A CASE STUDY OF TYPICAL PROBLEM OF BARRING

A CASE STUDY OF TYPICAL PROBLEM OF BARRING GEAR STALLING AND POST OVERHAULING VIBRATION IN GENERATOR IN 500 MW BHEL MAKE UNIT OF KODERMA TPS, DVC. � KAJAL BANERJEE: DY. CHIEF ENGINEER (MECHANICAL), OS, DVC- KOLKATA � MADHUKAR SRIVASTAVA: SUPERINTENDING ENGINEER (MECHNICAL), KODERMA TPS, DVC

ABSTRACT: �In Koderma TPS, DVC, 2 X 500 MW units were installed as Green Field Project. Commercial Operation of Unit-1 & Unit-2 KTPS was declared in the year 2013 & 2014 respectively. The Turbines of KWU design are manufactured, supplied and installed by BHEL. In unit-2, since its initial commissioning stage, stalling of barring gear used to take place after every tripping of the unit, when the shutdown period remained for more than 15 -20 hrs and the unit could only be brought back on bar at least after 3 days from time of tripping.

Though the said long lasting problem could be attended during turbine overhauling but the machine could not be synchronised immediately after overhauling due to new additional problem of high bearing and shaft vibration at Brg-6, i. e generator rear bearing. Before overhauling, there was no problem of such high vibration. The topic mainly covers the root cause analysis of barring gear stalling as well as high bearing vibration problem and ratification measures taken for the same.

INTRODUCTION � Since the inception of the unit, the machine suffered from stalling of Barring Gear after every tripping whenever the shutdown period exceeded for more than 15 -16 hrs. Normally, after every tripping machine used to come on barring gear but whenever HP Turbine shaft temperature dropped to around 480 OC to 475 OC, the stalling used to take place and barring gear used to be restored of its own at HP shaft temperature of around 310 OC. By adjusting rotor lift, it could not be attended though the gland segment clearances of LPT front & rear, HPT rear and to some extent of IPT rear were found adequate.

OBSERVATION �To attend the problem, it was decided to open all the turbine modules. After opening, rubbing marks observed in 11 L blade shroud of IP Turbine. IPT roll check reading found considerably disturbed at top/bottom (top-0. 8 mm & bottom-0. 2 mm) and left/right at diagonally opposite in front and rear. HPT rear side see-through gland sealing strips found in rubbed condition.

At the time of assembly, after co-axial fitment of IPT outer and inner casing, it was found that the radial clearances between left and right sides of IPT rotor and inner casing was significantly unequal. In one side, the gap was only 0. 2 mm and in other side it was around 0. 95 mm to 1. 0 mm. The roll check value of IPT also reflected the said clearances. The gaps were set right by scrapping the sealing strips of the inner casing top and bottom and roll check was carried out and desired value was obtained.

Other overhauling activities were completed along with slow speed balancing of HPT & IPT. It is understood that due to such deviation of clearances between IPT moving and stationery parts, rubbing on blade shroud and rubbing of HPT sealing strips jointly created the stalling problem. Whenever turbine temperature dropped to around 470 OC, the clearances used to become almost zero resulting stalling of barring gear.

HIGH VIBRATION AT GENERATOR REAR BEARING There was no history of high TG bearing and shaft vibration before overhauling of the unit. But after overhauling, when the machine was rolled to 3000 RPM, vibration of bearing -6 pedestal ( generator rear brg. ) and shaft reached as high as 130 micron & 180 microns ( Pk-Pk) respectively within 15 minutes after rolling at 3000 RPM and was creeping in nature. It was decided to inspect bearing-6 side pad clearances, recheck the alignment of LPT- Generator, Generator. Exciter, and Swing check of Exciter as a rectification measure for high vibration of Brg-6. After the said rectification, again during rolling at 3000 rpm, similar nature of high vibration was observed in bearing no-6. Balancing of exciter rotor did not yield any better result.

During this time, while rolling at 3000 RPM, the vibration of RHS secondary grouting of rear side Generator End Shield stool base (Fig-1) was found much higher than that of its LHS. The LHS secondary grouting vibration was similar to that of bearing no-5 area. With the above observation unit was boxed up. The vibration readings are shown below.

Fig-1 Vibration reading of Generator rear end shield (RHS & LHS) stool and secondary grouting

Vibration Readings

ACTION TAKEN Considering the high vibration of RHS secondary grouting of rear side Generator End Shield stool base, initially it was decided to go for pressure grouting of the said secondary grout. Moreover, the visible surface crack on the secondary grouting was chipped out but no propagation inside could be detected (Fig-2) Visible surface crack on secondary grouting Fig-2

During inspection of RHS secondary grouting of rear side Generator End Shield stool base, it was observed that 0. 1 mm feeler gauge is going through some area of the base plate shims, though proper loading was confirmed during assembly. So, the stretching of Anchor Bolts was checked and it was observed that only 0. 45 mm stretch is there in RHS Generator rear End Shield stool base and 0. 95 mm in LHS. Re stretching of RHS bolt up to 1. 70 mm was done, but after some time it came back to 0. 9 mm i. e stretching was not holding at a particular position (Fig-3) Stretching of bolt is not holding Fig-3

INFERENCE Though before overhauling, the vibration of TG bearings was well within limit but after overhauling vibration level of bearing -6 (generator rear) became high. The old bearing -6 was replaced during the overhauling due to several scratch marks on Babbitt surface and assembly of new bearing was also done maintaining all critical parameters of the bearing. It was understood that the minimum residual stability and rigidity of the secondary grout and Generator End Shield stool base got disturbed during removal of end shield load from it.

Though during assembly, proper loading on stool was confirmed. During rolling to 3000 RPM, loss of rigidity of that area took place and gap generated in between the sole plate and SS shims below it and as a cascading effect, anchoring bolt stretch value also reduced considerably. As such, RHS generator rear end shield along with its stool remained in practically hanging condition and as a result high vibration of creeping nature occurred in bearing -6 and bearing shell vibration level was continuously increasing at 3000 RPM from 65 microns to 130 microns before tripping the machine.

RECTIFICATION MEASURES At the time of stretching the bolt, downward movement of stool base plate up to 0. 12 mm also observed. With the said observation, it was decided to go for re-grouting of the secondary grout of RHS of rear end Generator End Shield stool base instead of pressure grouting. Load of Generator (Exciter End) end shield was initially taken on hydraulic jacks and later with appropriate support it was held. The secondary grout was removed totally (Fig-4). Several small pieces of packers were found inside the grout which were used for levelling of the 95 mm thick sole plate at the time of erection (Fig-5).

Secondary grouting removed Fig-4

Several small pieces of small and thin packers found inside the secondary grout which were used for levelling of sole plate during erection Fig-5

All the old small packers were replaced by new packers (two pieces at each side) - Fig-6 and levelling of the sole plate was done. After that anchoring bolt was placed and tightened without stretching. Finally secondary grouting with Conbextra GP-2 manufactured by Fosroc Chemicals (India) Private Limited was done. (Fig-7) Two pieces of packers at each side used for levelling of sole plate Fig-6

Fig-7 Progressive compressive strength of secondary grout using Conbextra GP-2

The desired value of compressive strength of the secondary grout was achieved on 8 th day, i. e more than 55 N/mm 2 (Fig -8). After achieving the strength of secondary grout, anchor bolt stretching was done and steady stretch value of 1. 50 mm could be achieved with no movement of sole plate. LHS, bolt stretching value was also increased to 1. 50 mm as per design

Conbextra GP-2 cube of secondary grout Figure shows a crushing force of 300 KN on the cube of size 70. 6 mm. X 70. 6 mm X 70. 4 mm i. e Compressive strength of secondary grout = 60. 19 N/mm 2 on 8 th day of curing Fig-8

FINAL BEHAVIOUR OF MACHINE Alignment of LPT-Generator, Generator-Exciter, Swing check of exciter etc were done again and finally machine was rolled successfully at 3000 RPM with vibration value of bearing and shaft well within alarm limit. Machine was loaded to 500 MW and all parameters were found normal. Moreover, the long persisting problem of barring gear stalling could also be resolved.

Mr Kajal Banerjee has graduated in Mechanical Engineering with Honours from NIT, Durgapur (formerly known as Regional Engineering College, Durgapur) in the year 1988. He joined DVC in 1988 as Graduate Engineer Trainee. He has worked for several years in Mechanical Operation, Commissioning & Turbine Maintenance section in 140 MW, 210 MW and 500 MW units of DVC. Now he has been posted at Corporate OS in DVC-HQ at Kolkata and looking after the Turbine related issues of all the plants of DVC. Email- kajal. banerjee@dvc. gov. in Phone- 9434745942 / 9647829487 Mr Madhukar Srivastava has graduated in Mechanical Engineering with Honours from MIT, Muzaffarpur in the year 1997. He joined DVC in 1999 as Graduate Engineer Trainee. He has worked for several years in Mechanical Operation, Commissioning & Turbine Maintenance section in 210 MW and 500 MW units of DVC. Now he has been posted at Koderma Thermal Power Plant, Koderma and looking after the Turbine Maintenance. Email- madhukar. srivastava@dvc. gov. in Phone- 7004265667

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