Throttle and relief valve

Throttle valve: A throttle valve is a valve that controls fluid flow by changing the throttle section or throttle length. Paralleling the throttle valve and the check valve can be combined into a one-way throttle valve. Throttle valve and one-way throttle valve is a simple flow control valve. In the hydraulic system of the metering pump, the throttle valve and the overflow valve cooperate to form three kinds of throttling speed control systems, that is, the throttling of the oil inlet throttle System, return throttle throttle control system and bypass throttle control system. The throttle valve does not have a negative flow feedback function and cannot compensate for the speed instability caused by the load change. Generally, it is only used for occasions where the load change is not large or the speed stability is not required to be high.

The performance requirements of the throttle valve are: • The flow adjustment range is large, and the flow-pressure difference changes smoothly; • The internal leakage is small, and if there is an external leak, the external leakage amount is also small; • The adjustment torque is small and the action is sensitive. The outline structure of the throttle valve is not different from that of the shut-off valve, except that the shape of the opening and closing part of the throttle valve is different. The opening and closing parts of the throttle valve are mostly conical and streamlined, and the flow and pressure can be adjusted by changing the cross-sectional area of ​​the passage. Throttle valves are used to reduce the pressure of the medium under extreme pressure drops.

The flow of medium between the throttle flap and the valve seat is so great that the surfaces of these parts are quickly damaged - the so-called cavitation phenomenon. In order to minimize the effects of cavitation, the valve flap is made of cavitation resistant material (made of alloy steel) and made into a streamlined cone with a tip angle of 140 to 180. This also enables the flap to have a large opening height. It is generally not recommended. Throttle under small gaps.

Throttle valve has the following features:

1, the structure is relatively simple, easy to manufacture and maintenance, low cost.

2, the adjustment accuracy is not high, can not be adjusted for use.

3, the sealing surface is easy to erode, can not be used to cut off the media.

4, poor sealing performance.

Throttle valve channel can be divided into straight and angular two; According to the shape of the opening and closing parts, there are needle-shaped, groove-shaped and window-type three. Throttle valve installation and maintenance should pay attention to the following matters: The valve often need to operate, it should be installed in the easy operation of the position. When installing, pay attention to the direction of the media and the direction of the arrow marked by the valve body. Throttle plug causes:

1. Mechanical contaminants in the oil, or colloids, asphalt, carbon slag, and other contaminants deposited by oxidation accumulate at the throttling gap.

2. Due to the aging of the oil or the extrusion of charged polarized molecules, there is a potential difference on the metal surface of the throttling gap, so the polarized molecules are adsorbed to the surface of the gap, forming a strong boundary adsorption layer, the thickness of the adsorption layer It is generally 5 to 8 microns, which affects the size of the throttle gap. When the above deposits and adsorbates grow to a certain thickness, they will be washed away by the liquid flow and then reattached to the valve port. In this way, the pulsation of traffic is formed.

3, valve port pressure is greater, due to the valve port temperature is high, the liquid squeezed to enhance the degree of metal surfaces are also more susceptible to friction and the formation of potential differences, so the pressure is large, easy to produce blockage.

Measures to reduce throttling of the throttle

1. Select a thin-blade throttle orifice with a large hydraulic radius.

2, precision filtration and regular replacement of oil.

3. Appropriately reduce the pressure difference before and after the orifice.

4, the use of metal materials with a small potential difference, select the oil with good oxidation stability, reduce the surface roughness of the throttle.

The application of the throttle valve Because the flow of the throttle valve depends not only on the size of the orifice area, but also related to the pressure difference before and after the orifice, the rigidity of the valve is small, so it is only suitable for the implementation of component load change is small and the speed is stable Sexual requirements are not high occasions. For throttling speed control systems with large component load changes and high requirements on speed stability, the throttle valve must be pressure-compensated to maintain the same differential pressure before and after the throttle valve, so as to achieve the flow stability relief valve: a hydraulic pressure Pressure control valve. In the hydraulic equipment, it mainly acts as a constant pressure overflow and safety protection. Constant pressure relief: In the metering pump throttle control system, the metering pump provides a constant flow. As the system pressure increases, the flow demand decreases. At this point, the overflow valve opens, overflowing the excess flow back into the tank, ensuring that the pressure at the inlet of the overflow valve, ie, the pressure at the outlet of the pump, is constant (the valve port is often opened with pressure fluctuations). Safety protection: When the system is working normally, the valve is closed. Only if the load exceeds the specified limit (system pressure exceeds the set pressure) open the overflow and perform overload protection so that the system pressure does not increase (usually the relief valve setting pressure is 10% to 20% higher than the maximum working pressure of the system). ).