1 Introduction

The Turbo generator (TG) requires lube oil system to lubricate and to provide adequate cooling to turbo generator bearings [1, 2]. The lube oil supply system also serves the purpose of providing motive oil for hydraulic turning gear which rotates the shaft system at sufficient speed before start up and after shutdown of turbine. During the operation of turbo generator, the seal oil system is required to seal the hydrogen inside generator where it is used as a cooling media of generator rotor and stator core. The jacking oil system is required during start up and shutdown of the turbine for lifting of rotor before rolling the rotor by motive oil and steam.

TG lube oil circuit has 2 × 100% AC auxiliary oil pumps (AOPs) to supply oil to the bearings and turning gear mechanism. For lifting the TG rotor, 1 × 100% AC Jacking oil pump (JOP) is available. If normal AC power supply fails, a battery-operated 1 × 100% DC Jacking oil pump (JOP) is also available to lift the rotor. In case of all AC power supply failure, one battery operated emergency oil pump (EOP) is available to lubricate and cool the bearings and protect them from getting damaged. All the above five pumps are located vertically on main oil tank (MOT). Two oil vapor exhausters (Each 100% capacity) are also located on MOT to remove oil vapor and the non-condensable gases from turbine bearing chambers and MOT. For generator seal oil system, 2 × 100% AC seal oil pumps are available for airside sealing and 1 × 100% seal oil pump (SOP) is available for hydrogen (H2) side sealing. In case of non-availability of AC power supply one battery operated 1 × 100% DC seal oil pump is also available for sealing the hydrogen to avoid escape of explosive hydrogen to atmosphere. 2 × 100% generator bearing chamber exhaust fans are also located below generator to remove oil vapor and the non-condensable gases from generator bearing chambers and seal oil storage tank (SOST).

To achieve continuous operation and maximum operating life of TG plant, oil used in oil circuits should be clean and free from all type of contaminants. If there is any presence of contaminants like dirt, silt and rust, they will enter in to the clean lube oil when oil is circulated through these systems. This simultaneously reduces purity of oil and thereby affects the equipment in the oil circuits such as pumps, bearing, filters, coolers and flow meters. It is necessary to flush the TG oil circuits before filling the clean oil [3].

In the literature, methods are available to flush the TG oil circuits and described elsewhere [4, 5]. The data on time of oil flushing, minimum quantity oil required for flushing are not easily available and accessible in the open literature. This paper deals with a method to find minimum quantity of oil required to start the different stages of flushing, reducing the time of flushing and an optimum oil flushing method based on the commissioning experience on flushing and analysis of results of 500 MW TG oil circuits.

2 Preparation for flushing of oil circuits in TG system

2.1 Lube oil system preparation

During flushing the contaminants present in lube oil circuits may enter in to the bearings which will create a serious problem. This can be avoided by bypassing all the bearings of TG with temporary loops. There are 7 numbers of lube oil supply line to Turbine Generator and each lube oil bearing supply line was connected to corresponding oil return line of that bearing pedestal with suitable temporary loop. Each lube oil supply line leading to the bearing pedestal has a throttle valve which can be opened or closed whenever required. The lube oil coolers were also bypassed by replacing the duplex 3-way valve of coolers with a suitable spool piece with drain pipe at its bottom. Lube oil temperature control valve (TCV) after the coolers was replaced with ‘T’ type spool piece. The internals in orifice after TCV and filter elements in duplex filters were also removed. The injector − 1 downstream oil supply line was disconnected from lube oil supply common header and connected to MOT tank riser section by temporary loop having a manual control valve. The nozzle box before the turning gear wheel at HP turbine front pedestal was replaced with flushing device and barring gear valve (BGV) was kept closed. The main oil pump (MOP) at the front pedestal was replaced with suitable spool piece. This is the highest location in lube oil circuit. In MOP spool piece, a vent line with valve was also provided which facilitates air venting during filling of lube oil circuit by EOP or AOP. One set of stainless-steel basket type strainers are available on the top of MOT. The bags made up of muslin cloth are fixed over these strainers. The contaminants from lube oil circuits will be collected in the bags during the flushing of oil circuits. Then the cloth bags can be removed and new one will be put on the basket strainer which facilitates easy cleaning of MOT strainer baskets and changeover. One spare set of strainers were also available for changeover of MOT strainers in case of choking occurred in the working strainers in MOT. The supply header and return oil header was connected by temporary loo** with a valve. The lube oil flushing schematic flow sheet was shown in Fig. 1

Fig. 1
figure 1

Lube oil system flushing scheme (Stage-1)

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2.2 Jacking oil system preparation

The JOP is a screw type pump. During initial stage of flushing, flushed particulate contaminants in MOT oil are expected to enter into the pump which may damage its internals or block the oil flow in screw pump. It may also cause jamming of pump shaft. Hence the JOP should not be operated during initial stage of flushing. For flushing the jacking oil line, both JOPs are bypassed and jacking oil discharge line after duplex filter was connected to the MOT injector 1 supply line from AOP common header by a temporary pi**.

The duplex filter in jacking oil line was also bypassed with temporary pi**. To avoid entering of oil contaminants during the initial stage of flushing in to the bearings from 1 to 4, the jacking oil pi** was disconnected from the bearings and let it directly in to the corresponding pedestal. The non-return valve internals were removed at all bearing supply lines. The jacking oil supply lines to bearings 5, 6, 7 were connected to corresponding lube oil return headers with temporary pi**. The supply header of jacking oil line was connected to SOST with temporary pi**. The jacking oil flushing scheme was shown in Fig. 2.

Fig. 2
figure 2

Jacking oil system flushing scheme (Stage-2)

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2.3 Seal oil system preparation

As JOPs, the SOPs are also screw type pumps. So, the entire pump rack was isolated during initial stage-1 flushing. The coolers for both airside and hydrogen side were isolated and temporary pi** arrangements were done. The suction header of SOPs was disconnected from SOST and connected to lube oil supply header through temporary pi**. The filter elements for both hydrogen side and airside filters were removed. The differential pressure valves for seal oil pumps, hydrogen side seal oil equalization valves at valve racks were also replaced with suitable spool pieces. The flow meters of both airside and hydrogen side and ring relief oil supply lines (Both turbine end and exciter end of generator) were also isolated.

Float valves in seal oil tank were removed to avoid any blockage of valve with flushed contaminants. The hydrogen side and airside seal oil supply lines were disconnected before the generator end shields and both are connected to hydrogen side seal oil drain pi** leading to seal oil tank (SOT) with common temporary pi** at both exciter and turbine end of generator. Vent lines were also provided at both exciter end and turbine end of the generator at highest elevations to release the air during the filling of seal oil circuits. After completing the temporary pi**, the air leak test was done by placing suitable dummies wherever it is applicable. The seal oil flushing scheme was shown in Fig. 3.

Fig. 3
figure 3

Flushing scheme of seal oil system (Stage-1)

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3 Air flushing and card board air busting of oil circuits

To remove free contaminants in the oil circuits, air flushing was carried out with compressed air. The compressed air at 6 kg/cm2 was derived from the compressor plant. For lube oil lines, initially supply header was flushed with air. Then lube oil supply lines to the bearings also flushed with air supply from each pedestal end. Thereby loose materials if any present in the pi** were removed.

Similarly jacking oil circuit was flushed with air by isolating the jacking oil pi** from duplex filter at both upstream and downstream. Upstream pi** of jacking oil is flushed with air by supplying the compressed air at pump end. For doing this, temporary pi** which is bypassing the JOPs was disconnected and the pump discharge up to duplex filter were flushed with compressed air was let out to MOT area. The downstream pi** after jacking oil duplex filter was flushed from exciter end of temporary pi**. Then individual supply lines to each bearing were also flushed from pedestal end. The flushed air was let out in to both MOT through return header and MOT area through the downstream pi** of jacking oil duplex filter.

In seal oil system, the compressed air was supplied at the vent points (located at exciter end and turbine end) and air flushing of supply lines of hydrogen side seal oil lines was carried out up to duplex oil filter (Hydrogen side) by disconnecting the pi** from filter. Similarly flushing was done for downstream pi** of air side duplex filter. Then flushing of duplex filter upstream pi** was carried out by passing the coolers (both air side and hydrogen side) through temporary pi** with compressed air supply at exciter end of air supply point. The flushed air was let out in to atmosphere for both downstream and upstream pi** of seal oil duplex filter.

After completing the air flushing for above oil circuits, cardboard air busting was carried out for oil lines wherever possible. The lube oil return header was entirely welded circuit and no provisions such as flanges available to isolate it and pressurize to required pressure during card board busting. But in lube oil supply header, provisions are there to isolate supply header from the return oil header. All throttle valves in bearings lube oil supply lines were closed. The spool piece for lube oil 3-way valve was removed and card boards of 5–10 numbers with thickness 2 mm were fixed at the duplex filter upstream pi** flange. The compressed air at 6.5 kg/cm2 was supplied at exciter end and the supply header was pressurized up to the pressure 4–5 kg/cm2. The card board busted at this pressure range and loose dust and rust of pi** was removed from the circuit. Several times, card board bursting was conducted till getting the clear air (verified with white cotton cloth fixed over the card boards). Since the lube oil circuit volume was greater, the busting time varies between 10 and 30 min depending upon the number of cardboards used at the lube oil cooler 3-way valve end.

For seal oil system the same procedure was followed. The compressed air at 6 kg/cm2 was supplied at the vent line of seal oil circuits. Since the volume of seal oil was less, with in no time (less than < 2 min) the busting of card board occurred. The busting was carried out for both hydrogen and air side seal oil lines of both downstream and upstream pi** of duplex filters by passing the pump rack and coolers, flow meters in the valve rack. Since the jacking oil lines are small, busting time will be small and cardboard busting will not be effective. Hence cardboard of jacking oil lines was not carried out. After completing the air flushing and card board air busting, the temporary pi** was normalized.

4 Filling the oil in to MOT and its calibration

Before filling oil, MOT was cleaned properly and inspected by statutory agencies. It was also ensured that all the drain valves of MOT remain closed. Required numbers of portable fire extinguishers (CO2, Foam, and dry chemical powder) were placed at turbine floor. Apart from this, CO2 flooding system for MOT was also made available. After clearance to fill oil, required numbers of 200 barrels were loaded in turbine operating floor. Each barrel contains 0.21 m3 of oil.

The lube oil can be filled in to MOT directly pouring the oil from barrel through the funnel arrangement on the top of MOT using overhead crane or by using an oil centrifuge. The later one is desirable because while filling the oil through a centrifuge in a clarifier mode, the oil is further purified by removing impurities like wax from the oil. The dimensions of MOT tank were 6120 mm × 3120 mm × 2610 mm. The MOT has magnetic type level gauge along with see through glass tube where oil level can be seen. The range of level gauge is − 750 mm to + 750 mm. The MOT calibration graph drawn for Number of barrels versus MOT level is given in Fig. 4. From this it was known that about 60–64 mm in MOT level corresponds to 1 m3. This variation may be due to space occupation of pumps, ejector and slightly inclined bottom section of MOT. An average value 62 mm (1 m3) was taken for volume calculations.

Fig. 4
figure 4

No. of barrel vs MOT level (MOT level calibration graph)

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5 Oil analysis for monitoring the flushing

During the manufacturing, installation and storage of TG oil circuit equipment the iron filings, welding slag, dirt, silt, cotton pieces, used cloths and other materials are introduced in to pi** and process equipment. Since the PFBR site is nearer to the sea, absorption of moisture and there is a possibility of development of both loose and fixed rust due to corrosion on the erected oil system components which is exposed to open to atmosphere till introducing oil in to the circuit. Since the TG oil circuit components are made up of carbon steel, they are ready to undergo corrosion and absorption of moisture from the atmosphere. When the clean oil is circulated through closed TG oil pi** during the flushing or normal plant operation, the above impurities are released in to the circulating oil from the circuits. Because of this, the pure oil becomes impure. The loose impurities are removed in stage-1 flushing by circulating the oil at turbulent flow condition. The fixed impurities are removed in stage-2 flushing by providing the thermal shocks with turbulent flow condition. The loose impurities > 250 µm are filtered by using basket filter at MOT riser section. The release level of loose and fixed impurities will indirectly indicate the oil flushing performance.

All the oil samples were taken from MOT oil sample point. The oil samples are collected during the running condition of flushing pump to ensure proper mixing and distribution of impurities in the oil phase. Before collecting the oil sample, the sampling section is sufficiently flushed and collected in clean and dried bottles with tight caps. The suspended impurities (< 250 µm) in oil is measured by analysis of amount of sediments in oil. The sediments were measured by filtering the known quantity of oil through a 0.45 µm Whatman filter paper and weighing it after drying in hot air oven. The amount of sediments expressed in ppm (mg of impurities per litre of oil). The moisture value is expressed in ppm (mg of water per litre of oil). The water present in the oil as water in oil emulsion state is measured by Karl Fischer Coulometric method. If the oxidation of oil occurs during circulation, it will degrade the oil quality and develops acid products in the oil which is measured by Total acidity number (TAN). The total acidity number (TAN) is measured in mg of KOH/g of oil sample by titration method.

The oil sample was directly taken from fresh oil barrel used as a control during the flushing process. The control sample was analysed for moisture, sediments, and TAN values. The values are 35 ppm, 0 ppm and 0.123 mg of KOH/g of oil respectively. During the oil flushing the change in sediments value is appreciable than change of values in moisture. Since the plant is not in prolonged operation, there is less possibility of change of TAN value during flushing. In this paper the sediments and moisture value of samples were analysed to monitor the flushing performance.

6 Flushing of oil circuits

The oil flushing was carried out in 3 stages in order to do effective flushing and to protect various equipment like pumps, coolers, filter, bearings and throttle valves before bearings. In stage-1 flushing, all coolers in lube oil circuit, jacking oil and seal oil circuits were bypassed [6]. The filter elements in lube oil circuit, jacking oil circuit and seal oil circuits were removed to avoid frequent choking of filter during stage-1 flushing. One set of basket strainers of 250 µm size were also installed in MOT return oil raising section. All TG bearings 1–7 were also bypassed by suitable temporary loo**. Oil centrifuge was also made available to reduce the sediment level in MOT. In stage-2 flushing, all oil coolers, filters were put in to service with thermal shocks. In stage-3 flushing, throttle valve internals in all bearing supply lines were replaced with line filters of size 37 µm and flushing was carried out to check any choke in these filters.

6.1 Stage-1 flushing

For flushing of oil circuits, both AOPs and an EOP are made available. Initially, lube oil circuit alone was taken for flushing. All the throttle valves in the oil supply lines to bearings were closed. The temporary oil supply lines from AOP header to jacking oil line and seal oil system were isolated by closing corresponding valves. Initial filling of lube oil circuit was carried out with the help of EOP. After ensuring the filling of entire circuit by releasing the air through the vent located on the top of MOP spool piece, AOP was started and EOP was stopped. The flushing oil flow must be turbulent which is given by AOP by circulation of oil. The oil was flowing only through lube oil supply header, flushing it by turbulent flow and returns to MOT through return oil header. The oil samples were periodically collected at MOT sample point for analysis of total acidity number (TAN), moisture and sediments [7,8,9,10]. The MOT strainers were periodically inspected to check whether there was any choking in the strainers.

After flushing the lube oil supply header, throttling valves in turbine bearing supply lines 5, 6, 7 were replaced with dummy discs. Then bearing supply lines 5, 6, 7 were taken in to service for flushing along with supply header. Since there was no throttling, full flow of oil through these lines took place. If flushing of all lines at a time with full flow is done, it may lead to trip** of AOP due to overload. Because of this, 2–4 numbers bearing supply lines (with full flow) at a time were taken in to service for flushing. The AOP flow to bearing supply header was adjusted by opening or closing (10–30%) the injector -1 upstream temporary line valve leading to MOT. After completing the lube oil system flushing, jacking oil system was flushed. The isolated seal oil system from the rest of oil circuits was leak tested against air excluding the seal oil storage tank. The seal oil tank was also leak tested with air separately. Then the seal oil system was put in to service by opening the temporary valve. First, overflow line only flushed. Then air side seal oil lines with seal oil ring relief lines, hydrogen side seal oil lines were taken in to service. The oil samples were also taken before and after putting seal oil system in to service for analysis. While flushing lube oil supply header alone, there was no appreciable increase in sediments in oil samples. The amount of moisture, sediments in oil are < 60 ppm & < 5 ppm.

But after 11th day onwards, there was a steep increase in sediments was observed. On this day, bearing supply lines 5, 6, and 7 were taken in to service for flushing.

When inspecting the basket strainers, many foreign materials like papers, nails, iron scraps, clothe pieces, used tea cups, few wood pieces were found. After stop** AOP, the basket strainers were removed and spare strainers were put in to MOT. The materials collected on the muslin cloth bags were removed. The strainers were air cleaned with compressed air. A new muslin cloth bag put on to cleaned strainer and kept as spare.

It was expected that the sediments value would shoot up while flushing was in progress. But there was a decrease in sediments level was noticed which is due to fresh oil filling (8 barrels) was done on 19th and 20th days as shown in Fig. 5.Similarly the seal oil system was also flushed. While charging the seal oil system, the change in volume of MOT for different loops of seal oil circuit was recorded to determine volume hold up of seal oil circuit loops. The two basket type strainers were periodically inspected to check its cleanliness. Since there was no choking of filter which is confirmed by clean condition of muslin cloth over the MOT basket strainer and no presence of dirt, silt, rust, foreign materials for the period of 24 h on continuous running of AOP, it was declared that stage-1 flushing was completed. At the end of stage-1 flushing the sediment value of MOT sample was 300 ppm (29th day). But there was no appreciable change in moisture.

Fig. 5
figure 5

MOT oil sample analysis for stage-1 and stage-2

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6.2 Stage-2 flushing

After completing stage-1 flushing, the spool piece for lube oil cooler was replaced with double side 3—way valve and coolers are ready to be charged. All the bearing lube oil supply lines were in closed condition and supply header is connected with return header. The jacking oil, seal oil systems were also in isolated condition and filter elements of above systems were in removed condition. The difference between lube oil level in MOT before and after charging one of the lube oil coolers will give hold up volume of cooler. By using MOT level calibration chart each cooler has hold up volume of 2.4 m3 which is matching with shell side cooler volume 2.4 m3 as given in manufacturer’s drawings. Then the second cooler also charged by changing the position of 3-way valve. Thus, it was confirmed that both lube oil coolers have total hold up volume of 4.8 m3. Oil charging of the coolers was done by using EOP.

After charging the lube oil coolers, one AOP was started and running EOP was switched off as done in stage-1 flushing. Since the pump was continuously running, the loose rust from the oil circuits slowly introduced in to the oil and oil sediments value reached maximum value 416 ppm on 58th day. This shows that effective flushing of oil circuits was in progress. The same day oil centrifuge was also brought in to service. Therefore, sediments value was reduced to < 5 ppm on 60th day. The oil centrifuge was stopped.

Thermal shocking of lube oil header was started on 70th day without installing the filter elements in Duplex filter. The sediments value was 45 ppm as on 70th day. The thermal shocking system consists of heating and cooling arrangements. A temporary heating setup consists of a tank of 6 m3 with 19 heaters (Capacity 12 kW per heater) and two numbers of hot water circulating pump. A temperature gauge was also available for measuring the tank temperature.

The suction of the pump is connected to hot water tank. The hot water circulated in tube side of cooler-1 heats the oil circulated in shell side of the cooler to 65 °C within 2–3 h. A double side three-way valve was available to facilitate changeover of coolers for heating and cooling. After heating the MOT oil to sufficient temperature (65 °C), the hot water circulating pump was stopped and the lube oil cooler-2 was brought in to service by changing the position of 3-way valve. Now the AOP is circulating the oil to shell side of the cooler-2. The cooling water was circulated on tube side of the cooler-2 which rapidly cools the lube oil to 45 °C within 30–60 min. The thermal expansion of rust will be different for heating and cooling process. Due to sudden and rapid cooling of components in the oil circuits and turbulent oil flow make the strong rust to peel off from the surface and was carried along with the circulating oil to MOT. The heating oil by cooler-1 and rapid cooling of it by cooler-2 constitutes one thermal shock of oil circuits [11]

During the plant normal operation, the lube oil will be cooled by normal service water. Since normal service water system for turbine building was not commissioned at that time, a temporary cooling system was arranged for cooling the hot oil. The temporary cooling system consists of 400 m3 tank, two cooling water circulating pumps and temporary pi**. The pumps suction was connected to 400 m3 tank and discharge was connected to lube oil cooler tube side inlet with temporary pi**. The cooler outlet was again connected to 400 m3 tank through a temporary pi** with feed and bleeding system. Since the cooling water outlet from the lube oil cooler was sent back to the 400 m3 tank, the temperature in the tank will rise gradually. To maintain the inlet cooling water temperature of cooler within the range, a feed and bleed system was also available to control the temperature of cooling water temperature well below 35 °C.

Thermal shocking initially was done for lube oil supply header and return header. Then 5, 6 TG bearing lube oil supply lines were flushed. Along with above lines, the jacking oil header was also in service. Similarly bearing 2 and 7, 1 and 4, 1 and 3 supply lines, seal system overflow line, seal oil hydrogen side pi**, seal oil airside side pi** with different combinations were flushed with thermal shocking process one by one with corresponding jacking oil bearing supply lines. At this time interconnecting of lube oil return and supply header valve was at closed condition. The Table 1 gives detailed thermal shocking data for lube oil, jacking oil and seal oil circuits. Oil samples were also taken periodically to monitor the sediments, moisture, TAN values. The centrifuge was also run intermittently in clarifier mode to decrease the sediments level in the system. The number of thermal shocks per day depending upon total time consumed per each shock. This will vary between 4 and 5 h (For both heating and cooling) per shock.

Table 1 Stage-2 Flushing data for thermal shocks
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There are 43 thermal shocks were given to the system without introducing the filter elements. 37 numbers of thermal shocks were given with introducing the filter elements into the Duplex filter and seal oil filters. This is given in Table 1. Before fixing the filter elements, the value of sediments in lube oil was around 177 ppm as on 84th day. Then filter elements were put inside the duplex filter. Since the sediments particle size greater than filter mesh size (37 micron) in duplex filter, due to filtration of the sediments its value started to fall. The sediments amount was reduced to < 10 ppm as on 104th day. If there is a choke in working filter, the standby filter of Duplex filter was brought into service. The choked filter was cleaned and kept as standby filter [12].

If the filters of lube oil system and seal oil system is clean and if there is no choking of filter for 24 h of running of AOP, and value of sediments < 100 ppm and moisture < 100 ppm, it will be declared that the stage -2 flushing was over. After 11 numbers changeover of Duplex filter 1 and 2, this criterion was achieved. At the end of stage-2 flushing, the value of sediments < 10 ppm, moisture < 26 ppm and total acidity number < 0.1 mg KOH/g of oil. It was also observed that there is no change in percentage choking in Duplex filter indicator and no change in pressure readings at the upstream and downstream pressure gauges even after 24 h. Hence it was concluded that stage-2 flushing was completed.

6.3 Stage-3 flushing

At the end of stage -2 flushing, the SOST drained and was inspected. As expected, small amounts of settled fine sediments were found at the bottom of SOST. Then, after ensuring quality of MOT oil, AC JOP and DC JOP were successfully commissioned. The jacking oil lines (5, 6, 7 and 1, 2 and 3, 4 combinations) were further flushed by running jacking oil pumps. During this time centrifuge was also in service at clarifier mode. Then the seal oil system (SOT and coolers, filters rack, valve rack and pi**) was drained by opening the drain valves of airside and hydrogen side duplex filters. About 3.1 m3 of oil was recovered from the seal oil system which is closely equal to the holdup volume of seal oil system calculated. The pi** at seal oil pump rack was cleaned by wire brush up to accessible areas and blowing the compressed air through the pi**. Then seal oil pumps (Hydrogen side, Airside) were normalized with filter and coolers rack after removing the temporary pi**. The SOT was cleaned thoroughly and inspected. The float valves are fixed inside the SOT. After that it was closed. Then equalization valve, differential pressure valves were reinstalled at appropriate places in the seal oil circuit. The MOP spool piece was removed and MOP was reinstalled. All seal oil pumps were commissioned.

The temperature control valve and orifice after the TCV was reinstalled back after removing the corresponding spool piece. The throttle spindle, cap nut, nut and cover in each bearing lube oil supply line was replaced with oil strainer (37 micron) and blind cover. The flushing device installed at each bearing was removed to allow the lube oil in to it. The AOP was started which supplies oil to all seven bearings. The oil was going through the oil strainers of bearing lube oil supply lines and lubricates the bearings and collected in pedestal. Then it returns to MOT through the return line connected between the return header and pedestal. The seal oil system was also brought into service along with installing the filters on both hydrogen side and air side. The pumps were stopped intermittently to check any chock in oil strainers in lube oil bearing supply lines and seal oil filters. If there is any chock in strainers, they will be cleaned and reinstalled. The change in pressure gauge readings at the upstream and downstream of the throttle valve can be used to find the choking of oil strainer in bearing supply line. If the surface of the strainer in each bearing supply line is clean and if there is no choking of strainer for 24 h of running of AOP, and value of sediments < 100 ppm and moisture < 100 ppm, it will be declared that the stage-3 flushing was over. At the end of stage-3 flushing value of sediments < 10 ppm, moisture < 26 ppm and total acidity number < 0.1 mg KOH/g of oil. Hence it was declared that stage-3 flushing was completed.

7 Analysis of flushing results and optimization

7.1 Optimizing the quantity of oil required for the stage-1 and 2 flushing

During the stage-1 flushing of oil circuits with different combinations as discussed earlier, the holdup volume of oil circuit was varying and the same was reflected in the MOT level which was shown in Fig. 6.

Fig. 6
figure 6

Change in level of MOT vs system flushed

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The supplier randomly has taken the different combinations of oil lines to be flushed. Around 176 numbers ( ̴ 37 m3) of barrels was used to complete the stage-1 flushing of lube oil, jacking oil and seal oil circuits. The stage-2 flushing was completed by adding 22 numbers of barrels. In total, about 198 numbers of barrels (̴ 42 m3) were used to complete the stage-1 and stage -2 flushing. But normal plant operation requires approximately 45 m3. This is given in the Table 2. The rundown level after starting flushing pump (AOP) during the flushing time should be equal to or not less than 21.42 m3 corresponding to the minimum operating oil level of AOP = 1200 mm from top of MOT = (− 200 mm) in MOT scale to avoid dry running of the pump. After completing the stage-1 flushing of lube oil, jacking oil and seal oil circuits, additionally 37.5 numbers (7.9 m3) of oil barrels required to start the stage-2 flushing.

Table 2 Oil requirement during plant normal operation and flushing
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To optimize quantity of flushing oil further, the lube oil systems are systematically to be flushed by using the change in MOT volume data as per Fig. 6 instead of randomly flushing the different combination of oil circuit lines as followed the supplier. The optimum methodology of flushing is given in Fig. 7. At stage-1 flushing, for safe operation of flushing pump (AOP), about 21.42 m3 (1200 mm from the top of MOT = − 200 mm in MOT scale) is filled in to MOT required as per pump manufacturer specifications. The pump suction point is 1920 mm which is well below the minimum negative scale value ( − 750 mm) in MOT scale. Since the difference between minimum operating level and suction level of pump is 720 mm which ensures sufficient cushion time (> 5 min) for the operator action to avoid dry running of flushing pump. From the Fig. 6, it is understood that the maximum fall in level while flushing the lube oil system alone is 395 mm corresponds to 6.37 m3. The oil that will retain in the circuit after first filling is 5.41 m3. In total the stage-1 filling requires 33.2 m3 which is 0.48 m3 less than flushing oil quantity used by supplier in stage-1 flushing and 4.84 m3 less than oil quantity used for normal plant operation.

Fig. 7
figure 7

Optimum oil flushing methodology for stage-1 and stage-2 flushing

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In stage-2 flushing, the seal oil system is to be drained and pumped back to MOT as shown in the Fig. 3. This step reduces the oil requirement about 2.7 m3 while taking the coolers in service for thermal shocking process. For starting stage-2 flushing, only 2.1 m3 (10 barrels) is to be freshly filled in to MOT. Totally 34.73 m3 of oil is required to complete the stage-2 flushing of lube oil circuit and seal oil circuit by this optimized method. This systematic approach of flushing of the TG oil circuits will reduce the flushing oil quantity to 34.73 m3 (165.3 barrels) which is 4.8 m3 less than flushing oil quantity used by supplier in stage-2 flushing and 9.64 m3 less than oil quantity used for normal plant operation.

7.2 Strategies to minimize the time of thermal shocking process in the stage -2 flushing

7.2.1 Enhancing the fraction of oil flowing through cooler

The stage-2 flushing took more than 55% of total flushing time. The time per thermal shock can be reduced by controlling the rate of cooling or heating of the lube oil circulated through the lube oil coolers. The rate of cooling or heating of lube oil is a function of mass flow rate of oil and temperature difference between lube oil at inlet and outlet of lube oil cooler and the quantity of lube oil heated in the system. Since the heating water and cooling water flow rates and quantity of oil in stage-2 flushing are fixed in the thermal shocking process, the rate of cooling or heating of lube oil is a function of mass flow rate of lube oil only. As shown in Fig. 1, the oil is flowing through the cooler and bypass line then combined flow of both goes to the duplex filter [13]. During the normal plant operation, the temperature control valve (TCV) will be in service. During stage-1 and stage-2 flushing, TCV was replaced with T type spool piece.

A small orifice plate may be fixed to divert more flow to the lube oil cooler shell side. The fraction of oil going through the lube oil cooler can be estimated by a simple energy balance across the T type spool piece. It is described here as follows. It is assumed that there is a good mixing of hot and cold oil in T type spool piece and there are no heat losses from the pipe lines to atmosphere, since shorter length between the temperature gauges. Energy balance across the TCV dummy piece gives

$${\text{m}}_{{1}} {\text{C}}_{{{\text{P1}}}} \Delta {\text{T}}_{{1}} + {\text{ m}}_{{2}} {\text{C}}_{{{\text{P2}}}} \Delta {\text{T}}_{{2}} = {\text{ m}}_{{\text{T}}} {\text{C}}_{{\text{p}}} \Delta {\text{T}}$$
(1)

where CP1—Specific heat of oil at T1 °C, CP2—Specific heat of oil at T2 °C, ∆T1—Change in temperature of oil with reference to standard temperature 25 °C, ∆T2—Change in temperature with reference to standard temperature 25 °C, m1—Flow rate of oil to lube oil cooler, m3/hr, m2—Flow rate of oil to cooler bypass line, m3/hr, mT—Total flow rate of oil to the duplex filter, m3/hr, T1 °C—Temperature of oil from Lube oil cooler outlet, T2 °C—Temperature of oil to the cooler bypass line, T °C—Temperature of oil going to the duplex filter, Substituting reference temperature as 25 °C in Eq. (1)

$${\text{m}}_{{1}} {\text{C}}_{{{\text{P1}}}} \left( {{\text{T}}_{{1}} - {25}} \right) \, + {\text{ m}}_{{2}} {\text{C}}_{{{\text{P2}}}} \left( {{\text{T}}_{{2}} - {25}} \right) \, = {\text{ m}}_{{\text{T}}} {\text{C}}_{{\text{P}}} \left( {{\text{T}} - {25}} \right)$$
(2)

Assume CP1 = CP2 = CP (it is reasonable if the temperature values between 45°and 70 °C), then Eq. (3) reduces as

$$\frac{{m_{1} }}{{m_{T} }} = \frac{{T - T_{1} }}{{T_{1} - T_{2} }}$$
(3)

Model case 1 For heating cycle

Substituting T1 = 71 °C; T2 = 63 °C, T = 64 °C, in Eq. (3), the fraction of oil flow through the cooler is

$$\frac{{m_{1} }}{{{ }m_{T} }} = 0.125$$
(4)

From Eq. (4) it is understood that, out of 100% flow of pump discharge only 12.5% of total oil is flowing through the lube oil cooler at the present flushing. The cooling time and heating time of oil can be controlled by fixing suitable orifice/control valve plate across the cooler bypass line. But this flow should not exceed the shell side design velocity of the cooler [1]. Otherwise, erosion of the tubes will take place. It is clear that it will be possible to control the time of heating and cooling of lube oil by varying the fraction of oil flowing through the lube cooler shell side. If the fraction of oil flowing through the cooler is high, the time of heating cycle and cooling cycle of thermal shocking process will be considerably reduced. So, number of thermal shocks per day can be increased and thereby flushing time for stage-2 will be reduced. This will help to complete the flushing of oil circuits quickly.

7.2.2 Avoiding inadvertent oil leaks during flushing

The oil leaks will also delay the flushing process. Time and man power will be wasted in cleaning the leaked oil and normalizing the system for further flushing. Hence it is advised to start EOP first for filling the circuit and venting the air in the pi**. Since the discharge pressure of EOP is lesser than AOP, starting EOP initially will help to identify and attend the leak points at initial stage itself and reduce the quantity of oil leakage if any. AOP should be started after achieving a stable level by EOP. It was observed that the stable level will be established in MOT within 3–5 min after starting EOP or AOP. If the level is not stable even after 5 min, there may be leak in circuit. It is to be addressed as quickly as possible. The MOT oil level should be continuously monitored. Any oil leakage in the circuit, it will be reflected in MOT level. It is highly recommended that the MOT oil level should be monitored in MOT scale for every 10 min while pumps are running.

7.2.3 Precaution to be taken during changeover of equipment

While AOP in service, to carryout changeover of coolers or filters during flushing, it should be ensured that the drain valves of standby equipment should be in closed condition. If the standby filter or cooler drain valve is in open condition, oil leak will occur. This may also delay the oil flushing for cleaning and makeup of oil into MOT.

7.2.4 Avoiding the parallel running pumps during the stage-2 flushing

In stage-2 flushing about 48 h, two AOPs were operated in parallel which caused damage of the filter elements frequently in duplex lube oil filter. This may release the trapped impurities in to the oil. Hence after installing the filter elements in stage-2 flushing, parallel running of AOPs should be avoided to reduce time for maintenance activities on Duplex filter.

7.2.5 Proper planning of the flushing operation and maintenance activities

It was found that total operating hours of pumps required to complete 3 stages of flushing was 644 h. In this study the entire flushing took approximately 4 months from the day one of MOT oil filling. The stage 1 flushing took 262 running hours of pumps (EOP = 12 h + AOP = 250 h). The flushing was done for 8 h per day. After completion of stage-1 flushing, there was a time delay happened due to delayed erection of thermal shocking system. Frequent power supply interruption, failure of heaters during stage-2 flushing also caused the delay. Stage 2 flushing took 358 running hours of pumps (EOP = 5 h + AOP = 353 h) for its completion. About 80 thermal shocks were given to achieve the oil sediments value < 10 ppm. Stage 3 required only 24 h to declare the completion of flushing. From the data of running hours of pumps, it is understood that actual operating time of pumps is within 30 days (644 h/24 = 26.8 days) and remaining period (approximately 90 days) is required for planning, manpower management and maintenance activities for carrying out flushing. With proper planning and manpower management, the maintenance time for stage -1 flushing can be reduced and AOP can be run for more than 8 h. The thermal shocking system should be erected before completing the stage-1 flushing. This will further reduce flushing time of stage-2 considerably.

8 Conclusion

It was observed that several factors like power supply interruption, delay in temporary equipment erection, equipment failure, oil leaks, and cooling water make up time and removal of temporary pi** system for normalization of oil circuits, draining the oil circuits and various site activities on turbine systems will influence the time of flushing. Based on the flushing experience of a 500 MWe Turbo Generator, the proposed methodology may reasonably reduce the quantity of oil and the time required for flushing which are directly having impact on cost factor of flushing process of TG oil systems. A slight design modification in lube oil coolers bypass line may decrease the time of stage-2 flushing considerably. It is concluded that with proper planning, it will be possible to complete the flushing of standard 500 MWe TG oil systems within 6 to 7 weeks or earlier instead of 17 weeks.