TB5000 LIGHT UP AND STARTING GUIDE.

Terms Used.

UV Flame Detectors. These detect ultraviolet radiation from a flame.

‘Inference’ Flame Detection. This is a software program to detect flame from PT Exit temperature rise.

1. IGNITION.

For ignition of the four pilot flames two three things are needed.

a.    Air supplied by the turbine gas generator.

b.    A correct flow of fuel into the igniter blocks.

c.    A good spark to ignite the igniter gas.

2.0 Air.

This is not adjustable on an electric motor start. The control system will not switch on the igniters until a gas generator speed of greater than 1800 rpm has been reached and the engine has had enough air through to be purged of any fuel gas. For gas motor starting or hydraulic, at least 1800 rpm is necessary by the time igniters are switched on (40 seconds norbit) or to start the core purge timer. If the starter reaches more than 2800 rpm before main fuel is on the governor can start ramping the throttle open causing stalling and high temperature shut down.

Pilot fuel will usually light OK.

3.0 Igniter fuel.

3.1  

Propane has been found to be the most reliable pilot fuel if good quality propane is available, it also has a standard set up. An alternative is using main gas fuel but this will need adjustments to be made to the igniter fuel system to increase flow. Some natural gas fuel is low in UV output so cannot easily be detected by UV detectors. Low calorific value gas fuel makes it difficult to detect pilot flames by temperature rise. 

Logic modifications to bring main fuel on four seconds after pilot fuel help the Ultra Violet or inference flame detection systems because main flames will be easier to detect than pilot flames. In this case even if one combustion chamber is not lit sufficient heat from the others will cause flame to be detected but the will shut down on deviation.

3.2  

Containers of propane need to be large enough and full enough to generate  sufficient gas at the necessary pressure during the light up period and for rapid restarts. Propane bottles will need to be joined in parallel when small ones are used or where they are located a distance away from the turbine or where there are additional pressure drops in the system.

3.3

 To provide the correct flow of igniter gas is done either by a PCV (Pressure Control Valve ) or an orifice normally located after the filter (See diagram 1) in the fuel valve module.

In the case of diagram 1, as well as a PCV in the fuel module, extra PCV’s are fitted on the bottles and an orifice is fitted in the main gas supply line. In this case the on skid PCV was wound full open and the pressure was regulated by the PCV’s on the propane bottles or by the orifice in the main gas supply line.

3.4

 Many sites will need the propane bottles to be heated to maintain sufficient pressure. Others in cold climates where the turbine is outside will need the pipe to the turbine trace heated and lagged to prevent the propane condensing.

3.5

 Tests have shown that for optimum recognition of a propane pilot flame by UV detectors, a pressure of 45-48 psig before the four 1/16^th inch orifices is required. Lower pressure can lead to late recognition on one pilot burner and higher pressures can lead to an unstable flame and the UV detectors losing sight of it. When inference detection is used, higher pressures will give more temperature rise and faster detection.

If a gauge is fitted it can be seen quickly whether the propane supply is sufficient and corrected as necessary.

NOTE Turbines can vary in the optimum pressures for light up. The object is to set pressures so that all four flames light quickly and stay stable.

 3.6  

Pressures for optimum main gas pilot fuel ignition will be higher than for propane. Tests on one onshore supply showed that approximately 60 psig before the 1.16^th orifices was needed for UV detection.  Other sites may need higher pressures.

High pilot fuel flow has been found to cause the gas generator to accelerate above 28%, 2800 rpm and ramp the throttle open before main fuel is switched on. This can lead to high temperatures or stall due to high heat input. (Affects turbines more when gas start and 4 flames detected needed for main fuel).

4.0 Spark igniters.

4.1  

The method of lighting the pilot fuel is by a spark plug. The spark plugs are connected to units(polarity sensitive) that input nominal 24 volts DC and output high voltage pulses sufficient to generate sparking at the plug.

Where there are long distances between the 24 volt dc supply and the igniter units, voltage drop on the supply cables can a problem leading to weak or intermittent sparks. Either an extra battery cell to increase the supply voltage or greater cross section cables can cure the problem.

4.2  

Gaps of 0.025 inch between the outer electrodes and the inner of the spark plug are recommended and should produce the optimum spark .

 4.3

It is possible for the spark units can fail or produce a weak or intermittent spark so reducing start reliability and for the flexible leads from the igniter unit to be damaged.

Replacement of these parts are the cure.

4.4

TB4000 and TB5000 igniter leads are not interchangeable, retrofit units have had problems when fitting TB4000 leads directly to the igniter unit. 

5.0 Ignition Fault Finding.

5.1

Find which pilot flame(s) are not being lit.

Use the control system to indicate which combustion chamber either did not

light or lit late. Late ignition indication on the control system may mean that

the pilot flame did not light or is receiving insufficient gas.

 Observing for presence of flame through the sight glasses on the combustion

chamber can be useful.

5.2

Check that there is plenty of pilot fuel. Propane bottles may be low in

 pressure, if so fit full ones.

5.3

Check that the 1/16 th orifices are clear on the suspect combustion chamber.

Blow out with air.

5.4

Check that the non return valves after the orifices are working. Replace if

necessary.

5.5

Check that the correct pressure is being achieved before the 1/16 th orifices.

If pressure incorrect investigate why.

5.6

 Check for leaks. Repair as necessary.

5.7

Check the spark plug, check gaps or for damage and replace if necessary.

5.8

Check for sparks. Replace spark plug or spark unit or leads.

5.9

Check for 24 volt supply to the turbine skid with igniters switched on.

If there is a large volt drop with the igniters turned on the cable from the supply will need increasing in size, or the voltage increased to compensate.

5.10

Check for condensing pilot fuel. Cold weather can cause this. Pipe from the propane bottles should be heated and lagged.

 5.11

Check for correct operation of the solenoid valve and blockages in the propane line. Replace or repair components as applicable.

6.0 Main Fuel ON.

6.1 When electric 4 to 2 pole motors are used the main fuel should cause the change from 4 to 2 pole by reducing the current through the motor. An undercurrent relay should be set so that the motor cannot change to 2 pole until main fuel is on and lit. Changing to 2 pole without main fuel on and lit can burn out motors and starters because the motor has insufficient torque to accelerate the gas generator and will stay on starting current which is at least 6 times full load current. 

6.2 Once the pilot fuel is lighting up reliably it may be that the turbine trips on deviation. A likely cause is dirty burners or low fuel flow at the point of  main fuel on. If flow is very low one combustion chamber may get less than the others and as the turbine accelerates and deviation limits close a deviation shutdown can happen. A simple solution is to increase the light up flow of fuel. Indications of this problem is that it is more likely when the turbine is cold. Bad deviation is an indication that burners are not clean and will also show as poor spread.

6.3 Stalling.

This is where the temperature rises without the gas generator accelerating.

Stalling can happen at main fuel on but the starter motor will continue to accelerate the gas generator. The stall is likely to sound unusual. 

If happening at light up the light up fuel demand can be reduced although this can lead to deviation shut down.

If the stall happens after light up there are two fuel rates that can be reduced to reduce the chance of stall. This may be necessary in the case of a turbine that uses solids cleaning when running just to achieve a start. In such cases the turbine is more likely to start cold.

A cold wash on a dirty engine can increase air flow and reduce chances of a stall.

7.0 STARTER OFF.

7.1 The starter dog is disconnected at a CT speed of 4800 rpm and the starter motor switched off. Also the fuel input changes to a slower ramp rate. A STALL can then be detected with slow or no acceleration of the gas generator and increase of engine exhaust temperatures. This can be due to over fuel which is due to a dirty or worn gas generator supplying insufficient air.

If fuel input is correct (ramp rate 1) and turbine is in good condition the amount of fuel at starter off should be sufficient to maintain the CT speed and continue acceleration smoothly but at a slower rate. 

7.2 IGV’s in open position. It should not be possible for the turbine to start with IGV’s open, check micro switch operation. IGV change over should be at above 9000 rpm CT speed.

7.3  Interstage BOV’s closed. This can cause stalling if stuck shut. They should be nearly closed at generator no load or very low load and close tight with more than 200-300 kW. On some turbines the interstage BOV’s close at no load. The purpose of the interstage BOV’s is to prevent surging by dumping excess air from the middle stages of the CT which cannot be coped with by the hp stages during the start. Also when load is put on the turbine from zero if surging takes place, later closing BOV’s can help prevent it.

7.4 Electric centre casing BOV’s open or leaking. If these leak or fail to close the result is likely to be high PT exit temperatures during this period of the start or during no load running.

Causes could be the diaphragm damaged, the valve seat damaged, blocked orifices, pipe leaks or leaking ASCO solenoid valves.

 Note: 4 off ASCO solenoid valves are fitted to TB5000 or up-rated TB4000 turbines.

8.0 Poor governing at no load.

8.1 More fuel than demanded. At no load the fuel should be in the region of 14-15 degrees for electronic actuator (approx. 3800kw for STAR systems). If the fuel is at 11 degrees or sitting on the blow out limits for long periods at no load then the fuel regulation is incorrect and it will be difficult to keep speed low. The control system will not allow less than the above fuel demands to prevent the flames blowing out. High regulated gas pressure for electronic actuator systems can cause this problem.

         

   For STAR actuator systems the normal no load heat demand must be higher than the blow out flow setting.

If more is being put into the turbine than asked for the speed can rise and the centre casing blow of valves will open. Usually set to open on 4% above demand speed.

Generators would have difficulty synchronising with others with this problem.

ADJUSTMENTS TO THE CONTROL SYSTEM SHOULD ONLY BE DONE  BY COMPETENT PERSONNEL.