Fluid Power Journal

Raising the bar: Flow Means Go? Not Exactly

When designing a fluid power system, it is important to separate the concepts of flow and pressure. The pressure determines the force available, and the speed of the actuator determines flow. There is no argument there. But we need to understand that we cannot completely separate flow and pressure.

Let me explain: What causes flow in a fluid power system? Go ahead. Shout it out! That’s right! There must be a pressure differential. Fluid always moves from higher pressure to lower pressure. Without pressure, there can be no flow. It is possible to have pressure without flow, but there can be no flow without pressure. The fact that there is flow tells us that units of fluid have been pressurized and are moving toward a lower pressure region. These units could be liters or gallons, cubic inches or cubic centimeters; it makes no difference. Flow is the rate at which energy units are moving.

This brings us to the next question: What determines power in a fluid power system? Just shout it out! Right again! Flow and pressure. Power equals flow times pressure. We may use different constants for metric (kW) or U.S. customary (hp), but power is always a matter of flow times pressure. Because flow requires a pressure differential and power equals flow times pressure, flow control is power control. 

Some of you may be asking, What’s the point? So what if flow control is power control? What difference does it make?

I’m glad you asked. Words mean things, and the way we use words has an impact on the way we think about things. Most of us do not equate flow control with energy. We think of flow control as the logical means of establishing the speed of an actuator. When we see the symbol in a circuit, we seldom view it in a negative light. It is just a necessary part of the circuit. 

However, when I think of the symbol as a power control, it is more likely to cause us to think about energy. When I see a power control in a circuit, I immediately know there is more power available than is necessary and some of it is being removed as heat. 

A power control is an energy consumer, a source of heat, and an installed inefficiency. It must be included in the list of components that produce parasitic losses in our fluid power systems. 

In a hydraulic system, a power control limits the number of energy units that reach an actuator by stealing some of the pressure as the units pass through. In a pneumatic system, the power control limits the rate at which the energy units reach the actuator without stealing the energy. Both systems waste energy, though they do so differently. The hydraulic system consumes energy all the time the unit volumes are passing through the power control. The pneumatic system wastes energy when the actuator deadheads and is then filled with full pressure.

I think it is important to rename flow controls as power controls. I have been criticized for using this unfamiliar term in some of my presentations because it might confuse people. My response is that they are already confused or at least misinformed. A power control is an energy waster and should only be used as a last resort when no suitable alternative can be found.

I’ll discuss suitable alternatives in future articles.

Fluid Power Journal encourages conversation about issues addressed in “Raising the bar.” Please use the Comment function at the bottom of the page to take part in the discussion with author Dan Helgerson.

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