Electric linear actuator for control valve 1

[PABW]

Hi,
I have a sliding stem valve application where running air will be difficult. I’m thinking of using a linear electric actuator similar to what we have on our turbine fuel valve. (Ball screw, or roller screw I think – we replaced an old hydraulic 10 months ago) My only concern is thread wear, which so far has not been a problem. (It will be inspected this fall – but I can’t wait that long) A local rep has shown me a design that includes an integrated controller in a nice small package, but he had no reliability data other than motion control applications. Anyone have thoughts on reliability with this type of actuator on a control valve?

PABW

 

[BigInch]

This seems to be typical Weibull data for motorized valves. You have to judge how severe your application is in relation to the "average" and change the characteristic life accordingly.

Motorized valves
Beta 1.1

Characteristic life (hrs)
Life low 17,000
Life avg 25,000
Life hi 1,000,000


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[PABW]

The data provided to me was based on the life of the roller screw, not the servo motor,and expressed in inches of travel not hour of operation. (Somewhere near 5 million inches) It's a 2 inch actuator.

PABW

 

[BigInch]

Its a problem with reliability data. Everybody's got their own measuring sticks and their own units. Breaking them down and making them match up into something meaningful is no easy task.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[JimCasey]

Inches of travel is not all that meaningful for a control valve. In real-life operation it will most likely spend most of its time dithering about a very small range. So you'll get wear on the favored spot equivalent to a bajillion inches of travel, while the other end of the stem may only have seen contact a few times.

Rexa makes an electraulic unit that is the standard of the industry. Jordan make industrial-quality electrics for rising-stem control valves. There are a lot of small electrics out there for HVAC-type applications, and there are a lot of gate-valve actuators (limitorque, Rotork, Auma, Biffi) than can be fittied with positioner cards. THe gate valve actuators doen't use roller screws but in my experience a bronze thrust nut with ACME thread.

 

[PABW]

I talked to my rep once again, and he mentioned that the actuator is a roller screw, not ball screw, so there is never a single point of contact on the threads, but a large contact surface area that is evenly distributed. I've considered the others you mentioned but am very intrigued by this servo solution, especially with the stiffness and stability is has shown in my other application. I did get a decent MTBF on the motor, but not the roller screw specifically.

PABW

 

[Ashereng]

PABW,

If it is only one valve, why not go ahead and try it. If the service is critical, why not try a valve on a less critical service.

In any case, it doesn't take that much time to change a valve/actuator.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[ScottyUK]

JimCasey has this right on the mark: a turbine fuel valve is in continuously modulating service and is typically operating over maybe 5% of its range except for startup. From a wear perspective that is why fluid actuators are superior, and the sponginess of an gas actuator (including air) normally makes a hydraulic servo the choice where accuracy and exceptional wear chracteristics are needed.

How big an actuator is this? Is this a aeroderivative machine? The fuel valve on an aero engine is a very different animal to the fuel valve on a frame machine, principally in the size and mass of the moving parts. Continous modulation will put a lot of dynamic force on the actuator as it accelerates and decelerates the valve. Depending on your loop tuning this may be quite aggressive to maintain adequate control of the turbine. Ask the vendor to warrant the life of the actuator for your specific application. The cycle life he quotes could be with little or no mass attached to the actuator.

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I don't suffer from insanity. I enjoy it...

 

[PABW]

ScottyUK
We have been using the electric actuator on our turbine fuel valve for over 10 months now. Since it does operate in essentially the same small area, I was concerned about thread wear, which we will check in October. So far we have seen no change in performance, and have been very pleased with the results. I was asking if anyone else had used an actuator like this one and had comments on its long term reliability. The technology is very different from what we are using so I’m not sure how to even schedule maintenance on it.

I’m going ahead with the second application, and will let you know how this turn out.

PABW

 

[ScottyUK]

PABW,

I'm following this with great interest because we're in the middle of a trial installation of a Rexa electro-hydraulic actuator to the IGVs of one of our turbines. Our fuel valves are all electro-hydraulic, as are the eight main valves on our steam turbines. We consistently find calibration errors of less than 0.1% on a 40mm stroke gas valve after a year in service. An error as large as 0.1% is unusual. We're hoping the Rexa unit will come close to these reliability levels - I'll post findings in a new thread next year once I have some data.

The ballscrew idea is something which we discounted on reliability grounds, perhaps a little unfairly, and I'm trying not to focus on only one technology. Your experiences are thus very interesting. Is the actuator a self-contained package? Would you mind posting a URL for the manufacturer?

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I don't suffer from insanity. I enjoy it...

 

[PABW]

ScottyUK,
Just to clarify - the actuator we are using on the fuel valve has a roller screw, not a ball screw. My understanding is that ball screws for valve control is not a good idea for the same reasons you stated - reliability.

The fuel valve actuator has a separate drive, not self contained. The actuator I'm considering for a different application is self-contained but I don't see info on the web yet. Rep tells me it's new.
Manufacturer for both is Exlar

http://www.exlar.com

PABW

 

[rmw]

I glanced at their stuff but did not look in detail. What you have to be concerned about is modulation rate. Ask your vendor to guarantee a modulation rate of 3600 starts per minute. That is one start per second. That is one thousand and move, two thousand and move, three thousand...I think you get the point. If the actuator has a modulating rating of anything less than 3600, run from it.

The foregoing assumes that yours is a modulating application. If it is a on/off application, then any old actuator will do. But if it is going to modulate, you better get an actuator from the class of devices that can meet that modulation rate. The Rexa certainly can, the Jordan can in most models, Beck can. Electrohydraulics are going to leak some day. Can't say when, but they will leak. If that is a problem, go strictly electric.

I know people who love any of the above, and I know people that hate all of the names I mentioned and that have been discussed here.

If you are going to operate at a fairly constant point, be concerned about what JimCAsey related about the wear at that point.

rmw

 

[Ashereng]

rmw,

Do you mean 3600 starts per hour?

I don't know of any actuators that can do 3600 starts a minute.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[rmw]

Good catch Ashereng. Thanks for straightening that out. Now 3600 starts per minute; that is a control system in bad need of tuning.

rmw

 

[JLSeagull]

Consider Rexa, or an equivelant electro-hydraulic actuator.

 

[PABW]

This actuator uses a continuous duty servo motor - not an intermittent motor. It's 'on' continuously, and moves with speeds up to 8 inches a second. (Which is why we used it on our turbine.)

Actuators like this are pretty costly, since they need a separate amplifier to ‘drive’ the motor via a resolver or encoder. (Basic motion control stuff) This new one however includes the amplifier in the same housing as the actuator, and the price is considerably less. It hasn’t shipped yet, but I’ll keep you posted on my results. If it works and doesn't show thread wear, I won't look at a electro-hydraulic again.

PABW

 

[rmw]

I don't consider the amplifier being mounted in the actuator housing necessarily to be an advantage. If an actuator is required to be in a hot place, if the amplifier can't be remote mounted where it is not so hot, then the actuator can be limited by the amplifier temperature limits. Inquire as to whether this actuator has the capability of having a remote mounted amplifier.

If temperature is not an issue, then this is unimportant.

rmw

 

[OCANDOA]

Does your application require a fail safe action? Can the roller screw electric actuator provide it?
It does look very interesting. Thank you for sharing this with us.

 

[PABW]

I am using a UPS for the failsafe. A mechanical spring was available, but the actuator/mount size would increase more then I wanted for the space I had. (I think they size the spring first - then the actuator) I noticed that Exlar now has this actuator on the web. It's called a Tritex. They also have a rotary version that interests me. Once I test the linear one out I may have to investigate further.
PABW

 

[goodvalve]

change it!safe is most important.