By Carl Dyke, Hydraulics Consultant at CD Industrial Group Inc. www.cdiginc.com
“It’s a PRV … a pressure control valve … a pressure regulator.” That’s what I sometimes hear when discussing symbols on a hydraulic schematic, or even when examining components. The acronym PRV really drives me crazy because it could stand for pressure relief valve or pressure reducing valve. These two types of valves aren’t the same at all. Let’s have a quick look at the differences. You can already see some of the differences in the symbols alone.
A pressure relief valve is often connected to the main pump outlet line. When used in this location, it is there to put a limit on the maximum system pressure. It does not make the system pressure go away. Nor does it unload the pump. That is yet a different type of valve. The relief valve simply supervises the pressure value in the main hydraulic supply line as it flows from the pump and reacts to keep it under a certain value as specified by the system designer.
When the system pressure climbs close to the level of the pressure relief valve setting, the valve will begin to open. “Cracking” is a term applied often to relief valves. It describes the pressure value needed to cause the smallest detectable opening where the valve is now passing some fluid back to tank. As the pressure value climbs higher, perhaps due to an overloaded lift cylinder, the relief valve will open farther as needed. If a lift cylinder is completely overloaded, the relief valve will be passing all of the pump’s flow to tank. The system pressure will be at its maximum. The relief valve itself is a pressure load on the system. It has at least one spring inside that is trying hard to close the valve.
A pressure reducing valve on the other hand is not often found near the pump. In most cases, it will be located downstream in the system, close to a component where pressure needs to be stepped down from a higher main system pressure. It may look like a relief valve, but it does not function the same.
If one cylinder in a metal stamping machine is there to punch and break through the stamped material, another cylinder may be used to clamp that material in place. Both cylinders may be fed by the same pump. The clamping cylinder, however, may only need a fraction of the pressure that the stamping cylinder needs. Too much clamping pressure and the material could be damaged. Or perhaps the clamping cylinders are of a light-duty design compared to the stamping cylinder. This design is common enough.
The pressure reducing valve takes a higher pressure on its inlet and then creates a lower maximum pressure for the remaining circuitry on its outlet. On the symbol, you see the dashed pilot line sensing and triggering on the outlet side.
Another interesting fact is that most of the hydraulic circuitry that follows after a pressure reducing valve is low-flow or zero-flow in design. At the very least, the sub-circuit is low or zero-flow at the moments when the pressure reducing valve is actually needed, such as when the clamping cylinder is busy holding material in place.