Moog Servo Valve Operation


Thread Starter


How does a MOOG servo valve can a controller get feedback?? what is the difference betwen a moog valve connected to a simplex controller and TMR based triple controller?? how does a voting occurs if one controller fails??

Bruce Thompson

You have many questions.

How is it that you are asking so many questions? It appears that you have found yourself in a situation that you are unprepared for.

What is your background and what are you trying to solve?

Here is a link to a cutaway view of the internals of a Moog servo-valve on Page 4, Fig. 6, 'Jet Pipe Servovalve (1957):

That servo-valve in Fig. 6 is very similar to <b>but not exactly like</b> the Moog servo-valves used on GE-design heavy duty gas turbines. There is one <b>KEY</b> component missing from the drawing which will be described below. But in almost every other respect the servo-valve shown in Fig. 6 is nearly identical to the ones used on GE-design heavy duty gas turbines.

Note that the servo-valve shown in Fig. 6 is a two-coil servo-valve, and that the servo-valve in Fig. 6 is shown in the position that <b>shuts off the flow of hydraulic fluid to and from a hydraulic actuator.</b> That's because the Control Ports of the servo-valve are blocked by the Valve Spool (or spool piece). In the position shown in Fig. 6, internal hydraulic forces acting on the ends of the Valve Spool are keeping the Valve Spool in the position shown.

An electro-hydraulic servo-valve converts electrical signals to hydraulic fluid flow to and from a hydraulic actuator, and also stops the flow of hydraulic fluid flow to an actuator (in the position as shown in the drawing in Fig. 6). The hydraulic actuator might be used to position a fuel control valve, or the IGVs (Inlet Guide Vanes) of a GE-design heavy duty gas turbine, or a steam control valve. These devices (valves and IGVs) need to be stopped in an infinite number of positions anywhere between fully open and fully closed. By controlling the flow of hydraulic fluid to and from the hydraulic actuator the device can be opened and closed. The valve is opened or closed by allowing hydraulic fluid to flow to or from a hydraulic actuator. And, by shutting off the flow of hydraulic fluid to and from the hydraulic actuator the device can be held at any of an infinite number of positions between fully open and fully closed.

(Some older GE-design heavy duty gas turbines use servo-valves to position valves that only fully open or fully closed. Isn't this fun? But not on newer units.)

The electrical signals sent to the servo-valves are DC currents. And they are "bi-polar"--meaning they can be any value between -10 mA and +10 mA. Negative current increases the flow of fuel or air or steam. Positive current decreases the flow of fuel or air or steam. "Zero" current stops the flow of hydraulic fluid to/from the actuator, causing the hydraulic actuator--and the valve (or IGVs)--to remain at its current position. Notice "zero" is in quotation marks.

The device at the top of the servo-valve in Fig. 6 is a two-coil "torque motor" (sometimes servo-valves are called torque motors, too). The amount of mechanical force produced when current flows through a coil is directly proportional to the amount of current flowing through the coil. If the current flowing through the two coils <b>in Fig. 6</b> is 0.00 mA, then the Valve Spool will be just as shown in Fig. 6, and no hydraulic fluid will flow through the Control Ports of the servo-valve to or from the hydraulic actuator.

When there are three coils, each driven by a single controller (processor), the magnetic forces are "summed" by the torque motor. If one Speedtronic controller fails, its servo-valve output current will (usually) go to a large positive value, to shut off the flow of fuel or air or steam. But, if the other two controllers are working properly they will each put out some negative current to overcome the forces developed by the single positive current and the flow of hydraulic fluid through the servo-valve will be reduced to zero--keeping the device being positioned by the actuator at the present position.

(Some Speedtronic servo-valve outputs have suicide relays with NC (Normally Closed) contacts that are connected across the output terminals. Under normal operating conditions, the suicide relay is energized and the NC contacts are open. When a serious problem is detected the suicide relay is de-energized closing the NC contacts and preventing any current from being applied to that controller's servo-valve coil. In a TMR control system, the servo-valve outputs with suicide relays (every output does not always have a suicide relay) will use a single relay for each controller's output.)

By applying negative current to the coils in Fig. 6, the Valve Spool will move to cause hydraulic fluid flow to or from the hydraulic actuator to increase the flow of fuel or air or steam. If the current goes back to 0.00 mA, the Valve Spool in Fig. 6 will move back to the position shown in Fig. 6 and the flow of hydraulic fluid will be shut off and the hydraulic actuator will stop moving and remain in its present position which will keep the fuel flow or air flow or steam flow or pressure steady at the present flow-rate or position or pressure. Applying positive current to the coils will cause the Valve Spool to move in the opposite direction, changing the flow of hydraulic fluid to or from the hydraulic actuator to decrease the flow of fuel or air or steam. Again, if the current goes back to 0.00 mA, the Valve Spool will move to the position shown in Fig. 6 and stop any flow of hydraulic fluid to or from the hydraulic actuator--holding the hydraulic actuator in a stable position.

When the reference (position or flow-rate or pressure) for the device being positioned by the actuator is equal to the feedback from the process (position or flow-rate or pressure), then the regulator driving the servo-valve output currents has a zero error--and puts out zero current. The feedback is usually subtracted from the reference to develop an error. If the feedback is less than the reference the error is positive, and the regulator puts out negative current (to increase the flow or fuel or air or steam or position or pressure). If the feedback is greater than the reference the error is negative, and regulator puts out positive current (to reduce the flow or fuel or air or steam or position or pressure).

The point of all this is: to increase the flow of fuel or air or steam from zero it's necessary to put out a negative current. Once the desired position or flow-rate or pressure is reached, if the current is reduced to "zero current" the flow-rate or position or pressure will remain at its present value. If it's desired to reduce the position or flow or pressure it's necessary to put out a positive current, until the desired position or flow or pressure is reached and then by putting out "zero current" the present position can be maintained. More positive current the faster the flow or pressure or position will be reduced. More negative current the faster the flow or pressure or position will be increased. And, holding the current at "zero current" will hold the present position or flow or pressure. Note again, "zero current" is in double quotation marks.

Now for the <b>BIG DIFFERENCE</b> between the servo-valve shown in Fig. 6 in the link above and the ones used on GE-design heavy duty gas turbines: <b>There is a <i>spring</i> at one end of the Valve Spool.</b> That spring is called the "fail-safe" spring. It's purpose is to move the Valve Spool to a position that ports oil to or from the hydraulic actuator to shut off the flow of fuel or air or steam in the event that hydraulic pressure is lost or all currents to the torque motor are lost. That spring (there's only ONE spring) is always applying a force on the Valve Spool that must be overcome by the torque motor when it's desired to keep the device being positioned at some position other than fully open or fully closed by shutting off the flow of hydraulic fluid through the servo-valve.

It was already said that when the reference equals the feedback the regulator error will be zero, and the output current will be "zero current." But, the Speedtronic panel is capable of adding, and does add (continuously!), a "bias" (extra) current to the output which will overcome the fail-safe spring tension and keep the Valve Spool at the position shown in Fig. 6. That extra current is called "null bias" current. And it's always being put out by the Speedtronic, whether or not the regulator error is zero, because there always force being exerted by the fail-safe spring on the Valve Spool regardless of the position of the Valve Spool.

In a TMR control system with three coils in the servo-valve torque motor the amount of null bias current needs to be split evenly between the three controller servo-valve outputs. The GE specification says that the fail-safe spring requires a total of -0.8 mA, +/-0.4 mA to overcome the tension and keep the Valve Spool in the position that shuts off the flow of hydraulic fluid to the hydraulic actuator, keeping the flow or pressure or position stable and steady (usually called the "steady-state" position). (Negative current because negative current is required to increase the flow of fuel or air or steam or pressure). -0.8 mA divided by three equals -0.267 mA, per controller. And, there is a range, and an upper limit and a lower limit to the range! The range is -0.133 mA to -0.400 mA, per processor, meaning that the null bias current for any controller should never be less than -0.400 mA nor more than -0.133 mA. And <b>the same amount of null bias current should be set for each controller supplying current to a particular servo-valve</b>.




Servo-valves used with TMR control systems have three coils; those used with SIMPLEX control systems have two coils (yes, two, for redundancy; if one coil opens or the circuit for one coil opens the other will still be working and keep the turbine running). On some control system retrofits ("upgradations"--gotta love that word!) from a TMR control system to a SIMPLEX control system, the servo-valves are NOT replaced. One of the three coils is shorted to prevent it from adversely affecting servo-valve operation. (I can just anticipate the next question: Can a SIMPLEX, two-coil servo be used in a TMR application? Probably, but not without some machinations and chicanery that I wouldn't want to do or even try to explain how to try to do.)

The amount of null bias current for the single controller of SIMPLEX for a two-coil servo-valve is -0.8 mA, +/-0.4 mA. Not -0.4 mA (half of the value), but -0.8 mA. The Speedtronic circuitry takes into account the two servo-valve coils and does the necessary "halving" of the null bias current.

The other difference between the servo-valves used on GE-design heavy duty gas turbines with TMR control systems and the one shown in Fig. 6 of the link above is that the coils won't be mounted in/on the torque motor exactly as shown in Fig. 6. But, the forces will still be "summed" in the torque motor, which is the "voting" that's commonly referred to for servo-valve outputs.

Now, there's a lot of other stuff that goes on inside the servo-valve (jet pipe and internal hydraulic flows and Cantilever Feedback Springs) but that's all "background" stuff and not important to the function of the servo-valve and how it works on the gas turbines.

I'm not quite sure about the question about how a controller gets feedback. Feedback from the servo-valve? Or feedback from the device being positioned by the hydraulic actuator the servo-valve is controlling the flow of hydraulic fluid to and from? The regulator providing the servo-valve current (one per output per controller in a Speedtronic) gets feedback from the device (either position, from an LVDT; or flow-rate, from a speed pick-up; or pressure, from a pressure transducer) and sums it with the reference (from the sequencing or application code running in the controller) and the error determines how much, and the polarity of the, current being applied to the servo-valve coil.

I think all of your other questions have been answered. And then some.

Hope this helps!
thanks for reply Thompson..

basically im from instrumentation background new to the power plant industry..

i'm unable to know how an moog valve operates and can u please xplain in detailed.

how moog valve responds to the controller and how it works if one of its controller fails in TMR mode??
Bruce Thompson,

It's very ... scary to see the changes in personnel hired to operate and maintain power plants in the last three decades I have been working in the power generation industry. There are no more "apprenticeship" programs to speak of, and very few locales have certification requirements for operators and technicians.

So, supervisors and owners hire the lowest-cost individuals they can find in most cases. Unless there has been significant problems requiring specially qualified people.

Many supervisors and owners are even loathe to send their personnel to training, because in many parts of the world that's tantamount to giving them a "ticket" to shop around to other power plant supervisors and owners who will pay a little more (I've seen people change power plants for as little as USD1.00 per week) for training and experience--because they now don't have to pay for training that individual! It's very cut-throat in some parts of the world.

I've been in power plants where the manuals provided with the equipment are locked away and unavailable to operators and maintenance technicians. Many of them would gladly consult the documentation if it were available, but it isn't. If you could see the looks I have gotten when I, as an OEM representative, have asked why the manuals are locked away and if I could be granted access to them, you would wonder in amazement.

And that's not just limited to second- or third-world countries. Which is even the saddest part of the whole situation.

Even in North America, I have encountered combined-cycle power plant operators with less than a high school equivalency certificate operating and maintaining multi-million dollar plants who don't know, and haven't been trained or expected to know, how RTDs work, and how to use a voltmeter, or how to read and use a P&ID.

Some of the principles used in operating an maintaining power plants are complicated and difficult to understand, but I've rarely seen an operator or technician, who when given some instruction and good answers, didn't improve their skills greatly.

The worst trend I see these days, though, is that contrary to years gone by when supervisors would hire people who were smarter than they were and were respected and admired for doing it nowadays that's not the case. And that's really sad, because some of these people were promoted to their supervisory positions because they were the last person left standing in line for a supervisory position and don't really have the qualifications to perform any of the duties they are responsible for overseeing and can't really judge anyone's qualifications for those jobs. They are just given a list of eligible candidates to review and pick a couple to interview and select from, the list having been vetted by someone primarily looking for a few keywords on a resume or application and who will work for less than some amount set for an upper limit by someone who doesn't really know what a qualified person should be making to perform the duties skilfully and responsibly.

There is also little loyalty on the part of employers or employees these days, which leads to high rates of turnover so there are a lot of "entry-level" people working for lower wages than they might be earning if they'd worked for someone for several years or more.

It's quite a changed landscape. And it's pretty scary.

Control systems are expected to protect the equipment, and when they malfunction even in the slightest for any reason, people just don't know how to react--except to force signals to get and/or keep the unit running. Which is unfortunately usually the first thing operators and technicians seem to learn these days. One of the most frequent questions I'm asked on sites is, "How do we force that?" or, "Can (Can't) we just force that?" Not, "What's causing that?" or "How do we stop that from re-occurring?" or "What do I need to know to understand why that's happening?" Just, "Tell me the minimum I have to do to get or keep the unit running until the next shift comes in." On any continent, even in unionized environments.

Lastly, there is precious little documentation available on this subject--much to many people's shock and amazement. Even the OEM people don't have much to work with, and a lot of experienced people have retired or are retiring. I'm seeing a lot of problems that should have never occurred because young, inexperienced designers with little or no hands-on practical operating and maintenance experience are designing and sending things to the field that "work on paper" but don't work as advertised, and they don't know what to do, except to say, "Well, it should work!"

Hope this helps to understand how people can be asking these kinds of questions--not just here, but anywhere they can. Some of them are working thousands of miles from home and family, and need to be sending money home for them to survive. Many didn't have access to the education many first-world people do, and have managed to learn enough English to use their common sense and abilities to get gainful employment and are just looking to improve their lot in life any way they can.

I fault the people who hire and supervise them, not the people asking the questions. The world should be a very different place than it is.

I have a Moog D661-4570E Servovalve for a Hydraulic Dynamic loading application. Recently, a test was stopped in the middle and after that the moog is not responding (unable to see its output in the software interface). But, i have checked the Output voltage in the corresponding terminal, it looks fine. Kindly, help me in sorting this issue.


Antonio Perez

Thanks alot CSA for taking the time to share your knowledge and experience with us, my respects to you Sir.
Dear CSA,

Thanks for your superb informative response. Your passion and sincerity to share your experiences amazed me. Thank you.
As expected from CSA. Thank you

I have one thing I fail to understand regarding this valve. First, it seems that this valve is an open center meaning it allows oil to go back into tank when there is no current applied to the coils. As you see in the link down, the pressure of system while there is no need to actuate any valve (GCV,IGV,etc) is 1350 Psig. The pressure compensator of the axial hydraulic pump is set at 1825 psig approximately (1825 Psig is the pressure of the pump discharge, not the hydraulic header). So based on my understanding, in case there is no need to actuate any valve (GCV,IGV,etc) the pump will always pump oil. Am I correct ? I fail to understand the importance of the compensator being set at 1825 psig. Is the swashplate angle of the pump is perpendicular meaning there is no oil being pumped?

this is about the moog valve

this is P&ID of the hydraulic system in question


The hydraulic pumps are positive displacement, axial piston pumps (each with a swash plate and a pressure compensator). The flow-rate through the hydraulic system defines the angle of the swash plate, which increases or decreases the piston stroke as require to maintain pressure as flow-rate changes.

I believe the pressure in the first .pdf is for an example only--it's NOT for the system in use on the turbine at your site. Even if GE put this in a training manual or a Service Manual it's not specific to your turbine at your site. The discharge pressure of the servo-valve--be it to an actuator or to drain, is going to be slightly less than system pressure at the time (because of losses flowing through the servo-valve).

The pump compensator is to be adjusted to make the Hydraulic Pressure system equal to the Device Setting value--whatever that is. There is usually a pressure relief valve (with an air-bleed check valve) downstream of the pump and before the system header. That is there to protect the system in case of failure of the compensator, and I'm confident if you look at the Device Summary the setting of the pressure relief valve is to be set ABOVE the desired system pressure. Again--it's just there to protect the hydraulic system in the unlikely event the pump pressure compensator fails.

If--and this happens a LOT--the pressure compensator setting is used to raise the system pressure above the pressure relief valve setting and the pressure relief valve is adjusted to make the system pressure equal to the desired system pressure then there is flow through the relief valve at all times which is stressing the hydraulic pump (just look at the current drawn by the hydraulic pump when the relief valve is relieving versus when it's not!), and the protection afforded by the relief valve is taken away.

To check/adjust the pressure relief valve it's necessary to use the pressure compensator on the pump to increase the pump discharge pressure until the relief valve relieves, and adjust the relief valve if necessary, then run the pump pressure compensator back down to the desired system pressure (sometimes the pressure has to be reduced below that to get the relief valve to stop relieving).

System pressure has nothing to do with how the servo-valve operates--it's really about flow through the servo-valve. Yes; pressure is necessary for the actuators to work properly, but there has to be flow, too. Many actuators will move with a lower pressure, but not as fast or won't work properly (produce the force required at the rate required). But, if the flow-rate is less than required even at a lower than desired pressure the device isn't going to respond as it should.

I don't have access to a good .pdf viewer at this writing, so I couldn't zoom in on the P&ID to get as close a look as I would have liked. But, one of the beauties of the auxiliary systems ofo GE-design heavy duty gas turbines is that they are pretty similar for most machines and have been for a long time. So, I feel confident there is a relief valve for each pump, and that each pump has a pressure compensator. And, I'm also pretty confident that the pressure shown on the servo drawing (it's well colored, isn't it?) is just for exemplary purposes, and that the drawing was copied from some other application/source and was not amended to be specific to your site. Now, I could be wrong about that--I have been wrong before, will be many times in the future--but the discharge of the servo-valve when there is no flow to the actuator is to "drain", which is to the L.O. tank. And, it doesn't make any difference what that discharge pressure (perhaps the pressure drop across the servo valve is as high as (1825-1350=) 475 psig. But it doesn't have anything to do with how the servo operates. All of the above is still correct (including previous posts); lots of GE-design heavy duty gas turbines operate with lower system pressures with the same servo-valves. I've never measured the discharge pressure either to an actuator or to drain; because it's not necessary to understand or troubleshoot (it might helpful, in thinking about it, to know the drain pressure--but then it might not, either!).

The pump pressure compensator(s) is(are) how the system hydraulic pressure is to be set and controlled. The relief valves are just there to protect the system in the (unlikely) event the pump pressure compensator fails (or the swashplate gets stuck), and it's setting is to be higher than system pressure. Using the relief valve to set system pressure is not correct (though it's frequently done); it causes higher than normal flows through the pump which cause the pump couplings to fail prematurely as well as excessive pump motor current--which in some cases cause the pump TOL (Thermal OverLoad) to actuate and annunciate an alarm. Most sites just go crank up the range of the TOL to maximum to be able to reset the TOL--but that's not protecting the pump motor. In some cases, the current drawn by the pump under normal system operation results in pump motor current at or very near the motor nameplate rating, so higher currents will cause more heat and stress in the pump motor, and therefore a shorter motor life.

Hope this helps!
CSA, Thank you for your time.

The following picture might help.

The Pressure Compensator is usually set @ 1800 PSI.

The Hydraulic Pressure Regulator is set @ 1625 PSI

In case of Hydraulic Pressure Regulator Failure, VR23-2 (called common relief valve) will open @ 1815 PSI.

The Relief Valve VR21-1 is for lift oil and will open @ 3500 PSI.

Now, based on my understanding the hydraulic regulator will close once the back-pressure on it exceeds 1625 PSI. the pump will continue pumping oil but there is nowhere for the oil to go. The discharge Pressure will rise up to 1800 PSI, which triggers the compensator to open and the pump will stop the flow of oil. Am I correct? If yes, then when does these case happens? is it during actuating or where there is no need for actuation?

I think that when there is no need for actuation. there is small amount goes into the lube oil tank through the Moog Valve. if these happens, the hydraulic pressure will drop which causes the hydraulic regulator to open. and then the pump will pump oil. These is my understanding. please correct me if I am wrong.

Quite right thing you explained in brilliant sentences.
You might have great experience to feel all these in your lifetime.

But as per today's high demanding time, people are very very selective to teach. i knew only that with how many complicated situation i passed thru during my first 5 yr in a power plant.

That's why i use to spend at least 10hrs a week to train the newcomers and trainees.

I appreciate your effort CSA for giving such immense help to people.

Lastly...I expected you could have elaborated a little the answers he asked for.

Yes; we kind of got side-tracked on this one....

I believe most of the questions were originally answered in one of the responses below. If you still have questions or need clarification, please post them here or in a new thread and we'll try to answer them.

Perhaps one of the questions that wasn't directly answered was what happens when one coil loses it signal or the coil fails or the circuit goes open (all of which will cause the torque from that coil to be lost to the torque motor). When all is good, the torque produced by each coil serves to overcome the fail-safe spring tension and keep the device in a steady-state condition. The sum of the null bias currents overcomes the fail-safe spring tension to keep the device in a steady-state, stable position/condition.

When one coil of a TMR servo or a two-coil servo loses it's signal for whatever reason, the torque from that coil is lost to the torque motor--which means the fail-safe spring will try to shut off the flow of air or fuel to the machine which will cause the position/condition to change which will cause the remaining processor(s) (two in a TMR; one in a SIMPLEX or DUAL redundant system) to have to put out slightly more current to produce more torque to overcome the fail-safe spring tension and return the device to desired position.

The "voting" of a multi-coil servo occurs in the torque-motor of the servo as a result of the torque produced by the current flowing in the coils. If one coil is lost the fail-safe spring tension will not be balanced and the device will move to try to shut off the flow of fuel or air to the unit. In this case, the remaining processor(s) have to put out slightly more current to overcome the fail-safe spring and return the device to the desired position/condition. This means, when looking at the servo current(s) to the coil(s) they will be slightly more negative (because negative servo current increases the flow of fuel or air to the unit!) than the null bias current value magnitude.

One last thing about null bias current: <b>It's a RANGE</b>, not an exact value. The total servo current (the sum of the two- (for SIMPLEX or DUAL redundant), or 3 (for TMR)) is to be -0.8 mA, +/-0.4 mA, so from -0.4 mA to -1.2 mA is the allowable range of current. Since it's not possible to know exactly what the precise servo current is that's required by a particular servo, the ACTUAL servo current being output to a servo may be slightly higher or slightly lower than the null bias current value specified in the turbine control system regulator output. Again, that's because there's a range of allowable, acceptable servo current for any particular servo.

And it's also important to know that for a TMR control system the total current is divided between the three processors, and for a SIMPLEX or DUAL redundant control system each control processor must output the total current. So, for a TMR control system, each control processor will output -0.267 mA (or 2.67% in GE Mark-speak since the negative polarity is handled in the control processor firmware), and a SIMPLEX or DUAL redundant control system will output -0.8 mA (or 8% in GE Mark-speak). These null bias current values are more than adequate for the majority of applications.

When looking at servo currents on a TMR servo when the unit is operating and the device is enabled and working, they <i><b>should</b></i> all be nearly equal in magnitude and polarity, but if they aren't exactly equal to the null bias current value that DOES NOT mean there is a problem. Again, the null bias current specification from the manufacturer is a RANGE, and it's virtually impossible to know precisely how much null bias current any particular singe servo requires, so in actual service the amount of current actually being sent to the servo may be slightly higher or slightly lower than the null bias current setting in the controller. That doesn't mean there is a problem with the servo or the controller--unless there is a huge mismatch in magnitude or polarity for a particular coil.

For example, if the null bias current setting of a particular regulator of a TMR control system is 2.67%, and when looking at the three servo currents <R>'s value was -2.05%, <S>'s value was -2.59% and <T>'s value was -2.45% this would be very acceptable even though the individual servo currents weren't exactly equal nor exactly balanced nor equal to the null bias current setting. Remember--servo currents are changing anywhere from continuously (Mark IV) to 128 times per second (Mark V) to 100 times per second (Mark VI and Mark VIe) (there are some unusual systems with slightly higher or slightly lower regulator scan rates), and the HMI display can't possibly change values that quickly.

Hope this helps! It's not rocket science, and it's not mysterious or magical or mythical or closed. It is not well documented, but it is pretty basic and pretty simple. (Servos have been used for decades in LOTS of applications.)

I was traveling and wanted to get home to print these schematics and have a look at the "big picture."

The hydraulic system is basically a static system--I say that because when the hydraulically-actuated devices (fuel control valves; IGVs) are in a steady-state position then no hydraulic oil is flowing into or out of the actuator, which means no hydraulic oil is flowing into or out of the servo to the actuator. That's for hydraulically-operated devices which are in service.

For hydraulically-operated devices which are not in service there is still some minimum flow through the device, but it's not much. And for devices which are in service and at steady state there still seems to be an infinitesimally small amount of hydraulic flow going through the servo (which could just be dripping oil coming out of the tubing).

The system in the P&IDs you sent is one of the newer F-class machines which uses the hydraulic pumps for bearing lift oil pressure. So, that's why the pressure setting for the pump is so high above the regulated hydraulic oil pressure which is used for control valves and the IGVs. And also why the relief valve setting for the pumps is so high.

I don't believe your idea about dumping hydraulic oil through servos when there is no flow is correct. The internal pressure compensator in the pump senses low flow and reduces the angle of the swash plate to keep pressure in line with flow; as flow increases and pressure would tend to drop the internal pressure compensator will increase the amount of oil entering the axial piston--which increase the amount leaving the hydraulic piston (which is what makes it a positive displacement pump) to maintain pressure as flow-rate changes. There is a line coming off VPR4-3, OH-14 (OH-15 from VPR4-4), that goes back to the tank (L.O. Tank--which is where the fluid for the hydraulic system comes from). Not ever had the pleasure or physically seeing one of these types of combined hydraulic/bearing lift oil systems I can't say how much flow goes out of that line, or if it's just a packing leak-off of some sort. But, I think it's more like a packing leak-off than anything. The variable pressure regulator is going to open/close to try to maintain downstream pressure (in this case, regulated hydraulic oil system pressure).

I still maintain the pressures shown on the servo cut-away drawing are typical pressures for exemplary purposes only, and are NOT for the unit at your site.

When the servo spool piece is in the middle position I believe--from personal eyewitness observation over many years--there is minimal flow through the servo to drain, but there is some flow. Again, that may just have been oil draining from the servo/tubing that I was observing, but the flow never stopped even if it was a very small trickle.

I do not believe the servo is used as any kind of pressure regulator or flow control device for the regulation of the hydraulic oil system pressure--even on this type of system.

There is a component labeled on "S" on 20QB-1 which I'm not familiar with, and it seems to direct some flow to VR23-2, which dumps to drain (the L.O. Tank) through OLB-5. I might think this is to establish a minimum flow through the hydraulic oil pump(s) when there is no lift oil flowing and there is little flow to the control valves/IGVs during normal running operation.

Do you have access to the Service Manuals provided with the turbines? There are individual sections for each system, and each section should have a write-up describing system operation, along with any vendor instructions/manuals for the various system components (such as 20QB-1, VPR4-3 & -4, etc.). Have you tried reading the System Descriptions and studying the manuals for the components? I don't have access to manuals for a unit with this type of combined/hydraulic oil system, so I can't say what may or may not be written in that description. But, again, I'm very confident the servo valves are not assisting in any way with regulating hydraulic system pressure at any time. The servos are just flow control valves directing oil to/from the hydraulic actuators of the fuel control valves and IGVs, and they can cause hydraulic system pressure to decrease (when they are sending hydraulic oil to an actuator) or decrease (when they are blocking hydraulic oil from getting to an actuator). The hydraulic accumulator will absorb some of those pressure fluctuations, and the internal pressure compensator in the pump(s) will operate to maintain pressure as flow-rates change.

Hope this helps--and sorry it took so long to respond. It's been a busy start to this new year.
Dear CSA, first you have my highest regards for your time and your will to help others.

Actually I wasn't meaning that when the spool is in middle position, oil will be dumped into tank. what I mean there will be small flow through the nozzle (the nozzle near the flapper in the servo cutaway). I believe the constant flow through the nozzle is necessary when the spool needs to be actuated.

When you apply current to the torque motor, it will move the flapper either into the right or the lift (depending on the polarity) which serves to create a pressure difference at the two ends of the spool, this difference will move the spool valve into the lower pressure side. The more current you apply, the more flapper will move, the higher pressure difference will be there, and the spool will move further. This is why i believe that there will be always small amount of oil through the nozzle.

Now the hydraulic pressure regulator is regulating the pressure downstream. let's assume the pressure regulator is set @ 1625 Psig. Now for the hydraulic pump to pump small amount of oil downstream the hydraulic the regulator, I can't imagine it to be other than the case I will mention now : When the actuated load (IGV, etc) finishes its travel, spool valve will be in middle position. Small amount of oil will flow through the nozzle back to tank (the spool valve has an open center [hole] where oil can pass through it). that flow through the nozzle will create pressure, that pressure is slightly less than the setting of hydraulic pressure regulator which makes the opening of regulator so small. Small opening means small flow, the pump will sense that and decrease the swash-plate angle.

*Zero flow means the hydraulic pressure regulator is fully closed and pump will sense that and will make swash-plate perpendicular (no flow)

The "S" on the 20QB-1 is there because the 20QB-1 (Lift Oil Supply Valve) is solenoid-operated valve. Actually I don't know the purpose of the line that goes to the tank (OH-5), but 20QB-1 should be closed (de-energized) unless lift oil is required. Also near the "S" there is a sensing line which feeds the high pressure compensator on the hydraulic pump. The hydraulic pump has two compensators. One for lift oil (High pressure compensator) and one for the hydraulic oil (Low pressure compensattor). When 20QB-1 is energized, there will be pressurized oil to the high pressure compensator which makes the pump to operate on the high pressure compensator setting.

The mechanism is shown in this link:

I have access to the manuals and I have studied them more than one time.