FUNDAMENTALS Josh Cosford • Contributing Editor
Hydraulic symbology 204 – flow control valves The name flow control valve broadly describes any hydraulic component capable of diminishing fluid volume downstream of itself relative to upstream. It goes without saying a flow control valve only reduces flow since the laws of nature remain unbroken. The method of varying flow varies considerably and depending on the choice of valve and its location, the effect can be substantial. The most elemental of flow controls is the fixed orifice, Figure 1. Drilling out a fitting makes a rudimentary orifice, so long as the reduced cross-section inhibits flow. An orifice is not a length of plumbing, which is a poor and inefficient way of controlling flow. An orifice will be as short as possible in depth while remaining strong enough to withstand the effects of pressure. Draw the fixed orifice in one of two ways as shown in Figure 1. The first, and most common method, shows the flow path surrounded by outward facing, gentle arcs. They signify a smooth compression of the fluid, but in reality, hydraulic components are rarely honed so smoothly. The second symbol with inward facing vertices depicts the less common method of drawing a fixed orifice, although it’s the one I personally prefer.
Figure 1. Fixed orifices
Fixed orifices are used typically used for factory settings in pumps, manifolds and valves, but offer no user adjustability. A variable orifice provides a method to control the size of the gap between the needle and its seat, thus changing the flow rate through itself. The symbol simply adds the diagonal arrow depicting adjustability in fluid power symbols. As with most symbology, the method varying flow in the physical valve is otherwise irrelevant in regard to the symbol. Additionally, neither does the adjustable symbol guarantee that flow rate will even be adjusted if there is no upstream provision 26
FLUID POWER WORLD
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to reduce or bypass flow otherwise allocated to the valve. This is, after all, positive displacement we’re talking about, and in a system with a fixed pump, fluid must go somewhere. Classic hydraulic theory teaches us it’s not a flow control valve unless and until there is a reverse flow check valve, such as in the last example of Figure 1. The check valve blocks upward flow through this valve symbol, pressing the ball into the seat when flow exists at the bottom port. Reverse flow allows the ball to lift and bypass the check valve, although a good portion of flow will still pass through the orifice, as pressure drop through both the orifice and check valve will be exactly equal to each other. The diagonal arrow shows us this flow control valve is a variable flow rate. Although this series is focused on symbols more than any principles of fluid power, it’s important to understand the relationship with pressure and flow. In any circuit where a restriction, orifice or flow control reduces flow, pressure increases. Also, in any circumstance where downstream pressure is high, the potential to flow through a metering device is reduced. The important term to remember is pressure drop, which is a comparison of upstream and downstream pressure through an object. Any change in flow or pressure drop can have consequence, either positive or negative, on system performance. The four symbols covered thus far represent valves that will flow at a rate dictated by pressure drop through them, and should downstream pressure rise or fall, flow will change inversely proportional. To get around this problem, a concept called pressure compensation was created, and it uses a clever technique to encourage flow when downstream pressure increases, thereby allowing a stable flow rate regardless of load or supply pressure fluctuations. The first symbol in Figure 2 depicts the simplified version of a pressure and temperature compensated flow control. This symbol comprises the orifice arcs, the variable arrow and reverse-flow check valve, just as with a standard flow control. However, the addition of the upward facing arrowhead tells us it is pressure compensated. I cannot tell you the etymology related to this choice of graphic, but it’s standard practice, nevertheless. More easily understood is the symbol for temperature compensation, which expresses itself as a sideways thermometer. Temperature compensation could also be called viscosity compensation because it’s just a feature that allows the valve to manage flow rate despite varying oil viscosity.
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