Bigger size class in floating ball valve 5

[NDS_21]

Hey!
Greetings!
I was going through catalogues of floating ball valve manufacturers and found a confusing thing that most of the manufacturers were offering floating 3-piece ball valve in smaller sizes (2" and less) in higher pressure class (800 and 900) but in bigger sizes (21/2", 3" and 4") the preesure class gets reduced to 300 and class 400. Why is that difference?
Is it like in bigger sizes for higher pressure class the floating 3-piece ball valve are not used because there is an option of tru-union design?
Or the market of 3-piece floating ball valve is smaller for those pressure class in big sizes?
Hope someone can clear this doubt.
Thanks!

 

[Danlap]

Hi,

I have seen floating ball valve 12"-600# three piece; 8"-300# two piece/side entry and some other sizes/classes as well. Most (if not all) of these similarities are all of them were specially engineered as per end user process requirement, have preferable direction (uni-directional), claimed to have special hard faced applied and operated with either gearbox or actuator.

Cmiiw, I assume you are referring to flange connection and replaceable seat ring and metal seated (without soft seat insert)

3-piece vs two piece vs top entry can be viewed from several perspectives: design and manufacturing ability e.g. short pattern or long pattern, full bore or reduce bore, weight, etc.; maintain-ability e.g. is it required to repair/replace ball and seat every now and then??; end user requirement e.g. as minimum leakage path as possible, etc.

Floating vs trunnion for relatively bigger sizes and or classes?? Not to be simplified, but the answer is due to engineering trade-off.
[ul]
[li]Assumed solid ball, meaning the bigger the size then the heavier the ball. When not supported (floating) and valve is horizontally mounted, ball's weight for most of the time will be concentrated at the bottom part of the seat, therefore at low pressure usually the upper part is more prone for passing.[/li]
[li]Uni-directional: Downstream seat is spring energized against the ball to provide sufficient sealing force. Spring(s)' k is determined by the Delta P of the line process condition. Unless requested bi-directional, usually manufacturer will offer unidirectional with no or minimum spring energized force on upstream seat. This to provide sufficient clearance in order not to damage ball once moved.[/li]
[li]Special hard faced --> higher hardness meaning more brittle. If the clearance is too tight between ball and seat(s) due to spring force + weight of ball + upstream pressure, then hard faced may worn out or shattered shall valve is operated.[/li]
[li]Thermal expansion factor must be considered as well for ball/seat shall exposed to thermal shock or highest temperature condition. Again if it is too tight, ball may got stuck or hard faced shattered.[/li]
[li]Etc.[/li]
[/ul]

Does all end user prefer floating?? in theory floating ball valve allows process medium to settle inside the cavity. Shall this happens, does the medium subject for polymerization and or clogging thus causing valve stuck??

There are many consideration as mentioned above and more to go to floating path. And only a handful of manufacturers have been proven successful (performance wise) in this field such as MOGAS, ValvTechnologies, etc.

Kind regards,
MR

https://nosuchvalve.com

All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected

http://www.eng-tips.com/faqs.cfm?&rat1=2&fid=376

 

[LittleInch]

NDS 21

I think you'll find it is simply application of physics and practicality.

In addition to the points above, a floating ball valve creates sealing force by pressure difference and hence force on the ball from the upstream higher pressure side.

At max rated differential pressure the force at 2" and below creates a force and hence pressure on the seal area which the seal ring (usually a soft seal of some sort) is able to handle.

However as the valve and ball size increases, the area increases as a square of the diameter and hence the force increases as a square of the diameter. However the seat circumference only goes up in direct proportion to the diameter.

Hence the pressure on the seal goes up relentlessly as size increases and the seals basically just can't handle it anymore unless they become very hard and strong in which case they tend not to seal very well and start to cost a lot. Two inch is just the happy medium for class 800/900 and 3-4" for class 300.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

LittleInch and Danlap bring up a couple of points that were new to me (I've taught valve choices for over 30 years) and very cogent.

Regardless of the orientation, the mass of the ball in a floating ball valve will tend to put more sealing force on the side of the valve closest to the center of the earth--with even a very small amount of slack in the valve chamber, this will tend to provide a reduced sealing surface at the "top". No big deal in 2-inch or even in 4-inch, but much bigger than that and this becomes significant, I've installed 8-inch floating ball valves and never been very happy with their shut-off characteristics, now I'm wondering if gravity acting on the ball itself is the problem.

I've also never considered that the sealing force is a square function while the seating area is a first order function. I've pulled out large floating ball valves and found the seating surfaces to be deformed (an extruded look) which could be explained by the square/first-order effect. I have never been able to explain this observation, but this seems to do it.

Thank you guys

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist