First decade of large-power gas turbines in Nigeria

Applied Energy 51 (1995) 213 222 Copyright c" 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved 0306-2619/95/$9.50

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0306-2619(94)00050-6

First Decade of Large-Power Gas Turbines in Nigeria H. I. Hart Mechanical Engineering Department, Rivers State University of Science and Technology, Port Harcourt. Nigeria

A BSTRA CT This is an appraisal of the perJormance of the four 75 M W gas turbines (GTs), i.e. the first large-power generating GTs installed in Nigeria. All the major parameters considered--availability, utilization and capability jactors--were much lower than acceptable. The J?equency of failures of some vital components ~?["the machines is also higher than normal. The general low performance is linked to some peculiar environmental conditions of the place and circumstances o[ the counto', which make the design qf certain parts ~[" the GTs inadequate.

INTRODUCTION Gas turbine (GT) sets were first used for the generation of electricity in Nigeria in 1963 when four units each with an installed capacity of 10 MW were commissioned at Afam Generating Station, 50 km north-east of Port Harcourt, which is the main oil city of the country. Before then, electricity was generated by the Electricity Corporation of Nigeria (ECN) and now known as the National Electric Power Authority (NEPA) with coal-fired steam-engine and diesel-engine generators. Between then and 1978, several more GTs were installed at Afam, ljora Power Station, Lagos and Delta Generating Station, Ughelli, in the present day Delta State. Several sets installed during this period proved to be inefficient and troublesome to run to the extent that some had to be abandoned in 1971 at Ijora after they had been in use for only a few years. The difficulty associated with running the GTs was due mostly to the fact that gasturbine technology was relatively new in the country and the operators had not acquired the requisite experience about GTs. It was therefore 213

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understandable that the installation of GTs was limited to small sets with low power-generating capacities. From 1972, the load forecasts started showing the need for the provision of large power plants. The first hydro-electric plant was commissioned at Kainji in 1969. It was, therefore, too early to embark on the construction of another such plant for many reasons: construction of new hydroelectric plants usually take long periods (Kainji took over 5 years); the effectiveness of Kainji was yet to be proven; the building of another big dam anywhere across the River Niger, on which Kainji is situated, could not be guaranteed; the non-availability of other viable rivers for large hydro-electric plants, etc. One of the few options left, therefore, was the trial of GTs with large power-generating capacities. In pursuance of this, four sets, each with an installed capacity of 75 MW were introduced at Sapele in 1981. The sets were of the same make and type. At the end of 1992, all the sets had been in existence for a little over 10 years. As at that time, none of them was operational. Some had experienced serious problems such as blade failures and ruptured combustion chambers after operating for very short periods. This paper originates from the appraisal of these sets during the first decade of their operation.

T H E POWER P L A N T Location The GTs were installed at Sapele, a town in the Delta State of the country in 1981. The major reason why Sapele was chosen was the availability of large reserves of natural gas, much of which was being flared at the time and so was lost. Sapele is in the part of the country known as the Niger Delta area. This lies between latitudes 4°N and 6°N and longitudes 5°E and 8°E and the vegetation of the area is equatorial rain forest. There are basically two seasons--the wet (April-September) and the dry (OctoberMarch). However, rain falls throughout the year. The mean annual rainfall is 3000 mm, while the mean monthly rainfall for the wet and dry seasons is 500 mm and 125 mm respectively. The mean daily temperature is 30°C while the maximum and minimum temperatures are 40°C and 20°C respectively. The relative humidity of the area is rarely lower than 60% even during the dry season. In the wet season, the south-west wind from the Atlantic Ocean is predominant while during a good part of the dry season, the north-east trade wind from the Sahara Desert deposits its dust as it sweeps through the area. Dust deposits, which are negligible during

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the rainy season, can be as high as 100 mg/m 2 of surface area. Between December and February, which is known as the Harmattan period, the dust concentration can be as high as 20 000 mg/m 2 especially during times of sand storms. Dust sizes may vary from fine to coarse and the aggressiveness of the particles could be quite high, especially because of the location of these plants in the oil-field areas, where the effect of gas flaring is quite considerable. The GTs The gas turbine sets were manufactured by the Brown Boveri Company (BBC) of Switzerland, which is a renowned giant in the manufacture and distribution of power plants. Each of the sets is of the model known as BBC Type 13 with an installed capacity of 75 MW. At the time of their installation, they had not been tried in an environment similar to that which exists at Sapele. Each set can be divided into major components as follows. Gas-turbine package The GT is a single-shaft (two-bearing) heavy-duty industrial unit with a 17-stage axial-flow compressor, a five stage axial-flow turbine and a single burner which burns only gas, unlike the smaller models which have double burners and can use both gas and automotive gas oil (AGO). The compressor casing has two circular channels from the 4th stage to the 7th stage. These channels are closed at the top of the casing by three air blow-off valves, which are open at shaft speeds of less than 2985 rpm to prevent the compressor from stalling. The set has individual black-start capability. Air-inlet system The elevated air-inlet system has two stages of filtration. The first is a porous wire mesh that prevents insects and objects with diameters exceeding 1 mm from being ingested. The second stage is made of perforated-steel baffles, held together in the form of overlapping louvres. The louvres pass through a bath of viscosine oil for cleaning as they are rotated at intervals controlled by a timer switch. The oil-soaked louvres trap dust particles as the air flows across. An elaborate silencer for sound attenuation completes the air-inlet system. The oil system The GT has a complex oil system which comprises the following: Lube, cooling, low-pressure, high-pressure, control, safety, emergency and set-

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point oil sub-systems. All these are interwoven to control and protect the GT from start to synchronous speed as well during normal operation.

Electrical package The major item in the electrical package is the AC generator which is an open self-ventilated, salient pole, revolving field, air-cooled machine with a brushless excitation system. Other items include the generator control boards, high-voltage switch gear, voltage regulator and generator neutralground equipment.

Enclosure A rigid frame type enclosure houses all the equipment mentioned above while the control room is air-conditioned.

ACTUAL PLANT PERFORMANCE The GT 1 was commissioned on 26 June 198l and it remained generally available until 1983, when it developed excessive vibration problems. A comprehensive check, which was carried out on it on 26 January 1984, led to its shut-down on 25 June 1984. It could not be rehabilitated for lack of spares and BBC manpower services for the rest of that year. The first overhaul of the machine was later completed in July 1985. After 2 years the set developed air cracks on the combustion chamber inner lining and this led to the reduction of the turbine's maximum temperature allowable by 20°C from 950°C in September 1987. The maximum generating capacity was also pegged at 70 MW. Also two major operational problems because of which the set was not available most of the time, were the compressor blow-off valves not closing as appropriate and high turbine-vibration. The latter problem led to the shut-down of the unit on 19 January 1991. Pre-rehabilitation inspection was completed in November 1991 with a compilation of the list of spare parts required, but the set remained unavailable and remains so (as at October 1994). The GT 2, which started operation on 15 July 1981, showed early signs that it would have a chequered history. In 1983, the intermediate shaft was damaged but it was replaced, and the unit was back on stream on 24 February 1984. Early in 1984 also, it was discovered that the inner liner of the combustion chamber had been damaged. The maximum turbine inlet temperature was consequently reduced to 850°C from 950°C. This reduction also pegged the power output at 55 MW. Many more problems, such as an inadequate cooling-water pump motor, startingequipment faults, pedestal-bearing high vibration and the collapse of the

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air-intake silencer kept the set out of operation for over 2000 h for that year. The continuing combustion chamber problem led to its modification. The unit was eventually shut-down in March 1987 for the first ever overhaul, which was started in October of that year. By the end of 1988, the mechanical aspect of the overhaul had been completed but the job was abandoned because of the unavailability of some electrical and electronic parts. A rehabilitation programme for the set was started in 1991 but was not completed until the last days of 1992, After a series of commissioning problems, it was synchronized into the national grid on 23 December 1992 for only about 7 h up to a maximum load of 20 MW. Several defects were detected and the job had to be suspended until January 1993 because the Asea Brown Boveri (ABB) personnel who were in charge of the rehabilitation were going away for the Christmas holidays. The GT 3 was commissioned on 31 July 1981. Its performance in the first years of installation was not much different from those of other sets. The combustion chamber developed problems early and the set was out of operation for 1042 h in 1984 on that account. The condition of the combustion chamber's inner casing necessitated a maximum temperature restriction of 850°C. External disturbances resulted in the collapse of the air intake silencers and that generated other problems such as excitationequipment failure and the flame-detection system malfunction during the same year. The set kept tottering in performance until it was overhauled in June 1986, but in November 1987 cracks developed on the turbine blades and the temperature restriction was further lowered to 820°C, fixing the maximum load to 50 MW. The year 1990 turned out to be perhaps the worst year for this set. It was out from 1 January 1990 to 20 February 1990 on account of a number of problems: 20 February 1990 to 14 March 1990 because of a synchronizing-equipment problem: and 26 March 1990 to the end of that year on account of complications after outages from system surges. The set had not been repaired by the end of 1992. The GT 4 which was commissioned on 22 August 1981 was forced out of use on 26 April 1990 along with GT 3 due to the damage to their common generator transformer. The set was operationally healthy and available for over 70 MW before the problem. The set had its first major problem on 1 January 1984 when it was forced out. The overhaul that was completed on 24 October 1984 included the installation of a new combustion-chamber inner-liner, new generator cooling-water cooler and intermediate shaft. The set was to go faulty again in 1987 as a result of high bearing vibration and exhaust-heat-shield failure, It was rehabilitated in June 1988. Early in 1990, a planned inspection was carried out and it was completed on 10 February 1990, but a breakdown of the

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TABLE 1 Plant performance statistics.

Unit/ plant

Date of commissioning

Run (h)

Total electricity

Availability

Utilization

factor

factor

Capability factor

generated

GT 1 GT 2 GT 3 GT 4 Plant

26/6/81 15/7/81 30/7/81 22/8/81

54 796 30778 47547 35 632

(103kWh)

As inst.

As avail,

As inst.

As avail.

2 592 4602 1 141 6 623

0.54 0-31 0.47 0.36 0.42

0.65 0.62 0.62 0.47 0.58

0-34 0-06 0.15 0.09 0.16

0.41 0.12 0.18 0.12 0.21

0.63 0-19 0.29 0.26 0.35

water-pump kept the turbine out for 2 more days. After running for just a month, the air-intake silencer collapsed as a result of a system surge. It took over a month to fix that, but a bigger system-disturbance induced surge occurred on 26 April 1990 and succeeded in knocking out the turbine completely. By 31 December 1992 (at the time of data collection for this study), it was yet to be rehabilitated.

PLANT P E R F O R M A N C E STATISTICS These are considered with respect to the following: (a) Unit availability factor--the ratio of the number of hours the unit is available for generation to the number of hours in the period considered. (b) Unit utilization factor--the ratio of the energy (MWh) generated by the unit to the installed capacity of the unit in MWh for the period considered. TABLE 2 Lost times of units.

Unit

Total lost time (months)

GT 1 GT 2 GT 4 GT 4 Average

64 95 72 137

Time lost in spare procurement (months) 33 68 33 41 43.75

Time lost on actual maintenance (months) 31 27 39 46 35.75

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TABLE 3 Maintenance history of units.

Unit

Date of last overhaul

Operating hours before last overhaul

GT 1

July 1985

15 308

Nil

39 488

54 796

3

1

GT 2

In progress June 1986

30 778

14 778

--

30 778

2

1

16 761

761

30 786

47 547

3

1

May 1988 20 676 (rehabilitation)

4 676

14 956

35 632

2

1

GT 3 GT 4

(c)

Excess Hours Hours Required Actual hours'after run since since number number due for last commissioning of oJ overhaul overhaul overhauls overhauls

Unit capability factor--the ratio of the actual generation capability of the unit to the installed capacity.

Table 1 shows these values for both 'as installed' and 'as available' situations. From Table 1 and the history of the units, the lost times for the sets are represented in Table 2.

PLANT MAINTENANCE HISTORY According to the recommendations of the manufacturers of the sets, the time of overhaul is influenced by four criteria but fixed by the one that occurs first. ~ The criteria are: years of operation, actual operational hours, life hours and the number of starts. The life hours are indicated by a counter and it is usually K times the value of operational hours, where K is a value that depends on a number of factors. The recommended requirements for an overhaul are also as follows: every 5 years, every 16 000 operating hours, every 24 000 life hours and every 800 starts whichever comes first. Based on the criteria above, the maintenance history of the units is given in Table 3.

OBSERVATIONS A N D DISCUSSION It is instructive that all the four sets developed the same problems, i.e. combustion-chamber liner deformation in 1984, when none of them was due for an overhaul according to the general criteria mentioned earlier;

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the operational hours were less than 16 000, the number of years of operation was less than 5, the life hours were below 24 000 and the number of starts was less than 800. It is known that the combustion-chamber liner receives heat by convection from the hot gases and by radiation from the flame: it is also known that it loses heat by convection to the cooling air (from the compressor) flowing along the outer surface and by radiation to the outer casing. Although there are empirical relations from which an energy balance to determine the liner temperature can be made, the accuracy of such calculations depends on many variables. The emissivity of the flame varies with the type of fuel and this tends to increase with the relative density. Also the liner is exposed to a greater heat radiation if soot particles are present, because it is the principal radiating component in luminous flames. So the problem may be caused by the type of fuel used as it is known that the composition of the gas used in the operation of the sets varies widely. On the other hand, the issue of combustion liner failures in the circumstance under discussion could be linked to the design of the equipment vis-fi-vis its operation in the peculiar environment of Sapele and indeed Nigeria. The use of the same cycle pressure ratio, but higher ambient temperature than that used for the design of the set, implies that the temperature of the air leaving the compressor will be higher than envisaged by the design. In this situation, where the pressure ratio is about 10 and the difference in the ambient temperatures is about 20°C, the difference in the compressor's outlet temperature even for isentropic compression is about 40 K taking the index of compression as 1.40. The loss in the cooling potential of the air is more than marginal, when it is realized that the mass of cooling air involved is about 200 kg/s at full load. Another factor that may have aided the untimely failures of the liners is the presence of contaminants which entered the gas turbine through the air stream and fuel system. Given the characteristics of the dust particles found in Sapele, the wire mesh and oil-bath filtration system is not adequate in cleaning the air before it enters the set. Any dirt that goes into the combustion chamber with the air can act as hot spots, which may cause high-temperature corrosion of the liners. If the dirt goes in with the cooling air, it would foul the outer surface of the liner and reduce the rate at which heat is transferred away from the liner. It is pertinent to add that, when GT 4 was overhauled in 1984, a different type of liner, made of Nimonic 80A, was installed. This is a confirmation of the inadequacy of the liner that came with the set. The development of cracks on some turbine blades of GT 3 in November 1987, only a year after the set was overhauled and the subsequent pegging of

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the maximum temperature and load to 820°C and 55 MW respectively is another indication of the high-temperature induced problems of the sets. It will be recalled that, in 1984, the temperature was also reduced by 100°C from 950°C to 850°C. Another issue that was prevalent was compressor surges that were brought about by disturbances in the national grid external to the plant. Compressor surges were responsible for a large quantity of outages and time loss because most of the time they happened violently, resulting in excessive vibrations which often caused the collapse of the silencers and the damage of other components of the machine. The excessive-vibration problems that all the sets experienced at various times are traceable to a large extent to these phenomena. F r o m all that has been said, it is not surprising that the sets have not fared well as Table 1 shows. All the parameters--availability factor, utilization factor both on 'as installed' and 'as available' as well as the capability - - a r e all dismally low. Another factor that added to the poor values of the parameters especially those of the 'as installed' figures is the length of time lost waiting for spares to arrive. On the average, about 44 months were spent waiting for spares for each set whereas only about 36 months were spent on actual maintenance and repair works (see Table 2). In fact between 26 April 1990 and 31 December 1992 none of the sets was on stream because there were neither spares nor personnel to carry out the necessary repair work. For GT 2+ the period of inactivity was from March 1987. The question of non-availability of spares definitely contributed to the delay of overhaul of the sets. In fact none of the sets was marked for overhaul at the appropriate time. They were all forced out of operation before they were due for overhaul. Hence none of the sets had been serviced as is recommended, as shown in Table 3.

CONCLUSION AND RECOMMENDATION It is obvious that the 75 MW GTs have not fared well in the first l0 years of their installation. This is partly because the sets were not fully adapted to operate in the Nigerian environment and national grid. The peculiar environmental conditions which have inhibited their operation include: (i) high ambient temperature: (ii) large amount of dust in the air during an appreciable part of the year: (iii) the instability of the national grid system.

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The fact that spare parts could not be procured in time, even for first overhauls, indicates that appropriate long-term plans were not made for the operation of the sets. The following suggestions are made: (i) A better heat-resistant alloy should be used for the combustion chamber liners. Liners made with Nimonic 80-A may solve the problem although those made with Hastelloy-B are better. Both Nimonic and Hastelloy have the same maximum temperature of rupture of 780°C under load but Hastelloy has a lower coefficient of thermal expansion 2 and therefore, is less susceptible to rupture. Some other alloys may also be considered. (ii) As a result of the local high relative humidity, an electrostatic filter should be added to the oil-bath filter in the air-filtration system. (iii) An appropriate under-frequency relay should be installed at load centres in the national grid to protect the sets from system disturbance induced compressor surges. (iv) Strict compliance with respect to the specifications of the fuel used in the combustion chamber should be applied. (v) In general, extensive studies on the environmental impact of the GTs should be carried out before they are installed. (vi) Long-term considerations of hard-currency sourcing and adequately trained and experience staff should be made before such sets are purchased in the future.

REFERENCES 1. Zaba, T. & Lombardi, P., Experience in operating of air filters in gas-turbine installations. Paper at ASME Gas Turbine Conference, 84-GT-39, 1984. 2. Baumeister, T. et al., Marks' Standard Handbook for Mechanical Engineers, 8th edn. 1978, pp. 6-94--6-101. 3. BBC, General instruction for maintenance of gas turbine plants, types 9, 11, 13, 4-095011.