We at Fluid Power World write many articles about fluid power components, but less often do we write about where these components are located in a system. You can find in our pages/on our websites what a transducer is, what a relief valve is and what a flow control is, but you will find fewer discussions on locating those components. For fluid power components, location is everything, and it makes the difference between well-designed system and a collection of parts.
In reality, some hydraulic components only work when installed in a particular location; hydraulic pumps, for example, can only pump when they are installed after and plumbed to a reservoir. Stationing a pump where a cylinder should be would create odd and unpredictable system performance, to say the least. Conversely, putting a cylinder where a relief valve should be would give your circuit the lifespan enough to fully stroke the cylinder before something blows. These examples are obvious, but where can you locate other components to take advantage of their unique qualities.
There are other items common to every system that can be optimized by a specific location. A hydraulic filter is most often in a return line, but is there a better location alternatively? Pressure gauges are often after the pump, but would they be better suited somewhere else? Does it make sense to only have a flow meter in the main pressure line?
One of my favourite attributes of hydraulics is the countless ways you can use components outside of their standard scope of operation. The lowly relief valve can do much more than regulate pressure; it can also control the motion of loads, operate functions in sequence, or dampen load induced pressure. You see, a counterbalance valve, a sequence valve and a crossover relief valve are all just versions of the relief valve with small modifications, such as check valves or external drains, to make them more suitable in a different location.
A system relief valve installed after a pump really just needs to be a spring and a ball or poppet, one port plumbed to the pressure side and the other to the tank side—and that’s about it. Because it drains straight to tank, it doesn’t need a dedicated drain line for internal leakage. Let’s move the relief valve to the cap port of a cylinder under compressive load. That relief valve is now a direct acting counterbalance valve, and will not open to allow the cylinder to lower until its pressure setting is achieved. Same valve, different location. Nevertheless, the counterbalance valve requires a reverse-flow check valve if you ever want to raise the cylinder again, and it would be a good idea to drain the spring chamber to tank so that trapped pressure doesn’t lock the valve closed.
Let’s flip that relief valve around so that it flows towards the cap port of the cylinder. Now the valve won’t open—and the cylinder won’t lift—until upstream pressure reaches the setting of the relief valve. We now have a sequence valve. Because this valve will see pressure on both ports, it needs to have a vented spring chamber. In these circumstances without a drain, any pressure in the spring chamber is additive to spring pressure, and the valve will quickly lock shut. Also, if you want the cylinder to lower, you’ll need to add that check valve again.
There is a way to use a relief valve unchanged from its original purpose. If you have a hydraulic motor, you sometimes need to dampen the inertia of the load being turned by the motor. If you simply slam a directional valve shut, pressure spikes in the outlet port of the motor as fluid has nowhere to go. Simply plumb the pressure port of the relief valve in a tee to the motor outlet line, and when the directional valve shuts, the fluid bypasses to the inlet side of the motor plumbing. This has the effect of absorbing that inertial energy, and if the motor is bi-rotational, you can add one plumbed backwards from the other port.
Relief valves essentially change their function by their location, but there are circumstances where you want to use a component the same way in a different location. A pressure gauge or pressure transducer, for example, can be mounted in various locations to provide differing metrics. I’ll use the transducer as an example, because its precisions and accuracy make it useful in ways pressure gauges cannot be.
Just like a relief valve, a pressure transducer can measure system pressure, letting you know the total load on your pump and prime mover. You can also measure pressure at a subcircuit of the machine, such as the clamping function of a press. A pressure reducing valve keeps downstream pressure lower than system pressure, and mounting the transducer downstream shows what load pressure is, even if it’s different from the rest of the system.
A pair of transducers can be placed on either side of a metering valve, such as a proportional directional valve, and in this combination you can compare the two signals to observe actual pressure drop, which tells you real flow rate. If a desired pressure drop—and therefore flow rate—is known by the PLC, it will adjust the signal to the proportional valve to maintain that pressure regardless of changes in load induced pressure.
A transducer can be placed in even more unlikely locations, such as the return line. Although no actual work is expected to be achieved on the tail end of system flow, measuring accurate tank line pressure helps diagnose other problems. A spike in tank line pressure is a symptom of other problems, and it’s possible to measure milliseconds, rather than stead-state pressure of an analogue gauge. Spikes in tank pressure could be because of decompression shock, which is the sudden release of energy when a high pressure, high volume of fluid is released at once, such as on a large shear. This shock can cause damage in hydraulic components, but can be hard to detect without accurate instrumentation.
Another tank line component popular in hydraulics is the return filter. Because return line flow is low pressure, it’s common to place an inexpensive filter assembly there to remove particles as they enter the reservoir. But is this the best choice of location? Think about it … why are we letting the particles travel the circuit in its entirety only to clean it as it enters the location most likely to ingest contamination anyway; the reservoir.
I’m a firm believer filters should exist in pressure lines, especially right after the pump. If fluid entering an expensive and complex machine is clean, components are more reliable and accurate, and only the pump itself risks contamination damage. There really is no way for oil to be too clean, so you can go ahead and use supplemental filtration in return lines and other key locations, but the days of a single return filter need to end.
I’ve seen complicated injection molding machines with five pumps; four to provide system flow, and one for pilot pressure. On one example, there were four 50 gpm flow meters attached to each pump’s outlet to see if they were providing their full-rating of 40 gpm each. These were thousand-dollar flow meters rated for 3000 psi, so you can imagine the importance of this machine.
However, I can save $3,500 of the $4,000 worth of flow meters, and more accurately observe the health of each of those pumps. These pumps were electronically controlled variable displacement pumps, and when they weren’t flowing as expected, the flow meters showed reduced flow. But, they didn’t tell us if the pumps themselves were the problem or the electronics had failed. My improvement would be to put four small, low-pressure flow meters on the pump drain lines. If historical leakage flow from the drain lines was 5 gpm, and this rose to 10 gpm over the course of a few days, I can tell you your pump is definitely the culprit. Any fluid in a pump has to go either out the drain line or the pump outlet, and if it’s the drain line, it’s wasted as pure heat rather than useful work.
These examples show you how standard components used thoughtfully in non-standard locations can provide valuable improvements to your hydraulic circuit. Furthermore, standard components can be used in standard locations with different results, proving that like in real estate, it’s all about location.
Filed Under: Mobile Hydraulic Tips