GE MK5 servovalve


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What is the difference between FAL and FQROUT which are used by MK5 to control fuel of GE frame 5 gas turbine?
FAL is the servo valve current as a percentage, FQROUT is really FSR1 to all intents and purposes.
FQROUT (Liquid Fuel Flow Reference - Output) is the signal name assigned to the BBL (Big Block Language) block output in the CSP (Control Sequence Program) which is the "final" determination of the Liquid Fuel Flow-rate reference.

This signal is assigned (in IO.ASG) to the servo-valve output regulator which will be used to control the Liquid Fuel Bypass Valve.

FAL (Fuel Current - Liquid Fuel) is the signal name for the measured amount of servo-valve output current which is being applied to the coil(s) of the electro-hydraulic servo-valve on the Liquid Fuel Bypass Valve from the servo-valve output of the Mk V.

NOTE: Servo-valve output regulators compare the CSP reference (FQROUT in this example) to the feedback (defined by Regulator Type in the I/O Configurator, usually Liquid Fuel Flow Divider Feedback for Frame 5s) 128 times each second and adjust the servo-valve output current as required 128 times each second to make the feedback equal to the reference. HOWEVER, the amount of current (FAL in this example) is only written to the CDB (Control Signal Database) 4 times per second. Many people try to use VIEW2 at the 32 Hz rate to monitor servo-valve output current and are surprised when the signal only appears to change 4 times in 32 scans. They think there's something wrong with the servo-valve output; there's nothing wrong with the servo-valve output, it's just that the designers didn't think it was important to write the servo-valve output current values to the CDB more often than 4 times per second....

<p>on the io.asg, FAL and FQROUT are assigned as follows:
Q_Q_SVO3A FAL PCT ;TCQA-REG-CUR Liq fuel bypass valve servo current
Q_Q_SVO3 FQROUT PCT ;Q -QTBA-033 Liquid fuel bypass valve servo command
<p>the three servo-valve wires are taken from QTBA cards of R, S & T (33&35 screw terminals), I am wondering why they use FQROUT as command if it is (as you have mentioned before) reference value? Another question, does the servo-valve return to the null position after the adjustment of by-pass valve position? because -as I know- it is three position, four way, directional control valve.
One thing everyone MUST learn about engineers and documentation and procedures: Engineers, who are "trained" to be logical and methodical, are the WORST people at writing documentation and procedures--including comments and notes in files--of nearly any professional. This author includes himself in this description! (He just recognizes the trait of the profession and tries a little harder--mostly because he's been a victim of poor documentation and procedures for too many years...)

Engineers who are tasked with writing technical documentation (a task which most engineers consider cruel and unusual "punishment") write thinking that everything that is obvious and intuitive to them is obvious and intuitive to everyone else--therefore, there's no need to mention it in the documentation.

When engineers write procedures they usually list the steps which weren't obvious and intuitive to them, AND they don't necessarily put them in order. Also, if they actually wrote down each and every step, the procedure (1) would take forever to write, and, (2) would be very long and indicate that not much forethought was put into the process.

How many times have we all begun using some technical procedure (usually before reading it through completely, right?!?!) and gotten to Step 7 of the procedure and enountered a "step" which says, "Do [this] before [Step 4]" or "Make sure [this] is done before [Step 5]"? This has happened so many times to this author that EVERY procedure is read through at least twice BEFORE ever attempting to use the procedure--which usually results in a hand-writeen procedure which includes relisting all the "steps" like above in the proper order, and any steps which seem to have been "overlooked" or "forgotten" because they must have been obvious and intuitive to the engineer/writer.

This caveat with respect to engineer's writing includes "comments" and notes in ASCII text files associated with the Mk V. Everything after the semicolon ( ; ) is a comment and ignored by the compiler. One can argue that FQROUT, in this example, is the "command" output of the CSP (which it is) as opposed to the command output of the processors available on the screws of the QTBA termimal boards....

The caveat also includes any information contained in LONGNAME.DAT, which should be used as a GUIDE not a GOSPEL (in other words, what you read on a CSP--which comes from LONGNAME.DAT--should prove useful to understanding the signal but may or may not be completely accurate). [One definition of gospel is "...something which is accepted as unquestionably true..."]

So that comment referring to FQROUT as a "command" is nothing more than a note. How would you describe FQROUT knowing what you know now? How would you describe the actual servo-valve output? It is kind of confusing, since FQROUT is an input to the regulator whose output is available on screws on the QTBA terminal board....

It's even more confusing when you consider that the servo-valve output current (FAL) is basically at "zero" or null when the output is steady, even though the reference/command (FQROUT) might be at something like 56%....that's just how it works, though.

For the regulators of servo-valve outputs of a Mk V, one must have a reference and feedback. The reference is defined in IO.ASG as the CSP signal which is to be used by the assigned servo-valve output regulator which is connected to screws on the QTBA terminal board. The feedback is defined by the type of regulator defined in the I/O Configurator and the feedback that is hardwired to the regulator's input (refer to the Mk V Application Manual, Sect. 7, for detailed drawings and descriptions of the servo-valve outputs and regulators and feedbacks).

The servo-valve, as it is drawn on the drawings provided by GE with GE-packaged units, is "enclosed" by two straight lines--one on each side. This "envelope" indicates the valve is "infinitely positionable". The servo-valve may be drawn as as four-way valve, three-position valve--but if it has the "envelope" it is infinitely positionable.

The same is true for the Liquid Fuel Bypass valve schematic representation, and the Gas Control Valve, and the SRV (Stop-Ratio Valve), etc.--the valves may be drawn as two-position, two-way valves, but we all know that they can be operated in any position from fully open to fully closed. On GE drawngs they usually have the "envelope" also to indicate they are infinitely positionable.

The only time the servo-valve is NOT in the null position when the device is in service (in this example, when the unit is running on Liquid Fuel) is when the valve is being commanded to move to increase- or decrease fuel flow. When the feedback (Liquid Fuel Flow Divider RPM for the Liq. Fuel Bypass Valve of a Frame 5) is equal to the reference/"command" (FQROUT), the servo-valve is in the null position and no high-pressure hydraulic fluid flows into or out of the servo-valve to the actuator. Hydraulic fluid only flows when the servo-valve is NOT in the null position.

Some servo-valves have a very tiny flow through a "weep" hole in the filter to maintain a "miminal" hydraulic flow through the servo-valve when there is hydraulic pressure. But this flows just goes to drain--not to the hydraulic actuator.


P.S. Any errors or omissions or mis-spellings or mathematical mistakes are the fault and responsibility of the author, who, being an engineer, is documentation-challenged. "He might know whereof he speaks, but he has a hard time communicating in writing!"