Turn everything off
The moving parts come to a halt

("Shutdown," by Ash, 2015)

 

WHAT HAPPENS WHEN A GAS TURBINE (GT) IS SHUTDOWN FOR SCHEDULED MAINTENANCE AND OVERHAUL 

Brilliant  PTOA Readers and Students  … 

meaning those who are reading the PTOA Segments in the intended sequential order …

just learned in PTOA Segment #199 how a Gas Turbine's (GT's) Advanced Control System greatly assists human beings known as GT Technicians and GT Operators during a GT Startup.

So who is surprised to learn that the GT's Advanced Controls System also assists during the normal GT Shutdown?

IF the Plant Manager has continued to invest in a proper Preventative Maintenance (P/M) program to inspect and perform relatively minor but crucially important GT repairs …

THEN the GT engine will run flawlessly for 25,000 to 50,000 hours before a complete engine overhaul is necessary. Shazam! That's almost 3 to almost 6 years of working day in an day out!

Gadzooks! Some natural gas fueled GTs with stable service are expected to operate reliably for 10 years without an overhaul!

Eventually, every GT that's been working like a dog will be scheduled for a total engine overhaul.

Naturally, the GT must follow a GT Shutdown protocol before any maintenance interval can begin.

The GT Shutdown steps described in this PTOA Segment are general in nature; all future GT Operators and Technicians must follow the written GT Shutdown Procedures for the specific GT Engine they are responsible for.

In this PTOA Segment #200 PTOA Readers and Students will learn the following about GT Shutdowns:

• The circulation of Lube Oil must continue any time the common Axial Compressor-HP Turbine Shaft is rotating.
• "Bowing of the Shaft" must be prevented.
• The last step of the Maintenance Interval is cleaning the Axial Compressor. 

 

 

GENERAL GT SHUTDOWN EVENTS

The PTOA Department of Redundancy Department reiterates that the content in the following paragraphs describe general GT Shutdown phenomena that occur during a Normal GT Shutdown … not an Emergency GT Shutdown.

Actions that approximate the following steps will be performed in every GT Shutdown … because each of the described steps has a meaningful purpose.

 

 

The GT Shutdown Begins: Achieving Thermal Equilibrium at Idling Speed

The GT Technician/Operator initiates the GT Shutdown by pressing the GT's STOP button, which more than likely is accessed via the HMI of the GT's Advanced Control System and not a physical button as shown in the nearby photo.

After the GT's STOP button is activated, the Advanced Control System will gradually decrease fuel to the Combustor until the Turbine attains "idling speed of the GT" …  somewhere in the range of 50% - 62% design speed.

 

The Turbine continues to idle at this reduced speed while the GT achieves "thermal equilibrium" … thus preventing the GT from experiencing thermal shock.

PTOA Readers and Students know all about how heat automatically moves from a hotter area to a cooler area while in pursuit of achieving  "thermal equilibrium" because heat transfer phenomena was explained in the PTOA Heat Transfer Focus Study.

The nearby graphic will remind PTOA Readers and Students that heat transfer only stops when the previously "hot" and "cold" areas are at the same PV Temperature … which is shown as "Room Temperature" in the nearby graphic.

 

The Auxiliary Lube Oil Pump Kicks On

All PTOA Readers and Students learned in PTOA Segment #199 that the Main Lube Oil Pump is more than likely an "Auxiliary Load" driven by the GT's Power Turbine.

Ergo, as the Power Turbine's speed declines, the Main Lube Oil Pump cannot generate sufficient discharge PV Pressure to continue circulating lube oil in the Lube Oil Loop.

The GT Advanced Control System automatically starts the Auxiliary Lube Oil Pump when the Advanced Control System senses decreasing PV Pressure in the Lube Oil Loop.

Since the Auxiliary Lube Oil Pump is driven by an external A/C motor it has no problem infusing the PV Pressure into the circulating Lube Oil!

Eventually the Turbine's rotor will stop rotating and come to a rest.

At a standstill the Turbine is still extremely hot.

The Auxiliary Lube Oil Pump continues to supply bearing lubrication oil so that heat can be transferred from the bearing into the circulating lube oil.

The circulation of lube oil continues until the bearings have had sufficient time to cool. The typical time interval between the GT Technician/Operator pressing the STOP button and the Auxiliary Lube Oil Pump disengaging is an hour.

The  Auxiliary Lube Oil Pump will eventually be shutdown by the Advanced Control System. However, lubrication is required throughout  "Ratcheting of the Shaft" which is explained below.

 

 

Why the GT's Rotor Will Warp During Shutdown

Way back in PTOA Segment #64, PTOA Readers and Students learned how natural convection occurs when hot gases and liquids rise up and cooler gases and liquids flow to lower regions (as is shown in the nearby graphic of a room with a hot area heater and a cooling area window).

Well, the hot air inside the Turbine will likewise rise to the upper half of the Turbine's casing.

The more dense cooler air will be pushed to settle at the bottom half of the Turbine's casing

And way back in PTOA Segment #104, PTOA Readers and Students learned how metals will expand when exposed to hotter PV Temperatures and contract when exposed to lower PV Temperatures.

Ergo ...

Who amongst the brilliant PTOA Readers and Students is surprised to learn that this stratification of temperatures within the Turbine's casing causes the bottom half of the Shaft to contract at a faster rate than the metal at the top, warmer half of the Shaft.

Left to cool in this state, Bowing of the Shaft will result. In other words, the Turbine Shaft will warp.

 

 

How to Prevent the Rotor from Warping During Shutdown

For the reasons explained above, the GT will have a "ratcheting mechanism" that will slowly rotate the Shaft until it cools.

Slowly turning the common Axial Compressor-HP Turbine Shaft with the help of an external mechanism is called "ratcheting."

 

"Slowly ratcheting the Turbine Shaft" means the "ratcheting mechanism" will slowly turn the Turbine Shaft at a rate of 30 degrees over 3 minutes. That's super slow!

 

Slowly ratcheting the Turbine Shaft evenly distributes heat within the Turbine's casing which prevents "Bowing of the Turbine Shaft."

 

 

Typically the hydraulically powered "ratcheting mechanism" is activated by the GT's Advanced Control System once the Advanced Control System instrumentation senses that the Turbine has stopped rotating.

Some GTs use the Startup Motor described in PTOA Segment #199 for ratcheting the Turbine Rotor.

In older model GTs  the "ratcheting mechanism" might involve the GT Technician/Operator manually completing the meshing connection between the worm gear at one end of the "ratcheting motor" and the spur gear in the Accessory Drive Assembly.

 

 

 

 

THE FINAL STEP OF THE GT MAINTENANCE INTERVAL:  CLEANING THE AXIAL COMPRESSSOR'S INTERNAL HARDWARE

The very last step of the maintenance interval will typically be cleaning the Axial Compressor.

Cleaning the Axial Compressor is scheduled to be completed just prior to beginning the GT Startup because the first steps in the GT Startup …  "Cranking" and "Warm Up" … insure that all water and debris will be displaced from the Axial Compressor before the GT engine begins its next long run of service.

 

 

Overview of Cleaning the Axial Compressor. 

The purpose of cleaning the Axial Compressor is to remove debris from its Stator and Rotor Blades.

PTOA Readers and Students learned in PTOA Segment #198 that the buildup of debris on the Axial Compressor's Stator and Rotor Blades can easily reduce the Compressor's Discharge PV Pressure by 5%.

A dirty Axial Compressor means that the GT Engine will never be able to attain full speed.

Cleaning the Axial Compressor takes place after the maintenance interval and prior to GT Startup. Therefore, the GT Inspectors have examined the Axial Compressor and are aware of the condition of the Axial Compressor's Rotor Blades. The GT Inspectors use their know-how to select the appropriate cleaning media. For example:

• A solution of detergent and distilled water that flows through the Compressor, removing dust and salt deposits.

 

• A cocktail of detergent, distilled water, and hydrocarbon to soak the Compressor's Rotor Blades and free them of harder deposits.

 

• An Abradant Cleaner … maybe even crushed walnut shells! … to remove salt water, oil vapors and oily dust contamination that will not budge from the Rotors using the other two methods.

 

All of the above methods are followed by a water rinse.

 

Preparation for Water Washing the Axial Compressor

The GT more than likely has a Water Wash Manifold/Wash Ring permanently located inside the Compressor's Air Inlet Duct.

The casing for the Axial Compressor will have holes which are intentionally used to drain the dirty wash water into a runoff-water manifold.

The dirty water runoff that exits the manifold could flow into hoses that are used to fill drums.

Before starting the Water Wash of the Axial Compressor, GT Process Operators and Technicians might assist in disconnecting, capping, and locking out the non-Compressor parts of the GT that would otherwise be impacted by the water wash.

The Startup Motor that is used to rotate the common Compressor-HP Turbine Shaft typically has a dedicated START WATER WASH BUTTON that the GT Technician/Operator will activate.

 

Water Washing the Compressor with a (Detergent + Distilled Water) Solution

The PTOA Department of Redundancy Department reiterates that each GT has a specific Axial Compressor Water Washing Procedure. The following paragraphs are for informational and instructional purposes only.

Typically, a dedicated Water Wash Cart will be rolled out for the Water Wash procedure.

The Water Wash Cart will have a compartment filled with the (detergent + distilled water) solution and a different compartment filled with just distilled water for rinsing.

The detergent solution and rinse water are delivered at 20 to 40 psi at a flowrate of 6 to 8 gpm.

 

Egads! The common Compressor-HP Turbine Shaft is about to be rotated by the Start Up Motor so that wash water can be drawn into the Axial Compressor via the Air Inlet Duct!

 

Prior to activating the START WATER WASH BUTTON on the  Advanced Control System, the Auxiliary Lube Oil Pump must be operating so that the PV Pressure can be added to lubrication oil circulating within the Lube Oil Loop.

Don't stress out, Fred!

More than likely the GT's Advanced Control System will not allow the Start Up Motor to engage unless sufficient PV Pressure is sensed in the Lube Oil Loop.

However, older versions of a GT Advanced Control System might actually rely on the GT Technician/Operator to "trick on" the Auxiliary Lube Oil Pump (as was described in PTOA Segment #199).

 

Whatever the exact procedure for the Axial Compressor Water Wash may be … the crucially important point being made is that the  Auxiliary Lube Oil Pump must be operating before the Startup Motor is engaged and drawing wash water through the Air Inlet Duct and into the Axial Compressor.

 

Furthermore … whatever the exact procedure for the Axial Compressor Water Wash may be … the Rotor Blades must be rinsed with distilled water so that any detergent film left on the Compressor's Stator and Rotor Blades will be removed.

Like the dirty wash water, the rinse water is also drained through the holes in the Compressor casing and must be disposed of using an environmentally approved manner.

The Startup Motor will continue cranking the common Compressor-HP Turbine Shaft for 30 minutes to purposefully confirm that:

• All the water has been drained from the Compressor
• The water that has been drained from the Compressor has been displaced by ambient air.

 

The common Compressor-HP Turbine Shaft will be briefly stopped while the GT Engine is prepared for the GT Startup.  And … of course … the GT Advanced Control System will require affirmation that each GT Startup preparation step has been completed prior to allowing the GT Startup protocol to continue.

The "Cranking Step" in the GT Startup procedure displaces the air left in the Compressor with yet more air ... and then the "Warmup Step" starts heating up that air!

 

Thank you Dinocu for another "PTOA You Tube and Chill" Time!

Hey, it is time for yet another "PTOA You Tube and Chill" time with this cool Water Wash of GE LM6000 You Tube uploaded by Dinocu. Be sure to "Like" the You Tube because it helps PTOA Readers and Students understand what Water Washing an Axial Compressor looks and sounds like!

More than likely Dinocu added an abundance of detergent to make it easier to "see" the water wash.

The video shows the Water Wash Ring and the effluent of dirty wash water flowing carelessly to grade. We must assume the dirty water is captured and dealt with in an environmentally sound manner.

Access the video below or HERE.

 

 

 

Cocktail Soaking the Axial Compressor

The preparation for soaking the Compressor with a cocktail of (distilled water + detergent + a hydrocarbon like kerosene) will require disengaging and capping all discharge lines from the Compressor, including the lines that introduce Primary, Secondary, and Tertiary air to the Combustor's ignitors and pilot valves.

The "Cocktail Solution" might be whipped up using the following recipe in a 10 gallon jug:

• 4 gallons of distilled water
• 4 gallons of kerosene
• 1 quart of detergent.

 

The "Cocktail Solution" will be drawn into the Axial Compressor via the same Water Wash manifold found in the Air Inlet Duct.The "Cocktail Solution" will be delivered at approximately 5 psi PV Pressure through the manifold.

As was explained above, the  Auxiliary Lube Oil Pump must be activated prior to any rotation of the common Compressor-HP Turbine Shaft.

After the Auxiliary Lube Oil Pump is activated, the Water Wash Button on the Startup Motor can be activated and something like the following steps will occur:

 

The Shaft will be slowly cranked for 2 minutes which allows the Cocktail to flow into the Compressor.

 

The Cocktail is allowed to soak the oily deposits from the Rotor Blades for 15 minutes ... and all the while the Auxiliary Lube Oil Pump keeps running.

 

The dirty Cocktail Solution is drained from the Compressor using an environmentally approved procedure.

 

More Cocktail Solution is cranked into the Compressor for 2 minutes, followed by a 15 minute soaking period. Then drained.

 

This procedure continues until all of the Cocktail Solution is used.

Naturally, the Axial Compressor is rinsed with distilled water at the conclusion of the last Cocktail Soak.

Rinsing the Axial Compressor with distilled water removes the film of kerosene and detergent from the Compressor's Rotor Blades.

Following the rinse step, the Startup Motor rotates the common Compressor-HP Turbine Shaft for 30 minutes to confirm that all the water has been drained from the Compressor and has been displaced by ambient air.

The rotation of the common Compressor-HP Turbine Shaft will then be stopped so that the GT can be lined out for the GT Startup procedure.

 

Abradant Cleaning of the Compressor

If the GT operates in an unprotected environment which includes salt water and oily vapors or dust, the GT's Axial Compressor may need to be cleaned with a material that is more abrasive so that the stuck-on debris can be filed off.

Since Abradant Cleaning of the Axial Compressor can easily damage the Compressor's Rotor Blades this type of cleaning is only done under the guidance of experienced GT Technicians/Operators using a cleaning procedure that is beyond the scope of the PTOA to describe.

Your Mentor has assisted in an Abradant Cleaning of an Axial Compressor; however, in this case the Axial Compressor was part of a Fluid Catalytic Cracker (FCC) process … a totally different application than a GT engine used for driving Rotating Equipment or electricity generators.

In the FCC process service, crushed walnut shells were sucked into the Axial Compressor while it was normally operating. This cleaning procedure was successful because the process flow scheme allowed the walnut shells to be easily collected downstream of the Axial Compressor without impacting FCC operations.

 

TAKE HOME MESSAGES: Properly maintained, a GT Engine will provide years of service between Preventative Maintenance intervals and total overhauls. Each PM interval and overhaul is preceded by a Normal Gas Turbine Shutdown. Just like the GT Startup Procedure, the Normal Gas Turbine Shutdown procedure is greatly assisted by the GT's Advanced Control System.

Important steps in the GT Shutdown procedure include:

• Idling to achieve thermal equilibrium.
• Automatic kick-on of the Auxiliary Lube Oil Pump when the GT's Advanced Control System senses low PV Pressure in the Lube Oil Loop.
• Ratcheting the Shaft to mitigate Bowing of the Shaft (aka warping of the Shaft).

 

"Ratcheting the Shaft" evenly distributes heat between the upper and lower regions of the GT Shaft. "Ratcheting the Shaft" means turning the Shaft very slowly. "Ratcheting the Shaft" is performed by a ratcheting mechanism that is specified for each GT. The ratcheting mechanism MIGHT be a hydraulic device or an additional duty of the Startup Motor.  The Advanced Control System typically engages the ratcheting mechanism once it senses that the common Compressor-HP Turbine Shaft has stopped rotating during the Shutdown.

Even at a standstill, the GT internals are extremely hot. The Auxiliary Lube Oil Pump continues to circulate lube oil until sufficient heat has been transferred from the bearings.

Clogged Axial Compressor Rotor Blades will reduce the Discharged Pressure of the Axial Compressor by 5%. A dirty Axial Compressor means the GT Engine will never be able to attain full speed.  

Cleaning of the Axial Compressor is the last step of the maintenance interval because the GT Startup Steps of "Cranking" and "Warmup" (which will soon follow) insure that all fluids used for cleaning will be displaced by air.

A Water Wash Manifold/Ring is permanently situated in the inlet Air Duct specifically for the purpose of cleaning the Axial Compressor. The Startup Motor is more than likely used to rotate the common Compressor-HP Turbine Shaft during the cleaning operation.

The internals of the Axial Compressor will be inspected during the GT maintenance interval. The conclusions of the inspection will be used to select the cleaning media for the Axial Compressor, for example:  

• A solution of distilled water and detergent.
• A "Cocktail Solution" of distilled water, detergent, and a hydrocarbon like kerosene.
• An abradant cleaner like crushed walnut shells. The success of this cleaning option is highly dependent upon the service of the Axial Compressor.

 

The Axial Compressor must be rinsed with distilled water to remove detergents and hydrocarbon films that would otherwise remain on the Compressor's Rotor Blades.

ANY TIME THAT THE COMMON COMPRESSOR-HP TURBINE SHAFT IS ROTATED THE AUXILIARY LUBE OIL PUMP MUST BE ENGAGED.  This statement means the Auxiliary Lube Oil Pump must be engaged prior to the "ratcheting the Shaft" after GT Shutdown and "cleaning the Axial Compressor" prior to GT Startup.

Many thanks to Dinocu for allowing use of Water Wash of GE LM6000 You Tube.

©2019 PTOA Segment 0200

PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Movers/Drivers - Gas Turbine Focus Study

 

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