Mii Lynnn MSV w SVBP

[byrdj]

I've got a strange one now...

Lynn MII Main stop valve. This is the constant orifice feed, and servo drain to close.

When the valve is opened, it will not go closed the last inch. The valve can be fully opened, then asked for closed and it travels smoothly, but stops 1" from closed.

When tripped (even from 1" open position) the valve will close.

When reset, it stays closed until servo current is brought to null

The cabinet is putting 40mA on 16mA servo (415). I've used a voltage source for VPU error to drive it with varying current levels



I've replaced servos twice, (used on of those on the IV scram and it worked there)

I've checked the feed orifice, drawing 0.040". I found 0.043 and installed a 0.032"

The smaller orifice did make a difference in opening time, but not in the 1" from closed stopping

I've checked the servo jet tubing

I've verified the actuator spring wire diameter, coils, and pitch are the same as the RSVs (and the same as sister unit)


The drain to the tank does not go through the coolers

I'm requesting a pressure guage installed to measure hystersis today and minimun pressure with maximun servo drain

Every had any thing similar or any suggestions?

 

[Trublu97]

First, I would verify the actual position of the valve stem by a visual on the stem if you haven't already done this.

Second, I would take a Hysteresis, which are are planning.

You have enough closing force when tripped to close the valve, but not enough to close it when using the servo.

The fact it will close when tripped shows the spring to probably be alright. ( I think is should be about quater inch overtravel, it should be on the drawing. varies per design).

So, it is possible the servo is not allowing all the oil to vent at the 1"point or
there is internal binding which the spring can not overcome without the help of inertia.

but, it will close when tripped at the 1" point, when the inertia is zero. Hmmmmmmmmm

The only thing left to conclude is the servo is not allowing the oil in the Hydraulic actuator to vent at the 1" point. The pressure gage will show this immediatly.

If it is binding the hysteresis curve will fan out at the end of travel. It is not uncommon for an old fossil stop valve to start to creep and deform. If it is binding you might want to sweep body to seat to make sure there are still in line along with the other typical checks.

I have listing of hysteresis charts for different failures from EPRI if you need help interpreting the results.

 

[byrdj]

Thanks

I calibrated the positioning loop so I could use it to position the valve with the warming pot to take the pressures under the piston. after I was assured i wasn't stiking and expeiraning spikes to HPU rated output (1800psig), I used a 300psig guage to get good data. the calulated spring rate and piston area agreed with the found lifting pressures (90 to 170 psig) . The under piston residual pressure at full servo drain was 10 to 20. all hystersis data showed smooth, except when lowering between 0 and 1" lift (any decrease in this range result in pressure dropping with no stem)

The ability to trip from inside that 1" opening range proved inconsistaint. to ensure closure, had to lift just above

At this point, it hit me...that is the operating range of the steam side inner valve disk. I'm thinking the main disk's stem bushing or its collar. As I type, clearance is being hung to remove MSV. I even plan to reinstall the original 0.043" feed orifie

 

[byrdj]

Conclusion...disassembled and cleaned the steam side of valve...now strokes proper.

 

[Trublu97]

Did you find chemical carry over in the bypass taking up th design clearance or just run of the mill scale? what kind of boiler chemistry do you use? Have you run on the bypass for any length of time? If I recall, magnesium oxides, iron oxides and copper comes out of steam in the high temp end, with boiler chemicals tending to come out in the PTZ. Most copper has been eliminated in the steam plant steam cycle. check the iron levels in the boiler chemistry logs to see if its trending higher or at the high range. I think Mg is from the make up water and due to poor demineralization.

If its depositing in the MSV, its probably in the HP section degrading performance.

For general information for all, these valves should be inspected every three years. In the fossil units using 2500psig at 1000F over 30 YO I've have been seening distortion of the valve body resulting in poor alignment of the component and poor sealing. The inspection of the seat to the body is not a typical inspection but probably should be on older high temperature applications. A maintenance engineer should request this extra item when putting out RFQ for valve inspections.

 

[byrdj]

The valve was rebuit by OEM at thier shop during this major. This inspetion and reassembly was also performed by OEM on site.

I was just passing through, assiting with EHC for the return to service.