Why is the lift so small on water pump ball valves?

Most plans and posts I've looked at typically give 1/32" lift for pumps with ball bearing type valves. Why so little when it's easy to arrange things so the ball is constrained and can be made to naturally fall onto a conical seat? It seems counter-intuitive for a free flow...

When a pump stroke ends, and the ball changes direction, there will be a small backflow. A short ball stroke will minimise this backflow and allow the ball to re-seat as quickly as possible. If feeding a boiler, for example, a quick re-seat would be a good thing to avoid losing boiler pressure (or backflow boiler contents). JD

If you have too much lift the ball has further to drop or be forced back down onto the seat. As to free flow I think if you calculated the flow area between the ball and the valve body above the seat of the clearance in most designs you will find it equal to or greater than the cross sectional area of the tubing internal diameter that feeds the valve.

For full flow the lift need only be the diameter of the seat divided by 4. In your case 1/32" lift would give full flow on a 1/8" seat diameter.

Ah, I see - thanks Bob. And thanks Reg for that handy rule of thumb :D

Reg has it on the money.

As an aside, and I realise this is straying from the original topic, why is a volcano shape conical seat better than a funnel shape which naturally channels the ball to the right place?

Perhaps the thin edge of a conical seat deforms to match the ball more effectively.

Neil

Ive never made or seen a funnel shaped seat - I would imagine the ball would sooner or later get stuck in it! I only ever make perpendicular seats, or flat to you and me. I tried the volcano conical type but any minute off centre turning means it doesnt seat. I used to swear by nitrile balls until one got sucked in!