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BHARAT HEAVY ELECTRICALS LTD.,BHOPAL

EFFECT OF HYDRAULIC DESIGN OF PELTON TURBINES ON DAMAGE DUE TO SILT EROSION AND ITS REDUCTION

BHEL Bhopal, INDIA

AUTHOR:V.K.PANDE Executive Director (TBG)


INTRODUCTION:BHEL is the major power equipment manufacturer in India responsible for establishing nearly two third power projects.In India, 80% of available Hydro resources are from Himalayan region.Silt erosion of Hydro turbines is a widely acknowledged problem.Problem is severe in the Himalayan region because the mountains are young and relatively fragile.


Silt and damage• Nature of silt

– Quartz rich with quartz content generally 65%-95%– PPM content high: 5000 to 20000 in monsoon– Freshly broken fine conical particles

• Damaged areas– Guide vane and Cheek plate– Runner inlet at blade skirt and crown– Runner outlet on pressure side – Pelton Nozzle & Spear– Pelton Runner bucket

Damage due to silt at site

Damage due to silt at site

3


Strategy to tackle silt erosion:

The silt menace can be effectively tackled by a three pronged approach :

Effective desilting to prevent silt from reaching the turbine.

Applying coating of hard material on various components for withstanding silt.

Development of design where silt erosion is inherently minimized.


Problem in Bhaba HEP

• These are 3X40 MW Pelton machines in Himachal Pradesh in Himalayas. Spear and nozzle are facing severe damage due to silt. So much so that they hardly last three monsoon months.

• All types of coatings viz plasma nitriding, Boronising, Ceramic coating using D-gun, HVOF, have been tried without success.

Eroded spear of Bhabha Hydro Power station after one monsoon.

Examples of erosion:


PRESENT WORK:

This paper discusses the approach for combating silt erosion by analyzing the flow dynamics behind it so that it can be avoided by modifying profile of erosion prone areas of Pelton turbines.

A systematic attempt to numerically simulate erosion in different nozzle-spear designs and its validation with site feed-back.


ACTION PLAN:

Use of recent advances in the field of numerical simulation of flow to predict performance & silt erosion.Experimental model testing/Data collection from site to validate numerical simulation.Evolving correlation between analytically predicted erosion severity and actual erosion at Site


Features of Numerical Tool:

Numerical tool used for analysis of flow in turbine components is CFD software CFX-TASCFLOW.Features of Tascflow CFD software

1. Full 3-Dimensional Navier- Stoke’s solution.2. κ-ε turbulence model with standard wall

functions.3. Multiple frames of reference for simultaneously

solving rotating and stationary components.4. All calculations have been made assuming

smooth walls.


CAPABILITIES DEVELOPED IN-HOUSE:

Macros for evaluation of performance parameters like efficiency, speed, pressure distribution & cavitation parameters, erosion severity have been developed.Validation of macros by experimental model testing at the state-of-the-art Hydro turbine Model testing Laboratory ,BHEL Bhopal and also with site feedback.To simulate the effect of silt on turbine components, Lagrangian particle tracking module with facility of user defined size and concentration of silt particlesis used.


APPROACH:The rate of erosion depends on -

1. The size, concentration, shape & hardness of silt particles (Petrographic & Petrofabric data) which are profile independent parameters. For a given site these can be safely assumed to be constant.

2. The velocity of the particles and angle of impact which are profile dependent parameters.

3. To compare silt erosion of different profiles a local erosion rate coefficient (ξ) is defined –

Erosion rate coefficient (ξ) α (velocity)n * f (θ)Where f (θ) is a function of impact angle θ.Value of exponent ‘n’ varies with material.

Physical validation of erosion

Actual damage on Francis runner

Predicted erosion pattern


Application of Numerical Tool:

Salient features of grid used for numerical analysis.

1. Structured grid with hexahedral elements.2. Grid domain includes short pen-stock,

distributor, nozzle, spear.3. Number of grid points

Distributor– 500,000 nodes.Nozzle assembly (each)- 200,000 nodes

4. To facilitate comparison of results of different variants, similar grids are generated using similar philosophy.

Computational domain for analysis

RESULTS : NOZZLE WITH EXTERNAL REGULATION


Vector plot – Flow velocities


Particle tracks


Erosion severity in nozzle


Erosion severity in Spear

RESULTS : INTERNAL NOZZLE (60-45 design)

Erosion severity in nozzle & spear


Particle track in nozzle & spear


Flow Velocity vector plot


Particle tracks


Erosion severity in nozzle & spear


Erosion severity in spear


Site Feed-back

St. Steel with Stelliting

13/4 St. Steel

13/4 St. Steel

HVOF/Borinising/Nitriding/Ceramic

Material/Coating

Nominal damage reported

Negligible damage reported

Moderate erosion damage reported

Severe erosion damage reported

Site feedback

Internal90/50

Baspa

Internal60/45

Bassi

Internal90/50

Malana

External60/45

Bhabha

Reg. Mech.

Project


Conclusion :

Nominal damage reported

Internal90/50

Baspa


0.16Internal60/45

Bassi


0.22Internal90/50

Malana

Severe erosion damage reported

0.38External60/45

Bhabha

Site feedbackErosion Severity Coeff(Numerical)

Reg MechProject


Conclusion :

Strong dependence of nozzle design on erosion characteristics.Externally regulated nozzle not conducive from

erosion as well as performance point-of-view.Trade-off between performance and erosion

characteristics in case of internal nozzles.


SCOPE FOR FUTURE WORK

Substantial damage reported

Internal90/50 degrees

Baspa



Bassi



Malana

Very little erosion damage reported

External60/45 degrees

Bhabha

Site feedback for damage to runner

RegulatoryMechanism

Project


THANK YOU

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