Hydraulic valves play a crucial role in directing and controlling the flow of hydraulic fluid within a system, ensuring that machinery operates smoothly and efficiently. Below, we delve into the different types of hydraulic valves, their functions, and where they are commonly used.

Understanding Hydraulic Valves

Hydraulic Check Valve

The main function of check valves in Hydraulic Power Packs and Systems is to permit fluid flow in one direction and prevent reverse flow. Essentially, they're installed in a pipe to stop oil from flowing backward. When required, the valve closes, halting any reverse fluid movement.

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Hydraulic check valves typically feature two ports: an inlet and an outlet. These connect with the motor, cylinders, and hydraulic pump, managing the fluid's flow for proper equipment performance.

These valves come in various designs—such as poppet, disc, ball, or plunger types—depending on their application and size requirements. Common applications include braking systems, construction tools, and lifting systems, where fluid backup could cause significant problems.

For instance, reverse oil flow in a pipe could drain a hydraulic system into the equipment reservoir. Even if the machinery is off, a hydraulic valve can prevent this, keeping the system ready for the next operation.

Dual Pilot Operated Check valves (P.O.C) can be opened by external pilot pressure. They function like standard in-line check valves by blocking flow in one direction but can be opened with external pressure applied to a third port. The necessary pilot port pressure is generally only a third of the cylinder pressure, controlled by the Pilot Ratio (3:1 and 4.5:1 are standard). These are often used with double-acting cylinders to lock the system when pressure is shut off, either intentionally or due to failure. They can be mounted directly between ports on a ram or integrated into a power manifold block. Direct mounting to a ram with "hard" pipework enhances device integrity. If only one side of a cylinder needs a pilot check valve, it can be aptly placed on the A or B sides.

Common uses for pilot check valves include commercial vehicle loading ramps, balers and compactors, and security access bollards and blockers. However, they are not ideal for applications where reversing loads may overrun.

Flow Control Valves

Flow control valves manage fluid flow and come in various forms:

Fixed Orifice

These are essentially holes in a tube or inserts in the hydraulic line, limiting fluid flow based on pressure.

Adjustable Orifice

These allow size adjustments of the orifice. Known types include inline and barrel valves and needle valves for fine adjustments on low-flow systems. Once set, the adjustments can be locked, making them ideal for lift or tipper applications with uniform loads.

Pressure Compensating

These valves adjust for pressure changes in systems with varying loads, ensuring consistent flow to hydraulic actuators. For instance, scissor lifts require high pressure initially, which decreases as mechanical advantage grows. The reverse occurs during gravity-driven lowering, making pressure compensating valves suitable here.

Reverse Flow Check

Single-acting power packs optimize the pump and motor combination to achieve the desired lift speed. These valves have an integral bypass line with a built-in check valve, allowing full flow in one direction. When lowering, the bypass line restricts flow for controlled descent.

Dual Flow Control Valve

This comprises two valves in one block, optimizing actuator speeds in double-acting rams by permitting full flow in one direction while controlling flow in the other. A common use case is the rear door of a horsebox, where slow closing is necessary to prevent shock and noise.

Relief Valve

Relief valves are essential control devices in almost all hydraulic systems, protecting them from excessive pressure that could cause mechanical failure. They are mechanical and require no external input other than applied pressure. Once the excess pressure is relieved, they reseat to allow normal operation. Typically, they consist of a spring and plunger pushing onto a seat. When pressure exceeds the spring force, oil spills to a volume, usually the oil reservoir. Relief valves can be housed in cartridges or directly designed into aluminum or steel hydraulic manifolds.

Multiple relief valves may be present in a hydraulic circuit: one at the power pack's end to protect the pump and another in a control valve circuit to alleviate load induced by external forces. Dual relief valve modules may address different pressure needs on the full bore or annulus side of a hydraulic cylinder. An example is a hydropower generation sluice gate, where an obstacle like a log prevents closure.

Key terms related to relief valve operation include:

• Overshoot: Pressure reading when the valve bypasses fluid.
• Hysteresis: Difference in pressure when the valve starts spilling flow and when it passes full flow.
• Stability: Pressure fluctuation during bypass at set pressure.
• Reseat pressure: Pressure at which the valve closes after operation.

Counterbalance valves, a type of relief valve, generate back pressure to prevent load runaway during lowering. Set at about 30% higher than the pressure induced by the load, these valves often feature built-in check valves for reverse flow. Note that both valve sides are subjected to full pressure, which not all relief valve designs can accommodate. Effective counterbalancing requires a low back pressure return path, as additional back pressure impacts the valve setting.

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