Hello everybody,

I need procedures for adjusting servo valve (MOOG) to Null bias value.

Thank you.

 

What is the MOOG servo-valve being used for?

How many electric coils are in the servo-valve?

 

If you are talking about the mechanical null balance, the procedure is, don't touch it. If it is the elctronic null bias let us know.

 

It is used to control the amount of fuel for GE gas turbine MS5001P (Frame 5). I believe there are two coils!

 

Electronic null bias?
Best Regard for all.

 

What makes you believe the null bias needs adjusting, either the mechanical null bias or the electronic null bias?

What control system does the unit have? TMR or SIMPLEX?

The rule of thumb for servo-valve current is -0.8 mA +/- 0.4 mA, TOTAL (the algebraic sum of the currents to each coil). The amount of curent to each coil should normally be fairly equal, and many people believe that if they are not equal that some mechanical or control system adjustment can be made to make the equal. If they're not equal, it's usually the result of some instrumentation or control system issue, but we don't know enough about the control system or the application. Adjusting the null bias without understanding the particulars usually results in more problems that are even harder to recover from.

As Bob Johnston said, don't even try adjusting the mechanical bias, particularly if one of the coil currents is not as it seems to be. There's only one spring, and adjusting it to bring in one coil's value will cause problems with the other coil(s).

 

Thank you CSA.

I will tell you the real problem, we have 4 units frame-5 and 2 units frame-6 GE gas turbine. Those units desgin to work at 50 degree C ambient.During summer at 40 degree C, all units stock up at 35 to 40% speed.At 25 degree C ambient all units reaches full speed smothly.We contacted one of the gas turbine service company and they said we have to check the null bais value of servo valve. Units have TMR.

 

In our line of business (Aerospace Testing) we make frequent adjustments of Moog servo valves. It is necessary anytime that a servo valve allows flow when its control cable is disconnected.

The adjustments are easy, but should be done with caution. Your best source for instruction (required tools, etc.) will be the vendor itself.

Begin your search for this information on moog.com

 

"Stock up"

Stock up.

Stack up?

Stop up?

Start up?

I don't understand "stock up".

Also, TMR control systems are made by a couple of companies. GE Mark IV Speedtronic? GE Mark V Speedtronic? GE Mark VI Speedtronic? Triconex?

Wouldn't it be easier to state what you want in your first question? And include the pertinent information?

 

Thank you, CSA. Sorry for misspelling. What I want to say, the speed doesn't reach 100%. It will stop at 40%. We have Mark-II system.

 

Thank you Bruce.

 

Some words of caution: Aerospace applications requiring servo-valves can be very different from the applications on GE-design heavy-duty gas turbines. GE does *not* recommend adjusting the mechanical null bias of any servo-valves used on GE-design heavy-duty gas turbines. This is usually clearly stated in the Control Specification document.

As for adjusting the null bias current value of a Mark II-controlled unit, you should be able to find the procedure in the Control System Adjustments portion of the Control Specification documents.

Mark IIs are not TMR control panels; they are more like SIMPLEX. TMR control panels use servo-valves with three coils; SIMPLEX and Mark II Speedtronic turbine control panels use two-coil servo-valves.

It's not clear what the organization who made the suggestion to check/adjust the null bias was told about the problem, nor why they made that particular recommendation. Servo-valves get improperly blamed for a lot of problems.

If the units are establishing flame during starting and are not accelerating past 40% speed on some occasions (which appear to be related to ambient temperature), I would suspect that something other than the null bias is the problem.

The purpose of using a two-coil servo-valve is so that if one coil fails the unit will continue to run on the remaining coil until the problem can be corrected. My recollection is that Mark II servo-valve outputs come from independent circuits (again, more reliability) and both of those would have to be misadjusted to cause a serious problem.

And, it's not clear why starting problems which appear to be related to ambient temperature would be associated with the servo-valve null bias.

Unless, someone's already been mucking with the mechanical null bias and so "adjustments" have been made to the null bias current settings. In my experience, and in Bob Johnston's also, it seems, adjusting the mechanical null bias of a servo-valve without the proper instrumentation and monitoring equipment with the proper instructions renders the servo-valve useless. An aerospace testing company could probably be expected to have the proper test equipment and instrumentation; most gas turbine power plants would not.

 

But if it's not servo-valve, what do you think causing the problem? Is it from starting motor, or torque converter? Also, we consider the mass of air as one of big factor causing this problem. One of our engineers suggests to increase IGV's angle but we refuse to do so. We installed our Frame5 gas turbines twenty years back, the problem started this year...

 

It's been longer than 20 years since I've worked on a Mark II, but if I remember correctly the startup acceleration was "open loop" meaning that the amount of fuel (VCE, I think it is) during acceleration is fixed and the rate of acceleration is presumed to be correct. In other words, the fuel isn't varied in order to achieve the desired acceleration rate.

You say you have multiple units and that the same problem occurs on all units. Is that correct? Can we then presume that it's something which is common to all units--like the fuel? Have you recently change fuel suppliers or have the characteristics of the fuels changed?

Cooler ambient temperatures mean the air is more dense. Denser air means it requires more energy to turn the compressor. Yet, you say that at 25 C all the units reach rated speed with no problem. 25 C is less than 40 C or 50 C. Something's not adding up here.

Later Speedtronic turbine control panels (Mark IV, Mark V, and Mark VI) all use something called temperature-corrected speed (TNHCOR) for calculating fuel valve positions. On colder days the fuel flow is biased up slightly to take into account the extra energy that's required to compress the denser air. On hot days, the fuel is reduced since less energy is required to turn the compressor since the air isn't as dense. This is all part of the "low cycle fatigue" reduction to reduce thermal stresses during starting.

I don't know if your version of Mark II uses temperature corrected speed (I doubt it, but it's possible) so there may be something wrong with that circuit.

But, for the servo-valves of all the units to be behaving similarly just doesn't seem to be a real possibility. I don't think we know enough about the problem.

If it were the torque converter, one would also think that all the units would not be similarly affected. Also, cooler air is denser requiring more work at lower ambient so if it were the torque converters it would seem they would have more problems at lower ambient than at higher ambient.

Increasing the IGV angle during starting at any ambient will increase the air flow through the compressor will require still more energy to turn the compressor. Also, increasing the IGV angle can contribute to compressor stall and/or surging which can mean catastrophic compressor failure. Not recommended in any case.

So, yes you have a problem. But if all four units are behaving similarly, it sure doesn't seem likely that all the servo-valves just started misbehaving similarly. (Again, servo-valves are improperly blamed for lots of problems, because they are so misunderstood.)

Have you tried replacing the servo-valves on one unit? What happened?

 

We haven't changed the fuel supplier since installation. Two days ago, we changed one servo-valve for one unit but the problem still there. So, we start to analyze the problem another way. We think there is no enough torque to drive the compressor,that means, there should be more fuel. Or the acceleration card should be revised.

 

You haven't changed fuel suppliers, but has the fuel changed?

Which servo-valve did you replace?

If your previous posts were correct, you are having trouble accelerating when the ambient is high (40 C) but no problem when the ambient is lower (25 C). The units require *more* torque when the ambient is lower because the air is more dense and the compressor has to work harder to push more air at lower ambient.

Increasing fuel when at 40 C will also increase the exhaust temperature during acceleration. Warmer ambient temperatures mean less air moving through the compressor and with the same amount of fuel the exhaust temperature will be higher. And, this could be part of the problem already. If the exhaust temperature is too high during acceleration (at the Isothermal temperature limit) the fuel will be reduced, which may be causing the unit to not be able to accelerate.

The fact this is happening on multiple units is the odd thing for me. Unless the units were recently "tuned" to the same values it wouldn't seem the control systems would be close enough to the same characteristics to cause all the units to behave similarly.

And, if the amount of torque from the starting means isn't enough during starting at higher ambients *and* the fuel causes the exhaust temperature to be so high that the amount of fuel is being limited to limit the exhaust temperature then this could be the problem. But, again, one would not expect the torque converters of all the units to be experiencing the same degradation.

Do you know that the exhaust temperatures are during starting at the lower ambient and the higher ambient?

If you choose to increase the fuel to accelerate the turbine, be careful to observe the exhaust temperatures. The thermal stresses caused by high exhaust temperatures during starting can be very hard on combustion parts (liners, nozzles, buckets, and exhaust components (diffuser, exhaust duct, etc.)).

 

Before you start looking at servo null bias I think we need to have a closer look at the start-up and where you get stuck. You are getting what is called a "hung start". When the unit stops at 40% speed, is the VCE control on Temperature Control. If it is, you are over fuelled, and temperature control is limiting the VCE and hence the fuel and you cannot get the speed up to get more CPD and start to cool the machine down a bit, hence the expression "hung-start". Make sure that all your VCE settings are as per the Control Spec. particularly ACC VCE on the SSKC card. If you are not on temperature control at the point of getting stuck, let me know and we can have a look at another few scenarios, but lets try and get the most obvious one out of the way first.

Remember, the higher the ambient, the less CPD you have and hence less cooling air so a hotter machine all round, that's why it will start at 25 Deg. but not at 40

Please do not stay at the hung start point for too long as it is very bad for the machine and can cause permanent damage if you stay there too long.As soon as you can see no increase in speed for a couple of minutes maximum, shut the machine down. Basically, you are running very hot with little cooling air and can start to do some thermal damage.

Good luck and report back your findings.

 

Do you have bogdown relay? it should function to shutdown the turbine if it is not accelerating.