By Josh Cosford, Contributing Editor
The words flow control valves broadly describe 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 the 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.
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 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, the fluid must go somewhere.
Classic hydraulic theory teaches us they do not flow control valves 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, the 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 pressure and temperature compensated flow control. This symbol comprises the orifice arcs, the variable arrow and reverse-flow check valve, just as with 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 the flow rate despite varying oil viscosity.
Splitting pump flow will supply two sub-circuits, which is where the priority type flow control comes in handy. Also known as a “3-Port” flow control, it will send fluid from port 1 to 2 at a fixed rate based on the orifice setting, and all excess fluid is sent as bypass through port 3. This fluid can be dumped to the tank or used for actual work. What’s important to note is flow at port 3 can only be maintained when the incoming flow is higher than its set value. For example, if the flow is set to 8 gpm at port 2 with 10 gpm incoming flow, 2 gpm will bypass to port 3. However, if the incoming flow drops below 8 gpm, all flow will now travel to port 2, leaving nothing for the bypass at port 3.
The final symbol showing a detailed representation of a pressure compensated flow control is where things get complicated, but if you stick with me, you’ll understand. The variable orifice and check valve are self-explanatory, but the compensator symbol added downstream has a lot going on. Port 1 upstream of the orifice is connected to the b-side envelope of the compensator, which shows the “T” symbols to block flow at both ports. Port 2 is connected downstream of the variable orifice and feeds its pilot line to the b-side envelope of the compensator, but this shows it as normally flowing in neutral. Port 3 of the valve simply connects the whole assembly and bypasses the fun bits and provide free flow in reverse; a true flow control.
The compensator is shown as a 2-position valve, but it’s more of an infinitely variable spool valve that meters between flowing more or flowing less. The compensator is offset by a spring that provides 90 psi of effort, additive to whatever is transmitted from port 2. When flow occurs through the valve, the compensator compares pressure at the ports 1 and 2 of the variable orifice. Pressure will always be higher at port 1, so pilot pressure forces the compensator closed until port 1 pressure matches the 90-psi spring valve. Flow through the variable orifice will always represent whatever 90-psi worth of pressure drop will achieve through itself, regardless of its setting.
If we use an example of a pump capable of 12 gpm and a 3,000-psi compensator or relief valve, the pressure at port 1 will see 3,000 psi. Let’s assume we want 10 gpm at 90-psi pressure drop, so we adjust our orifice to suit. Because the compensator is installed and wants to see a 90-psi difference between ports 1 and 2, the pressure at port 2 will close the compensator to block flow until port 2 pressure is 2,910 psi. At this point, 10 gpm will be flowing through the valve while the pump is either dumping 2 gpm over the relief valve or reducing its swashplate angle slightly.
If downstream pressure rises to 1,500 psi, pilot pressure at port 2 will increase, forcing the valve open and compensating for the downstream pressure increase. What would normally result in less flow potential over a given “Delta P,” now results in the compensator opening to reduce downstream backpressure. The compensator works like a pressure reducing valve in reverse; as pressure increases, it opens wider to allow more flow that would be normally lost to reduced pressure drop.
Pressure compensators can be added to any valve of a hydraulic circuit that controls flow rate, including proportional valves. Later in this series, I’ll talk about some advanced concepts based on pressure compensators, which are sometimes called “hydrostats.” Check back soon for the next article in this series, this time covering the symbols of hydraulic pumps.