Ball valve torque confusion

[NDS_21]

hey
Greetings!
I am a bit confused here. I have bought 2 three-piece ball valves with same pressure class and same features but different manufacturers. One is smooth to operate while other requires more efforts to operate it in same line. Why is that happening?
Two reasons I can think of:
1. The valve which operates smooth has seats with less co-efficient of friction and ball perfectly lapped.
2. The valve which operates with more efforts has more interference between the seats and ball.
Which one can be said a better valve? Perfectly matched contour of ball and seat is good OR some amount of interference is always required? And is it good that valve operates smoothly?
P.S. I have took many trails to conclude that one is smooth and other is not so here I am talking about operating torque and not the one which is applied while operating the valve for the first time (Break torque).
Thanks.
Learning something new everyday!

 

[Danlap]

Hi NDS_21,

There are several details that you have not told us. What is the size/class, trunnion or floating, is it with lever or gearbox, unidirectional/bidirectional as per manufacturer recommendation and what is the DeltaP while someone operate the valve. And also, whether you check only one sample or several sample from each batch of different manufacturer
Valve torques can vary for different manufacturers and shall be checked to ensure appropriate selection of gear operators based on the maximum and sustained force requirements
Assume it is lever (with limited mechanical advantage shall compared to gearbox), the torque/twist moment upper threshold is depending on mainly two things:
-MAST
-And maximum cracking force required to (crack) open the ball valve with relatively short handwheel at maximum DeltaP (upstream maximum-downstream atmospheric) is for example 445 N refer to adult people ergonomical yet less convenient maximum force. if it is higher than this value, gearbox might be required

Not to oversimplify, but your assumption is almost complete:
-Ball (for simplification) have four sides: Sealing side A; Sealing side B; and two non-sealing sides in between A & B
Either by machine or by hand lapping is a repetitive process of polishing side A and side B against the Seat with “8 shape” method. The other two side is often not lapped since it is not essential sealing area, however it is very unlikely that the other two side is not round and cause excessive friction due to the “non-round” shape. But never say never
-Three-piece have three body parts: end A, body and end B. End A to body is often bolted connection as well as End B to body, with gasket placed in between each connection. Maybe the seating stress for gasket and friction coefficient (for bolts) applied is different thus one manufacturer X is “tighter” compared to another Y. However, both valves might still not be showing external leakage as well as no internal leakage found. This do not necessarily mean that manufacturer X is better than Y or the other way around. it is just their approach.
The tighter it is the more force required to turn the ball
-DeltaP while both valves operated is not the same, and other small process factor may be different one another
-If this is trunnion, the seat sealing force against the ball is by spring either corrugated or coil. Might be the spring coefficient applied is different.
-Etc.
Or combination of all

As long as it is still movable by hand (not cheating bar), no scratch found on ball after you operate the valve, no external leakage through End-A/B to body connection and internal passing is tolerable or zero, it is ok.

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]

Stem seal type and lubrication (if any) can also have an impact, but without further details as noted by Danlap, it's a very vague question so a very vague response.

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

 

[NDS_21]

Sorry for less information but yeah here the details of both the ball valves I have:
Floating type - 3 piece ball valve - class 800 - socket welded - DN 25 - Hand Lever operated - bi-directional - no lubrication
So your views remain the same?
Thanks!

 

[gerhardl]

1. Operating force is a combination of friction and adhesion of contact between all surfaces.

2. A repeatable and thight closure might as well be reached through small, but precise machined surface contact and gliding type of ball and stem sealings giving smooth operation, as from other constructions that require more force.

3. In my experience 'best' valves (cost/lifetime) comes from manufacturers with exact and precise machining processes, with very small limits for variation in end products, and good quality control. Operation af a good valve might then as well be smoth as less smooth.

 

[Danlap]

In general, the views remain the same. DN25/class 800 - assumed this is metal seated? and what is the distance between the socket weld ends towards the (replace-able or maybe integral) seat(s)??
Also to take into consideration welding process that may causes the metal seats to expand. This for sure will add friction factor.

A bit intriguing, one of the idea of three piece is easiness for dismantling the valve for maintenance purpose. What is the reason of socket weld then?? For sure the leakage path for three piece welded ball valve is three (two ends vs body + gland) shall comparable with top entry flange ball valve (also three: top and its two pipe flanges), or two entry ball valve (four - gland, two flanges and body to end connection). However, the last two types is easier to be maintained.
Just out of curiosity.

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]

https://www.bray.com/ball-valves/3-piece-valves/series-7000-8000

Assuming your valve looks something like this - click on the BOM table and scroll down to see an exploded view then the margins are quite low for body to end tolerance.

If the seats end up being squeezed a little more in one design than the other that will make all the difference, ditto how tight the thrust washer and packing is done up.

potential impact of socket welding and slight distortion or seal change caused by excess heat might make a difference as well.



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

 

[NDS_21]

So basically both valves can be said the best if they serve the purpose of leakage proof line and being smooth or tight while operation varies from manufacturer to manufacturer.
P.S. @Danlap valve is soft seated one and not metal seated. Sorry not to mention that before.