One of our readers wrote me with this question:
"I'd like to hear your opinion about isolation valves on pump intake lines. Within our company there is an ongoing debate over whether we should use a more expensive ball valve or a cheaper butterfly valve?"
At the root of this question is the negative effect of turbulence in the pump intake line. The argument for using a ball valve as an intake line isolation valve is, when it's open, the full bore of the valve is available for oil flow. So if you have a 2" ball valve in a 2" intake line, when the valve is open, from the oil's point of view at least, it's as if it wasn't there at all.
A butterfly valve on the other hand, is not full bore. Even when fully open, the butterfly remains in the bore. So the butterfly presents a partial restriction which is irregular in shape. This causes turbulence - rapid variation of fluid velocity and pressure. The result is dissolved air can come out of solution in the intake line. If this happens, these air bubbles will collapse when exposed to pressure at the pump outlet. In other words, a butterfly valve may cause gaseous cavitation.
So which is best: ball or butterfly valve? Well, like a lot of issues in hydraulics, it depends. In a perfect world I would always chose a ball valve ahead of a butterfly valve. And for intake line diameters up to about 3" there's virtually no cost penalty involved in doing so.
But when you get into 4", 6" and 8" diameters, ball valves are VERY expensive in comparison to their butterfly counterparts. And they take up a LOT more space - particularly in overall length. So, in a mobile application for example, not only may the cost of a large diameter ball valve be prohibitive - there may not even be enough space between the tank outlet and the pump inlet to install it!
But there is third alternative - which I am a big advocate of. The whole issue of intake line isolation valves is one where people tend to get blinkers on. By that I mean, like suction strainers, they wrongly believe they are essential. When in reality, but for a few exceptions, they are not.
So my preferred approach is: fit neither ball valve nor butterfly valve if you can get away with it. If you have to have one, use a ball valve if cost or space aren't an issue. But if either of these things are a problem, then a butterfly valve is the only choice you have left.
Bottom line: installing a pump intake isolation valve, or the wrong type of valve, can be a mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.