High axial shift in 110 MW turbines,01-02.pdf

7/28/2019 High axial shift in 110 MW turbines,01-02.pdf

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FEED BACK No. 2

PROJECT: ENNORE UNTT-4 - 110 MW (RAM)

SUBJECT: HIGH AXIAL SHIFT IN 110 MW TURBINE

OBSERVATION

Unit No.4 which was running at a load of 40 MW with a high curt is stagepressure o f 62 KS C t r ipped a t 16 .01 Hrs . on 09 .04 .2001 on h igh ax ia l sh i f t

(As ind ica ted in the meter on tha t day , the ro tor showed i ts movement

towards generator) . Simultaneously al l other differential movements showed

a rotor movement of 0.40 mm towards generator. These observations were

made from the trend curve as taken at the t ime of tr ipping. Also from the

trend curves (refer Curve No.1) taken on 08.04.2001 it was found that the

axial shift which had gone to +0.5 mm (movements of rotor towards HP) was

found to have slowly gone to -0.3 for which no reason could be immediately

found out. The trends of TSI on 08.04.2001 showed that whenever a

movement of rotor was there (may be due to load changes) as shown by the

axial shift, the same was reflected on the DEs of HP, IP and LP. (Refer

Trend Curves 2 & 3).

ACTION SUGGESTED

From the above observation, it was concluded that since there is movement

of rotor by 0.4 mm as indicated by turbovisory instruments, the bearing

No.2 may be opened and inspected. Simultaneously the axial shift pick up

also may be calibrated. Before the trip, the HPDE instrument was showinghigh differential expansion readings. Hence it was suggested that the HPDE

also may be checked for calibration availing the shutdown.


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04ACTION TAKEN

Accordingly the bearing No. 2 was opened on 14.04.2001 after stopping barring gear. No abnormality

was found on the bearing except for some rub marks on the thrust pads of the non-working side. Thesemarks were removed by scraping and the bearing No.2 was assembled back. M/c was put back onbaring gear and found OK. The axial shift pick up was taken out and was calibrated. The pick up wasfound to be OK except for the cable joint fault. This lug joint was replaced with soldered joint. Metallichose was provided fully covering the pick up cable. The zero of the axial shift reading was adjusted at thestandstill position of rotor after stopping barring gear motor on 17.04.2001. All the differential expansionpick ups were recalibrated and adjusted for zero with rotor in standstill condition after the metaltemperatures of m/c come down below 50

0C.

ANALYSIS AND CONCLUSION OF THE OCCURRENCE

- As per report by BHEL/SAS, the rotor had moved towards HP side when themachine was on load and was normally working on the non-working pad side.This movement was due to high curtis stage pressure. The same wasindicated by TSI also during normal working of the Unit which was indicatedas 0.5 mm as against a designed value of 0.1/0.2 mm.

- While calibrating and inspecting the axial shift pick up, the following observationsare made:

a) The cable joint which was provided by providing straight lugs had gotoxidized.

b) The PVC insulation tape and the Teflon tape wound on the joint had gotburnt.

The reasons for the above cable failure is attributable to the high temperature steamimpingement on 20.02.2001 when the flange in the W pipe of MP inlet failed. The

steam impingement caused a reduction in life of the joint. This was acting on the joint asslow poison and weakened the joint gradually.


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05This weak cable joint causedvoltage drop and an error in the axial shift

reading on 07.04.2001 and the reading slowly drifted from +0.5 to -0.3. The

machine was working with this drift till 09.04.2001 when another drift of 0.3

mm occurredsuddenly and caused the tripping of the Unit. (Refer CurveNo.2) at the set value of - 0.6 mm.

In fact, the rotor was actually at 0.4 mm position due to high curtis stage

pressure. But returned to its neutral position on tripping. Since only an

axial shift indication is used for protection also system reliability problem

experienced.

TEETHING PROBLEM :

The turbovisory panel, supplied by M/s Global Weighing Ltd (earlierknown as

Philips) through EDN, had reachedsite without the completion of internal

wiring of the cards and instruments. These works were carried out at site

with short duration of time since the supplier engineer who reached site also

was not aware of the panel condition. The unit had teething problem in the

TSI system and Unit had tripped on axial shift/HP DE due to malfunctioning

cable joints and cards. The defective cable connectors were replaced with

straight lug joints at site in consultation with supplier. Hence site suggested

two out of three logics protection system during November, 2000.

RECOMMENDATION

1. As per the modem control concept, all the trips are to be provided

through 2/3 logic. The same is suggested for axial shift also. This

logic is provided for axial shift at Jindal (2x130 MW sets) by

BHEL/Hyd.

2. If at all any cable joint is to be done in this measuring circuit, it

should be a soldered joint only to avoid leakage of joint.


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High axial shift in 110 MW turbines,01-02.pdf