I would like to share a small problem which has started to bug me recently after the replacement of GCV/SRV in one of my unit (GE F9e with Mark V TMR).

Recently the GCV/SRV (combined vlv) was replaced with a spare refurbished valve after the SRV spring broke. However after the replacement, calibration and operation for 1 day, the core for 96SR1 (LVDT#1) of the SRV broke.

The core was found to break at the top at the before the thread. The core broke again after replacement within 1 day.

Observation during unit shutdown (SRV fully closed) as follows:
(i) Holder for LVDT is tightly secured to the vlv stem.

(ii) LVDT assembly is tightly secured to the base of SRV and LVDT holder

(iii) The holder for LVDT could be moved to the left and right. Since the holder is secured to the valve stem, the SRV stem could also rotate when the holder is moved.

Observation when SRV is opened at various percentage (using AUTOCAL) during unit shutdown:

(i) The holder for LVDT could NOT be moved at all.

Observation when unit is in operation (SRV open):
(i) The holder for LVDT could NOT be moved at all.

(ii) The whole valve assembly vibrates.

Observation of another unit of same type:
(i) The holder does not move either when unit is shutdown, SRV open during shutdown or during unit operation.

(ii) The whole valve assembly does not vibrate.

My assumption so far is that the movement of the LVDT holder and vibration of the valve could cause the LVDT core to break. The fact that 96SR2 which is installed on the same holder does not break, I think is due to the distance from the valve stem is closer as compared to 96SR1 (slight movement causes most displacement at furthest distance).

Currently the unit is operating with 1 LVDT (96SR2) only without any problem as the replacement of the LVDT core without resolving the root cause would cause it to break yet again.
Appreciate if anyone have experience similar problem to advice on the possible root cause.

 

The most disconcerting point here is the vibration of the SRV shaft, to which the LVDT cores are attached. By vibration, it's presumed you mean up and down, not side to side. It should be possible to balance a coin with smooth edges on its edge on the bar to which the LVDT cores are attached while the unit is running. So, if the shaft is vibrating, then something is definitely amiss.

It would seem there is a problem with the servo-valve, or how the servo-valve is connected to the actuator, or with how the wiring from the Speedtronic is connected to the servo-valve.

Did you verify the polarity of the current being applied to the servo-valve coils prior to LVDT calibration? For each individual servo-valve coil?

It's difficult to imagine any other possible component causing the valve stem to vibrate other than a servo-valve problem.

As for the LVDT core problem, it would seem there is some kind of mechanical misalignment which, coupled with the excessive valve stem vibration, is causing the core to break. I've even seen incorrect LVDTs, with the wrong physical dimensions (length of core, for example) used on refurbished assemblies.

This might also be the problem with servo-valve; an incorrect part number was used. Or a refurbished servo-valve was used.

Sometimes when changing hydraulic components, dirt can either be released from fittings or elbows or introduced during the change-out.

 

Thanks for the feedback CSA. The problem was found and solved. We found that few of the base bolts were not tightened properly after the installation. This caused the abnormal vibration and the failure of the cores. Now the unit is operating normally.

 

Thanks, sd.

Feedback is the most important contribution!(c) here at control.com. It's what lets others who read these posts know if the information provided was helpful, or a complete miss, and what the ultimate problem and resolution was determined to be.

Sometimes, it's the most obvious things that get overlooked. (And, the Mechanical Department can get very rushed in such situations and so their work should always be reviewed with a "fresh" set of eyes prior to putting such a device into service. Many a time flange bolts or compression fittings have not been suitably tightened and have led to serious fuel leaks. And, of course, instrument and controls tech's also forget to tighten terminals in a rush, but that's to be forgiven. ;-) )

Thanks again for the feedback!