Hydraulic Valve Systems: Pressure, Flow, and Check Valve Operations

Posted on Jun 10, 2025 in Technology

Pressure Control Valves in Hydraulic Systems

Function of Pressure Control Valves

The control of hydraulic power in hydraulic systems is primarily achieved through the use of control valves. The specific control requirements are dictated by the function of the system. Parameters of the mechanical power delivered to the load are managed hydraulically by controlling the pressure, flow rate, or the direction of flow.

Classification of Hydraulic Pressure Valves

Pressure valves used in hydraulic power transmission systems include:

  • Direct-Operated Relief Valves
  • Pilot-Operated Relief Valves
  • Pressure Reducing Valves
  • Sequence Valves

Operation of Direct-Operated Pressure Relief Valves

Relief valves are connected to both high-pressure and low-pressure (return) lines. Their primary function is to limit the maximum operating pressure within the high-pressure lines. A direct-operated relief valve typically consists of a poppet loaded by a spring. The spring pushes the poppet to rest against its seat in the valve housing. The spring pre-compression force can be adjusted using a spring seat screw or by inserting distance rings.

The poppet is subjected to both the spring force and the pressure force. The poppet remains seated as long as the pressure force, FP = P * AP, is less than the spring force, Fx = k * xo. These two forces become equal when the pressure reaches the cracking pressure, Pr. For further increases in pressure, the poppet is displaced, allowing oil to flow from the high-pressure line, P, to the return line, T.

Operation of Pilot-Operated Relief Valves

A pilot-operated relief valve consists of a main valve (1) loaded by a spring (2). This valve is designed with a large diameter for the main poppet (1) and a small stiffness for the main valve spring (2), which helps to decrease the override pressure. The spring is pre-compressed, but its pre-compression force can be overcome by a small pressure difference (approximately 4–10 bar) between the input pressure, P, and the spring chamber pressure, Ps.

The operation of the main valve is controlled by installing a pilot stage (3). The pilot valve is a direct-operated relief valve, connected to the input high-pressure line through two nozzles, N1 and N2. The diameters of these nozzles are typically less than 1 mm, resulting in a very small flow rate through them. The direct-operated relief valve (3) is used to impose an upper limit on the spring chamber pressure (2). The pilot stage has small dimensions and a very stiff spring; however, the override pressure is negligible in this valve due to the very small flow rate.

When the supply pressure is less than the cracking pressure of the pilot stage, its poppet remains seated, and the pressures in the valve input line and spring chamber, C1, are equal. In this condition, the pressure forces acting on the main poppet (1) are compensated, and the spring (2) acts to close the main valve. When the pressure, P, becomes greater than the pilot valve cracking pressure, the pilot poppet valve opens. Consequently, the pressure in chambers C1 and C2 becomes equal to the relief pressure of the pilot stage, Pr. As the pilot valve flow rate increases, the dynamic depression in nozzles N1 and N2 creates a sufficient pressure difference (P – Ps) between the inlet chamber and chamber C1. The main valve poppet then moves to allow fluid to flow to the return line. Pilot-operated relief valves enable greater flow rates with smaller override pressure compared to direct-operated relief valves.

Unloading Function in Pilot-Operated Relief Valves

Pilot-operated relief valves can be utilized for system unloading. When a hydraulic system is idle or waiting, a normal relief valve would maintain maximum system pressure, leading to high energy consumption. To prevent this, a system incorporating a directional control valve, typically controlled electrically, can be used. When activated, this valve directs the flow rate directly to the tank, bypassing the relief valve. This results in no pressure in the system and, consequently, no energy consumption during idle periods.

Applications of Hydraulic Pressure Reducing Valves

Pressure reducing valves are employed when a subsystem (e.g., a cylinder) needs to operate at a lower pressure than the main system pressure.

Operation of a Pressure Reducing Valve

The pressure in the exit port (A) is connected to the control chamber, located on the right-hand side of the spool, via line (6). This pressure acts on the spool, opposing the spring (3). If the pressure in the exit port (A) is less than the value corresponding to the spring pre-compression force, the spool shifts to its extreme right-hand position. The pressure line (P) is then connected to the exit port (A). As the pressure in port (A) increases, the force acting on the spool also increases. When this force overcomes the spring force, the spool moves to the left, throttling the connection between P and A. In its final position, the spool lands separate line (A) from both the pressure and tank lines, except for radial clearance. If the pressure increases to values greater than that preset by the spring, the spool moves further to the left. This spool displacement connects line (A) to the tank, which decreases the pressure in line (A).

Flow Characteristics of Pressure Reducers

Figure 5.12 (external reference) illustrates the flow characteristics of a pressure reducer for different preset pressure levels. If the exit pressure, PA, increases, the spool moves to the left, allowing fluid to flow from port (A) to the tank line (T), creating a negative flow rate. In this manner, the valve functions as both a pressure reducer and a relief valve for line (A). A built-in check valve (2) allows for free flow from line (A) to line (P) whenever needed.

Functions of Pressure Sequence Valves

Sequence valves are used to establish a specific sequence of operations based on the pressure level within the hydraulic system. The pressure in the control chamber acts on the spool against the spring force. If the pressure forces overcome the spring force, the spool displaces to the left, connecting line (P) to (A). The valve can also be externally controlled through port (B); in this case, the connection of port (P) with the control chamber should be blocked. Optionally, the valve may be equipped with a check valve to allow for free reverse flow. Sequence valves are also known as multifunction valves and are utilized in various configurations to control sequencing, braking, unloading, load counterbalancing, and other hydraulic functions.

Check Valves in Hydraulic Systems

Differences Between Direct-Operated and Pilot-Operated Check Valves

Some hydraulic applications, such as the locking of hydraulic cylinders, necessitate the installation of check valves. In certain operating modes, it is desirable to open the check valve to permit free fluid flow in both directions. Pilot-operated check valves are specifically designed to fulfill this requirement, whereas direct-operated check valves typically allow fluid flow in only one direction.

Function of Pilot and Direct-Operated Check Valves

Spring-Loaded Direct-Operated Check Valves

These valves consist of a simple poppet valve with a spring-loaded poppet (see Fig. 5.49, external reference). The poppet rests against its seat, obstructing flow from port (B) to (A). It allows fluid flow in the direction from (A) to (B) if the pressure difference (PA – PB) is greater than the cracking pressure, Pr. The cracking pressure is defined as the pressure difference that produces a pressure force equal to the spring force, and it is usually less than 10 bar for check valves.

Direct-Operated Check Valves Without Springs

For applications requiring a very low cracking pressure, check valves are designed with springs of very low stiffness or even without springs (see Fig. 5.50, external reference). These valves operate with a cracking pressure typically less than 0.2 bar. Their symbolic representation is drawn without the spring.

Pilot-Operated Check Valves Without External Drain Ports

As mentioned, applications like hydraulic cylinder locking require check valves that can be opened to allow bidirectional flow in certain operating modes (see Figs. 5.51 and 5.53, external references). These valves permit fluid flow in one direction (A to B) and are piloted to allow for reverse flow (from B to A). A pilot-operated check valve typically comprises a valve housing (1), main poppet (2), spring (3), pilot piston (4), and an optional decompression poppet assembly (5).

In the checked direction (B to A), the main poppet (2) and the decompression poppet (5) are seated by the spring (3) and by the pressure in port (B). When pilot pressure is applied to port (X), the pilot piston (4) moves to the right. The decompression poppet (5) opens first, followed by the main poppet (2). This design facilitates rapid and smooth decompression of the fluid. Figure 5.52 (external reference) provides an example of a pilot-operated check valve application for hydraulic cylinder position locking.

Function of Hydraulically Piloted Check Valves

Figure 5.54 (external reference) illustrates a double pilot-operated check valve of sandwich plate design. This valve ensures leak-free closure of the two actuator ports (A2 and B2) during idle periods. Free flow (from A1 to A2 or B1 to B2) is permitted, while flow in the opposite direction is not allowed. Flow (from A1 to A2 or B1 to B2) applies a pressure force to spool (1), which moves to the left (or right), unseating the opposite poppet (2). Oil then flows from (B2 to B1 or A2 to A1). To ensure correct seating of the poppet valves, the ports of the Directional Control Valve (A1 and B1) should be drained, meaning they should be connected to the tank when the Directional Control Valve is in its neutral position.

Types of Check Valve Symbols

Common types of check valves for which symbols are used include:

  • Spring-Loaded Direct-Operated Check Valve
  • Direct-Operated Check Valve Without Springs
  • Pilot-Operated Check Valve with Internal Drains
  • Pilot-Operated Check Valve with External Drains
  • Double Pilot-Operated Check Valve
  • Mechanically Piloted Check Valve

Flow Control Valves in Hydraulic Systems

Main Types and Differences of Flow Control Valves

The main types of flow control valves are:

  • Throttle Valves: These are pressure-dependent, meaning the flow rate through them changes with variations in pressure difference. Simple throttle valves do not precisely control the fluid flow rate. They can be either viscosity-dependent or viscosity-independent (also known as sharp-edged throttle valves).
  • Pressure-Compensated Valves: These are pressure-independent, providing a consistent flow rate regardless of variations in the system’s pressure difference. Sub-types include series pressure-compensated flow control valves, parallel pressure-compensated flow control valves, and flow dividers.

Operation of Pressure-Compensated Flow Control Valves

Figure 5.59 (external reference) illustrates the hydraulic circuit of a system incorporating a series pressure-compensated flow control valve (FCV), also known as a two-way FCV. This valve consists of a sharp-edged throttle and a pressure compensator connected in series. The pressure compensator is installed downstream of the throttle and comprises a spool valve loaded by a spring. The pressure difference across the main throttle (P1 – P2) acts on the spool via the force Fp = As(P1 – P2), opposing the spring force Fx. The compensator maintains a constant pressure drop, ΔPt, across the main throttle, typically ranging from 4 to 10 bar. In a steady state, this pressure difference produces a force equal to the spring force.

The two-way flow control valve operates as follows:

  • In the steady state, the pressure difference across the main throttle reaches its required value, ΔPt. The pressure and spring forces are in equilibrium, and the spool is in its steady-state position.
  • If the pressure difference increases (P1 – P2 > ΔPt), the flow rate initially increases. Simultaneously, the spool moves downward, against the spring, to decrease the area of the spool valve restriction. This action reduces the flow rate through the main throttle and, consequently, the pressure difference across the main restriction. The valve then returns to a steady state where P1 – P2 = ΔPt.
  • If the pressure difference decreases (P1 – P2 < ΔPt), the flow rate decreases. The pressure force acting on the compensator spool becomes less than the spring force. The spring pushes the spool upward, increasing the restriction area of the spool valve. This increases the flow rate through the main throttle and the pressure difference (P1 – P2) until they reach the required steady-state values.

The pressure compensator continuously acts to offset the effect of variations in supply and load pressures. In the steady state, the spool of the pressure compensator remains in equilibrium.

Applications of Throttle Valves and Flow Control Valves

Throttle valves are primarily used to restrict fluid flow in both directions. Flow control valves, on the other hand, are employed when precise control over the fluid flow rate is required.

Function of Flow Dividers

Flow dividers are used to split a fluid flow rate into two or more parts, either equally or according to a specific division ratio. The two main classes of flow dividers are displacement types and spool types. Displacement flow dividers consist of two or more hydraulic motors mounted on the same shaft, ensuring they rotate at the same speed.