Dear All,

We have noticed problem with Moog servo valve once it cleaned with thinner.

Background:
In the pass two major inspection we had problem with startup, Speed /stop ratio valve (SRV) was hunting and impact on load. It was rectified after replaced Moog valve with new one. Earlier it was working normal. We did nothing except servo filter replacement and cleaning with thinner.

Is it good practice to clean Moog valve with thinner or what is your opinion for Moog servo valve service?

take care
G.Rajesh

 

Dear Rajesh,

You asked for opinions, so that I what I will offer. My opinion is to not attempt to clean any servo with any type of cleaning product. There is the risk that internal seals and coil insulation could be damaged by cleaners that are not compatible.

The service life of any servo is going to be impacted mainly by the quality of the control oil in use and by the type of service the servo is used in.

The most common failure I am aware of for most servos is contamination from oil varnish, water or other oil degradation or lack of cleanliness.

The second factor that influences service life of the servo is how it is operated. A servo in use by a turbine running 24/7/365 at baseload will not really be doing much since most valves will remain more or less in one position, this assumes steady gas pressures etc. This is in contrast to a unit that starts and stops daily, or cycles its output up and down throughout the day. This would have the tendency to cause more wear on the flapper and control oil nozzles, as the servo modulates the GCV's and IGV's etc.

As far as maintenance practices for servos I have not seen a definate answer as to how to approach this. And since each plant is different it is hard to have a hard and fast rule that works for everyone. If your plant runs 24/7/365 and is critical then some time based maintenance of servos might work best for you. If you do not run often and can afford to shut down to replace a malfunctioning servo then your practice would be different.

But in short I would not attempt to clean a servo. Changing filters is about as far as I like to go. When a servo needs maintenance we use a vendor who will recondition them for us to like new status, and provide a detailed report on what was needed so we can attempt to better predict the life and health of other servo's in use.

 

Good answers MIKEVI. Servo maintenance and life has been an eternal problem on GE turbines. One thing is sure, servo problems are directly related to lube oil quality, as Mike says, lacquering is the biggest problem and this is accelerated by low TAN No. oil. I have never heard of anyone trying to clean servos with any kind of solvent and would strongly advise against it. Again, Mike is right, how you deal with servos depends on the critically of your plant. We are running an LNG plant and downtime is mega expensive. We replace all our servos every maintenance outage and have found it to be cost effect.

 

Dear MIKEVI,

Thank you for your feedback,

Well, we will avoid to clean with thinner in future.

Since we have replaced many servos, we would like to have servo valve test kit. In near future we may buy one kit.

Will the test kit useful to detect any problems and correct it?

Thanks in advance

Take care
G.Rajesh

 

Dear Rajesh,

I do not have any experience with, nor was I aware of any type of tester for servos so I can offer no advise or experience to that subject.

As mentioned before and from Mr. Johnston as well, oil contamination is one major source of servo failure. If you are truly failing many servos I am sure you are curious to find out why. We have machines that are still running using the same servos installed back in 1985.

My suggestion would be to send a faulty servo to a reputable facility for inspection/diagnosis and repair and require a detailed report from them. Hopefully from the findings you can decide how to proceed. If you are failing servo's due to oil contamination then you can come up with a plan of attack of how to repair.

We did a test similar to this. We removed two servos in service for 5 years that were still working properly with no problems.

We sent one in for repair(to a company that sounds like OOG) by the way. We told them exactly what the situation was, and told them we wanted a detailed report of condition(mechanical and especially any signs of varnish), and a estimate to repair. We were very disappointed in what occurred with them. They disassembled the servo, reported that control wires were cut, the flapper and ports were completely worn out and servo was not worth repairing, and should not have even been working. We got back a useless bag of parts.

The second servo went to a company that specializes in repair of all types of electrohydraulic components. We gave them the same instructions. They disassembled the servo and reported back that the servo was in very good condition and recommended a basic level rebuild and noted that there were no signs of varnish or any type of oil contamination. What we got back was a reconditioned servo that went right back into service and worked as new.

My point is not to bash any company. What we learned was to find a reputable company, and provide totally clear instructions for what we wanted and what our expectations were. We wanted to find a way to estimate the health of our servos and oil system, and come up with an educated plan to maintain them that was cost efficient and minimizes downtime.

I wish you luck in coming up with your own plan that accomplishes what you want and need!

Cheers.

 

That story of going to a major vendor (perhaps an OEM????) sounds very familiar for many components. In my experience it appears that independent repair shops that specialize in repair/overhaul of components are usually able to provided excellent value by keeping a usable component in service for many more years--many times when the OEM says it is "no good".

An independent usually MUST provide good value and service to stay in business. A very large OEM...........

Of course people often says the OEM guarantees the product better; but in real life I've yet to see this work out this way. Usually the independent is the one that will answer the phone when the big guy forgot who you are LOL.

On another note, I wonder if synthetic oil would alleviate many problems with servos. I haven't heard of varnish problems on the aero-derivative side and they run synthetic oils.

 

Thank all for your replies.

We used to sent servo valve for repair, in many cases the amount is equel to new one or it may not work in some cases. Hence we are going for the new one.

Now we have decided to get servo valve test kit. If anybody having experience please share with us.

Take care
G.Rajesh

 

G.Rajesh,

I've had a look at the Moog site and found two different servo testers/test kits.

Exactly how would you use this test kit? One is pretty much a bipolar output device (neither of the available tester output impedances matches that of the typical servo-valve used on GE-design heavy duty gas turbines).

And if you are going to disconnect the Speedtronic leads and connect the tester leads to drive the servo, what exactly will you be proving by doing so?

Have you ever recorded a problem with the Speedtronic servo-valve output being failed high or low or not changing (except when suicided at zero output)?

Aren't most of the problems being experienced with servo-valve related to varnishing or filter blockage or just age/wear--of either the servo-valve or the actuator the servo-valve is porting oil to and from?

How would a tester help with any of the above problems--unless you have had repeated problems with the Speedtronic outputs and you are trying to prove the problem is with either the Speedtronic outputs or the servo-valve's ability to control device position?

You don't need a fancy, bipolar output device. Any DC power supply capable of pushing +/- 10 VDC into 1000 ohms will work if you just want to verify the servo will work with a source other than the Speedtronic. You only need one servo coil to be driven by a current with the proper polarity to be able to see the if the device will move and maintain position (if you can control the power supply output to maintain a suitable null bias current).

Heck, most AC mains-powered DC power supplies can supply much more current than required by all three coils of a servo-valve, and all three servo-valve coils could be driven by connecting them in parallel to one power supply. (Polarity is still critical!)

These servo-valves are not rocket science.

This has been said before on control.com, at least once. The oil companies have subtly changed their turbine lubricating oil formulations over the past decade-and-a-half. They have improved the lubricating characteristics immensely, but for those machines that use lube oil for hydraulic systems, too, this formulation hasn't worked out so well if temperature and cleanliness are not properly maintained. (And lube oil cleanliness is MUCH more than simple filtration!)

I believe it's Castrol, or BP-Castrol, that have developed a suitable lube oil. If I recall correctly it was called Perfecto or Perfecto XPG or something like that. GE worked with owner-operators in the UK to test the oil in several units in the UK with excellent success. I was told that GE was going to be promoting such a lube oil to owners/operators of GE-design machines.

So, the real problem isn't the mystical, magical, mind-bending servo-valves themselves. It's the type of oil being used, but in my opinion it's more about the cleanliness of the lube oil. There are LOT of new machines, and older machines with these new lube oil formulations, that don't experience servo-valve problems like some sites continually seem to experience. What's the difference? Maintenance practices, and specifically, pro-active maintenance practices.

I just recently consulted with a site which had installed a new turbine control system on a 20+ year-old turbine-generator about a year ago and everything was running fine until a few weeks ago when servo-valves started "failing" at an alarming rate. Of course, the last thing to have changed was the turbine control system, so it MUST be the new turbine control system causing the problem. Right?

When the owner/operator was asked when was the last time they had done any maintenance on the actuators on the gas valves and the IGVs, they replied that in 20+ years of operation they had never done any maintenance on the gas valve or IGV actuators.

When asked about lube oil maintenance the owner/operator replied they had never changed the lube oil or had it "polished" (run through some kind of centrifuge or extremely fine filter system); they had only changed lube oil- and hydraulic filters when the differential pressure indicated the filters needed to be changed.

When asked when was the last time they had sent a lube oil sample to a lab for analysis, they replied they had no record of ever doing so, though one control room operator recalled having done so about 15 or 17 years prior. And this was a base load plant in a critical process industry.

I subsequently heard that when a L.O. sample was tested it was found to have tens of multiples of the "allowable" amount of total dissolved solids (smaller than the smallest filter could remove).

Further, they took a weekend shutdown and found excessive wear on one of the gas valve actuators, so much wear the servo-valve couldn't supply enough oil to reliably control the valve position at a particular position--which was where the unit most commonly operated. So the turbine control system was putting out an excessive amount of servo current to pass a large volume of oil to the actuator which was leaking out of the actuator causing the actuator to not be able to maintain position causing the turbine control system to put out much higher than normal servo currents, particularly at that one operating position.

And, this was a turbine control system problem! According to the Customer. Of course, the old control system (which had been replaced slightly over a year ago) never had the same problems, so it had to be the new control system causing these servo problems. Right?

Lube oil maintenance is a multi-billion USD industry, but it's more than just an industry. It's a necessary practice. All of this talk of failed or failing Moog servo-valves with the same or similar results in a very short period of time only serves to improve the profit margins of Moog Corporation. If people would look an the entire system and analyze all the components (including fluids) they would start to understand how these little servo-valves can fail.

And it's <b>extremely</b> rare for servo-valves to bail because of the Speedtronic control panel servo-valve outputs. I've seen Speedtronic control panel outputs damaged by failed servos, or more correctly, by well-intentioned but mystified maintenance technicians trying to jumper this or that or "force" something to happen.

But, G.Rajesh, please tell us how you would be using either of the two servo-valve testers to prevent, reduce or detect servo-valve failures?

 

Dear CSA & All,

Thank you very much CSA for your detailed reply

Well, we intend to have servo valve tester because we are having 40-50 servo valves which replaced in the past. (We have 3 frame-6B & 2 frame-9E).

We are not sure all may not bad. To test all the servo's the kit may helpful to identify which is really damaged.

We are replacing oil at regular interval and sending for analysis. Earlier we used Shell oil and now using Total oil.

In many occasions which is working good giving trouble during startup. Hence we are thinking, are we doing over maintenance on servo? (by cleaning with thinner). So we keen to know what is best practice of service for servo is concern.

In frame-6B we are doing fuel change over test from gas to liquid. In most of the liquid fuel change over test VC3 won't open and it'll be alright once servo replaced. Here no doubt for wear & tear as in distillate we are running for short time and it's ideal for month. So what we replaced it may be good.

If plan to send all servo valves (40-50) for repair that cost will equal (or more) to test kit and moreover test kit will be an asset in future.

If anybody having an idea for best service practice for servo valve/servo valve test kit, please share with us.

Take care
G.Rajesh

 

G.Rajesh,

We understand that you want to use some means to verify if a servo-valve has actually failed or not and that you want to purchase a servo-valve tester to aid in this effort.

I am specifically asking how you will use the tester to accomplish this task. What do you think or what have you been told the tester can be used to indicate?

Will the tester be connected to the servo-valve in place of the Speedtronic servo wiring while it is still installed on the turbine?

Or, will you remove the servo-valve from the turbine to be tested? If so, how will the tester be used to confirm proper operation of the servo-valve? Does the tester have a hydraulic oil reservoir and pump and a means of measuring flow-rate through the servo-valve?

I m really trying to understand how this tester will be used and how it will help to confirm proper operation of the servo-valve.

If you are buying a servo-valve tester from Moog or one of its representatives have you asked them if there is a suitable fluid which could be used for cleaning servo-valves periodically without causing harm to the valve components?

Have you been periodically replacing the servo-valve internal filters?

Finally, if you have received any kind of information or documentation about the servo-valve tester from the manufacturer or supplier of the tester can you post a copy of it to a web-sharing/hosting site (for example, tinypic.com, or www.speedyshare.com, or www.2shared.com) and then post the URL to this thread for others to be able to examine the information?

Thanks!

 

Dear CSA & all,

Please view the following link for more details about servo valve tester. if have any idea please share again.

http://dietzautomation.com/download/Valve Expert 01 - English.pdf

Take care
G.Rajesh

 

G.Rajesh,

Thank you for the link to the document.

It must be nice to work for a company that has so much money to be able to afford such a servo tester.

In addition to the cost to purchase the servo tester, there is going to be the cost to train at least one person (and what if that one person leaves the company or retires, or worse?) to be able to use the servo tester.

There will be the cost to set up the various test manifolds to be able to test all of the servos (some servos are used with single-acting actuators, some with dual-acting servos).

And there will be the cost to actually test all of the servos.

Each time there is a suspected problem with a servo valve installed on a turbine it is going to have to be removed to be tested. Either a known working servo will have to be installed at the time the suspect servo is removed in order to be able to re-start the unit or there will be lost production/generation while the servo is removed for testing, tested, and then either re-installed or replaced with a known working servo.

I think MARKVI had a better idea: Find a vendor who can test these servos for you and provide you with a detailed report (based on the criteria you supply and wish to see the results of the tests for), and can rebuild the servos if possible and return them to you. Personally, I have not had good luck with servos rebuilt by the original manufacturer. Perhaps MARKVI can provide the name and contact information of the vendor his company uses for others.

And, I still argue that something other than the servos themselves are the problem. Certainly, cleaning them with thinner doesn't seem to be working, for whatever reason. There is an underlying reason for these servos to be "failing" in such numbers as you have described and unless you are buying refurbished servos of questionable quality then a root-cause analysis would certainly seem to be more cost-effective.

There will be maintenance and upkeep that will need to be performed on the servo tester. A suitable location for the servo tester will need to be found. AC mains power will need to be provided for the tester and the PC which will be a part of the "kit". There are a lot of "incidental" costs to be considered in this endeavour.

Please let us know how you decide to proceed, and keep us up to date with your findings and progress.

 

And I incorrectly identified one of our best GE-design heavy duty gas turbine posters....

My (REALLY) bad!

MIKEVI, not MARKVI.

Red-faced apology to MIKEVI!

 

Dear CSA & All,

Once again thank you for your information.

The quoted amount for the servo tester is 23K Euros (almost equal to 12 new servo valve price). Hence we decided to purchase BUT still we are analysing the situation as you mentioned.

We'll keep you update in future.

Take care
G.Rajesh

 

Dear G.Rajesh

Nice to see you.

i work in a servovalve manufactory in Shanghai, so i am very interested in your topic of servo valve repair. We clean a lot MOOG valves (sometimes including filter replacement) in a year.

As we all know that servo valves are accurate products. It always needs high quality of oil in the system. To my experience, 90% problems of servo valve is contamination. (as MIKEVI said).

Usually we clean the valve with thinner like petrol. And a test report and computer plot is included with the repair.

It is recommended that send your valve to the professional repair company when it has problems.

By the way, do you consider that send your MOOG valve back to the MOOG company when it needs check or repair?

Scarlett

 

G.Rajesh,

Can you provide some update about the servo-valve test kit your site was purchasing and going to use to test the many servos which had been replaced?

 

CSA,

We are not yet purchase the servo valve test kit. It is waiting for budget approval. We'll let you know once it concluded.

Thank you
G.Rajesh