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by Eric, Technical Support for FlexPVC.com a division of PVC Distributors LLC, Pahrump, Nevada
Nominal Ball Valves
First of all, what does "Nominal" mean?Nominal means "in name only." For example a 1/2" pipe is not 1/2". Just like at the lumber store a "4x2" is not 4" x 2". These are nominal names. A 1/2" Sch 40 pipe is almost 5/8" ID (inside diameter) and almost 7/8th OD (outside diameter, see http://dims.flexpvc.com) So why do they call it 1/2"? It's just tradition. It's about 1/2", but the tolerance is the OD (Outside Diameter) not the ID (Inside Diameter). What that means is the OD is EXACT. In the case of a 1/2" pipe the OD will be exactly .840" +/- .002". But the inside, well it could be .625", .640", .6", .680", etc. You'd have to see how pipe is made to understand it but it's stretch extruded through a die. The die confirms the outer diameter and the stretch creates the ID. Confusing, I know.
So just like a 4x2 (lumber) is 3.5" x 1.75" a 1/2" pipe does not mean it's 1/2" on the ID. It's pretty close, but not really 1/2". If you look at the chart at http://dims.flexpvc.com you will see sometimes the "nominal" size is exactly the ID of the pipe, but not always. And of course the OD (the thing most people see and actually measure) has nothing to do with the size of the pipe. But of course it's the only thing that is made to a specific dimension! :-)
So what does this have to do with Nominal Ball Valves? Well they are worse. A 1/2" Nominal Ball valves is about 1/2" ID. But 1/2" pipe is .625-.640" ID. A 2" Nominal Ball valve is about 1.5" ID. If you don't believe me, go look at some. It does not matter who makes them or who you buy them from. They are all about the same. Most people just assume when you buy a certain size ball valve it'll pass the same flow as the same size of pipe. This is a mistake even pros make. If you run thinwall pipe to get the maximum flow possible for a given pipe size, the ball valve could be way less.
Here is a table of nominal ball valves, the Sch 40 pipe ID, the Class 200 pipe ID, the actual Ball Valve ID, and the percentage of loss over the pipe without the ball valve.
I know those number seem like very small differences, and they are on their face, but when you look at the reduction in the percentage of cross sectional area (and therefore flow) they can be huge losses!
So what is the solution? It's pretty easy. Just buy one size larger ball valve and then use reducer bushings to take it back down to the pipe size you are actually using.
Ball Valves Found Here (As well as gate valves & blade valves which generally don't have this problem.)
Slip/Spigot Reducer Bushings Found Here (For slip socket ball valves.)
MPT/FPT Reducer Bushings Found Here (For female pipe threaded ball valves.)
We also did a video showing this information on our youtube channel.
I hope you found this informative.
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