Ball Valves: importance of the Ball

Some are surprised over the price variation of the Ball Valve.  Yes, Ball Valve can vary much dependent on several factors such as:
1)  Body type – forged or cast.  3-piece design of forged steel body is generally more expensive.
2)  Ball material.  For carbon steel and stainless steel ball valves, the ball is usually stainless steel for reason of corrosion protection and long-term leakage prevention.  Stainless steel 316 ball is indeed more expensive than stainless steel 304 ball.
3)  Ball type.  There are 3 types:  solid forged steel ball, solid cast steel ball, and hollow welded steel ball.  Solid forged steel ball is the most expensive whereas hollow welded steel ball is the cheapest.
4)  Ball plating and coating.  Ball could be plated with chrome or electroless nickel.  Ball could be coated with Stellite, chrome carbide, tungsten carbide or Teflon.  Non-plated, non-coated ball is the cheapest option.
5)  Facility grade.  What is the grade of factory and machinery used for making the balls and ball valves?  Grade A factory has CNC spherical grinders and 5-axis CNC lathe equipped with rotating table to manufacture the balls.  Valve seats are also machined using CNC lathe and CNC vertical machining centre.  On the opposite end, Grade C factory uses manual machining method whereby accuracy and precision is not guaranteed.
6)  Full-bore or Reduced-bore.  Full-bore ball valve is always more expensive, larger and heavier than the reduced-bore ball valve.

Ball valve is the most important of all mechanical valves (ball valves, gate valves, globe valves, gate valves, check valves, butterfly valves, etc.).  Ball valve is often used as a control valve in the pipeline.  So, don’t be a cheapskate.  Do buy Grade A ball valves.  Yes, Grade A ball valves are much more expensive than Grade B ball valves, but ball valves are crucial and daily used.  Ball valves need to be the best in quality, reliability and durability. 
Forget the Grade C ball valves, unless you intend to be a Cheat, a hit-and-run businessman.

In this blog post, let us look at the Quality aspect of the Ball.

Solid forged steel ball is the best option.  Steel is forged out of a solid, round steel bar.  The solid forged steel ball is denser than the cast steel ball.  Steel grains are close together in forged steel, and there is no porosity in forged steel.  Even in high pressure ball valve (Class 900 and greater), the liquid medium could not penetrate into the forged steel ball.

Solid cast steel ball is an economical option.  Cast steel ball may contain some extent of porosity, and is unsignificantly less dense than forged steel ball.  In high pressure ball valve (Class 900 and greater), the liquid medium may penetrate into the tiny porosities of the cast steel ball.  In the fully-close position, for high pressure ball valves, there may be chance of micro leakage due to these tiny porosities of the cast steel ball.

Hollow welded steel ball is the sinful option.  These are used in Grade C ball valves, and sometimes employed in Grade B ball valves.  Welded steel ball is made of curved steel plates which are welded together.  Hence, the welded steel ball is hollow in side.  Easy way to differentiate the welded steel ball from the solid steel ball is the light weight of the welded steel ball.  Welded steel ball is not good because the liquid medium could enter into the hollow portion of the welded steel ball.  This would cause minor leakage of the ball valve in the fully-close position.

Before machining and finishing, we could easily identify the solid forged steel ball, solid cast steel ball and hollow welded steel ball.  However, after machining and finishing, all these 3 types of ball look visually similar.

Quality of the ball is also affected by the machining of the ball.  The best machining method is the CNC spherical grinder.  Second best option is CNC lathe fitted with rotating table, which would give a 5-axis machining capability.  CNC machining would enable the accuracy and precision of every single ball.  CNC spherical grinder is better than CNC lathe because the grinding will produce better smoothness and better roundness.  Grade A balls are always machined using CNC spherical grinders or CNC lathe with rotating table. 
Grade C ball are machined using manual lathe machine, which could not guarantee the consistency and precision; hence, slight dimensional variation among the balls; so some balls will be good, some balls may cause leakage.

Finishing of the ball should be mirror finish.  All Grade A balls have this mirror-like finish, which is easily achieved using CNC spherical grinder. 
On the opposite end, Grade C balls will be hand lapped, using lapping compound, after the manual machining is completed.  Manual machining also produce uneven roundness.  If the person doing the hand lapping is unskilled, the uneven roundness will be continued.  Grade C balls, being unevenly round and not mirror finished, will have tendency toward minor leakage.

With these explanations, I hope buyers could understand the importance of quality of Balls in the Ball Valves.  Quality of Ball affects significantly the price of the Ball Valve.  Quality of the Ball also plays a major role to determine the quality, reliability and durability of the Ball Valve.

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