Triple Offset Butterfly Valve Shaft Operation & Failure 4 - Eng-Tips

Hi
I have some questions about triple offset butterfly valve.Could somebody pls confirm if my understanding below are correct or be kind enough to correct me if I'm wrong?

(1) When valve is under good condition (nothing is broken), the disc can only turn 90deg (max) from fully closed to fully opened.

(2) Under good condition, can the disc turn freely up to 180deg from fully closed position? Can it turn 360deg freely?

(3) What stop the disc from turning more than it should be? Is there some sort of device for that function?

(4) Under what kind of failure will cause the disc turning freely?

Thanks in advance

Hello Jimnat,
Triple offsets are quite different from double offsets & centered, in answer to your points.
1) Triple offsets are torque seated, not position seated, hence the disc can rotate from fully open to fully closed + 4 or 5° (this due to machining tolarances in valve seat/ disc seal ring).
2) The disc can rotate freely to almost 180° (the disc being offset to the shaft will not allow 180° rotation), as per 180°, 360° impossible due to design.
3) The valve disc "should" stop in the seating area even without sealring (this depends on manufacturer)assembled in disc.
4) A disc turning freely....I suppose this is what you see from the outside of the line (actuator/gear + shaft turning through 90°), if so, the most common (not very common if all operating parameters are correct) failure is in shaft to disc connection failure, there are several types of connection... or shaft has sheared inside valve and anti-blowout prevents shaft from coming out of body...as it should, this means, one half of shaft rotates freely without changing (or holding) disc position.
However, after taking all of the above answers into consideration, your best working practice would be to check the IOM from valve manufacturer, these normally give detailed trouble-shooting prodecures.

A detailed answer will depend on make and construction of the valve, operation element for the valve (lever gear or actuator), and sealing (metal or soft) and detailed sealing construction.

a) The disc at thight closed BFL valves, also double and triple offset BFL valves, has usually some slight variation in position from 100% 90 degrees, and some slight adjustment possibillities in closing position. This to allow correct sealing, but not too high loosening torque. This adjustment has to be done manually, and endstops corrected accordingly. (Exception is perhaps direct leveroperated smaller valves where the lever is positioned normally by a thoothed plate)

b) Even if offset valves do not get stuck as easily as centric valves, it can and will happen if the disc is forced too hard into the seat.

c) Most common if disc is 'stuck' is that the disc position is to much forced in closed direction, and/or has remained too long in closed position. If electrical actuator: check limit switches position and that actuator stops on limit switches and that actuator turns in the right direction (phase connections correct). If pneumatic actuator check that actuator has endstops and adjust endstops correctly. Same for manual gear. Check valve seat and sealings for damage.

d) Disc can be turned in wrong direction when open but not more than about 90 degrees from fully open and will not seal.

e) It is unlikely that the disc can be forced past 90 degrees in correct position (but perhaps not absolutely impossibel?)

Jimnat,

Attached is a further file to check, similar to the one desertfox already posted.
I believe you may have confused "valve" with "valve disc" direction, the valve can be in two directions, the "preferred direction" and the "non-preferred" direction when bi-directional, or "wrong direction" if uni-directional.
The valve should have an arrow on body that indicates the preferred direction of flow.
Best if you advise on valve manufacturer.
Ciao
 http://files.engineering.com/getfile.aspx?folder=ece5-df76-4cd6-800f-b95bab42af11&file=MaintenanceManual.pdf
gerhardl,
triple-offset metal seated valves are for sure torque seated, there's no doubt about it.



itscot,
about "preferred" and "non preferred" directions, I would add that distinction must be made between flow and tightness (or pressure).

When the valve is completely open, the Cv is almost the same in both directions, so the valve may be considered (almost) symmetrical as regards flow.

When the valve is closed, due to the triple-offset design, there is one side on which pressure action helps seating (and then less torque is needed to reach the required tightness level): this is the so-called "preferred" direction, the opposite (where pressure tends to open and more torque is needed) is the "non preferred" one.
Sometimes (when seat tightness is not the major concern; for example when the valve main mission is quick opening and/or control...), it is also possible to install this kind of valve in the "non preferred" direction with good results...



Hope this helps, 'NGL

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