Emergency Stop Valve Exercising

[ilan7]

We have a problem with some Turbine stop valves that after some weeks or months staying open they fail to close at the original speed. Is there any rule regaring exercising the valve daily or weekly? Thanks

 

[byrdj]

DAILY

 

[byrdj]

I don't know of a industry regulation, it might be an isurance requirement, but the OEM makes the recomendation.

The Primary purpose is to besure (and an attemp to remove) any deposits that have formed between the stem and bushing. This is greatly effected by water chemistry. Some plants have gone to extended periods (greater than daily) with the reasoning their water quaility and operating tempertures do not cause rapid deposit build up.

A secondary purpose is to keep the hydrulics free

If you have single SV and the test is only partial, you might be building up a deposit area and thus will it test ok, it still fails to close when tripped.

During valve inspection, it is critical that stem runout and stem to bushing clearance is acceptable. if the unit is 15/20 years old, you need to verify that the OEM might have increased the bushing ID from original drawings. Try bars are a MUST.

I'm assuming your symptom observed is stoke speed, ie how many seconds to close and then open. This might just be a calibration error or a drift in the controling components. If the change in test times was noticed following an inspection, It could be your problem

If EHC, the servo valve can take a null shift and the test times change greatly. the null shift can either be internal mechanical or a plugging of the jet strainer.

If MHC, it could be worn linkage connections resulting in lost motion to the pilot valve positioner

 

[stgrme]

Basically, I agree with byrdj. Testing frequency is usually recommended by the manufacturer, but may need to be made more frequent if the steam chemistry is poor and "blue blush" and/or deposit build-up occurs.

Besides the mechanical aspects noted by byrdj, some manufacturers and/or after-market suppliers now offer new materials for valve stems which resist "blue blush" and deposit build-up. You might want to check with your OEM and/or TurboCare and Turbo Parts.

http://www.turbocare.com/index.html

http://www.turbopartsinc.com/

Best of luck!