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IGV Servo Positions Explanation
- Thread starter asarhan
- Start date
asarhan,
Servo-valves are not four-position valves; they do not only have four discreet positions and can be in only one of four positions.
The electro-hydraulic servo-valves used by GE for all hydraulically-operated devices (fuel control valves and IGVs, primarily) are infinitely positionable valves--and the horizontal straight lines drawn above and below the four positions "make" the device (the servo-valve in this case) infinitely positionable--meaning that it can be in any position between full flow to/from the actuator. Those two horizontal lines above and below the servo-valves are referred to as an "envelope" and, again, make the device they enclose infinitely positionable.
If you look at the fuel control valves, for example, those appear to be two-position valves, but we know they are not. And, most people miss the envelope which encloses those valves (which are drawn as open/close, two-position but we all know are infinitely positionable valves).
The rate of opening/closing of the device (the IGVs or control valves) is a function of the magnitude (amount) of the current applied to the servo-valve coils. More current (higher magnitude) means the servo-valve moves to permit more hydraulic oil to flow to/from (a higher flow-rate) through the servo-valve to the hydraulic actuator; less current (lower magnitude) means the servo-valve moves to reduce the flow-rate of hydraulic oil through the servo-valve to the actuator. And, zero* current applied to the servo-valve coils holds the servo-valve in a position that shuts off the flow-rate of hydraulic oil to/from the hydraulic actuator. [*"Zero" current refers to null-bias current, which is required to overcome the fail-safe spring in the servo-valve to "null" the flow-rate of hydraulic oil through the servo-valve.]
The direction of hydraulic oil flow through the servo-valve is controlled by the polarity of current applied to the servo-valve coils. Negative current increases the flow of fuel or air; while positive current decreases the flow of fuel or air. And, zero* current shuts off the flow of hydraulic oil to/from the hydraulic actuator--which holds the hydraulic actuator in a steady-state, or stable, position (position hold, as you described it). [*"Zero" current refers to null-bias current, which is required to overcome the fail-safe spring in the servo-valve to "null" the flow-rate of hydraulic oil through the servo-valve.]
And this doesn't just refer to the GE-design Frame 9E heavy duty gas turbine--it refers to the hydraulic servo-valves used for all Frame sizes of GE-design heavy duty gas turbines.
So, the servo-valves are not four-position devices--they are infinitely positionable devices, meaning they can be in any one of an infinite number of positions from one end to the other (from full flow in one director to no flow to full flow in the other direction). The "slow flow" position is defined by a small amount (magnitude) of servo current; a fast flow position means a large amount (magnitude) of servo current. And the direction is defined by the polarity of the current (positive or negative). And, "position hold" is defined by zero* current. [*"Zero" current refers to null-bias current, which is required to overcome the fail-safe spring in the servo-valve to "null" the flow-rate of hydraulic oil through the servo-valve.]
Hope this helps! (Great question, by the way!) There should be a document provided with the Operations and Service Manuals, usually in the same tab where the P&IDs are located, called the 'Piping Symbols Drawing.' The Piping Symbols Drawing shows all of the various symbols used on the P*IDs with a description of each, including the "envelope" symbol (the horizontal lines drawn above and below a device which "enclose" the device to indicate the device is infinitely positionable.)
Servo-valves are not four-position valves; they do not only have four discreet positions and can be in only one of four positions.
The electro-hydraulic servo-valves used by GE for all hydraulically-operated devices (fuel control valves and IGVs, primarily) are infinitely positionable valves--and the horizontal straight lines drawn above and below the four positions "make" the device (the servo-valve in this case) infinitely positionable--meaning that it can be in any position between full flow to/from the actuator. Those two horizontal lines above and below the servo-valves are referred to as an "envelope" and, again, make the device they enclose infinitely positionable.
If you look at the fuel control valves, for example, those appear to be two-position valves, but we know they are not. And, most people miss the envelope which encloses those valves (which are drawn as open/close, two-position but we all know are infinitely positionable valves).
The rate of opening/closing of the device (the IGVs or control valves) is a function of the magnitude (amount) of the current applied to the servo-valve coils. More current (higher magnitude) means the servo-valve moves to permit more hydraulic oil to flow to/from (a higher flow-rate) through the servo-valve to the hydraulic actuator; less current (lower magnitude) means the servo-valve moves to reduce the flow-rate of hydraulic oil through the servo-valve to the actuator. And, zero* current applied to the servo-valve coils holds the servo-valve in a position that shuts off the flow-rate of hydraulic oil to/from the hydraulic actuator. [*"Zero" current refers to null-bias current, which is required to overcome the fail-safe spring in the servo-valve to "null" the flow-rate of hydraulic oil through the servo-valve.]
The direction of hydraulic oil flow through the servo-valve is controlled by the polarity of current applied to the servo-valve coils. Negative current increases the flow of fuel or air; while positive current decreases the flow of fuel or air. And, zero* current shuts off the flow of hydraulic oil to/from the hydraulic actuator--which holds the hydraulic actuator in a steady-state, or stable, position (position hold, as you described it). [*"Zero" current refers to null-bias current, which is required to overcome the fail-safe spring in the servo-valve to "null" the flow-rate of hydraulic oil through the servo-valve.]
And this doesn't just refer to the GE-design Frame 9E heavy duty gas turbine--it refers to the hydraulic servo-valves used for all Frame sizes of GE-design heavy duty gas turbines.
So, the servo-valves are not four-position devices--they are infinitely positionable devices, meaning they can be in any one of an infinite number of positions from one end to the other (from full flow in one director to no flow to full flow in the other direction). The "slow flow" position is defined by a small amount (magnitude) of servo current; a fast flow position means a large amount (magnitude) of servo current. And the direction is defined by the polarity of the current (positive or negative). And, "position hold" is defined by zero* current. [*"Zero" current refers to null-bias current, which is required to overcome the fail-safe spring in the servo-valve to "null" the flow-rate of hydraulic oil through the servo-valve.]
Hope this helps! (Great question, by the way!) There should be a document provided with the Operations and Service Manuals, usually in the same tab where the P&IDs are located, called the 'Piping Symbols Drawing.' The Piping Symbols Drawing shows all of the various symbols used on the P*IDs with a description of each, including the "envelope" symbol (the horizontal lines drawn above and below a device which "enclose" the device to indicate the device is infinitely positionable.)
K
kwaku
Dear CSA,
Kindly clarify the following to clear my confusion please :
>The rate of opening/closing of the device (the IGVs or
>control valves) is a function of the magnitude (amount) of
>the current applied to the servo-valve coils. More current
>(higher magnitude) means the servo-valve moves to permit
>more hydraulic oil to flow to/from (a higher flow-rate)
>through the servo-valve to the hydraulic actuator; less
>current (lower magnitude) means the servo-valve moves to
>reduce the flow-rate of hydraulic oil through the
>servo-valve to the actuator.
Does the magnitude of current applied to the servo-valve corresponds to the rate(how fast)of opening/closing or how far the corresponding controlled device(IGV,Control Valve)must move?
for example, if a current of say -2mA is applied to the coils of a servo-valve as opposed to a current of say -1mA,would the -2mA make the corresponding valve move(open) faster than the -1mA or it is the distance/Angle covered that would be the difference with a constant rate of movement.
Thank you.
Kindly clarify the following to clear my confusion please :
>The rate of opening/closing of the device (the IGVs or
>control valves) is a function of the magnitude (amount) of
>the current applied to the servo-valve coils. More current
>(higher magnitude) means the servo-valve moves to permit
>more hydraulic oil to flow to/from (a higher flow-rate)
>through the servo-valve to the hydraulic actuator; less
>current (lower magnitude) means the servo-valve moves to
>reduce the flow-rate of hydraulic oil through the
>servo-valve to the actuator.
Does the magnitude of current applied to the servo-valve corresponds to the rate(how fast)of opening/closing or how far the corresponding controlled device(IGV,Control Valve)must move?
for example, if a current of say -2mA is applied to the coils of a servo-valve as opposed to a current of say -1mA,would the -2mA make the corresponding valve move(open) faster than the -1mA or it is the distance/Angle covered that would be the difference with a constant rate of movement.
Thank you.
kwaku,
The amount of opening is NOT controlled by the amount or polarity of the current. The electo-hydraulic servo-valves used on GE-design heavy duty gas turbines are NOT like other, more common types of positioners where an increase of 4 mA in a 4-20 mA positioner will cause a change of 25% of stroke (if it's a 4-20 mA for 0-100% stroke device). The common type of 4-20 mA positioner is not bi-polar, either; it's unipolar (meaning the current is one directional, positive).
The amount (magnitude) of current flowing through the servo-valve coils affects how far the spool piece in the servo-valve moves, which affects the flow-rate through the servo-valve which affects how fast the hydraulic actuator moves.
The polarity of the current controls which direction the spool piece moves, which affects the direction of travel of the hydraulic actuator (opening or closing).
The regulator in the Speedtronic compares the reference to the feedback and adjusts the amount and polarity of the current as required to achieve and maintain position. In Mark V, Mark VI and Mark VIe turbine control panels, this is done at rates as high as 128 times per second, sometimes higher (depending on the type of turbine).
So, if the actual position/flow equals the reference position/flow the current is zero* (null bias current, actually). If the device has to open from the current position then some negative current is applied, and depending on the error between the actual and reference the amount (magnitude) is determined. As the actual approaches the reference, the magnitude is reduced until the current reaches zero* (null bias current) again. Since null bias current must overcome fail-safe spring tension trying to reduce the air or fuel flow the null bias current is a slight negative (magnitude).
Hope this helps clear the confusion!
The amount of opening is NOT controlled by the amount or polarity of the current. The electo-hydraulic servo-valves used on GE-design heavy duty gas turbines are NOT like other, more common types of positioners where an increase of 4 mA in a 4-20 mA positioner will cause a change of 25% of stroke (if it's a 4-20 mA for 0-100% stroke device). The common type of 4-20 mA positioner is not bi-polar, either; it's unipolar (meaning the current is one directional, positive).
The amount (magnitude) of current flowing through the servo-valve coils affects how far the spool piece in the servo-valve moves, which affects the flow-rate through the servo-valve which affects how fast the hydraulic actuator moves.
The polarity of the current controls which direction the spool piece moves, which affects the direction of travel of the hydraulic actuator (opening or closing).
The regulator in the Speedtronic compares the reference to the feedback and adjusts the amount and polarity of the current as required to achieve and maintain position. In Mark V, Mark VI and Mark VIe turbine control panels, this is done at rates as high as 128 times per second, sometimes higher (depending on the type of turbine).
So, if the actual position/flow equals the reference position/flow the current is zero* (null bias current, actually). If the device has to open from the current position then some negative current is applied, and depending on the error between the actual and reference the amount (magnitude) is determined. As the actual approaches the reference, the magnitude is reduced until the current reaches zero* (null bias current) again. Since null bias current must overcome fail-safe spring tension trying to reduce the air or fuel flow the null bias current is a slight negative (magnitude).
Hope this helps clear the confusion!
K
kwaku
Dear CSA,
That was a great one!!!!
Confusion "over cleared"
I am most grateful.
>The amount (magnitude) of current flowing through the
>servo-valve coils affects how far the spool piece in the
>servo-valve moves, which affects the flow-rate through the
>servo-valve which affects how fast the hydraulic actuator
>moves.
That was a great one!!!!
Confusion "over cleared"
I am most grateful.
>The amount (magnitude) of current flowing through the
>servo-valve coils affects how far the spool piece in the
>servo-valve moves, which affects the flow-rate through the
>servo-valve which affects how fast the hydraulic actuator
>moves.
>Dear CSA,
Just want to understand is there any new approach here with the terms slow close and fast close or not?
as you have mentioned with a positive current sent to the servo valve the hydraulic actuator would travel to close position till the SP = PV so the null bias would be the current sent to servo to hold position.
So is there any automatic gain change at the regulator that would change the magnitude of the current to have a fast close travel (gain bigger than the original one) else why we are calling it fast close?
thanks in advance
Just want to understand is there any new approach here with the terms slow close and fast close or not?
as you have mentioned with a positive current sent to the servo valve the hydraulic actuator would travel to close position till the SP = PV so the null bias would be the current sent to servo to hold position.
So is there any automatic gain change at the regulator that would change the magnitude of the current to have a fast close travel (gain bigger than the original one) else why we are calling it fast close?
thanks in advance
isulamu,
The new terms are those of the original poster; they're nothing I've ever seen before in more than 30 years in the business.
Nothing like an automatic gain change. What part of the magnitude of current affecting the servo valve position which in turns controls the flow-rate of hydraulic oil to the hydraulic actuator which controls the rate of movement of the device (IGVs or control valve) do you not understand?
If the error between the actual position/flow and the reference position/flow is high then the amount of current will be high which will move the spool piece farther. As the error gets closer to zero, the current will get closer to zero* (null bias current) and the spool piece will move to the position that shuts off the flow of hydraulic oil to the hydraulic actuator which will hold the device (IGVs or control valve) in a stable position.
No magic here, or there. What you see is what you get; nothing more, and nothing less. Very straightforward; no smoke and mirrors.
Servo-valves are NOT magical, mythical devices--contrary to popular (and mistaken) belief. Even 4-20 mA positioners do the same thing in the end, making the actual equal to the reference. It's just "simpler" because there is a unipolar signal that is proportional to position (in the majority of cases). But, the actual position must still be compared to the reference and the actuator must be driven to the proper position, to make the actual equal to the reference.
I know it's all in what you learn, but this is what I learned--and, to me, it's very simple. No magic; no myths.
The new terms are those of the original poster; they're nothing I've ever seen before in more than 30 years in the business.
Nothing like an automatic gain change. What part of the magnitude of current affecting the servo valve position which in turns controls the flow-rate of hydraulic oil to the hydraulic actuator which controls the rate of movement of the device (IGVs or control valve) do you not understand?
If the error between the actual position/flow and the reference position/flow is high then the amount of current will be high which will move the spool piece farther. As the error gets closer to zero, the current will get closer to zero* (null bias current) and the spool piece will move to the position that shuts off the flow of hydraulic oil to the hydraulic actuator which will hold the device (IGVs or control valve) in a stable position.
No magic here, or there. What you see is what you get; nothing more, and nothing less. Very straightforward; no smoke and mirrors.
Servo-valves are NOT magical, mythical devices--contrary to popular (and mistaken) belief. Even 4-20 mA positioners do the same thing in the end, making the actual equal to the reference. It's just "simpler" because there is a unipolar signal that is proportional to position (in the majority of cases). But, the actual position must still be compared to the reference and the actuator must be driven to the proper position, to make the actual equal to the reference.
I know it's all in what you learn, but this is what I learned--and, to me, it's very simple. No magic; no myths.
hello,
any one to give for us help
the servovalve does not follow input command signal ,and i want to purge this servovalve please any one to give for us the procedure how to can we purge without remove it from the line
any one to give for us help
the servovalve does not follow input command signal ,and i want to purge this servovalve please any one to give for us the procedure how to can we purge without remove it from the line
chitourh01,
What turbine control system does the turbine have?
If the turbine control system is a GE Mark*, can you tell us what Diagnostic Alarms are present that are related to the IGV servo output and LVDT feedback? (It might just be easier to send a screen shot of the Diagnostic Alarm display--they can be attached to thread responses on Control.com. Just please make sure if you take a photo of the display that it is a clear photo.)
What Frame size is the turbine?
What does the turbine drive (a generator or a centrifugal compressor)?
Have you checked/changed the filter at the IGV servo manifold block?
What is the oil quality in the turbine? Is it tested regularly? Do the test results meet GE's recommendations?
What is the differential pressure across the hydraulic system filters?
What is the hydraulic system pressure?
What should the hydraulic system pressure be (can be found by looking up the setting of the hydraulic pressure pump compensator in the Device Summary)?
Have you tried using the turbine control system to move the IGVs when the unit is not running (at rest, or on Cooldown)? If so, what happens?
Have you verified the accuracy of the IGV LVDT calibration? (I'm NOT asking if the IGV LVDTs were recently "calibrated"--I'm asking if the accuracy of the IGV LVDT calibration has been verified by using the turbine control to move the IGVs to some position, say 54 DGA, and then measuring the IGV position (using the pointer on the side of the compressor casing or a machinist's protractor in the inlet) to see what the ACTUAL physical position of the IGVs are, and compare that to the LVDT feedback on the screen?) LVDTs rarely drift, unless they become mechanically loose or damaged, so re-calibration is rarely necessary. BEFORE re-calibrating LVDT feedback, the accuracy of the existing calibration should ALWAYS be verified as described above, and possibly at two or three positions (54 DGA, 70 DGA, 84 DGA), and then after analyzing the results if the calibration accuracy is insufficient then, and only then, should the IGVs be re-calibrated.
I sense English is not your primary language, so, please try to answer the questions (all of them!) to the best of your ability. We can't help without more information.
Most of the electro-hydraulic servo-valves used on GE-design heavy duty gas turbines do have a very small diameter filter which can be removed and cleaned and re-installed. BUT, before doing so, one needs to make sure they are turning the proper screws to remove the filter AND the area where the servo is located should be cleaned VERY WELL so that dirt and contaminants don't get accidentally into the internal servo passages when the filter is removed or re-inserted.
If you decide to replace the servo-valve, know that it IS NOT necessary to calibrate the LVDTs if all you did was replace the servo-valve. BUT, it is necessary to perform a polarity check of the servo currents being applied to the servo to be sure they are all correct (this procedure has been detailed MANY times in previous threads on Control.com and can be found using the 'Search' feature of Control.com). In fact, if you're having servo issues, it's entirely possible you might want to perform a servo current polarity check just to make sure they are all correct; it's too common to find one coil with the incorrect servo current polarity driving the coil--which can make small problems appear worse.
Help us to help you by providing as much information as you can. We look forward to hearing back from you.
What turbine control system does the turbine have?
If the turbine control system is a GE Mark*, can you tell us what Diagnostic Alarms are present that are related to the IGV servo output and LVDT feedback? (It might just be easier to send a screen shot of the Diagnostic Alarm display--they can be attached to thread responses on Control.com. Just please make sure if you take a photo of the display that it is a clear photo.)
What Frame size is the turbine?
What does the turbine drive (a generator or a centrifugal compressor)?
Have you checked/changed the filter at the IGV servo manifold block?
What is the oil quality in the turbine? Is it tested regularly? Do the test results meet GE's recommendations?
What is the differential pressure across the hydraulic system filters?
What is the hydraulic system pressure?
What should the hydraulic system pressure be (can be found by looking up the setting of the hydraulic pressure pump compensator in the Device Summary)?
Have you tried using the turbine control system to move the IGVs when the unit is not running (at rest, or on Cooldown)? If so, what happens?
Have you verified the accuracy of the IGV LVDT calibration? (I'm NOT asking if the IGV LVDTs were recently "calibrated"--I'm asking if the accuracy of the IGV LVDT calibration has been verified by using the turbine control to move the IGVs to some position, say 54 DGA, and then measuring the IGV position (using the pointer on the side of the compressor casing or a machinist's protractor in the inlet) to see what the ACTUAL physical position of the IGVs are, and compare that to the LVDT feedback on the screen?) LVDTs rarely drift, unless they become mechanically loose or damaged, so re-calibration is rarely necessary. BEFORE re-calibrating LVDT feedback, the accuracy of the existing calibration should ALWAYS be verified as described above, and possibly at two or three positions (54 DGA, 70 DGA, 84 DGA), and then after analyzing the results if the calibration accuracy is insufficient then, and only then, should the IGVs be re-calibrated.
I sense English is not your primary language, so, please try to answer the questions (all of them!) to the best of your ability. We can't help without more information.
Most of the electro-hydraulic servo-valves used on GE-design heavy duty gas turbines do have a very small diameter filter which can be removed and cleaned and re-installed. BUT, before doing so, one needs to make sure they are turning the proper screws to remove the filter AND the area where the servo is located should be cleaned VERY WELL so that dirt and contaminants don't get accidentally into the internal servo passages when the filter is removed or re-inserted.
If you decide to replace the servo-valve, know that it IS NOT necessary to calibrate the LVDTs if all you did was replace the servo-valve. BUT, it is necessary to perform a polarity check of the servo currents being applied to the servo to be sure they are all correct (this procedure has been detailed MANY times in previous threads on Control.com and can be found using the 'Search' feature of Control.com). In fact, if you're having servo issues, it's entirely possible you might want to perform a servo current polarity check just to make sure they are all correct; it's too common to find one coil with the incorrect servo current polarity driving the coil--which can make small problems appear worse.
Help us to help you by providing as much information as you can. We look forward to hearing back from you.
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