Similarities and Differences of Three Pressure Control Valves (Part 1)

May. 11, 2020

Hydraulic transmission is based on pressure oil or compressed air as the working medium for energy conversion, transmission, and control of the transmission. The hydraulic transmission system is generally composed of four parts: the power part -- hydraulic pump; Executive part -- hydraulic cylinder; Control part -- various hydraulic valves; Auxiliary parts -- oil tank, pressure gauge, pipeline, etc.

Step-Down Control Valve

Step-Down Control Valve

In the hydraulic transmission system, used to control the level of oil pressure or pressure change to achieve a certain action of the control valve is called a pressure control valve, referred to as a pressure valve. There are many kinds of pressure control valves, which can be divided into an overflow valve, step-down control valve, sequence valve, and pressure relay according to their different USES. They generally work on the basic principle of balancing the pressure of the oil with the force of the spring.

Manufacturers of industrial process control valve think: because of the overflow valve, relief valve, sequence valve of the three internal structure, working principle, the function of valve symbol there are a lot of similarities, correct understanding of three kinds of valve working principle and functions of symbols, master effective failure analysis method, the hydraulic system of the whole research has very important significance.

The function of a valve:

The main function of the overflow valve has two aspects: one is the overflow and maintains the pressure stability of the system (or circuit); The second is to prevent system overload, plays the role of voltage limiting protection. It is used in almost all hydraulic systems, and its performance has a great influence on the normal operation of the whole hydraulic system.

Reducing the valve is a pressure control valve that makes the outlet pressure lower than the inlet pressure. Its function is to reduce the hydraulic pressure of a circuit in the hydraulic system, the use of an oil source can provide two or more different pressure output.In addition, when the oil pressure is unstable, a series of reducing valves in the loop can obtain a stable lower pressure. Commonly used is a constant value output pressure reducing valve.

The function of the sequence valve is to take the system pressure as a signal to the action of the sequence of multiple actuators for automatic control of the valve. When the fluid pressure at the inlet of the sequence valve does not reach the regulated pressure of the valve, there is no fluid flowing out of its outlet. When the inlet pressure reaches the setting pressure, the valve port will be opened and the channel will be connected by itself so that each executive element can operate in the specified order.

Working principle of a valve:

The common characteristic of the pressure control valve is to balance the liquid pressure and spring force on the valve core. They have a direct moving type and pilot type of the division, direct moving type of the structure is simple, the working principle is also relatively easy to understand, here we mainly compare the three-valve pilot type.

2.1 pilot overflow valve

The pilot type overflow valve is divided into the main valve and pilot valve two parts.The pilot valve is a small - scale cone - valve - type direct - acting overflow valve, its spring is used to adjust the overflow pressure of the main valve part. The spring of the main valve does not regulate the pressure, only to overcome the friction force so that the main valve core in time to return to the setting. There is a damping hole at the bottom of the main spool. The system pressure oil reaches the lower part of the main spool through the oil inlet and reaches the right end of the pilot valve through the damping hole.

(1) when the acting force on the pilot valve is not enough to overcome the acting force of the pilot valve spring, the pilot valve is closed, there is no oil flow through the damping hole, so the pressure at both ends of the main valve core is equal, under the action of the soft main valve spring the main valve core is at the lowest position, the oil inlet and outlet is cut off, there is no overflow.

(2) when the force on the pilot valve increases enough to overcome the forerunner, the power of the valve spring, the pilot valve is opened, the pressure oil can through damping hole, the pilot valve flow back to the fuel tank, due to the effect of damping hole, make the main valve core top pressure is p1, p2 is less than the bottom pressure differential Δ p = p1 and p2. When the Δ p force is less than the spring force of the main valve core and valve weight, the main valve to close. When Δ p is greater than the power of the dead weight of the spring force of the main valve core and valve, the main valve opens, regulated the overflow or safety protection.

2.2 pilot type output pressure-reducing valve with a fixed value

Pilot type reducing valve is also composed of the main valve and guide valve. The bottom of the main spool of the pressure reducing valve is also open

There are damping holes, the hydraulic oil pressure from the inlet P1 through the pressure relief port into P2, and then through the channel into the main valve cavity, and then through the damping hole into the main valve cavity and pilot valve front cavity, the pressure is P3, and then through the cone seat after the damping hole on the pilot valve.

(1) when the force generated by P3 is less than the pre-tightening force of the pilot valve spring, the pilot valve is closed, and no pressure difference is generated between the upper and lower ends of the main valve;

(2) when the pressure in front of the valve increases, the pressure P3 after the valve increases accordingly. When the force generated by P3 is greater than the pre-tightening force of the pilot valve spring, the pilot valve opens, the pressure difference between the upper and lower ends of the main valve is generated, and the main valve core is promoted to reduce pressure.

2.3 pilot sequence valve

The pilot sequence valve is also composed of a pilot valve and main valve. There is also a damping hole at the bottom of the main spool. The hydraulic oil entering the system first reaches the middle part of the spool and passes through the damping hole through the oil hole inside the valve to reach the right end of the guide valve.

(1) the valve port is closed all the time before the pressure reaches the setting pressure of the valve;

(2) to achieve the setting pressure, pilot valve open drainage, oil through the damping orifice throttling effect, make the main valve core and produces pressure difference Δ p, Δ p increases with the increase of inlet pressure, when Δ p is enough to overcome the resistance of main valve core, the main valve core and make the oil in and out of the mouth, so the valve actuator to go into the pressure oil, began to act.

Conclusion: the similarities of three kinds of valves can be obtained from the above description

1) three kinds of valves are composed of the main valve and guide valve.

2) the main valve core has a damping hole and is the main valve core and forms the opening differential pressure of Δ p directly.

3) when Δ p more than the main valve spring force (valve weight and resistance to ignore), the main valve core to open it.

4) when the pressure condition changes, the displacement of the three kinds of valve cores is the same, but the valve opening degree changes differently.