Hydro Turbine Bearing Challenges

BEARING CHALLENGES AT

KARIBA NORTH BANK POWER STATION

Author:

Mahaku Maloza - Site Project Engineer, ZESCO Limited, KNBPS

CONTENTS

ABSTRACT 3

1.0 INTRODUCTION 5

1.1 The Kariba Hydro Electric Scheme 5

2.0 POWER STATION REHABILITATION AND UP-RATING PROJECT 8

2.1 Rehabilitation Project 8

2.1.1 Turbine Rehabilitation 8

2.1.2 Generator Rehabilitation 8

2.2 Up-rating of Generating Units 9

2.2.1 Electrical Circuits 9

2.2.2 Turbine Equipment 10

2.3 Completion of Works 11

3.0 UNIT 3 BEARING CHALLENGES FOLLOWING REHABILITATION 12

3.1 Bearing Arrangement 12

3.2 Rehabilitation Works 13

3.3 The Combined Bearing Problem 14

3.3.1 The Fire Accident 14

3.4 Resolution of the Bearing Problem 15

3.4.1 December 2009 (05/12/2009 - 14/12/2009) 15

3.4.2 March 2010 Outage (10/03/2010 - 01/05/2010) 17

3.5 Remedial Works 20

3.5.1 Oil Coolers Modifications 20

3.5.2 Thrust Bearing Support Modifications 21

3.5.3 Guide Pad Modification 23

3.6 Results 24

4.0 CONCLUSIONS 26

Bibliography 27

ABSTRACT

In 2002, ZESCO Limited embarked on the rehabilitation and up-rating of all four generating units at its Kariba North Bank Power Station from 150MW to 180MW.

The rehabilitation of Units 1, 2 has been completed but the up-rating has only been done on electrical circuits. The original turbines have been retained and will be replaced at a later stage. Rehabilitation works and full up-rating of Unit 3 was completed in September 2008. Rehabilitation and up-rating of Unit 4 is still underway and will be completed in April 2012.

The generating units are vertically arranged and supported at two points by friction type bearings. On the generator side the shaft is supported by the combination of thrust and guide bearings (a combined bearing). The shaft on the turbine side is support by a 2 segment guide bearing.

Unit 3 had high combined bearing guide pad temperature problems initially at commissioning but these were thought to have been overcome by re-adjustment of the guide pads. The unit operated with normal bearing temperatures for about seven months when it started exhibiting high thrust pad temperatures on two pads.

In September 2009, the power station experienced a fire accident which swept through the roof of the powerhouse and all three machines tripped as result of loss of auxiliary supplies. On running up the machines the Unit 3 bearing temperatures remained high.

It was decided to take out the Unit to resolve the high bearing temperatures in December 2009. However, the condition worsened upon restarting the unit as more thrust pads exhibited high temperatures.

One thought attributed the high bearing temperatures to the machines shutting down without high pressure oil lubrication during the fire. According to the suppliers of the bearing these machines are able to shut down without auxiliary power supply without any detrimental effect on the bearing. Subsequent outages to resolve the problem did not produce desired results. The combined bearing guide pads are designed to have a clearance of between 0.15mm and 0.18mm. The guide pad clearances on Unit 3 ended up being adjusted to between 0.45mm to 0.50mm. This was done to enable the unit to run at a constant 150MW, de-rated by 30MW, whilst waiting for a resolution of the problem.

This paper describes the challenges the Power Station faced and the solutions implemented.

1.0 INTRODUCTION

The Zambian power industry is composed of ZESCO Limited, a national utility involved in the generation, transmission Distribution and Supply of electricity, Copperbelt Energy Corporation (CEC) a private company involved in the transmission of power to the mining loads on the Copperbelt Province and the Lunsemfwa Hydro Company, a private company involved in the generation of power.

ZESCO owns and operates the following power stations:

* Kafue Gorge power station with an installed capacity of 990 MW

* Kariba North Bank power station with an installed capacity of 720 MW

* Victoria Falls power station with an installed capacity of 108 MW

* Four (04) Small Hydro power stations with a combined installed capacity of 23.75 MW

ZESCO's total installed capacity is about 1800 MW

1.1 The Kariba Hydro Electric Scheme

The Kariba Hydro-Electric Scheme at Kariba was constructed between 1955 and 1962. The scheme comprises an arch dam 128m high, with a crest length of 617m and underground power stations on the north and south banks. The Hydro-electric Scheme is situated on the Zambezi River which at this point forms the border between the Republic of Zambia and Zimbabwe.

Kariba North Bank Power Station, located on the north bank at the Kariba Dam, is an underground Power Station and is the second largest Power Station in Zambia. The station consists of four vertical Francis turbine and synchronous generators with an initial total generating capacity of 600MW. The power station was commissioned in 1976.

There are four separate turbine intakes with trash screens and hydraulically operated main intake gates (each intake serves a single penstock and turbine). One emergency gate, operated by a gantry crane, is available to isolate one intake at a time, enabling work to be performed on the main intake gates. The penstocks are concrete lined except for the 30 m length immediately upstream of the machine hall where a steel lining is provided. Steel lined