Hydraulic Pump Intake Valves: Ball, Butterfly or NOTHING At All?

500HPU3-cropOne of many on-the-drawing-board dilemmas for hydraulic system designers is whether to install a more expensive ball valve in the pump inlet line, a cheaper butterfly valve, or no valve at all.

At the root of this dilemma 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 almost 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 pressure and velocity. 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 can cause gaseous cavitation.

So faced with these two choices, the designer would ideally always chose a ball valve ahead of a butterfly valve. And for intake line diameters up to 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. Plus 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 fit it in!

The third alternative, no valve at all, is often not seriously considered. But here are a few of the benefits:
1. The cost of the valve is saved.
2. The distance between the tank and the pump can be shortened.
3. And best of all, the pump can NEVER be started with the intake isolation valve closed!

This does of course mean the tank must be pumped out, or alternatively, a vacuum applied to the tank head space when the pump needs to be changed out. But in the majority of applications, this is totally do-able. And to prove my walk matches my talk, pictured inset is the pump on a hydraulic power unit I designed recently.

The tank’s oil volume is 1,000 liters (264 USgal). And the pump is a 260 cc (16 cu.in) per rev axial piston unit with integral impeller pump for supercharging its inlet. The thing to notice is how short, direct, and unrestricted the 4″ diameter intake line is. This combined with the flooded inlet pretty much guarantees this pump won’t see any cavitation.

To summarize, installing a pump intake isolation valve can be costly, damaging and unnecessary. 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.

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