FT8 Turbine GG & PT – FT8 & Solar – Turbine Technical Information

ART217 – FT8 – Sixth stage bleed valve training video

Donald Murphy — Thu, 20 Jun 2024 12:55:54 +0000

ARTICLE REF – ART217

This is a training video on the Sixth Stage Bleed Valves on the FT8. This gives a detailed description of operation and control of these valves.

ART069 – FT8 – Gas fuel hose failures

Donald Murphy — Wed, 19 Jun 2024 12:33:40 +0000

ARTICLE REF – ART069

This article discusses the potential issue with gas fuel flexible hoses. The early FT8 turbines came with rigid stainless steel tubing, but for some reason P&W moved to a flexible hose to connect the injector to the gas manifold. This happened around 2010. The rigid stainless type take some additional time to install but work very well. The new style work well in gas only service, but in a dual-fuel configuration, running on liquid fuel there have been issues.

The hose has a bellows type inner metal tube that is reinforced by a stainless steel braded exterior. Without the stainless brade the hose is weak and will fail. On several occasion the failure occurred at the end fitting.

Below you can see part of the gas fuel system and also the interface to the liquid purge. When running on gas fuel there isn’t any issue because the gas flow keeps the hoses cool. When running on liquid fuel the gas hoses heat up a little due to the close proximity to the metal surface of the diffuser – combustor area. The hoses are well capable of handling these temperatures. The problem arises when there is a back-flow of gases from the combustor entering the gas path through the injector (running on liquid fuel). This has happened in the past due to the failure of gas check valve CV1101. The red arrows in the diagram shows the path of the back-flow to the flow divider or through SOV1402. SOV1402 is a solenoid actuated drain valve that is connected to the gas purge system. It opens whenever the as fuel system is non-operational to drain any liquid that might leak past CV1102 (while the engine is running on liquid fuel). The valve is arrange to be a low-point so that any liquids will drain to the valve rather than drain into the gas header. Opening this valve also serves to de-pressurize the gas header and the gas purge path up to CV1102. De-pressurizing the line also helps to fully seat both CV1101 and CV1102. Orifices reduce the flow of the gases but there is enough flow to heat up the hoses with very hot gases. You can identify if this is happening as the colour of the stainless steel brading will change (similar to the change in colour on the cold buffer air supply hose which is carrying thirteenth stage air).

PWPS have changed to a different manufacturer. Below is a photo of the older type in blue CT113015 and the new type on the left CT117015.

When there is a back-flow TE1103 is supposed to rise and alarm to indicate the problem. There is a tunable to adjust the alarm between 250 – 300 F. The one time I seen this happen the temperature increased only as far as 175 F.

Another issue with back-flow of gas – if there are any contaminants in the liquid fuel they may end up sticking to check valves, fuel pipes, injectors etc.

End of article.

ART068 – FT8 – Start Motor Repair – 3 Starter Achille’s heel

Donald Murphy — Wed, 19 Jun 2024 12:32:31 +0000

ARTICLE REF – ART068

Achilles Heel of the starter

This freewheel assembly which is the drive between the start motor and the gearbox is the “Achilles Heel” of the start motor. This is a very complex yet reliable start motor. Most of the failures you get with this motor is not being able to accelerate the turbine sufficiently to help get to self-sustaining speed. I say help because it is working in conjunction with the turbine which has lit-off and is also contributing to the effort.

Most of us associate a freewheel with the freewheel on a bicycle. It allows us to stop peddling while allowing the drive wheel to continue turning. The freewheel of the starter allows the start motor to stop while the gearbox which it once was driving to start the unit continues to run. So one part of the flywheel continues to rotate while the other stops.

The photos below show the part of the freewheel which allows the disconnect between the shafts. It resembles a bearing with an inner and outer race, but that is where the similarity ends. There are kidney bean cross section pieces of metal which are located between the inner and outer ring. These can rotate about 70 degrees allowing the metal pieces to lock themselves between the two shafts or lie down allowing a disconnect of the two shafts. The photo on the left shows the metal pieces standing up in line with the center of the shafts, whereas the photo on the right shows the meal pieces lying down which would allow the gearbox to rotate as it does in service and the outer shaft connected to the starter to stop.

In the photo below you can see the flywheel splined into the start motor on the bottom and to the gearbox on it top part. The top part also contains a carbon seal to prevent oil leaks. Once cover is removed the freewheel can be removed by hand. The lubrication of the freewheel is not adequate on older units and they wear at the point where the metal kidney bean pieces of metal rub against the gearbox shaft. Once it wears a little the freewheel will start to slip and then it will not allow the turbine reach self sustaining speed. There are a lot of start motors lying around workshops which could easily be repaired. The part numbers of the freewheel is given at the end of the article. And while the starter is a complicated machine, the replacement of the freewheel is straightforward. Considering the price of a new starter it might we worth taking it apart and taking a look.

Below you can see the two parts of the freewheel. The smaller one is the drive to the gearbox, which spins with the turbine. The freewheel does not rotate in service, only when rotated by the start motor.

The photo below is the shall shaft of the freewheel with the part allowing it to over run removed. You can see the wear caused by the kidney bean metal parts rubbing and not having sufficient lubrication.

The part number for this shaft is CT116346‐SP15.

The part number for the bigger part of the freewheel is CT116346‐SP37.

ART067 – FT8 – Start Motor Repair – 2

Donald Murphy — Wed, 19 Jun 2024 12:31:25 +0000

ARTICLE REF – ART067

This is another typical location for an oil on the starter motor. The swashplate is a device that changes the stroke of the variable multi piston displacement motor. It is supported on two Swashplate support pins inserted opposite each other on the starter casing. With care this job can be done without removing the starter.

You can see the access you have to the lower swashplate pin in the photo. Not the best location to be working but if it means not having to remove the start motor it would be worth it.

ART066 – FT8 – Start Motor Repair – 1

Donald Murphy — Wed, 19 Jun 2024 12:30:27 +0000

ARTICLE REF – ART066

This is a typical leak location for the starter. It is a simple repair that requires the replacement of a paper gasket.

This gasket regularly leaks so one was made up one locally. Because the motor shaft was supported in a vice, oil started to come out the other end. This is because we supported the motor by the drive shaft and when we took off the cover, the starter casing moved down due to its own weight. This caused to carbon seal to open and spill oil.

We took the mechanical seal out and it was was found to be worn and chipped.

To dissemble the drive shaft carbon seal you need to first remove the drive coupling. Remove the spring ring then the coupling will slide out. Check for wear and corrosion on this coupling as it is an item that wears.

The motor drive end has a splined shaft. The removable plate on the left houses the stationary steel face of the carbon seal. The carbon is spring loaded against this face. The PN for the carbon seal is CT116346-SP23.

This is the carbon seal CT116346-SP23 that is found on the drive shaft. The kit includes all in the photos above except the cover plate which houses the polished steel surface. This is pressed out with a socket. Once the plate is removed the assembly can come out. The reality is that once in use the assembly has to be prized out. This assembly includes the rubber lip seal.

PWPS part numbers:

CT116346-SP11 ….. drive coupling

CT116346-SP10 ….. drive coupling retainer

CT116346-SP13 ….. gasket end cover

CT116346-SP12 ….. starter compensator

CT116346-SP23 ….. input drive seal kit

ART063 – FT8 – Turbine Cooling Air – TE076

Donald Murphy — Wed, 19 Jun 2024 12:27:15 +0000

ARTICLE REF – ART063

Turbine Cooling Air TCA is 8th stage air tapped off to cool the casing of the LP Turbine and #6 bearing support struts. After cooling the casing it then flow out of turbine and is directed into the Power Turbine to provide some cooling to it.

The LP casing is constructed of light gauge metal which expands and contracts fast. The location cooled is show in the blue circle.

EC7 is a solenoid valve that provides 8th stage air to open or close the two LPT TCA valves. When the turbine is starting up EC7 is de-energized, therefore no cooling air is supplied to the LPT. The LPT turbine casing is of thin metal construction so it will grow quicker than the LP turbine blades. This will ensure that the turbine blade tips will not rub against the casing seal. The blades then grow to their operational size and then EC7 is energized. Both LPT cooling valves open and cooling air is supplied to the cavity which will reduce the diameter of the casing. This will close the gap between the LPT tip and the casing seal for better performance.

Some of the air will exit just forward of where the cooling air entered and is directed to the PT to do some cooling. This is where TE076 is fitted.

In service you can get a driveback alarm if the temperature of TE076 increases to 730 F. Normally the problem is due to one of the 8th stage TCA valves stuck closed or partially open.

PWPS have allowed some sites to increase this value after some studies of site data.

PWPS made a change to the PTTB logic some time ago (SB 12M08) which opens the PTBB valve a little more to relieve some additional forces on the thrust bearing #8 of the PT. This additional air has a back pressure effect on the cooling air coming into the PT from the GG side. This can cause the unit to go into a TE076 Driveback also.

ART060 – FT8 – Cold Air Buffer

Donald Murphy — Wed, 19 Jun 2024 12:23:48 +0000

ARTICLE REF – ART060

As the power of the FT8 was increased by PW Engineering upgrades, the heat internally in the turbine increased also. As you increase the oil temperature in a lubricating system it first starts to varnish the sides of bearing housings and return lines, then as the oil gets hotter it forms a very hard substance called COKE. COKE started to form in the lubricating system at the hottest bearings of the FT8, bearings 4 – 5 & 6. The Cold Air Buffer System (CAB) was introduced in 1996 and was offered as an upgrade to existing units with Service Bulleting 96M07. Air from the last stage of GG compression (13th stage air) is tapped off and cooled to the required temperature to cool the bearings. If ambient air was used to cool the bearing locations the metal would loose some of its properties due to thermal shock.

Air coming off the 13th stage is very hot and under pressure – typical values 350C & 350 psi. The heat discolors the stainless steel wire brading used to protect the flexible hose, so take care. You can see the discoloration of the hose in the photo below (hose 1).

13th stage air leaving the diffuser splits into two and then pass through orifice plates. These plates regulate the flow so that the system can operate within given parameters. When a turbine is commissioned the commissioning engineer will start the unit with standard size orifices. Check Procedure 24 of the Commissioning Manual for orifice sizes. There are plates for different size piping (1.5 and 2″ lines) and also different sizes for FT8-1 -2 and -3. Once the turbine is started there is a calculation to be completed to verify that the orifice plate size are acceptable. If the calculation calls for a different orifice size you need to install a smaller or larger one. More on this check later in the article. This check has also to be carried out once per year after the annual audit. Below you can see the split of the discharge air and then the two orifice plates. A set of plates are provided by PWPS at commissioning time and should be somewhere in your stores system. It is a bit surprising that the orifice size check uses pressure as the basis of determining whether the orifices are suitable or not. You can see the tapping on one of the return pipes in the photo.

CT116811-x for the 2 inch piping.
CT116812-x for the 1 1⁄2 inch lines.
IPD22389-x
x = orifice diameter in thousands of an inch.
They may be listed as kits IPE24263-GR2 or IPE24351-GR2 or –GR3.

There has been a recent bulletin SB 16M01 issued by PWPS on the type of spiral gaskets used on the flanged joints as seen in the pipework in the photo above. Considering that the typical pressures and temperatures of 350C / 350 psi it is not surprising you need special gaskets.

After flowing through the orifices the air enters a heat exchanger which has two elements of different sizes (see photo below). Between different size orifices and different size heat exchangers the air leaving the heat exchanger is approximately 350F for cooling bearing 4&5 and somewhere between 150 to 230F for bearing 6. The most critical temperature is for bearing 4&5 and therefore it is this value that is controlled. Apart from orifice and cooler size the other item that can effect the temperature of the air is the speed of the cooler. It is driven by a VFD and it controls the temperature of bearing 4&5. Bearing 6 will then fall within the range of 150 to 230F if everything is normal.

On each of the return lines back to cool the turbine bearings you have an RTD that measures the air temperature. RTD TE1601 measures the cooling air to bearing 4&5 whereas RTD TE1602 measures the cooling air to bearing 6. TE1601 is used in the feedback loop to control the speed of the VFD. Both RTDs are used for alarms, drivebacks and shutdowns.

The cooling air passes over the bearing housings and then exit with the rest of the combustion gases after passing through aerodynamic seals etc.

There has been a couple of VFD used in this application. The one shown in the photo below is an Allen Bradley 1305. Some of these are still in service today although pars must be difficult to get hold of. This particular VFD did have an issue with grounding requiring that the control signal be isolated from the main control.

The PowerFlex 4 replaced the AB1305. TPMD 416 is the PWPS document that is gives information on how it is used in the FT8 application with programming parameters provided.

Navigating the menu is not obvious:

If you keep pressing the “Esc” button you will come back to the default display parameter “d002” which is the output frequency of the drive. This is what is normally displayed on the front panel.

Therefore if you press “Esc” until you come back to the “HERTZ” value on the display, then press “Sel button” and “d002” will come up with the “d” flashing. If you press the “Arrow up or Arrow down” you will cycle through the different parameter tyes available ie. D – display group – P – basic program group & A – Advanced group. If you want to accept “d” then press “Sel” and the curser will move over to the parameter number and start flashing. You can move up and down the parameters until you get to the one you want, then press the “Sel” button. Once the parameter value is changed to the value you wish, press the “Enter button – arrow left” to save the new value.

Operation

The CAB (cold buffer air) is enabled when NH reaches idle speed (8000 RPM approximately), and when TE1601 reaches 2OO° F or TE1602 reaches 175° F. Once activated the fan is commanded to minimum speed.

The CAB is shut down when “T3 synthesis” falls below 300° F. Anytime you see synthesis used in PW it means the value is calculated form other values. T3 stands for temperature at station 3 of the turbine which is the entry to the combustor. T3 is also used for alarms, drivebacks and shutdowns.

Loss of TE1601 and TE1602 will result in a controlled shutdown. Both sensors must be working in order to start the gas generator.

Inputs used to calculate T3 Synthesis:

TE010 – air intake temperature
PT007 – combustor burner pressure
PT006 – compressor inlet pressure

Bleed valve position is also taken into consideration.

Alarms

Alarms TE1601

The Low Alarm and Low Shutdown are based on the T3 Synthesis curve below. These are low alarms because you want to avoid thermal shock of the metals.

There is a High Driveback which is set at 365F. This is to prevent damage to instruments and also to ensure that there is adequate cooling. There isn’t any graphic showing the operating point with reference to the curve and it would be nice to have one. This would be a good example of what you can do with your own monitoring – see article on Smart Monitoring & Diagnostics.

Alarms TE1602

The Low Alarm and Low Shutdown are based on the T3 Synthesis curve below.

There is a High Driveback which is set at 250F.

Normal operation is 150 to 230F.

Checking orifice plates

Reference the Maintenance Manual Section 22.01 for this procedure.

The procedure is also found in the Commissioning Manual.

Troubleshooting

There is a good troubleshooting section in Service Bulletin 96M07.

Installing FT8-1 instead of FT8-3

The following modifications must be performed in order to adapt the GG8‐3 systems to a GG8‐1.

These tunables need to be changed … best ask PWPS engineering for the value as they may change.

CONFIG_1A.ENG_RATING.IN
CONFIG_1A.FT8D3_ENG.IN
CABLOOP_A.CABPID_FF.Y_1_1
CABLOOP_A.CABPID_FF.Y_1_2
CABLOOP_A.CABPID_FF.Y_1_3
CABLOOP_A.CABPID_FF.Y_1_4
CABLOOP_A.CABPID_FF.Y_1_5

Cold Air Buffer #4&5 Bearing Hose
GG8‐1 has a 1 ½” hose connection for Cold Air Buffer #4&5 bearing (2” for GG8‐3). A flange adapter can be used as in the photo below. Another option is to install a threaded adapter on the threaded side of the 2” hose. In this case, you can use the existing 2” hose (GG8‐3) but you must install a 1 ½”female / 2”male adapter on the GG8‐1 #4/5 CAB connection.

Cold Air Buffer heat exchanger shield (P/N IPD24531) must be removed from CAB air cooler. It must be reinstalled for the GG8‐3.

NOTE:- When you calibrate the CAB by verifying the orifice size, you can still run into problems. The VFD should be able to control the air for 4&5 by varying the speed of the VFD. But you may get some alarms for the cooling air for bearing 6. If this happens you may need to adjust the sheet metal position installed on the top of the small heat exchanger.

ART053 – FT8 – Honeywell Solenoid

Donald Murphy — Wed, 19 Jun 2024 12:03:08 +0000

ARTICLE REF – ART053

Subscriber looking for information on Honeywell Solenoid Valve PN 815V700-2

These solenoid valves are used in the aircraft turbine industry and are very expensive. If anyone has information on where spare parts to repair this solenoid or a repair facility where you have repaired them I would appreciate if you can share the information.

I was sent a cool video on the disassembly and assembly of this valve – it is 28 MB so I am not putting it on my site. Anyone who wants a copy send me an email.

ART039 – FT8 – Power turbine lean back issue

Donald Murphy — Wed, 19 Jun 2024 01:46:05 +0000

ARTICLE REF – ART039

Power turbine lean back.

In 2019 there was a failure of a Power Turbine in Georgia (Europe) due to what is know as “Power Turbine Lean Back”. This issue has been around sine the first PT failed in Chile in 2011. Basically what happens is the first stage static vanes lean back due to deformation of the support for #7 bearing and they clash with the base of the first stage blades of the PT. This ends with a lot of damage to the PT and associated hardware such as coupling. A special tool was designed called a Pirate’s Wheel which is used to measure the distance from the front flange of the inlet case (P-flange) to the front face of the 1st stage vane inner platform front face.

Below you can see the measurement being taken with a depth gauge. The Pirate’s Wheel is in grey.

The Pirate’s Wheel is the base from where the readings are taken and this has to be done with the GG removed. Apart from the lean back issue there is also a problem with the first stage seal plate and the second stage inner air seal distorting. You use a borescope with measuring capability to evaluate the seal separation. It is not difficult to do this inspection, but you need to be trained by someone experienced. This inspection is possible with the GG installed, although this is far more difficult to do. With the GG installed you may not be able to take the twelve readings that you take with the GG removed.

There have been many modifications by PWPS engineering to eliminate these issues and depending on what modifications and hardware is used on your PT, it will be given a Stage number, such as Stage 0 – Stage 1 – Stage 2. The stage number of your PT will determine what inspections are needed and how often they need to be done.

The question being asked by the operator in Georgia is why the vibration system did not give any alarms in the three days leading up to the failure. You can see in the graphic below that there was vibration on the days leading up to the failure but the duration of the event was not long enough to trigger an alarm. Process Analyst captured one event with a value of 3.7 mils, which was in excess of the 3 mils p-p alarm set point (see the graphic below), but there is a one-second delay on the alarm. The alarm has to be in excess of 3 mils for one second to trigger the alarm. The argument against reducing the alarm duration would have to be the prevention of nuisance alarms, but the data in this case supports the idea that a smaller time frame might be prudent.

The graphic below shows how an alarm might even be over the shutdown level yet not even register as an alarm. As explained above the vibration must be detected for one full second for an alarm and 0.1 second for a shutdown. Control Assistant which records historical data samples one time per second so a short time vibration may or may not show up in history. It is important not to confuse the two points. If the alarm is present for more than one second then it will be added to the alarm list yet it is possible that the historical data might miss this.

Card number 7 in the Bently Nevada 3500 system is the Relay Ouptut Card. If the vibration from “S” Flange is over 4 mils for 0.1 seconds, Channel 3 of the Relay Card will be de-energized. This opens the circuit in the 86E causing the whole Unit to shut down. The whole unit shuts down because you can’t have one side turning the other (both PTs are connected to the generator).

Testing by this user in Georgia found that the scan rate in GAP was 240 milliseconds and after reducing this to 50 milliseconds the best update they were able to get was 100 milliseconds. This should allow you to detect an alarm should you reduce the delay time from one second to as low as 100 milliseconds.

Every block in GAP is assigned an update rate. The reason is that the low level status of oil in lubrication system tank for example does not need to be read into GAP every ten milliseconds, whereas the EGT needs to be read in as quickly as is possible. This allows for higher update rates for important channels.

The BN 3500 vibration system is tied into the 86 Relays by hard wire, but it also communicates via modbus to the CPU and this communication takes time. For this reason 100 millisecond update rate was the best that he could get. But this is a backup to the relay which will shut down the unit quicker.

The recommendation to reduce the alarm delay time makes sense considering what happened in Georgia. Every operator of FT8 should know the “Stage” number of each of their Power Turbines and from this be able to determine what inspections are needed and when (Letter L-FT8-13N01 from PWPS has this information).

ART048 – FT8 – Air system – bleed valve lubricant WC 393

Donald Murphy — Wed, 19 Jun 2024 01:27:53 +0000

ARTICLE REF – ART048

Sticking bleed valves was an article issued recently which focused on the tool used to check the valves and how to determine if they are sticking.

Comment from one our participants:

WC 393 is the product recommended by PWPS for lubricating the bleed valves and it works very well.

Loosen the air line and squirt 2 to 3 squirts of WC393 once a year and bleed valve sticking magically disappears.

It costs a little over $41 for a 12 oz bottle and is available from AP+M in the USA.