Floating & Trunnion Mounted Ball Valves 1

[Southcoaster]

Guys,
With regards to floating and trunnion mounted ball valves operating in natural gas.

Looking through the manufacturers catalogues, floating ball is limited to 600lbs for some reason and trunnion mount goes to 2500lbs. Asking questions about this seems to get vague answers.

My question is;

Is there a cut off pressure or diameter where one type would stop and the other begins? i.e would floating ball be used up to 600lbs at a maximum diameter of 6", 8" at 300lbs etc.. and above this trunion. I would appreciate informed comment with regards to a possible rule of thumb.

 

[zdas04]

Floating ball valves seat by the dP across the valve shifting the ball into the downstream seat. The ball in large ball valves (above about 10-12 inches) is just too heavy to shift reliably. This movement is very small, but it is required to get a good seal. Trunnion ball valves use a series of springs to shove the seat into the rigid ball. They work about the same at any dP.

I just looked in a couple of catalogs and both manufacturers stop making floating ball valves around 12-inch for ANSI 300 and around 8-inch for ANSI 600.

I've found that the cost savings above about 3-inch are a false economy and I simply don't spec flanged floating ball valves (i.e., if the line is too big to thread the valves in, get a trunnion). Virtually every ball valve problem I've ever had has been with a floater.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[C2it]

IMHO floating ball valves should be avoided at the higher end of their manufacturer's specified range. I've seen a number of cases where the seal as been sheared off by rotation of the ball, resulting in massive leakage. As David says, it's false economy, use a trunnion ball instead.

 

[bcd]

The size and pressure cut-off for floating versus trunnion will varu by manufacturer depending upon the type of seals used. With floating ball, upstream pressure pushes the ball into the downstream seat and giving very effective shut-off. However, if the pressure is too high, the resulting force on the ball can crush the downstream seat and / or create so much friction that you cannot turn the valve to open it. This is why they have lower pressure ratings. Trunnion valves are more expensive to make and tend to have more external joint that are potential leak points. Normally, trunnions are upstream sealing, but not always. Some are still downstream seat sealing and use the trunnion to limit the forces on the seat. The trunnion bearings inside the valve withstand the force from pressure across the ball and enable a lower friction material, smaller radius to enable lower operating torque against higher pressures. Both are good provided you use them within their limits. If you spec. trunnions for all applications, you are wasting money. If you use floatings balls anywhere near their limits, you will suffer reliability problems.

bcd

 

[Southcoaster]

Thanks Guys for your valuble input on this subject, most helpful.