Problem Faced in Turbine Governing System During Commissioning of TDBFP

PROJECT : SIPAT UNIT - 4 PROBLEM : PROBLEMS FACED INTURBINE GOVERNING SYSTEM,DURING COMMISSIONING OF TDBFP- 4A HP CONTROL VALVE: 1. HP control valve could not be adjusted for its start of opening at 1.5 ksc of secondary oil pressure. 2. The pilot valve stops spinning, leading to erratic behavior of the valve. The pilot valve was opened and serviced. The holes for jet oil were thoroughly cleaned. The flow path (drilled passages in the body from control oil to jet oil inlet to the pilot) for the jet oil was inspected. It was found that one of the plugs provided in the drilled hole ends was found to be obstructing the flow of oil. The plug length was reduced by cutting and proper oil flow was ensured. The jet oil was adjusted with with the adjusting screw and free rotation of the pilot was observed. After rectifying the problem of pilot rotation, the start of opening of the valve was adjusted and it was found that there was no response of the valve for the secondary oil pressure of less than 1.7 ksc. The spring tension could not be adjusted further, as it was restricted by the top cover. The top cover was machined by 3 mm as shown in the figure, to allow for further adjustment. Subsequently the start of opening adjustment adjustment was made at 1.5 ksc of secondary oil pressure. TRIPPING DEVICE:

The tripping device was getting automatically reset without admitting holding oil. The tripping device is normally reset by the holding oil generated either from over speed trip device tester or by the remote engagement solenoid. Once the tripping device is reset, the trip oil will be generated and subsequently the tripping device is held in reset position by the trip oil. The holding oil supply to tripping device should be closed and its pressure is killed by draining through the hole provided in the tripping device. As the tripping device was found tight for tripping and it gets automatically reset. As it was suspected that the holding oil is not getting isolated, one dummy was provided in the holding oil line to tripping device. The tripping device was found not getting automatically reset and tripping behavior was normal with minimum force. The tripping device was opened and it was observed that the hole for draining of holding oil was not found. The same was checked in the tripping device of TDBFP - 4B and draining hole was available there. One 2 mm hole was drilled at site similar to TDBFP - 4B tripping device. Dummy provided in the holding oil line was removed and the behavior of tripping device was found to be normal. OVER SPEED TRIP GOVERNOR: During solo run of TDBFP drive turbine, oil injection test was carried out at 5400 RPM. The over speed pin acted at 5.5 ksc of test oil pressure. Actual over speed test was carried out and it was found that the trip pin is not acting even at 6100 RPM. Turbine was stopped and the over speed governor assembly was dismantled for servicing. Lot of machining chips were found inside the hollow shaft, which were removed and cleaned. Also, the eccentric weight (adjustable pin, item number 6 in the enclosed drawing) was not found at the end as recommended in

the drawing. The assembly was done keeping the pin at end. Again during trial run, the trip device acted at 5400 RPM. Over Speed Trip Device was dismantled. The original pin (M12X1.5) was having length of 47mm. Spare pin of 37.5 mm length brought from BHEL,Hyderabad was assembled and over speed trial was taken. Further the same pin was cut and the over speed trip was adjusted. For achieving the trip value of 6030 RPM, three trials were taken. Trials 1 2 3

pin length 37.5mm 32mm 26mm

trip rpm 5682 5350 6030

RESIDUAL PRESSURE IN TEST OIL LINE: During oil injection test, the test oil is admitted through limiting device and through regulating valve. It was observed during operation of turbine at rated speed that the test oil line is getting pressurized to 1 ksc even after keeping the test oil supply valve in close position. This may lead to early operation of tripping device. The residual test oil pressure is suspected to be developed from lube oil inside front bearing pedestal. This pressure is killed by keeping the test oil regulating valve full open. The oil returns to the drain through this regulating valve and test oil pressure remains zero. Hence BHEL site advised the customer to close the oil injection ball valve during turbine running and to keep the oil injection-regulating valve fully open to drain the test oil line.