Soft Seated Ball Valves

Ball valves come in an array of sizes, styles, configurations and quality. They tend to be treated very much like a commodity in the marketplace, a mistake that costs industry large sums of money supporting levels of maintenance that are not necessary, given proper valve selection. Why use a ball valve in the first place?

PROS

• it’s a quarter turn device making it much easier to reduce emissions and cheaper & easier to automate.
• it has a relatively large free flow area even in the standard/regular port configurations

CONS

• on every cycle to the closed position the process media is trapped inside the ball and the body cavities which can “freeze” or otherwise have detrimental effects on operation, including increasing valve torques substantially.
• high cycle life with tight shutoff is almost impossible with a mediocre quality ball valve, without being maintenance intensive

What makes a quality ball valve – it starts with the sphericity of the ball – a good ball valve has a sphericity in the order of ± 0.0004 inches and a surface finish of 4 RMS. This provides outstanding smoothness and roundness, resulting in a prime contribution to low torque and zero leakage. The seats are the next item to be addressed – the softer the seats (i.e. as they approach Teflon) the less demanding the quality of the ball. But, of course, the softer the seat, the more susceptible to damage and wear it is and the higher the maintenance. So generally, one is driven towards harder seats and hardened balls How the ball is held and how it mates with the seat is the next issue, which takes us to the two basic styles with a myriad of variations on the theme: floating ball and trunnion designs. Almost all of the high-end ball valves are trunnion mounted or a variation thereof, such as a heavy/extended stem guiding system.