgrading had not touched the guide grid, so the level of vibrations on 30Hz frequency remained the same.
The oscillations on frequency 60Hz are also generated by vortex on guide grid, so they remained intact.
There are also vibrations of high frequencies (higher than 100Hz) with maximum on 160
170Hz which are generated by the hydrodynamic forces acting in the runner and with eigen frequencies of the rotor-stator structure (2,5Hz, 9,3-9,8Hz, 3,0Hz ect).
As it is seen from the table 3 the mechanical behavior of the agregate remained the same and for certain cases even ameliorated after upgrading.
In addition to dynamic forces the static stresses in the runner element were calculated for proper evaluation of upgrading influence. The calculations have been fulfilled for the most heavy working conditions under Head H=10,5 m with maximum guide vanes opening ao =350 mm. The flaw calculations were done for both initial and modernized variant for flow streams shown in Fig 1a. For every stream surface the pressure drop DP = PP - PT was determined in 21 points, where PP and PT are pressures on blade working and back side correspondingly.
The results obtained shows the difference in pressure drop not more than 10-15%, so both variants are practically equally loaded.
Among all operating HPS in Russia the Volkhov HPS has the longest service period. The period of working in power generating regime achieved 500 x 103 hours. So the assessment of residual operational life of the runners is assumed to be the most important task, though the modernization has lead to some reduction of acting forces. The initiation of cracks in welded Francis runners takes place as a rule in the zones of connections between blades outlet edges with runner hub and rim where there exists the maximum level of working static and dynamic stresses as well as welding residual stresses.
But in case of Volkhov HPS the other situation was created.
The cracks are appearing here on the blades inlet edges close to the rim, where high stresses are absent.
The main reason for crack appearance here is explained by the presence of high hydrodynamical forces acting on the frequencies 30 and 60Hz, transmitted from the guide-vanes on the runner blades. The upgrading of the runners does not eliminate these sources of dynamical forces.
The appearance of many cracks on the blades outlet edges occured, after 340 . 103 - - 350 . 103 hours of operation.
The subsequent cracks are usually initiated occasionally in the zones of welded up defects. Thus during the last general ovehauls three cracks were revealed only in turbine of Unit 1 (28.07.97).
The cracks of 800 mm length were found along outlet edges of working blades in the places of old welded defects, so the crack origin may be rather ascribed to low quality of repair works.
The estimation of residual operational life of runners, blades consisted of following procedures: determination of standard strength factors (s0,2, sB, d5, Yx
), investigation of brittleness characteristics (critical temperatures, impackt strength, stress intensity), of critical characteristics of cyclic strength (fatigue limit s-1, crack resistance factor k+h , crack growth rate, structure and chemical composition).
Three zones of running blades material having different levels of working stresses were studied. The structure and chemical composition of blade metal correspond to the steel Ρς3ορ (Russian Code).
Inside surface layers of blades the metal structure distortions were observed (outlet edge) caused by decomposition of perlitic component and forming of special substructure under the influence of cyclic stresses.
The strength characteristics at +20 0C were as follows: sB = 335
480 MP; s0,2 = 175
260 MP; , d5 = 30
42%; Y = 59
73%. These values may be estimated as safficient and not seriously affected by long time operation. As to impact strength characteristics, the investigations have revealed the abrupt transition of the metal into brittle state with relevant increase of stress concentration factor, especially for most stressed zone. Impact strength k1c = 341
377 kg/mm3/2, fatigue strength s-1 = 140
180 MP.
So it may be concluded that running blades metal have not exhausted its service resource, though low crack resistance capacity (Kth = 32
48 kg/ mm3/2) requires high quality of all repair works with revealing and subsequent elimination of all crack-type defects.
The experience in modernization of Volkhov HPS, described here is rather peculiar and shows the real way for upgrading of equipment with very long period of operation. Up to thin time, the 2/3 of power generating equipment at Hydro- Power Stations in Russia have exhausted rated service life and is not planned for recent replacement. Under such conditions the upgrading of turbine runners on the basis of modern science and technology is a very effective measure, the effectivity of which being the higher, the older is the modernized equipment.
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