LAPINO POWERPLANT PENSTOCK FAILURE By ...

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Jul 7, 2001 - Three people were killed as a result of flooding in the machine ... Oneida Station Hydroelectric Plant (United States), both caused by valve ...
CASE STUDY: LAPINO POWERPLANT PENSTOCK FAILURE By Adam Adamkowski1 ABSTRACT: The paper presents the results of investigations carried out in connection with a penstock rupture at a small hydropower plant in Poland. The investigations comprised material tests of the ruptured penstock shell, analysis of the stress in the shell of the ruptured penstock section, analysis of hydraulic transients under conditions of failure, and testing of the penstock and gensets after repair. In the case under consideration, excessive water hammer caused by rapid flow cut-off was recognized as the direct cause of the penstock burst. Low strength of the penstock shell because of low quality weld joints and lack of strengthening in places of large stress concentration also contributed to the penstock failure. Results of the case investigation highlight some problems that should be taken into account during the process of design, modernization, and operation of hydropower plants in order to ensure their safe operation.

INTRODUCTION The amount of scientific and engineering literature on penstock failures in hydropower plants is not very impressive. This is due mainly to the fact that information of this type is not willingly disseminated. Unfortunately, such a situation does not facilitate the publication of literature on prior failures that could be used to reduce the number of failures in the future and raise the operational security of hydropower plants. The paper by Bonin (1960) describing damage to a water turbine in the Oigawa Power Station, Japan in 1950 caused by a penstock failure seems to be a rare exception. The failure at Oigawa was caused by a water hammer due to a sudden closure of a butterfly valve. Three people were killed as a result of flooding in the machine chamber. The case is often referred to in the literature on threats associated with water hammer (Chaudhry 1979; Pejovic et al. 1987; Wylie and Streeter 1993). Recently, Arrington (1999) presented several very serious failures—a rupture of the lower needle valve body at the Bartlett Dam (United States) and a breach in the steel penstock at the Oneida Station Hydroelectric Plant (United States), both caused by valve slamming creating pressure surges. As a consequence, five people were killed. This paper describes the most important results of investigations carried out in connection with a penstock rupture having less severe consequences. The rupture took place at the Lapino hydropower plant operated by the ENERGA Gdansk Energy Company. The power plant was built in 1927 on the Radunia River, 15 km southwest of Gdansk, Poland. It is operated both as a run-of-river and storage hydropower plant of 2 MW total power, rated head 13.8 m and discharge 22 m3/s. Water from the upper reservoir is supplied to the turbines by a penstock branching off in the powerhouse substructure (Fig. 1). The upper section of the penstock, upstream of the first bifurcation, is a concrete pipeline; whereas, the lower section under the building is a welded steel structure. The power plant is equipped with two identical gensets (Fig. 2) each consisting of two horizontal Francis turbines and a generator. Turbine runners are mounted on a common shaft with the generator. Turbine wicket gates are coupled by a common control shaft. Genset No. 1 is equipped with the original mechano-hydraulic governor; whereas, genset No. 2 is equipped with an electrohydraulic governor of a new design. 1 Head of Dept. of Res. and Diagnostic Hydr. Machinery, Inst. of FluidFlow Machinery of the Polish Academy of Sci., ul. Fiszera 14, 80-952 Gdansk, Poland. E-mail: [email protected] Note. Discussion open until December 1, 2001. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on September 29, 1999; revised March 2, 2001. This paper is part of the Journal of Hydraulic Engineering, Vol. 127, No. 7, July, 2001. 䉷ASCE, ISSN 0733-9429/01/0007-0547–0555/$8.00 ⫹ $.50 per page. Paper No. 21966.

In December 1997, during acceptance tests of the new governor, the steel penstock supplying water to the turbines ruptured. The substructure of the powerhouse was entirely flooded. The machine hall was flooded up to a height of 2.5 m above the floor level. Just prior to the failure, a load rejection test of the genset No. 2 from a load of about 50% of the rated power output was carried out. Genset No. 1 was disconnected from the grid at this time. In order to evaluate the technical state of the penstock after the failure and to determine the causes of its burst, the failure was subjected to an extensive investigation covering the following: • nondestructive tests of the remaining penstock shell, with a particular focus on the weld joints (Ostrowski et al. 1998), • material tests of the ruptured penstock shell (Krzysztofowicz et al. 1998), • analysis of the stress in the shell of the ruptured penstock section (Krawczuk 1998), • analysis of hydraulic transients under conditions of failure (Adamkowski and Henke 1998), • elaboration of guidelines for repair of the penstock and further operation of the power plant (Adamkowski 1998a). Following the repair of the penstock and the preparation of all machines and technical devices for operation, the acceptance tests of the gensets were carried out. Primary attention was given to fulfilment of the required penstock standards. The findings from the Lapino investigation highlight some problems that should be taken into account during operation of hydropower plants and their modernization in order to prevent future penstock ruptures. VISUAL INSPECTION AND NONDESTRUCTIVE TESTS OF THE PENSTOCK SHELL The main destruction, that is, the failure of the penstock shell, took place in the left branch of the penstock (looking downstream) at the point where the penstock branches to supply water to genset No. 1 (Fig. 1). As a result of the burst, the conical surface of the penstock section (2 m long with minimum and maximum diameters of 2.1 and 2.4 m, respectively), and part of the inlet pipe supplying water to turbine set No. 1 were split along their crown seams and flattened out on the ground (Fig. 3) (The wall thickness of the broken penstock section was 8 mm). A characteristic feature of the destruction is a breach in the continuity of the upper longitudinal weld joint and circumferential joints in two cross sections. The appearance of the breach surface along the weld joints is characteristic of brittle fracturing (Fig. 4). Besides the destruction in the left branch of the penstock, JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001 / 547

FIG. 1.

FIG. 2.

Layout of Flow System at Lapino Hydro Power Plant

Overview of Genset No. 2 at Lapino Power Plant

the same penstock section in the right branch was subject to cracking. A crack was found in weld joints at the corner connection between the penstock and the inlet pipe to turbine set No. 1 (Fig. 5). This crack appeared along the longitudinal weld joint (0.7 m long) as well as in the main material of the inlet pipe (0.3 m long). It is worthwhile to note that this point coincided with the area of plastic deformation of the broken section of the penstock’s left branch. This point indicates that connection of the conical section of the penstock with the inlet 548 / JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001

pipe and was the point of inception of the burst caused by exceeding the bursting pressure. Following sandblast cleaning, the unbroken penstock shell was subject to a visual inspection, during which primary attention was paid to the weld joints. The inspection revealed numerous defects in the weld joints. These defects consisted mainly in the lack of postwelding at the inner surface of the penstock and inappropriate weld geometry. About 25% of the preserved weld joints were subject to nondestructive testing by

in force, the welding material was selected properly—it contained less carbon than the native material itself. Values of yield stress Re and tensile strength Rm obtained from mechanical (strength) tests (values averaged from three samples) were as follow: breaking in the direction of rolling, Re = 280 MPa and Rm = 401 MPa; breaking across the direction of rolling, Re = 294 MPa and Rm = 431 MPa. Material samples of the penstock shell were subject to recrystallization annealing so as to reconstruct the shell’s initial state. Values of Re and Rm after recrystallization for samples subject to breaking in the direction of rolling were Re = 293 MPa and Rm = 408 MPa. For all samples, values of the ultimate strain A5 ranged between 22% and 24% (According to Polish standards, the ultimate strain A5 is defined as the maximum relative elongation of a cylindrical rod of diameter D and L = 5D length during a tensile strength test). The average value of KCU2 obtained from four impact resistance tests was 115 J/cm2. It follows from the strength tests that the penstock material maintains all normal values of Re, Rm, A5, and KCU2 , expected values for low-carbon structural steels. No unfavorable effects of long-term operation (fatigue and ageing) of the penstock material on its strength and plastic properties were observed. Strength investigations of the welded seams in the penstock shell revealed a decreased tensile strength by about 40–50%, compared with the main material of the penstock. Further macroscopic and microscopic metallographic investigations of the main material and welding seams showed the following characteristics:

FIG. 3.

Broken Segment of Penstock Left Branch

means of the wet magnetic method. Numerous longitudinal cracks of the weld seams were discovered, mainly in the region of fusion into the penstock material. In conclusion, it was stated that in light of current safety requirements, the joint defects were not permissible and could not guarantee the safe operation of the power plant. MATERIAL TESTS OF THE PENSTOCK SHELL The investigation revealed that the penstock shell was made of ordinary structural carbon steel with 22% of the maximum content of carbon. The material used for gas welding the penstock shell was structural carbon steel of low carbon content (0.014%), with decreased contents of manganese, sulphur, potassium, and silicon. According to the requirements currently

FIG. 4.

• The main material had a pearlitic/ferritic structure with the permissible amount of non-metallic inclusions. • The weld-joint material contained more defects, including gas bubbles, cold droplets, and oxides, in the region of the first fusion into the native material. • There were microcracks both in the native material and in weld seams; however, it is difficult to univocally establish whether these microcracks appeared during the technological processes of penstock manufacturing, during regular operation of the penstock, or during its burst. • In most cases, structural characteristics for the heat affected zone were not revealed in the investigated weld joints (this indicates that too little heat was delivered to the material during welding). On the basis of metallographic tests and measurements of wall thickness of the penstock shell, the effect of surface corrosion can be estimated as surface uniform and taking place at a moderate rate. A higher rate of corrosion was observed in some places of the weld joints; however, those places did not coincide with locations of cracks. ANALYSIS OF STRESS IN PENSTOCK An analysis was carried out on a penstock segment where the continuity of the shell material was broken in both

Macroscopic Picture of Cracked Weld Joint JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001 / 549

FIG. 6. Distribution of Equivalent Stresses in Broken Segment of Penstock

FIG. 5. Cracking of Penstock Right Branch in Region of Connection of Conical Part of Penstock Shell with Inlet Pipe to Genset No. 1—Inside View

branches of the penstock. A commercial computer code, ADINA, based on the finite-element method (FEM) was used to analyze the state of stress. The calculations drew on measured geometry of the penstock segment and strength properties of the material found from material tests. A wide range of internal pressure was analyzed. Components of the calculated stress field were substituted with the equivalent stress according to the Huber/Henkey/von Mises criterion theory (ASCE 1993). The obtained results show a very unfavorable distribution of stresses in the analyzed segment of the penstock shell (Fig. 6). The connection of the inlet pipe with the main branch of the penstock is distinctive by a significant concentration of stresses. The stress concentration coefficient, a ratio of the maximum stress to the stress in a uniform conical segment of the penstock, ranges between 5 and 7. This easily correlates with values of the coefficient for similar structures cited in the literature (Beczkowski 1964; Helguero 1986). Taking into account the fact that the tensile strength of weld joints amounts to about 50% of that of native material (This is approximately the result for tests of samples of weld joints), the critical pressure at which the penstock could rupture was estimated to be 250–300 kPa. It is notable that the estimated critical pressure cannot be compared with the permissible pressure assumed for the design and construction of the penstock because of the loss of technical documentation during World War II. HYDRAULIC TRANSIENTS UNDER FAILURE CONDITIONS To establish the causes of failure, a computer simulation of hydraulic transients occurring during the rupture (i.e., during 550 / JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001

load rejection of genset No. 2 from power corresponding to a 50% opening of its wicket gates) was carried out. The simulation was performed using the HYDTRA (HYDraulic TRAnsients) computer code developed by the author to simulate water hammer in flow systems of hydraulic turbomachinery such as impeller pumps, water turbines, and hydraulic reversible machines (Adamkowski 1993, 1996). Prior to the simulation, the data on the flow system were assembled (Fig. 1). In the steel part, besides the parallel branches, the penstock is characterized by a length-wise variable inner diameter. For computational purposes, this part of the penstock was modelled by a number of segments of constant cross-sectional area, with the diameter given by a multistep function. Similarity conditions were preserved during transformation from the real penstock to equivalent segments (Wylie and Streeter 1993). Data concerning turbines were formulated in the form of accessible dependencies of the power and flow rate on the wicket gates opening, evaluated for the nominal rotational speed of the runners and constant head. On this basis, and also on the conditions of dynamic similarity of hydraulic machines (Jackowski 1971), variations of turbine operational parameters during calculated transient states were estimated. The calculations were carried out for several conditions of flow cutoff. Because of a lack of appropriate data, the time history of wicket gate closure was assumed arbitrarily to follow a linear, a quadratic linear, and a quadratic law. Closure in accordance with the quadratic linear law was performed in such a way that the initial stage of wicket gate closure (the first half of the closing period) from y = 0.5 to y = 0.35 proceeded in accordance with the quadratic law; whereas, the final stage from y = 0.35 down to 0 was in accordance with linear law. Additionally, in order to univocally determine the quadratic function for the initial closure stage, it was assumed that its peak point is determined by t = 0 and y = 0.5 coordinates. In the case of closure following the quadratic law, it was assumed that the curve determined points A (t = 0, y = 0.5), B(t = T/2, y = 0.35), and C(t = T, y= 0). The choice of the quadratic linear and quadratic functions follows from the need to analyze, at least qualitatively, the effect of inertia forces in the wicket gate driving system and also the effect of hydraulic forces exerted by the water flowing through the machine on closure history. In reality, this process takes place at a lower rate at the initial stage of closure. The calculations performed take into account the effect of

pressure (250–300 kPa) can be exceeded if the closure time of the wicket gates from 50% opening is less than values given for the tested closure methods (Fig. 8): closure according to the linear function, T < (0.3–0.35) s; closure according to the quadratic linear function, T < (0.4–0.55) s; closure according to the quadratic function, T < (0.5–0.65) s. DETERMINATION OF CAUSES OF PENSTOCK FAILURE The new governor was designed for the minimum time of servomotor closure from a 100% opening equal to 2 sec. This time was assumed on the basis of tests performed of load rejection from the operation of gensets equipped with old governors (Adamkowski 1998a). Laboratory tests of the new governor confirmed its operational rate; however, the tests were carried out without external load of the servomotor piston, neglecting, among others, the effect of moment of force exerted by flowing water on the wicket gates and the effect of inertia of the guide wheel’s movable elements. The time of wicket gate closure from 50% opening, for which the evaluated critical breach pressure (250–300 kPa) can be exceeded and found from calculations, is significantly lower than that of laboratory tests on the new governor. When establishing the reasons responsible for the failure, one can consider the following cases linked with the improper operation of the oil feeding (supplying) system of the governor servomotor: 1. Failure of the oil system supply line 2. Lack of a throttling orifice in the oil system supply line 3. Improper deaeration of the oil system before starting the tests FIG. 7. Exemplary Transients Calculated for Failure Conditions (Heavy Line, Linear Closure Law of Wicket Gates; Light Line, QuadraticLinear Closure Law of Wicket Gates)

FIG. 8. Evaluation of Effect of Wicket Closure Method on Maximum Pressure in Breach Section under Conditions of Failure

damping in the movement of the servomotor piston at small openings, as a result of which the last stage of closure from 5% downward last 4 s. Such damping of the servomotor piston movement was applied in the new governor of genset No. 2. Sample results of the calculations (i.e., hydraulic transients corresponding to the failure conditions) are presented in Fig. 7; whereas, Fig. 8 brings together the obtained results in the form of a relationship between the maximum pressure at the breach section of the penstock and wicket gate closure time T for the liner closure considered. The results imply that the estimated values of critical breach

In all of the above-mentioned cases, a sudden closure of the wicket gates was possible. Such situations are known from the accessible references (ASCE 1995, 1998). The first case did not take place. The governor’s manufacturer decisively rejects the possibility of the second one. The most probable case is the third one. The decreased throttling of aerated oil, as compared with that of the pure oil, results in raising the speed of servomotor piston displacement. Additional factors affecting acceleration of wicket gate closure are conditions allowing the flow induced forces to act in the wicket gate closing direction. Such conditions have been confirmed on the basis of test results discussed in following sections. It is also worthwhile to stress that during more than 70 years of operation of this power plant, there were frequent power decays in the electric power system, following simultaneous shut-downs of both gensets. Under these conditions, which appear the most dangerous for this power plant (taking into account the older governors and the fact that water was supplied to the two gensets by one penstock), no failure happened. Therefore, the failure should not be attributed entirely to the state of the penstock. In light of the above considerations, severe water hammer caused by excessively fast flow cutoff during a load rejection test of genset No. 2 was recognized as the direct cause of the penstock burst. It should be stressed that the low strength of the penstock shell because of the low quality of the weld joints and a lack of strengthening in places of a large concentration of stresses also contributed to penstock failure. RECOMMENDATIONS To ensure safe operation of the power plant, it was considered advisable to increase the strength of the penstock, compared with its state before the failure. It was further recommended that flaws and defects in old weld joints be repaired, JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001 / 551

to reinforce some of the older joints by additional welding, and also to make new weld joints. To prevent dangerous situations caused by possibly excessive loads, it was recommended that reinforcement rings be installed to strengthen the connection between the inlet pipe to genset No. 2 and the conical penstock. It is worthwhile to note that the reinforcement of wye branches is now considered a necessary provision to decrease stress concentration (Technical 1991; ASCE 1993). The recommended displacement speed of the servomotor piston was determined on the basis of the analysis of load rejection conditions, considered to be the most unfavorable for the machines and using the literature data on the most frequently assumed permissible values of the maximum increase of pressure in the flow system and the maximum rise of the turbine runner speed under these conditions. Transients during load rejection were computed (with the aid of the HYDTRA code) for various times of linear closure of the wicket gates, both for one or two operating gensets. In the case of two gensets in operation, the same method of flow cutoff was used for each turbine set during load rejection from rated power. The computed maximum pressure increase in the turbine spirals and in the breach section of the penstock as well as the maximum rise of the runners rotational speed as a function of closure time (with the linear law of wicket gates closure assumed) are shown in Fig. 9. The maximum pressure rise caused by water hammer in hydraulic systems with Francis turbines operating under relatively low heads usually does not exceed 50% of the rated head; whereas, the maximum increase of rotational speed is not larger than 30% (Guidebook 1991). Absolute values of the Lapino power plant are 67 kPa and 75 rpm. It follows from the calculations that the first value refers to the time of linear closure from full opening higher than 3 s, while the second value refers to closing time smaller than 3.8 s (Fig. 9). On these grounds, it was recommended that the new governor be set so as not to allow wicket gate closure from full opening in a time period shorter than 3.2–3.5 s (understandably, an initial stage of delay in closure cannot be taken into account).

FIG. 9. Maximum Rise of Pressure and Rotational Speed As a Function of Time of Linear Wicket Gate Closure Calculated For: (a) Full Load Rejection From Genset No. 2 (During Stillstand of Genset No. 1); (b) Simultaneous Full Load Rejection from Both Gensets 552 / JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001

PENSTOCK REPAIR A new segment was installed in place of the broken one in the penstock’s left branch. The segment was made of 8 mm thick steel. The same material was used to replace inlet pipes supplying water to gensets No. 1 and 2 in both branches of the pipeline. Cracked elements in the right branch were also repaired. The connections of the inlet pipes to gensets No. 1 and 2 with the penstock branches were reinforced using steel clamping rings 22 mm in diameter. Before replacement of the inlet pipes, the cracked main valve of genset No. 1 was regenerated, and the destroyed flange joint in the left branch was repaired. The strength of the weld joints in the penstock segments that were not replaced was increased by the replacement of weld joints or additional welding from the inside. After welding, the new or regenerated weld joints were scrutinized and subjected to nondestructive investigations to detect possible defects. During the repair of the turbine supply system, water was removed from the machine chamber, and generators, electric security systems, and turbine control systems were dried and brought back into use. Repair work was finished in the middle of 1998. INVESTIGATIONS OF PENSTOCK AND GENSETS AFTER REPAIR After the repair of the penstock and the preparation of the machines and technical equipment for start-up, test investigations were carried out prior to recommissioning of the gensets (Adamkowski 1998b). In order to secure safe operation of the power plant, the rate of operation of the governor of genset No. 2 during load rejection was thoroughly checked. Also, the levels of stresses in the shell of the repaired penstock were checked over a wide range of operational conditions. For each genset, a measuring system was installed to facilitate the investigation. The measured quantities were as follow: • position of the servomotor piston in turbine sets No. 1 and 2—y1 and y2 • rotational speed of gensets No. 1 and 2—n1 and n2 • power output of gensets No. 1 and 2—P1 and P2 • pressure in spirals of gensets No. 1 and 2—pt1 and pt2 • longitudinal and circumferential stress at four points of the penstock shell—␴x1, ␴y1, ␴x2, ␴y2, ␴x3, ␴y3, ␴x4, ␴y4.

FIG. 10. Locations of Measurement Points to Determine Deformations of Penstock Shell

The stresses in the shell of the newly installed segment of the penstock were evaluated by means of strain gauge measurements—(Fig. 10). At each selected point, strain values were measured in two perpendicular directions. Strain gauges fixed (with glue) streamwise served to measure longitudinal strain TABLE 1. Pressure zu 99.6 m

Equivalent Stresses in the Penstock Caused by Static ␴r1

␴r 2

␴r 3

␴r 4

86 MPa

73 MPa

19 MPa

16 MPa

FIG. 11. Measured Quantities During Simultaneous Load Rejection from Nominal Power of Gensets No. 1 and 2 TABLE 2.

of the conical penstock and the inlet pipe; whereas, those fixed in the cross stream direction measured circumferential strain. Strain gauges fixed at the upper part of the segment (points 1 and 2 in Fig. 10) were located straight at the clamping ring that was strengthening the connection (weld joint) of the penstock with the inlet pipe. The investigations revealed that this place is distinctive by the highest concentration of stresses. Strain gauge measurements of shell deformation showed longitudinal and circumferential components of stresses, which were then recalculated to equivalent stresses according to the Huber/Hencky/von Mises hypothesis (Fig. 10). The two-axial state of deformation was postulated, and it was assumed that the measured longitudinal and circumferential stresses are principal stresses. The stresses caused by hydrostatic loads were measured several times while filling the penstock with water and while emptying it. The maximum values of stresses measured under these conditions are shown in Table 1. The significant concentration of stresses at the joint of the penstock with the inlet pipe deserves particular attention. The stress concentration coefficient that ranges between 3.8–5.4 is, however, lower than that resulting from the calculations carried out for the prebreakdown conditions, probably caused by the strengthened connection of the penstock with the inlet pipe. It is noteable that the changes of the stresses measured before and after removal of the leakage through the flange connections in the penstock did not exceed 5 MPa at any measuring section. During tests of regular start-up and shut-down procedures of both gensets, no considerable variations of pressure in the turbine spirals nor resulting changes of stresses in the penstock shell were observed. The maximum pressure variations at the turbines inlets did not exceed 10 kPa; whereas, variations of stresses at the place of their highest concentration were below 5 MPa. Investigations of load rejection were carried out for each genset separately as well as for the two sets simultaneously. Fig. 11 shows an example of transient curves of load rejection from the maximum power of the two gensets. Maximum values of the measured quantities in the investigated states during load rejection are shown in Tables 2, 3, and 4. It is remarkable that the flow cut-off in genset No. 1, equipped with the old governor, takes place relatively fast. The time of wicket gate closure from full opening (⯝1.8 s) for this set is much shorter than the recommended time and the time obtained for genset No. 2, after fixing the new governor (3.6 s). Therefore, the maximum pressure rises caused by water hammer and the resulting variations of stresses in the penstock shell during tests of load rejection from genset No. 1 are much larger than those of genset No. 2. The reverse applies to the

Values of Characteristic Quantities During Load Rejection from Genset No. 1

P1o (kW)

y1o (%)

T1 (s)

n1 max (rpm)

⌬pt1 max (kPa)

␴r1 max (MPa)

␴r 2 max (MPa)

␴r 3 max (MPa)

␴r 4 max (MPa)

280 540 910 1,060

39 51.5 75 92

1.1 1.35 1.6 1.84

262 273.5 280

38 42 50 55

104 110 113 116

87 94 97 98

22 24.5 25 26

19 21 21 21.5

TABLE 3.

Values of Characteristic Quantities During Load Rejection from Genset No. 2

P1o (kW)

y1o (%)

T1 (s)

n2 max (rpm)

⌬pt1 max (kPa)

⌬pt2 max (kPa)

␴r1 max (MPa)

␴r 2 max (MPa)

␴r 3 max (MPa)

␴r 4 max (MPa)

260 575 1,000 1,080

27 50 70 100

1.0 1.9 2.8 3.6

254.5 270 287 312

17 18 19 20

26 28 30 31

98 100 101 101

83.5 84 85 85

21 21.5 22 23

18 18 18.5 19

JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001 / 553

TABLE 4.

Values of Characteristic Quantities During Simultaneous Load Rejection from Two Gensets

P1o P2o (kW)

y1o y2o (%)

T1 T2 (s)

n1 max n2 max (rpm)

⌬pt1 max ⌬pt2 max (kPa)

620 560 1,090 1,060

55 50 100 100

1.3 1.8 1.9 3.6

255 273 282 316

60 61 62 56

FIG. 12. Comparison of Wicket Gate Closure Rates Recorded under Conditions of Dewatering and Load Rejection from Genset No. 2

variations of runner rotational speed. The simultaneous flow shutdown of both gensets gives rise to the largest pressure increase in the penstock and the largest change of stresses in its shell. The maximum pressure increase was equal to about 62 kPa (46% of the head); whereas, the maximum equivalent stresses equalled 122 MPa at the place of highest concentration of stresses, that is, at the connection of the penstock with the inlet pipe. This stress value amounts to 50% of the yield stress of the penstock material. Taking under consideration the fact that the safety coefficient for constructions of this type is equal to 1.8 with respect to the yield stress (Technical 1991), the obtained equivalent stress of the investigated material can be assumed to be acceptable. However, it was recommended to also reduce this level in the future by extending the time of servomotor closure from full opening in genset No. 1 up to a value not lower than 3.2 s. During investigations prior to recommissioning of genset No. 2, the displacement speed of the servomotor piston in the new governor was checked by performing a series of ‘‘dry tests,’’ that is, with wicket gate closure in air. The comparison of closure history recorded during one of these tests with that simulated under conditions of the shut-down leading to the penstock rupture is shown in Fig. 12. It follows from the comparison that the flow of water through the wicket gates can decrease the time needed for full displacement of the servomotor piston by as much as 0.5 s. On these grounds, it can be deduced that the wicket gate closure rate of the new governor during the breakdown was more than that indicated by laboratory tests. In conclusion, the real rate of closure was large enough to give rise to the excessive pressure that lead to penstock failure. CONCLUSION The investigation indicates a number of problems that should be thoroughly looked into during the process of design, modernization, and operation of hydropower plants (including small ones) in order to ensure their safe operation. The problems include: (1) the rate of flow cut-off and the resulting maximum pressure rise in the flow system; (2) increased concentration (intensification) of stresses in the geometrically ir554 / JOURNAL OF HYDRAULIC ENGINEERING / JULY 2001

␴r1 max (MPa)

␴r 2 max (MPa)

␴r 3 max (MPa)

␴r 4 max (MPa)

121

102

26

22

122

103

26.5

22.5

regular elements of the flow system, for example, in the penstock bifurcations (wye branches); and (3) quality of the welded or riveted joints. In the case under consideration, the excessive pressure rise due to water hammer after a rapid flow cut-off resulted in the bursting of the penstock. The low strength of the penstock, mainly caused by the low quality of weld joints (old welding technology) as well as the lack of reinforcement at the points of high stress concentration, were also conducive to penstock burst. The Lapino penstock failure should be a warning that in hydropower plants, even those equipped with short penstocks, there is a real danger of a breakdown caused by water hammer that can lead to serious consequences. To prevent these situations, it is desirable to check the quality of material used and to analyze various conditions of operation by using current computational methods and strain measurements. REFERENCES Adamkowski, A. (1993). ‘‘Waterhammer in pumped-storage power plants: The calculation model and its experimental validation.’’ Trans. IFFM, Vol. 95, Gdansk, 73–104 (in Polish). Adamkowski, A. (1996). ‘‘Theoretical and experimental investigations of waterhammer attenuation by means of cut-off and by-pass valves in pipeline systems of hydraulic turbomachines.’’ Copybooks of IFFM, 461/1423/96, Gdansk, 154 (in Polish). Adamkowski, A. (1998a). ‘‘Causes of penstock breach at the Lapino hydro power plant.’’ Rep. IFFM 51/98, Gdansk, 52 (in Polish). Adamkowski, A. (1998b). ‘‘Investigations of turbine sets at the Lapino hydro power plant after post-breakdown repair.’’ Rep. IFFM 205/98, Gdansk, 59 (in Polish). Adamkowski, A., and Henke, A. (1998). ‘‘Evaluation of the technical state of the steel penstock at the Lapino hydro power plant.’’ Rep. IFFM 35/98, Gdansk, 28 (in Polish). Arrington, R. M. (1999). ‘‘Failure of water-operated needle valves at Bartlett Dam and Oneida station hydroelectric plant.’’ Proc. 3rd ASME/ JSME Joint Fluid Engrg. Conf., July 18–22, San Francisco, Calif., 1– 5. ASCE Task Committee on Guidelines of Aging Penstocks. (1995). Guidelines for evaluating aging penstocks, ASCE, New York, 175. ASCE Task Committee on Inspection and Monitoring of In-Service Penstocks. (1998). Guidelines for inspection and monitoring of in-service penstocks, ASCE, Reston, Va., 256. ASCE Task Committee on Manual of Practice for Steel Penstocks. (1993). Steel penstocks, Manual and Rep. No. 79, ASCE, New York, 432. Beczkowski, W. (1964). ‘‘Power industry pipelines. Part I. Design and calculations.’’ 2nd Ed., WNT, Warsaw, 361 (in Polish). Bonin, C. C. (1960). ‘‘Waterhammer damage to Oigawa power plant.’’ Trans. ASME, J. Engrg. for Power, Series A, 82(2), 111–119. Chaudhry, M. H. (1979). Applied hydraulic transients, Van Nostrand Reinhold, New York. Guidebook (or guidelines) for small hydro power plants. (1991). Nabba, Warsaw, 266 (in Polish). Helguero, V. M. (1986). Piping stress handbook, 2nd Ed., Gulf Publishing, Houston, Tex. Jackowski, K. (1971). ‘‘Hydro power plants.’’ WNT, Warsaw, 406 (in Polish). Krawczuk, M. (1998). ‘‘Evaluation of the state of stress in the steel penstock at the Lapino hydro power plant.’’ Rep. IFFM 21/98, Gdansk, 36 (in Polish). Krzysztofowicz, T., Krzysztofowicz, K., Nadolny, L., and Targan, M. (1998). ‘‘Material investigations of the penstock supplying water to turbines at the Lapino power plant.’’ Rep. IFFM 31/98, Gdansk, 24 (in Polish).

Ostrowski, B., Kasprowski, M., and Latczyk, A. (1998). ‘‘Investigations of penstocks supplying water to gensets 1 and 2 at the Lapino power plant.’’ Rep. ZRE, Gdansk, 19 (in Polish). Pejovic´, S., Boldy, A. P., and Obradovic´, D. (1987). Guidelines to hydraulic transient analysis, Gower Technical Press, Bradford-on-Avon, Wiltshire, England, 144. Technical inspection requirements. Pressure devices. Strength calculations. DT-UC-90/W0-0. (1991). Technical Inspection Office, Ed. Wydawnictwo Poligraficzne, Bydgoszcz, 149 (in Polish). Wylie, E. B., and Streeter, L. V. (1993). Fluid transients in systems, Prentice-Hall, Englewood Cliffs, N.J., 463.

NOTATION The following symbols are used in this paper: A5 = relative longitudinal strain of steel samples during burst; D = penstock inner diameter; E = elasticity modulus of penstock wall material; H = pressure head; Hp = turbine pressure head; KCU2 = steel impact resistance (from Charpy hammer tests); n = turbine runner rotational speed; P = power output; p = pressure; pr = pressure at penstock burst section; pt = pressure at turbine inlet; Y = displacement of main servomotor of turbomachine;

y = dimensionless displacement of servomotor piston, Y/Ymax); Q = discharge (volumetric flow rate); Re = yield strength (stress); Rm = tensile strength; T = wicket gates closure time (or servomotor closure time); t = time; x = distance coordinate; zd = water level in the lower reservoir; zu = water level in the upper reservoir; ␯ = Poisson ratio; ␳ = liquid density; εx = relative longitudinal deformation of the penstock shell; εy = relative circumferential deformation of the penstock shell; ␴x = normal longitudinal stress; ␴y = normal circumferential stress; and ␴r = equivalent stress according to Huber-Henkey-von Mises criterion (theory).

Subscripts 1 2 n max o k

= = = = = =

genset No. 1; genset No. 2; nominal value; maximum value; initial value; and final value.

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