TIGER™ CASE STUDIES

Tiger™ Case Studies

There are over 750 incidents that Tiger has helped to identify and resolve. Many of these problems were previously unknown or did not readily lend themselves to rapid diagnosis. In most cases the problems were unknown to the operators and each represented a potential incident, resulting in unscheduled down time and loss of revenue.

It has been noted that on sites where Tiger is fitted with its remote access capability, the engineering level of the operational staff can be lowered as the knowledge based system is continually offering solutions to identified problems. This, plus the facility for skilled staff to connect to the system through a modem and give instant advice to site engineers, reduces down time significantly.

Tiger has assisted with over 750 incidents. Below is a summary of 10 incidents, detailing the situation, how Tiger was used to assist with the situation and the benefits that resulted. Together they represent the wide range of Tiger's capability. Not only do these examples describe real incidents, but they also give an overview of the wide range of areas in which Tiger can provide assistance.

Faulty Controller Card

The customer phoned the Tiger team and asked for assistance with a problem on the gas turbine. Intermittently large numbers of alarm messages were being generated on the distributed control system. The Tiger team connected remotely into Tiger at site, and confirmed through the Tiger diagnostics that there was an intermittent fault generating a wide range of diagnostic problems.

By using the Tiger to get an overview, they could see the frequency and duration of the intermittent fault, and verify a number of times when the fault was occurring. The only signals to be affected were the in the contact input DTBA for the <C> processor, hence the Tiger team was quickly able to diagnose that the fault was a bad TCDA card causing a large number of alarm fault messages from the Mark V controller at intermittent intervals.

Without the facilities available in Tiger, the customer would have to go and investigate the alarms generated by the Mark V controller and conduct an investigation. Given the fault was intermittent, the customer would have to wait until the fault occurred, or sift through the alarms printed out by the Mark V controller.

In this situation, the gas turbine continued to operate, so no downtime was incurred, however, through the use of Tiger, the Tiger team was able to reduce the diagnosis time by 3-4 hours, and facilitate sending a field engineer to site with the correct replacement card to rectify the fault.

Sticking Bypass Valve

On this oil platform, the gas turbine is always started on distillate fuel oil. The engineer on the platform had noticed that the gas turbine was unstable during start-up. This problem had been continuing for sometime. The engineer notified the Tiger team after another unstable start. Tiger remotely connected to the oil platform and checked the diagnostic information collected during the start-up. It was quickly determined that the liquid fuel oscillates between 0 and 5 MWatt power levels.

Using the information available in Tiger and the remote support provided by the Tiger team was able to remotely diagnose that the problem was as a sticking bypass valve. This problem would have been very difficult to diagnose on the platform in their environment without Tiger, particularly for the site engineer, who would have had to change a number of components and then attempt another restart to see if the problem disappeared. These components are not always available on the platform. If the turbine failed to start due to these oscillations, which was a very high risk, then the platform would have lost this power source. By being able to diagnose and correct the problem, the use of Tiger's was able to avoid a probable future trip and shutdown, which would cause at least 5 hours downtime on the platform.

The remote diagnosis by the Tiger team was accomplished very quickly. However, for an engineer to change out components and attempt a local fix without the Tiger information would have required 3-4 hours.

Faulty Gas Compressor

An oscillation of 1½ bar was detected in the P2 pressure of the gas fuel system that was causing 5% swings in the gas valve position. Fast load swings resulting from these fuel fluctuations can cause long-term maintenance problems. Through the use of the information available in Tiger, it was determined that the gas valves were operating correctly  (i.e. controlling the gas fuel flow for the oscillating P2 pressure). The fault was in the associated gas compressor.

Determining When to Wash

Initial checks on the gas turbines performance indicated that there was approximately 1 MWatt drop in the base load output, when ISO corrected. This information was used as part of reviewing the washing regime of the gas turbine and determining when to optimise the wash intervals.

The information supplied via Tiger verified that it was appropriate to conduct an offline wash at that time.

The loss of 1 MWatt power output would be continuous and also depreciating if the unit continued to run. The use of the information also allowed a planned offline wash saving 15 hours for an unplanned shutdown of the unit.

Faulty Speed Ratio Valve

The Tiger diagnostics indicated that the speed ratio valve was exceeding 42% open for the running conditions and load. In addition, its associated servo was at negative saturation. The information available in Tiger showed that at full speed, no load, the servo was already at -18%, which is outwith the specification of the servo. It was also shown that on a 40 MWatt load, the valve would open to 100%, and the inlet pressure was not up to specification. Using Tiger it was established that the speed ratio valve's servo needed to be checked. By detecting the problem early it saved a potential trip of the turbine and allowed them to plan maintenance, potentially saving a 6 hour unplanned shutdown.

Faulty Compressor Bleed Valve

Tiger detected that a fired shutdown changed to a turbine trip when a compressor bleed valve did not open in the required time. Such trips during shutdown affect the maintenance lifetime of the machine.

The customer was advised to investigate the bleed valves and switch. The resolution to the problem was that it was the switch that failed, which was replaced. It was also noted that the switch was directly in line with some hot gases, this lead to the deterioration of the switch.

Sticking Stop Valves

The daily check of the Tiger diagnostics detected that both the gas fuel and liquid fuel stop valves were sticking for abnormal lengths of time - the gas fuel stop valve was sticking for 5 seconds and the liquid fuel valves were sticking for 10 seconds. The sticking of these valves can cause problems in fuel transfers, including a gas turbine trip when a fuel transfer was attempted and unsuccessful. The site was advised to change the appropriate fuel valves.

This problem was again detected 10 days later when during a fuel transfer to gas fuel; the speed ratio valve did not open until approximately 1 minute after the signal was given by the servo. This caused the fuel transfer to fail and the system remain on liquid fuel longer than expected. It was shown that the cause of the sticking was because the gas fuel stop valve stuck open for approximately one minute.

This incident was repeated 5 days later where the gas fuel stop valve stuck open, which stopped the Stop Ratio Valve from functioning, however the gas control valve continued to open, expecting a supply of fuel. The gas control valve continued to open to 100%, then the gas fuel stop valve closed, this caused a surge in fuel flow. This ultimately led to the exhaust temperature rising to 585oC, which tripped the unit.

It was reported that the same incident occurred the following day, but due to a low load it did not trip the unit. The last incident, which tripped the unit, could have been avoided if the customer had followed the recommendations to change out the appropriate fuel valves when Tiger first detected the problem. 6-8 hours down time would have been saved if the customer utilised Tiger's early detection of the fault.

 

Incorrect Calibration of Lube Oil Header Temperature Switches

The gas turbine tripped on a high lube oil temperature. The Tiger team used Tiger to remotely verify the fault and determine that the trip should not have occurred. An oil header temperature trip should normally be at 80^oC, however, the trip occurred when the temperature was only 63^oC. It was rapidly established through a remote to Tiger that the gas turbine could be safely restarted, but that the lube oil header temperature switches needed to be recalibrated.

Sixteen days later the gas turbine tripped again on high lube oil temperature. Tiger information was used to rapidly verify that the problem was the same as before, that is, the sensors were incorrectly calibrated and it was an unnecessary trip. Although the site had been told to recalibrate the sensors after the first incident, they had not done so, showing that if they had acted on the Tiger data, they would have prevented a trip.

Three months later the gas turbine tripped again for high lube oil temperatures. Tiger data was again used to verify that the cause of this was an incorrect calibration of the switches and that the gas turbine could be rapidly restarted. It was determined that an incorrect calibration procedure was being used. Tiger was used to rapidly verify the cause of the trip, and confirm it was safe to restart the turbine, as well as to identify that the site was not carrying out a procedure correctly.

Faulty Vibration Sensor

The daily check by Tiger indicated that one of the bearing sensors had a momentary spike of 16mm per second. The site was advised that this had occurred and the problem was monitored daily through Tiger.

Three weeks later the same transducer was now spiking approximately once per hour with a magnitude of 8mm per second. The site was advised that the sensor was faulty and needed replacement. This early detection of the problem, monitoring its evolution, and advising of a correction, prevented a potential high vibration trip.

Early detection by Tiger and remote diagnosis was used to verify that it was a sensor problem and prevent an unnecessary trip.

Faulty Relay in Fuel Clutch Circuit

A phone call was made from site to the Tiger team requesting assistance. They had just replaced the flow divider for the liquid fuel and wanted the Tiger team and engineers to monitor the gas turbine on the subsequent start-up. Three attempts were made to start the gas turbine, all of which failed.

The engineers, using Tiger, were able to observe that on all three attempts, no fuel flow was achieved through the new flow divider. Through discussion with the site they were able to guide the engineer to establish that the cause was that the fuel clutch was not engaging due to a faulty relay in the clutch circuit.

By using the engineering support available at base to assist the person at site, they were able to identify the cause of the failed starts much more quickly. The problem was a very obscure and uncommon fault, and it is unlikely that the engineer on the platform would have been able to identify it on his own in the situation.

Through the use of Tiger and the remote help of the engineers, they were able to get the machine up between 12 hours and one day quicker.

Tiger is a Trademark of Intelligent Applications Ltd

August 2001

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