Ip Control Valve 2 Was Closing When Machine Running on Load - Feb 2008

FEBRUARY

2008 

FEEDBACK:1 PROJECT: RTPP, Muddanur, Stage II, 2 x 210 MW PROBLEM: IP Control valve 2 was closing when machine running on load in unit 4.

Problem 1:RTPP unit - 4 was commissioned on 20 th November 2007 and the unit was restarted on 21st of December 2007 for continuous operation. The machine was operating at the load of 40 - 80 MW. The behaviour of IPCV-2 was erratic. The valve opens normally during startup and suddenly closes while machine on load from around 60% position to 0%. On slight tapping over the servomotor body, the valve again opens to normal position. To resolve the problem, the pilot valve was serviced and freeness of the pilot was ensured. The holes for jet oil were found healthy and pilot rotation was ensured during calibration of the valve. There is no foreign material etc observed in the hydraulic circuit. In cold condition the behaviour of this valve is normal. On 5-1-2007 the pilot valve was once again opened and serviced. The damping device was also inspected and serviced. During servicing it was noticed that a lot of oil vapour and air rushing out of the servomotor drain. To reduce the same the vapour extraction circuit was optimized and still the vapours are more compared to IPCV 1.Once the pilot chamber of IPCV2 was boxed up, the top cover of damping device in the Secondary oil circuit was

opened. Air was found rushing out from the downstream piping continuously from the servomotor end. Similar problem of IPCV closing suddenly on load and opening on tapping was observed in North Chennai due to air lock in the secondary oil circuit. The same was rectified in North Chennai project by proper rerouting of secondary oil piping. The piping layout at Muddanur was in order and the layout of Oil Piping in unit - 3 and in unit – 4 are identical and are as per layout drawings. No such behaviour has been reported in unit - 3. To check free rotation of the pilot during operation a transparent sheet was fixed in the front cover. It was observed that the pilot is getting stuck in close direction sometimes during hot condition. The problem was referred to BHEL,Hardwar and the clearances between pilot and bush was measured and the following were the observations: 1. The diameter of pilot at different locations and hole dimensions on the bush are given in the enclosed drawing. It was observed that the bottom portion of pilot was having less clearance 0.04 mm against the requirement of 0.1 to 0.149 mm. the clearances in other portion are within range. 2. Also, the pilot dimensions at the bottom shows a step of around 0.04 mm. 3. No other high spots are noticed. To increase the clearance as per requirement given in the drawing, buffing of the Bush in the workshop and polishing of the Pilot by fine emery paper were carried out. The machine was restarted and the behavior of the control valve was normal and the problem of sudden closure of the

valve was resolved. The pilot was stuck sometimes during hot condition due to lesser clearance at the bottom portion as indicated in the drawing.

Figure 1 IPCV Feedback pilot

Figure 1 IPCV Feedback pilot

Problem 2: Misbehavior of EHTC (Electro-hydraulic Turbine controller) during machine on load, leading to sudden load variation and changing to speed controller mode.

RTPP unit 4 was commissioned during November 2007. The unit was restarted on 21st December and was on EHTC mode of operation all the time. During 9th and 10th February 2008, it was experienced that the unit load was suddenly dropping to less than 10 MW from around 120 MW. The load controller changes to speed controller and fluctuation of load were observed. The unit was stabilized with manual intervention. As this phenomenon was observed atleast three times in one shift, the unit was changed over to hydraulic mode of operation. The behaviour on hydraulic controller was found normal. Analysis: During machine running on hydraulic governing mode with EHTC isolated in governing rack, the EHTC position set point was kept at around 50% and the feed back of EHTC convertor piston (Collins feedback output)was observed from trends, for any variation. The balance voltage on EHTC coil was found to be -3.6 volts against the setting of -1.5 volts during commissioning and it was put on trend temporarily for observation. Sudden change in the EHTC convertor piston feed back was noticed from trend, from 50% to nearly 0% and returning to normal in few seconds. At the same time the balance voltage on coil also varied. The position set point was same without change. This confirmed that the EHTC pilot is actually moving down without command to the EHTC coil. It was decided to service the pilot and following works were carried out.

1. The EHTC pilot was removed and cleaned, but no sticking was found. The pilot rotates freely while charging control oil for rotation. After removing pilot,

the control oil was charged and the sleeve was flushed. 2. While removing the coil, it was found that the control sleeve locknut was not there. The missing lock nut is indicated in the enclosed figure of EHTC. The sleeve was rotating by hand on the coil rod. One nut was arranged locally and the sleeve was locked. The change in balance voltage during operation was suspected to be due to the loose sleeve and missing lock nut. 3. The trip oil line to EHTC was removed and air blown. The orifice in this line was inspected and found healthy. 4. The trip oil pressure regulator to EHTC was serviced. Contaminants found in this regulator was removed and cleaned. The trip oil inlet pressure was set to 5.0 ksc. 5. The above activities were carried out online while unit was running on hydraulic mode. The unit tripped on other reason. During this opportunity, the EHTC was calibrated after setting the coil balance voltage to – 1.5 V. The unit was restarted on EHTC mode and no problem was experienced. The problem of EHTC was resolved.

Figure 2 EHTC