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PROBLEM FACED DURING COMMISSIONING OF LPBPSYSTEM IN UNIT 2

PROJECT : SIMHADRI 2 X 500 MW ,

PROBLEM : At Simhadri Unit-II, 500 MW LPBP system characteristicsadjustment was carried out during last week of July 2002.While adjusting LPBP stop valve 1, open/close characteristics with respect to LPBP signal fluid pressures were notable to get consistent values. Few occasions the stop valve did notopen even at 5.0 kg/Sq.cm signal oil pressure instead of normal

opening pressure of 2.88 Kg/Sq.cm. Some time the stop valveopened full while closing the signal oil pressure isolating valvecompletely.(Ref. Fig. in page 11)

OBSERVATION: The pilot valve was removed and serviced and put back. Still behaviourof the valve did not change. After that the liner (Item No.25) wasremoved. It was found that three number 'O' rings (Item No.40)

located between supply and drain ports were not available.Moreover the liner secured pin (Item No.27) was also found inshorter length (19 mm against the required length of 25 mm). Hencethe liner was not secured fully in position and was moving in up and down along with the pilot valve. Normally the liner should not move.Because of these problems, the servomotor characteristics behavederratically.

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13

RECTIFICATION:

The main servomotor is opened by control fluid against the springforce. A pilot valve item, No.24, will admit the control fluid toopen or close side of servomotor. This pilot valve will be normally indownward position because of the spring compression (ItemNo.6). When the signal fluid pressure raises at the bottom of thepilot valve then the pilot valve moves upwards against the springforce and admits control fluid into the open side of theservomotor and servomotor opens full. When the signal fluidpressure reduces, then the pilot valve will move down from upsideposition because of self weight and spring force then the controlfluid will be admitted into the close chamber of servomotor and atthe same time open side oil will get drained into the drain header.

New "O rings, three numbers and correct length cylindrical pin

Item No.27 (12 x 25mm) was machined at workshop and liner wasassembled. As per design characteristics when the LPBP signal fluidpressure of stop / control valve reaches around 2.88 Kg/Sq.cm, stopvalve will full open and when the signal fluid pressure drops below2.80 kg/sq.cm, the stop valve will close. If the isolating valve of thesignal fluid is closed then the stop valve will get closed fully. Then valvecharacteristics were checked repeatedly and consistent value of open /close found and thus the: problem was resolved.

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BY DETERMINING THE CORRESPONDINGDIAMETERS OF HEM NO. 24)

3 HOLES FOR GRUB SCREW{ITEM 35) TO BDRilJLEO AND TAPPEO OUKING ASSLY.

,6 UAxf cig.ca/ / / /\

CLEARENCt:. OH(1) &. (} SHA1L ALWAY> 3c LARGtK lH*r< Trie CLEARENCE ON (ft) k @ (IWiS MAY OE ACHIEVED

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