For official use only
DOC: WRD 22(577)C Feb 2012 BUREAU OF INDIAN STANDARDS Draft Indian Standard
GUIDELINES FOR PLANNING AND DESIGN OF RIVER POWER HOUSES INTEGRATED WITH BARRAGES PART 2 DESIGN (As part 2 of IS 14592) (Not to be reproduced without the permission of BIS or used as standard) --------------------------------------------------------------------------------------------------------------------------Last date for receipt of comments is 20-03-2012 --------------------------------------------------------------------------------------------------------------------------FOREWORD (Formal clauses of the foreword will be added later) The criteria for the design of low-head river bed power houses integrated with barrages are to a considerable extent, similar to the criteria for the design of surface hydro power houses. However a few special considerations are necessary for such designs, which are covered in this standard The main points to be kept in mind are that, these power houses are sometimes founded on soft pliable river bed material, in that case dynamic stability of the structure as per IS 1893: 2002, susceptibility to liquefaction of foundation strata and scouring of the bed material should be given due consideration. Differential settlements and sliding may also create foundation problems. Exclusion of heavy sediment inflow, including coarser particles, from entering and damaging the turbines is the next important issue to be considered. Hydrology of the river, which is highly variable from season to season and from year to year, has to be carefully analyzed for the power generation and peaking studies. In relatively large Indian rivers, however, sufficient storage can be provided to meet the daily load fluctuations by marginally raising the pond level for the required storage. When these power stations are located in plains, keeping in view the flat slopes special provisions/consideration should be taken into account to provide the minimum required storage. Planning and design of barrage power houses is formulated in two parts. Other part covers investigation, planning and layout. For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated expressing the result of test or analysis, shall be rounded off in accordance with IS 2: 1960 ‘Rules for rounding off numerical values (revised)‘. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.
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1
SCOPE
This standard covers the hydraulic and structural design of river bed power houses integrated with a barrage. 2
REFERENCES
2.1 The Indian Standards listed below contain provisions which through reference in this text constitute provisions of this standard. At the time of publication, the editions indicated were valid. All standards are subject to revision and parties to agreements based on these standards are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below: IS No.
Title
277: 2003
Galvanized steel sheets (Plain and Corrugated)- Specification (sixth revision)
456: 2000
Plain & reinforced concrete– Code of practice (fourth revision)
457: 1957
Code of practice for general construction of plain and reinforced concrete for dams and other massive structures
1786: 2008
High strength deformed steel bars and wires for concrete reinforcement – Specification (fourth revision)
2062: 2011
Hot Rolled Medium and High Tensile Structural Steel – Specification (seventh revision)
4247(Part 1): 1993
Structural design of surface hydroelectric power stations: Part 1 Data for design - Code of practice (third revision)
4247(Part 2):1992
Code of practice for structural design of surface hydroelectric power stations: Part 2 Superstructure (second revision)
4247(Part 3):1998
Code of practice for structural design of surface hydroelectric power stations: Part 3 Substructure (second revision)
4410 (Part 10):1988 Glossary of terms relating to river valley projects: Part 10 Hydroelectric power station including water conductor system (first revision) 4461: 1998
Code of practice for joints in surface hydro electric power stations (second revision)
6966(Part 1): 1989
Hydraulic design of barrages and weirs -Guidelines Part 1 Alluvial reaches (first revision)
7207: 1992
Criteria for design of generator foundation for hydroelectric power stations (first revision)
9761: 1995
Hydropower intakes - Criteria for hydraulic design (first revision)
10751: 1994
Planning and design of guide banks for alluvial rivers – Guidelines (first revision)
11130: 1984
Criteria for structural design of barrages and weirs
11388: 1995
Recommendations for design of trash racks for intakes (first revision)
12800(Part 3): 1991 Guidelines for selection of hydraulic turbines, preliminary dimensioning and layout of surface hydroelectric power houses: Part 3 Small, mini and micro hydroelectric power houses 13495: 1992
Design of sediment excluders - Guidelines 2
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TERMINOLOGY
For the purpose of this standard, the following definitions should apply 3.1
Run of the River Powerhouse Integrated with Barrages
These are low-head run of the river Powerhouses located in one or some of the bays of the main barrage itself or located in a short by-pass channel or tunnel connecting the upstream pond with the downstream tail water of the Barrage. 3.2
For the other definition refer to IS 4410 (Part 10).
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MATERIALS
4.1
Concrete
The plain and reinforced concrete should conform to IS 456. Minimum M25 grade of concrete should be used where the structure comes in contact with water. Mass concrete should conform to IS 457. 4.2
Structural Steel
The structural steel should conform to IS 2062. 4.3
Reinforcement
The steel for reinforcement should conform to IS 1786. 4.4
Galvanized Corrugated Steel Sheets
Galvanized corrugated steel sheets should conform to IS 277. 5
DESIGN
The different design aspects involved are grouped into a few major sub-sections for convenience. These are (a) Hydraulic design of the barrage, (b) Structural design of the barrage, (c) Design of the guide bunds, (d) Design of the intake, approach channel and the transition (e) Structural design of the powerhouse substructure, (f) Structural design of the power house superstructure, (g) Design of the by-pass channel power house (h) Available energy evaluation. Design criteria for most of the above are already dealt with in several existing Indian Standards as mentioned in clause 2. Only special provisions, wherever necessary, are mentioned below. 5.1
Hydraulic Design of the Barrage
5.1.1 For the barrages founded on alluvial rivers IS 6966 (Part 1) should be referred to. 5.1.2 For barrage in rocky bed the following modifications from IS 6966 (Part 1) are recommended: 5.1.2.1 Retrogression
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Where the bed is composed of exposed bed rock of average strength (unconfined compressive strength 20 MPa and above) no retrogression is to be considered downstream. 5.1.2.2 Waterway In rocky bed (even with a shallow overburden), the permissible minimum waterway of the barrage may be adopted on the basis of the rock strength and permissible maximum unit discharge at high flood stage as given below: Strength of rock in the river bed
Permissible Max unit Discharge (m3/sec) per meter length of waterway
Type
Unconfined Compressive Strength (MPa)
Rock Quality Designation Index (RQD)
Massive
>100
75%
150
Jointed
50 to 100
25 to 75%
120
Highly Jointed
20 to 50
10 to 25%
80
Weathered
<20
10%
20
In deciding the waterway, other factors like afflux, pondage required, flushing velocity, energy dissipation, flow obliquity and flow concentration, cost of protection etc. should also be duly considered. 5.1.2.3 Uplift pressure a) In weathered and highly jointed bed rock, uplift pressure is to be calculated by any accepted practice. b) If the bed rock is reasonably impermeable (coefficient of permeability variation 5x10-4 to 1x10-7 cm/sec) but moderately jointed and the major joints are more or less parallel to the river flow direction, two rows of curtain grout (cement or chemical) may be injected about 3 to 5m downstream from the upstream end of the pucca floor, for a depth equal to the maximum pondage head. The uplift pressure may be assumed as 50% of the full head at the grout curtain line, reducing to in the minimum tail water level at the downstream end. As an additional measure pressure relief pipes can be provided under the stilling basin. c) If the bed rock is reasonably impermeable (coefficient of permeability variation 5x10-4 to 1x10-7 cm/sec) but moderately jointed and the major joints are perpendicular or nearly perpendicular to the river flow direction, the uplift pressure may be assumed as 25% full head on the upstream end reducing to the minimum tail water level on the downstream. d) If the barrage is on dense rocky bed (coefficient of permeability variation, below 1x10-7 cm/sec and above), the uplift pressure may be assumed as varying from 15% of full head on the upstream end to minimum tail water depth on the downstream end. 5.1.2.4 Energy dissipation In order to avoid erosion and other problems energy dissipation arrangement should be adequate to ensure that undesirable residual kinetic energy of flow does not pass downstream of barrage. If the river bed rock downstream of the stilling basin is relatively dense (specification strength and RQD), it is not necessary to provide the maximum length of stilling basin required for full hydraulic jump under maximum pond condition. The basin length required for the jump under minimum 2 m gate opening is considered sufficient. For lesser openings, the residual energy from the incomplete jump will not be harmful to a relatively dense rocky bed (unconfined 4
comp. strength > 50 MPa and RQD > 50%). Hydraulic jump being unstable in the range of prejump Froud’s number of flow (F1) varying from 2.5 to 4.5, efforts should be made to avoid this range by suitably adjusting waterway and pond/afflux. If it is not feasible to avoid, appropriate stilling basin (as per relevant Indian Standard) should be adopted. 5.1.2.5 Sediment Excluder In rocky bed, generally sediment excluders are not needed, unless the bed is mobile and there is sandy or pebbly bed load movement over the parent bed rock. Exclusion of mobile gravels and boulders from power bay is essential to prevent damage to turbines. Maximum site of sediments which can be allowed to move through power bay will be governed by head and type of materials. 5.1.2.6 Scour In rocky bed of average strength & RQD and moderate joints, Lacey’s scour formula does not apply. A nominal depth of cut-off of 2.5 m under the downstream end of the pucca floor is recommended. No downstream flexible protection is necessary. If the bed rock is highly jointed or weathered, some scour may take place and lacey's scour formula may be adopted, assuming the bed rock ultimately disintegrating to a mixture of gravel and coarse sand (approximately 5 mm dia.) condition. The scour in alluvial reaches particularly bouldery should be worked out by using Thomson & Gambell formula. The depth of cut-offs to limit exit gradient of sub-surface flow should be governed by existing analytical methods as also electrical analogy, and flow nets etc. 5.2
Structural Design of Barrages
5.2.1 The provisions covered in IS 11130 should be referred to. 5.2.2 Where movement of boulders and gravels may occur over the barrage bays, special protective measures like granite blocks, etc. over the concrete surface should be used along with high performance and special concrete with polymers for protection against abrasion and impact. 5.3
Design of Guide Bunds
5.3.1 The provisions covered in IS 10751 should be referred to. 5.3.2 On the river bank adjacent to the proposed powerhouse, the guide bund should be carefully curved from a long distance, so that no oblique flow develops in the approach channel of the powerhouse. The upstream abutment should be continued with vertical face for a distance equal to the length of the first block of the in-stream powerhouse. Thereafter, the wall can be gradually sloped to merge with that of the upstream guide bund. To avoid cross-flow and eddy, a divide wall may be used to separate the power bays from the weir bays by the guide bund and divide walls may be finalized for hydraulic model studies. 5.4
Design of the Intake Structure and the Approach Channel
5.4.1 The main components of an intake structure for the low head run-of-the-river powerhouse are:
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a) Intake flume with transition from rectangular to circular opening at the entry and a diffuser at the exit to minimize head losses b) Trash rack and supporting structure c) Sediment flushing device and some trapping arrangement for boulders at upstream location d) Anti- vortex device if necessary e) Stop-log and head gate slot enclosures 5.4.2 In cases where there is a considerable movement of boulders, stones and sand in the downstream direction, the intake should be so arranged that the effect of such movement will not lead to a partial restriction or blockage of the intake. Hydraulic model studies may be necessary under special conditions. 5.4.3
Intake flume
a) The length of the intake flume is determined by the requirements of space for the installations within the passage, and should have sufficient space to accommodate, trash rack, stop-log, head gates, necessary transition lengths etc. b) To prevent vortex formation, the central line of the intake flume should be located below the minimum draw-down level such that the minimum cushion of water over the roof of the flume at the entrance is 0.3 he, where he is the entrance height of the flume (Fig 1). c) In case of oblique flow, this water cushion may be raised upto he. d) The requirement of water cushion may be reduced with the provision of anti-vortex devices. e) The flume can sometimes be inclined, depending on the need of sediment or boulder exclusion devices such as shingle flushing channel/ conduit, sediment excluder etc. on the upstream (See fig. 1). f)
The normal contraction of 30 percent should be used for low-head installations to ensure uniform flow through turbines.
g) In the diffuser transition, the side walls should not expand at a rate greater than an angle of 50 with the axis of the main flow. h) All slots for gates and stop logs should normally be outside the transition zone. 5.4.4 Trash racks a) For low-head powerhouses, a straight trash rack is usually preferred, slightly inclined (700 to 800 with the horizontal) for easy raking. IS 11388 should be referred for the design of trash racks. b) Sometimes, a skimmer wall is also provided, submerging to a depth of 0.5 to 1.0 m below minimum drawdown level, to retain material floating on the water surface. c) In the Power houses located in by-pass channels, the skimmer wall is not necessary. d) The normal velocity of flow through the racks structure is indicated below: For units with hand raking v = 0.75 m/sec For units with mechanical raking v= 1.5 m/sec e) Trash rack bars should be so spaced that the clear spacing between bars should be at least 5 mm less than the minimum opening between turbine runner blades as per IS 9761. 6
f)
To prevent entry of logs/trash in to the intakes provision of log booms at an upstream location should be considered.
5.4.5 Sediment flushing a) For flushing the sediments, boulders, etc, a sediment trap near to entrance sill (low baffle wall in the river bed) of 0.5 m to 3.0 m height may be provided on the upstream of the intake, to divert this rolling bed material to the adjacent barrage bays. Sediment flushing conduits (See fig. 2) are also sometimes necessary. b) It is not necessary to remove suspended sediments as these do not normally cause any damage to the turbines running under low head. 5.4.6 Special provisions a) Anti vortex devices like floating timber frames or perforated breast wall may help in reducing the minimum water cushion necessary for preventing the vortex formation. b) Wherever the intakes are situated at high altitude above snow line, de-icing arrangements like bubbler system or heating arrangements may be provided for arresting ice-formation on rack bars and gates. c) Stop log provisions should be kept both on the upstream and downstream of the machines. d) The approach apron should not be placed closer than 30 percent of the intake height (he) from the lower edge of the intake orifice as per IS 9761. The upward slope of the apron can be 1:1 or flatter. e) The up-slope of the exit channel should not be steeper than 6(H):1(V). Adequate bed protection, preferably of concrete may be provided for the full length of the sloping apron. 5.5
Structural Design of Powerhouse Substructure
5.5.1 The stability analysis of power house as well as barrage structure should be carried out prior to processing the structural analysis. Detailed investigation of the engineering properties of the foundation material should be carried out by suitable field tests as per relevant Indian Standards. The main design concepts of the sub-structure of low-head river bed powerhouses being basically the same as those of a surface hydro power station, the criteria stipulated in IS 4247 (Part 3) should be followed. For the design of the generator foundation for machines with vertical axis, IS 7207 should be referred. 5.5.2 Some special features mentioned below are to be taken care of, when the river-bed powerhouse is designed in continuation of a barrage structure. 5.5.2.1 Structural stress analysis a) When the powerhouse is to be constructed on soft river bed composed of boulders, sand or sandy silt, its substructure should be designed by analytical/numerical method. Either (i) as an elastic raft foundation, or (ii) as a rigid frame on uniform soil reaction, or (iii) as a rigid box structure resting on caissons or wells. Pile foundations should be avoided.
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b) Depending on the thickness and rigidity of the concrete mass up to the machine hall floor level (considering all the openings) and the relative elasticity of the natural foundation media, the design can be Either (i) as an elastic structure on elastic foundation or (ii) as a rigid structure upon elastic foundation (See Fig. 3) c) In bouldery bed, the base being relatively more steady, the powerhouse blocks can be divided by vertical joints into groups. The foundation pressure can be assumed as uniform. The analysis of each block can be done as a plane frame (See Fig. 4). d) In rocky bed, the load from the superstructure may be assumed to go down directly to the bed through the Piers. The bottom slab can be separated from the Piers and side walls, but suitably anchored below and provided with temperature reinforcements. e) Where the foundation grade is silty sand, silt or plastic clay, evenly distributed ground pressure are desirable. This may be achieved by suitably modifying the position of the internal openings or applying extra cavities etc. f)
The Foundation concrete mass should also be checked for vibration effects to eliminate the possibility of any resonance.
5.5.2.2 Settlement Relative settlement between the two ends of a powerhouse block is of vital importance. In hydropower structures the limit of unequal settlement should not be more than 0.0003 L where L is the distance between the points settling unequally. This should however be governed by serviceability requirement of the structure. To reduce differential settlement, consolidation grouting of the foundation and enclosing the foundation periphery by sheet piles may be useful. Where the foundation strata is weak and heterogeneous in nature use of compacted soil matrix below the foundation raft can also be considered based on the analogy of concrete faced rock fill dams. 5.5.2.3 Sliding The criteria mentioned in IS 4247(Part 3) should to be adhered to. 5.5.2.4 Floatation The criteria mentioned in IS 4247 (Part 3) should be followed. It may be useful to drive a row of sheet piles or cut-off all around power house foundation to reduce uplift pressure below the upstream end of the powerhouse raft. The measures against sub-surface flow/scour, finalized for the barrage should also be extended beneath the power house structure. In general as the thickness of power house raft is quite large and is also subjected to heavy machine loads, it is safe from uplift considerations. It may also be useful to provide on the upstream approach channel bed, for some distance, a thin concrete layer covered with compact clay. The top of this clay layer is to be protected by precast cement concrete blocks, with a thin intervening layer of graded filter. 5.5.3
Approach and exit velocities
The flow velocity in the approach channel of the powerhouse may be restricted as follows:
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H (Generating head) m
2
4
6
V (Approach velocity) m/sec
1.0
1.1
1.3
8
10
15
1.5
1.7
2.1
At the exit end, the outflow channel may be carefully protected from scour for a considerable distance, to prevent the formation of cavities under the tail pool bed slab. In soft foundation, end sheet piles or concrete cut-off walls should be used. Suitable protective paving, preferably with cement concrete, should be provided in the bed and sides on the downstream. In the soft foundation strata, confinement of structural independent units by means of shallow cut-offs all around may be considered to impart additional stability. Properly designed divide walls of adequate length should separate the approach and exit channels of the powerhouse from the adjacent barrage bays. 5.5.4
Expansion and contraction joints
The provisions given in IS 4461 should be referred. When the power house is in conjunction of existing barrage the joining pattern of the barrage should be followed. 5.6
Structural Design of Powerhouse Superstructure
5.6.1 The main design concepts of the superstructure of a low head river bed powerhouse being basically the same as those of a surface hydro power station, the criteria and guidelines stipulated in IS 4247 (Part 2) should be followed. 5.6.2 IS 12800 (Part 3) should be referred for dimensioning and layout of small, mini and micro powerhouses. 5.6.3 Some additional features mentioned below are required to be considered while designing the in-stream powerhouses integrated with a barrage. 5.6.3.1 Continuation of the road bridge over the power house The in-stream powerhouse being in continuity of the main barrage structure, the road bridge over the barrage has to cross over the power house. This bridge is most conveniently located over the draft tube. The draft tube structure should therefore be designed considering the road bridge load over it (See Fig. 1) 5.6.3.2 Crossing of the barrage gate gantry crane Vertical lift gates are generally provided in the barrage bays, which need high trestles and mobile gantry crane for operation and maintenance. These cranes generally require approaches from both the river banks. Therefore, it has to travel over the in stream power house also. In outdoor and semi-outdoor type power house, this crane can be combined with the power house gantry. 5.6.3.3 Submersible type power house In narrow gorges, where the in-stream power houses occupy considerable waterway, the spilling capacity of the barrage can be increased by allowing part of the flood to pass over the power house. The down-stream river bed has to be properly protected against scour. Side channel, chute or shaft type spillways may also be considered to augur spillway capacity. 9
5.7
Design of By-pass Channel Power House
5.7.1 The design criteria of the by-pass channel power houses are practically the same as those of the river-bed power houses and most of the guidelines mentioned above are applicable. 5.7.2 It is desirable to locate the power house near the tail end of the by-pass channel, for economy in the cost of excavation. 5.7.3 A head regulator is necessary at the entry point of the channel, provided with suitable trash rack arrangement. 5.7.4 If desilting is considered necessary, sediment ejector may be provided as per provisions contained in IS 13495. Note: Normally, in low head power houses exclusion of suspended sediment is not required, as it does not harm the turbine blades under low head.
5.8
Available Energy Evaluation
5.8.1 Daily or ten-daily average discharges available in the river at the proposed power house site throughout the year has to be collected for as many years as possible. At least 10 years reliable data should be collected. 5.8.2 From the above data, the discharge figures for an average year are to be selected and arranged in descending order. With these data, frequency of occurrence (on percentage of duration) is found. For any given discharge or above it, the discharge (Q) and Percentage Duration curve is plotted (see Fig. 5). 5.8.3 On the same graph, the average tail water stage - duration curve (tail water level corresponding to a given discharge is obtained from the stage-discharge curve) is plotted. 5.8.4 The pond level for the barrage is decided from practical considerations like, peaking required firm power discharge available, submergence, cost of embankments, shoal formation etc. (refer 5.10). 5.8.5 The effective pond level is obtained after deducting the total head losses due to contraction, trash, entrance, rack, gates, stop log, expansion and exit velocity head) from the pond level (see Fig. 5). 5.8.6 Deducting the top water level (T.W.L.) from the effective pond level, the net available head (H) is found and losses due to friction in the system corresponding net head-duration curve is then plotted (Fig 5). 5.8.7 On the net head duration curve, a rated head (HD and the corresponding rated design discharge Q for the turbine, quite close to the maximum available power (Pmax) (see Fig 5 and clause 5.8.8 ) has to be decided. The turbine and the generator are then to be selected, to give maximum efficiency for these parameters (that is, rated head and rated discharge). 5.8.8 From the above plots of Q & H and by using the formula P = 9.81 η QH (kw), the Power (P) duration curve is plotted (Fig 5). Overall efficiency ( η o) is the product of the efficiencies of the turbine (of η T), generator (of η G) and transformer (of η Tr) i.e. η o = η T. η G. η Tr 10
The maximum power output Pmax will be equal to 9.8 η o Qr Hr where Qr and Hr are the rated discharge in cumec and rated head in metre respectively. Pmax is the plant power capacity. 5.8.9 From the power- duration curve, the total energy that can be generated in an average year is then calculated as E = ∫ pdt .
∑
5.8.10 In the absence of the actual efficiency coefficients for the machines and the losses, the following figures may be assumed for the preliminary assessment of the available energy: Rack loss
= 5 to 20 cm approx.
Entrance loss
= nil
Exit velocity head
= 5 to 10 cm approx.
Turbine efficiency
= 0.85 to 0.92
Generator efficiency
= 0.92 to 0.97
Transformer efficiency = 0.94 to 0.98 5.9
Fixation of the Pond Level / Afflux
5.9.1 In a relatively plain topography, the pond level/afflux of the barrage cannot be fixed much higher for following reasons: a) Extensive submergence of land upstream due to backwater effect, b) Costly marginal/afflux embankments to train the river and flood protection, c) Shoal formation in the pond due to slack flow in the backwater reach, and d) Insufficient approach velocity needed for flushing of sediments deposited in the pond. However, the effect of a low pond level is that, no power can be generated during the flood period, because the net head (H) will tend to be zero (Fig. 5). In deciding the design pond level an economic balance has to be struck, considering the revenue earned from energy on one hand and the cost to raise marginal bunds and occasional stagnation of the low-lying areas in the country side depressions on the other side. The local temporary water logging can be cleared either by pumping or by a parallel drain discharging downstream of the barrage. For creating enough flushing velocity height of masonry part of the barrage should be limited and ponding should be achieved by constructing high head gates. 5.9.2 In the hilly regions, however there is not much problem of sub-mergence or marginal embankments, even if the pond level is kept considerably high than the high flood level. But the pond must be periodically flushed by regulating gates so that the requisite drawdown of the pond can be achieved. Storage being of little importance in the run-of the river power station accumulation of boulders in the pond and shoal formation may be permitted. Sediments so deposited may be mechanically removed by shovels and dozers, once a year and also by periodic flushing of pond by gate regulation during monsoon. 5.10 Peaking If the backwater of the pond extends for a considerable distance upstream, a little extra head of in the pond will help in creating considerable storage of water which will allow peaking 11
operation during the peak hours, by storing for several hours and then releasing the stored volume during the peak demand hours, depending on the typical load-demand curve of the region.
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