Pressure Control Valves

I. Relief Valves

The function of a relief valve is to start relieving pressure when the system pressure reaches its set value, keeping the system pressure basically stable at a certain set value. Properly functioning relief valves are critical for managing hydrostatic energy, preventing system damage from excessive pressure buildup.

According to their pressure regulating performance and structural characteristics, relief valves can be divided into two major categories: direct-acting relief valves and pilot-operated relief valves. Both types serve the same fundamental purpose but achieve pressure regulation through different mechanisms, each with its own advantages in managing hydrostatic energy under specific conditions.

Figure 1: Relief valve operation showing pressure regulation and hydrostatic energy management

In operation, relief valves continuously monitor system pressure and open proportionally to maintain the preset pressure level. This modulation of flow helps stabilize the system's hydrostatic energy, preventing pressure spikes that could damage components. Relief valves are often used as safety valves in systems where pressure must be strictly controlled, such as in hydraulic presses, injection molding machines, and heavy machinery.

II. Pressure Reducing Valves

The function of a pressure reducing valve is to make its outlet pressure lower than the inlet pressure and to allow the outlet pressure to be adjustable. In hydraulic systems, pressure reducing valves are used to reduce or regulate the pressure in a certain branch of the system to meet the needs of certain actuators. This precise control of pressure differentials is essential for managing hydrostatic energy in complex systems with multiple components requiring different pressure levels.

Pressure reducing valves are commonly used in clamping circuits and lubrication systems. Their ability to maintain consistent lower pressure in specific branches while the main system operates at higher pressure makes them invaluable for optimizing hydrostatic energy usage throughout the system.

According to their regulating performance, pressure reducing valves can be divided into three types: fixed-pressure reducing valves, fixed-ratio reducing valves, and fixed-differential reducing valves. Each type offers unique advantages in managing hydrostatic energy under different operational requirements.

• Fixed-differential reducing valves can maintain an approximately constant difference between the inlet and outlet pressures of the valve, ensuring consistent hydrostatic energy differentials.
• Fixed-ratio reducing valves can maintain an approximately constant ratio between the inlet and outlet pressures of the valve, providing proportional hydrostatic energy control.
• These two types of valves are generally not used alone but are combined with valves of other functions to form corresponding combination valves. Due to space limitations, they will not be analyzed separately here but will be studied together when discussing the corresponding valves.
• Fixed-pressure reducing valves, referred to simply as reducing valves, can make their outlet pressure lower than the inlet pressure and can keep the outlet pressure approximately constant, offering stable hydrostatic energy regulation for specific system branches.

Like relief valves, pressure reducing valves are also divided into direct-acting and pilot-operated types. Both designs effectively manage hydrostatic energy but differ in their precision, response characteristics, and applications.

1. Pilot-Operated Reducing Valves

Figure 5-34a shows a traditional pilot-operated reducing valve. It consists of two parts: a pilot valve and a main valve. In the figure, P1 is the inlet port, P2 is the outlet port. Hydraulic oil passes through the oil groove a at the lower end of the main valve spool 4, the damping hole b in the main valve spool, enters the upper chamber c of the main valve spool, and then enters the front chamber of the pilot valve through hole d. This complex flow path allows for precise control over hydrostatic energy conversion within the valve.

Figure 2: Pilot-operated reducing valve showing internal flow paths for hydrostatic energy regulation

When the outlet pressure P2 of the reducing valve is less than the set pressure, the pilot valve spool 2 is closed under the action of the spring. The pressure in the upper and lower chambers of the main valve spool 4 is equal, and under the action of the spring, the main valve spool is in the lower position. At this time, the passage gap e between the inlet and outlet of the main valve spool 4 is the largest, the main valve spool is fully open, and the inlet and outlet pressures of the reducing valve are equal. In this state, the valve allows maximum flow with minimal restriction to hydrostatic energy transfer.

When the outlet pressure of the valve reaches the set value, the pilot valve spool 2 opens, and the hydraulic oil generates a pressure difference through the damping hole b. The pressure in the upper and lower chambers of the main valve spool is unequal, and the pressure in the lower chamber is greater than that in the upper chamber. The difference overcomes the action of the main valve spring 3 to lift the spool. At this time, the passage gap e decreases, the throttling effect is enhanced, so that the outlet pressure P2 is lower than the inlet pressure P1 and remains at the set value. This precise balancing act ensures consistent hydrostatic energy delivery to downstream components at the required pressure level.

III. Sequence Valves

A sequence valve is a hydraulic valve that automatically opens or closes a certain branch circuit using pressure as a control signal. Because sequence valves can control the sequential action of actuators, they are called sequence valves. These valves play a crucial role in managing the timing of hydrostatic energy distribution throughout a hydraulic system, ensuring proper operational sequence of components.

According to their control methods, sequence valves can be divided into internally controlled sequence valves and externally controlled sequence valves. Internally controlled sequence valves directly use the inlet hydraulic oil of the valve to control the opening and closing of the valve, generally called sequence valves; externally controlled sequence valves use external hydraulic oil to control the opening and closing of the valve, also known as hydraulically controlled sequence valves. This flexibility in control methods allows for versatile management of hydrostatic energy in various system configurations.

According to their different structures, sequence valves can also be divided into direct-acting sequence valves and pilot-operated sequence valves. Each design offers specific advantages in terms of response time, pressure range, and precision in controlling hydrostatic energy distribution.

1. Direct-Acting Sequence Valves

Figure 5-40a shows a high-pressure direct-acting sequence valve, which is in the internally controlled state as shown. Hydraulic oil enters from the oil inlet P1, passes through the passage on the valve body 3 and the passage on the lower end cover 5, and enters the lower chamber of the control piston 4. This design allows the valve to sense system pressure directly, enabling rapid response to changes in hydrostatic energy.

Figure 3: Direct-acting sequence valve showing pressure control mechanism for managing hydrostatic energy

When the inlet pressure is not high, the spring 1 presses down the valve core 2, so that the oil inlet P1 is not connected with the oil outlet P2. In this closed state, the valve blocks hydrostatic energy transfer to the downstream circuit until the preset pressure condition is met.

When the inlet pressure increases to the set value of the valve, the control piston 4 lifts up, pushing the valve core 2 to connect the oil inlet P1 with the oil outlet P2. Since the oil outlet of the sequence valve is connected to the system, the oil drain port must be connected to the oil tank separately. This connection allows for proper venting of excess hydrostatic energy while maintaining system pressure.

Figure 5-40b shows the graphic symbol of the internal control sequence valve. Removing the lower end cover 5 in Figure 5-40a, rotating it 90 degrees and then reinstalling it (the lower end cover is square, and the screw holes are symmetrically distributed) and unscrewing the plug 6 to connect the control oil circuit forms an external control sequence valve. This versatility in configuration allows for flexible management of hydrostatic energy in different system designs.

At this point, the opening and closing of the valve is controlled by external oil pressure. This external control method provides additional flexibility in system design, allowing hydrostatic energy from one part of the system to control operations in another part, enabling complex sequential operations.

IV. Pressure Relays

A pressure relay is a component in a hydraulic system that converts the pressure signal of hydraulic oil into an electrical signal. This conversion of energy forms a critical interface between hydraulic and electrical control systems, enabling automated management of hydrostatic energy based on pressure conditions. Pressure relays are divided into plunger type and diaphragm type, each offering specific advantages in different applications.

Figure 5-43a shows the structure of a diaphragm-type pressure relay. Its working principle is: the control oil enters the oil inlet P of the pressure relay and acts on the diaphragm 2. When the control pressure is insufficient to overcome the spring force, the spring keeps the plunger 3 at the lower end, and the micro switch 13 does not act. This state continues until the hydrostatic energy in the system reaches the threshold required to trigger the relay.

Figure 4: Diaphragm-type pressure relay showing conversion of hydrostatic energy to electrical signal

When the control pressure reaches the set pressure, the spring 10 is compressed, the plunger 3 moves upward, and the inclined surface on the plunger pushes the steel ball 7 to move to the right, and the lever 1 is pushed to the right by the steel ball. At this time, the lever swings counterclockwise around the fulcrum (pin 12), and the left end of the lever presses down the contact of the micro switch 13 to close the circuit and send an electrical signal. This precise conversion of hydrostatic energy to an electrical signal enables automated system responses to pressure conditions.

When the pressure at the control oil port decreases to a certain value, the spring 10 presses down the plunger through the steel ball 8, and the contact of the micro switch 13 pushes the lever 1 to reset by its own elastic force, and the circuit is disconnected. This return action ensures that the relay only sends signals when sufficient hydrostatic energy is present, preventing false signals during pressure fluctuations.

Screw 14 is used to adjust the relative position between the micro switch and the lever. Spring 5 is used to adjust the control pressure difference (called the return interval) that closes and opens the circuit. This adjustment allows for precise control over the hysteresis of the relay, ensuring stable operation despite minor fluctuations in hydrostatic energy.

Screw 11 is used to adjust the pre-compression amount of spring 10, thereby adjusting the signal pressure of the pressure relay. This adjustability makes pressure relays versatile components that can be tailored to specific system requirements, ensuring accurate monitoring and control of hydrostatic energy across a wide range of applications.

Figure 5-43b shows the graphic symbol of the pressure relay.