The use of pressure relief valve constant pressure overflow effect: In the metering pump throttling regulation system, the dosing pump provides a constant flow. When the system pressure increases, will reduce the demand for traffic. At this point, the relief valve is opened to allow excess flow to overflow into the fuel tank to ensure that the relief valve inlet pressure is constant, ie the pressure at the pump outlet is constant (the valve port often opens with pressure fluctuations). Safety protection: When the system is working normally, the valve is closed. Only when the load exceeds the specified limit (the system pressure exceeds the set pressure), the overflow will be opened and over load protection will be implemented to prevent the system pressure from increasing (usually, the setting pressure of the relief valve is 10% ~ 20% higher than the maximum working pressure of the system. ). Used as a remote control valve unloading valve for high and low pressure multi-stage control valve for the sequence valve for back pressure (string in the return line). 1, direct acting relief valve 1), cone valve direct acting relief valve cone valve direct acting relief valve icon for the cone valve direct acting relief valve. The left end of the cone valve 2 is provided with a bias plate 1 to hold the spring 5. The right end of the cone valve has a damping piston 3 (the damping piston plays the role of damping when the cone valve is opened or closed on the one hand to improve the stability of the working of the cone valve. On the one hand to ensure that the cone valve will not tilt after opening). Inlet pressure oil (pressure P) can enter the bottom of the piston through the radial clearance of the piston, creating a left-hand hydraulic force F = P · A (A is the piston bottom area). When the hydraulic pressure F acting on the bottom is greater than the spring force, the valve opening of the cone valve opens, and the oil overflows from the cone valve opening to the oil tank through the return opening. As long as the valve opening, there is oil flow through the relief valve, the pressure relief valve inlet to maintain a basic constant. By adjusting the rod 4 to change the pressure spring 5 preload Ft, you can adjust the relief pressure. (5-21) where K and X0 are the spring rate and the amount of precompression (m), respectively; G is the self-weight of the valve core (self-weight when the valve core is placed vertically, and not considering the weight when it is placed horizontally) N); Ff is the friction between the valve core and the valve sleeve (in the opposite direction to the movement of the valve core) (N); F5 is the steady-state hydraulic power. Since the connection between the damping piston and the cone valve is cone, Therefore, F5 = 0; Fj is the jet force, an annular groove is formed on the drift disk at the end of the cone valve for the purpose of Changing the flow direction of the outlet of the cone valve produces a jet force in the direction opposite to that of the spring force. When the flow through the relief valve increases, the spring force increases due to the increase of the valve opening of the cone valve. Jet forces in opposite directions increase at the same time, offsetting the increase in spring force, ie. Considering F5 = 0 and Fj = Kx, equation (5-21) becomes (5-22) From equation (5-22), the inlet pressure P of this valve is not affected by the change of flow rate, By the valve opening x size of the impact. Controlled pressure P change is very small, constant pressure and high precision. 2), ball valve type direct acting relief valve ball valve direct acting relief valve is shown as a ball valve type direct acting relief valve. It also has a damping piston 3, but unlike the cone valve structure, the piston is not rigidly connected to the ball valve 1, but the piston is brought into contact with the ball valve by the damping spring 4 (hydraulic pressure balance between the pistons). Due to the damping effect of the piston, the ball valve which is always connected to the piston can be moved smoothly. (Pa) (5-23) where A is the ball seat hole area (m2); K1 and K2 are the stiffness (N / m) of the main spring 2 and the damping spring 4 respectively; x10 and x20 are the values ​​of the main spring 2 and The pre-compression amount (m) of the damping spring 4; x is the ball valve opening amount (m). From (5-23) we can see that due to the increase of the damping spring, the stiffness of the main spring is increased by K2, and the precompression is reduced by K2x20 / K1, which helps to improve the static characteristics of the valve. 2, pilot-type relief valve by the main valve and pilot valve of two parts. Pilot valve similar to the direct acting relief valve, but usually cone valve (or ball valve) shaped seat structure. The main valve can be divided into a concentric structure, two concentric structure and three concentric structure. Figure 1, pilot-operated relief valve Figure 1 pilot-relief valve. As the main valve core 6 and the valve cover 3, the valve body 4 and the main valve seat 7 are three places with concentric requirements, it is three concentric structure. The pressure oil enters from the oil inlet P in the middle of the valve body 4 and enters the main valve cavity through the damping hole 5 on the main valve core 6. The passage a in the valve cover 3 and the small hole in the cone valve seat 2 Role with the cone valve 1. The pilot valve is closed when the pressure p1 of the inlet port is smaller than the setting value of the pilot valve pressure-adjusting spring 9. Since the effective area ratio (A2 / A1) of the upper and lower sides of the main valve is 1.03-1.05, Large, the role of the main valve on the pressure difference and the main valve spring force are the main valve port closed, no overflow. When the inlet pressure exceeds the pilot valve set pressure, the pilot valve is opened, resulting in the capital port P well main spool orifice 5, the pilot valve port, the main spool center hole to the lower part of the valve body 4 outlet Outlet) O of the flow. The flow loss at the orifice causes the fluid in the primary and lower chambers to create a pressure differential that increases with increased pilot flow and overcomes the overall pressure differential across the lower active side of the main spool Main valve spring force, main valve weight G and friction Ff, the main spool open. At this point the oil inlet P and the outlet (overflow) O direct communication, resulting in overflow to maintain system pressure. Figure 2, the structure of two concentric pilot relief valve Figure 2 is a concentric pilot pilot relief valve structure, the main valve spool with a cylindrical cone valve. In order to make the main valve closed with good sealing, requiring a main spool of the cylindrical guide surface and the conical surface and the valve sleeve with a good, the two concentricity requirements are higher, so that the two concentric. There is no damping hole on the main spool, and the three damping holes 2, 3, 4 are respectively arranged on the valve body 10 and the pilot valve body 6. Its working principle and three concentric pilot-based relief valve the same, but the oil from the main valve to the main valve cavity on the lower chamber, to go through three damping holes. Drain holes 2 and 4 only produce a pressure difference between the lower main cavity of the main valve and the pilot cavity of the pilot valve, and act on the main cavity of the main valve through the orifice 3 to control the main spool to open. The orifice 3 also serves to increase the stability of the main spool. (Pa) (5-24) Where, Ac is the area of ​​the pilot valve seat hole (m2); Ky, Kx are the stiffness (N / m) of the main valve and the pilot valve spring; , X0, respectively, the main valve and the pilot valve pre-compression (m); y, x, respectively, the opening of the main valve and the pilot valve opening (m); Ff for the main valve and valve body friction G main spool weight (N).