Globe valve mental model
Globe valve mental model
I am in the process of Globe valve failure investigation, process crude oil: circa 200 degC, pressure circa 20 barg, which was relatively new (1 year), used as bypass valve for same size Control valve.
Long story short the Control valve blocked and damage, so we use the bypass valve for the last 6 months. It was start Vibrating last week --> leaking on gland --> bigger leaking and plant shut down. This line is exactly identical with other 7 lines in series (each line has 1 Control valve and 1 bypass valve).
Finding: what is not identical is that this 3”-600# Globe valve is with Gearbox, while others (relatively older valve and no failing ever reported) are with Hand wheel.
Check with the supplier and others in general manner, some are with Gearbox and some with hand-wheel. And provide me with drawing as follow
My mental model (refer to strip-down experiences and the way I was taught years ago) are:
-Under the Plug flow, size 4” or below (relative), Class 600#, even at full DP, is sufficient enough to be operated with Hand-wheel
-Most process engineers (without detail valve background) consider Globe valve as linear characteristic
-Even though not as pointy as Choke valve’s plug surface, however it should not be as (almost) flat surface. Plus with quite wide sealing area
My findings which are also in-conflict with above mental model are:
- Nowadays, Gearbox preferred without detail engineering justification (of Rim-pull, thrust, etc.). driven by manufacturers’ offer
-Refer to all above drawing, I think it is more for quick opening characteristic rather than linear. But then again this is relatively cheap valves shall compare to the control valve, And end user "don’t reserve" the right of asking Cv, flow characteristic for such cheap and mass produce valve.
-All the plug is almost flat surface (I’ve strip down 3 of above drawing), and the contact between Plug and Seat is literally a 0,5 mm line or thinner
Note that due to this miss-conception:
-The operation treat this as Control valve for throttling function between 20 to 40% position
-Galling found as evidence that operator miss-handle it due to ease of Gearbox by applying excessive force
-Should we be more specific in sizing Manual Globe valve to manufacturer --> which eventually increase the price due to engineering "hours" from both side
-Is my mental model wrong?
-Are manufacturer not convenient enough that their valve would not shut down without gearbox? Or is this some kind of conspiracy between gearbox manfct and valve manf.
The flatter it gets the more thrust required to close the valve, and shall torque required is greater than let say 350 Nm, then gearbox is preferred. Or does this tress-hold value is shifted to cover wide range of Operator ethnicity/strength (not meaning to be racist)
-Is it cheaper / faster to make the disc nearly flat rather than conical? Or does the trend is shifting this way?
Thank you in advance for the support and information. My excuse shall the topic is wide and not directly correspond one another.
All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected