TA-2015 GE Frame-V Field Instrumentation Overhaul

TG701 Field Instruments Overhaul

TA - 2015

Field Maintenance Team: Usama Tariq Khan/Haris Muhammad/Muhammad Asad Ullah

Prepared by: Muhammad Asad Ullah

Reviewed by: Raja Naveed Lehrasab


Page 1

Introduction

Gas turbine overhaul is always a complex and exciting task. Plant start-up after TA is always linked to the

availability of gas turbine after its overhaul, which makes the job critical and challenging.

The job in TA-2015 was special as a number of critical jobs were being carried out in parallel. The control

system was being replaced with Mark-VIe, turbine rotor was being replaced and the generator rotor had

to be removed for inspection. These activities necessitated better planning, improved coordination with

other teams and perfect execution of the job.

An elaborate plan was prepared before TA in view of the importance of field devices’ proper

overhauling/calibration to ensure that all the above mentioned modifications/replacements achieve

desired results in terms of machine performance and avoid unnecessary delays.

Planning

A thorough planning process was executed to create a detailed job plan. Various discussion sessions for

information and experience sharing were held with engineers and staff. Field visits were also carried out.

Following major activities were concluded:

Creation of work procedures for overhauling of each category of instruments

Creation of checklists to record all the parameters of removal, calibration and re-installation.

Fabrication of new thermowells for proper calibration of Temperature switches.

Arrangement of special tools for working on gas turbine.

Management of TG inventory to ensure the availability of critical spares

Arrangement of consumable spares.

Arrangement of proper storage location for instruments handling during TA.

Racks placed inside a specially arranged container for storage of removed instruments


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TA Activities

During the Turnaround, the sequence of working was such that initially the instruments were removed

according to the machine dismantling process and placed in workshop where they were checked and

calibrated. Later, they were re-installed along with turbine reassembly. Some of the instruments were

checked on-site.

A total of 171 instruments were checked amongst which some were removed for the first time. A summary

of the instruments is given below.

Sr. No. Instrument Type Checked / Calibrated

1 Pressure Switch 16

2 Temperature Switch 16

3 Level Switch 02

4 Limit Switch 15

5 Transmitters 05

5 SOV 19

7 Thermocouples 42

8 Analog IO/Misc. 06

9 Seismic probes & vibration switches 07

10 Bentley Nevada Probes 08

11 Magnetic Pickups 03

12 Control Valves 07

13 Gauges 25

Besides the normal jobs, there were some activities which required special preparation. A brief summary

of these activities is give below.

1) Modification at Bearing No. 1

The radial and thrust vibration measurement of turbine rotor is done by 2 VTs and 1 XT installed at the

bearing No. 1. Replacement of turbine rotor was planned in this Turnaround. However, the new rotor

design had a keyphasor notch at the position where the VTs were installed originally. Due to this reason,

GE changed the location and type of VTs and shifted the XT to the active side of the thrust bearing. The

installation mechanism for all three was changed as well.

This arrangement required special VTs which were not available with us. To cope with this situation,

arrangements were made with the help of Machinery department to ensure correct installation, probe

safety and oil sealing.


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After dismantling, the bearing cover was brought to the machine shop and a mandrel was placed inside it

to simulate the turbine rotor. The new probes’ adapter assembly was installed according to this

arrangement.

In the field, new JBs were installed and the extension cables were routed through tubing up to the main

JB.

Probe Adjustment Job in Workshop

2) New Transmitters Installation

With the up-grade of control system from Mark-II to Mark-VIe, the existing 4-wire transmitters were

replaced by 2-wire transmitters. Three new transmitters were added to the system as well.

- 20CD: Compressor Discharge pressure transmitter (Replaced)

- 20FG: Gas Fuel Pressure Transmitter (Replaced)

- 96HFD-1: Hydraulic Oil Filter DP transmitter (New Transmitter)

- 96HQ: Hydraulic Supply Pressure (New Transmitter)

- 96QA: Lube Oil header pressure transmitter (New Transmitter)

3) 20CB installation:

There are two anti-surge valves installed on the gas turbine which bleed compressor air up to 95% speed

of the turbine. 20CB is the SOV which supplies air to these valves.

The SOV’s ports are located in the turbine compartment while its solenoid assembly is installed through a

metal plate in the wall adjoining accessory compartment. The SOV is installed in one corner under the

turbine and its access is blocked by 20AA, 20PL, VA-17 and some other piping and instrument power

connections tubing.


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The SOV’s working was tested automatically during TG shutdown, when it opened the bleed valves below

95% speed. During the Turnaround, the coil health of the SOV and other field checks were performed and

were found ok. During the test startup checking, however, the SOV developed fault and started passing

which would result in bleed valves closure before the desired speed.

It was decided to replace the SOV with a new one. Keeping in view the intricate location, time constraint

and ease of future maintenance, the new SOV was installed outside the turbine compartment.

New 20CB

4) Wheelspace & Exhaust TCs:

There are eight wheelspace thermocouples in the gas turbine. Most of them were found stuck and were

removed with difficulty. Three wheelspace thermocouples were replaced as they were found damaged.

Tagging of the wheelspace TCs was corrected as well.


Page 5

Exhaust thermocouples were also removed and checked in the workshop. Alternate TCs were replaced

with the new ones preventively.

Wheelspace TCs Exhaust TCs

5) Boiler and Bypass damper limit switches:

New cables were laid for both boiler and bypass damper limit switches. Support for the Boiler damper

limit switches was reinforced to avoid vibration resulting in nuisance tripping. Contact area of bypass

damper lever was increased with the help of Equipment-UTY to ensure engagement with limit switch.

6) GCV LVDT replacement:

One LVDT of GCV was found malfunctioning during turbine start-up. It was replaced with a new spare.

7) GCV & SRV servos replacement:

Servos of GCV & SRV were replaced preventively.


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8) 06-LLS-102 level switch replacement:

06-LLS-102 is a side mounted float type level switch at steam drum of B-601. The switch was found in rusty

condition, so it was replaced with a new one.

06-LLS-102

Future Actions:

In light of our experience in this Turnaround, following are the recommended actions for improvement.

1) Spares present in the warehouse since commissioning should be checked for physical health.

2) Test setups should be developed for checking SOVs in field and/or workshop.

3) Externally adjustable GE vibration probes (identical to the ones installed in FFBL) should be procured

for installation at bearing # 1, as per original GE design for the new rotor.

TA-2015 GE Frame-V Field Instrumentation Overhaul

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