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Okui (Petroleum)
27 Nov 08 4:16
if there is high dp across the ball valve, will i have difficulties in opening the valve manually? some say yes..some say no....
gerhardl (Mechanical)
27 Nov 08 5:36

..and all are right. This will depend on necessary opening torque, starting movement of the ball. The opening torque is the largest torque, torque will be smaller when the ball is moving.

Opening torque may vary as much as 1:10 (and more), all else being eqaul on construction and size and pressure for the ball valve, but type of fluid, drying caracteristics, stickiness, abrasive flud rest on ball, compability of fluid to sealing, time closead (deforming of seals over time) etc, etc varying, and giving variation to required opening torque.

If you have clean, compatible fluids, valve constructed for the purpose and operational data within construction limits, gearing adapted to highest torque possible, it should be no problems with hand operation.

What is large dp? What is size? Even with larger sizes (one meter and above,) and fairly large pressures (64 bars), or 160 bars up to 300mm, there is no problem with hand operation with stem gearboxes for double eccentric ball valves constructed for water/clean fluids.

Check that the construction you want to use are suitable for your operational requirements!
 

zdas04 (Mechanical)
27 Nov 08 7:04
Floating ball valves seal by the ball physically shifting toward the downstream seat.  The movement is not far, but it is a contributor to high opening torque.  Trunion ball valves use springs to seal the seat against the ball both upstream and downstream, but (especially with fluids that can condense within the ball cavity) this can result in high dP across both seats and a high opening torque.  Reduced port floating ball valves tend to have the highest opening torque with clean, compatible fluids.  When the fluids get nasty or can swell the sealing surfaces then either kind of ball valve can have high torque.

David
belowzero (Mechanical)
27 Nov 08 12:04
zdas04 (Mechanical)-
Don't mislead the guy by stating that all 'Trunnion' ball valves have springs as you did in thread408-134074: Trunion Mounted ball Valve. Take a 'Jamesbury' 10" 6000 series, these purely use polmer seating technology and a very unique design to load the seat onto the ball despite being trunnion ball valves.
Also I personally would have said a full bore valve rather than reduced would/is harder to turn as the seat is larger and the ball bigger, which taking load over area means that the pressure on the ball is greater!
 Get the right sized gearbox and you won't have any difficulty what so ever.
B
zdas04 (Mechanical)
27 Nov 08 17:48
I guess my offending statement was:

Quote (zdas04):

I've never seen any of the standards define the difference between floating ball valves and trunion ball valves.  To me the difference is that a floating-ball valve relies on the ball shifting into the downstream seat to seal and a trunion has components to prevent lateral ball movement from either direction and has spring-loaded seats.

I suppose I should have said "pre-loaded seats" instead of spring-loaded seats.  But you know what?  Who (outside of possibly Jamesbury trying to differentiate themselves or trying to save the cost of a spring) really cares.  The point is that floating ball valves seal by shifting the seat and the seal in trunion valves is started by indegral equipment.

You are right that full port valves have the potential for more starting torque because of the larger area that the dP is acting on--it has just been my experience that I have had more torque problems with reduced port valves, I don't know why, that is just what I've observed.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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Helpful Member!  JimCasey (Mechanical)
3 Dec 08 13:41
Well, it was a vaguely worded question and I fear we are overanalyzing it, but the only accurate answer I can think of to the actual question is:
Valves are harder to open when there is more differential pressure across them.

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