What type of valve is this?
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Excuse me, what kind of valve is this? What are the functions of the check valve and globe valve in the small square box? Valves are used to control the pressure in the lubrication circuit
To prevent the check valve from failing, should another stop valve be installed?
The stop valve and check valve inside the box together form a pneumatic component, namely a speed regulator; you can take a look at SMC’s symbols for speed regulators – they resemble it somewhat. By adjusting the speed regulator, it is possible to control the timing of the valve’s opening and closing.
This needs to be understood by referring to the actual device; it is likely a self-regulating pressure control valve with a pilot function.
The throttle valve adjusts the response speed of the control valve, while the check valve enables rapid oil drainage
This is a stand-alone pressure relief valve used to better control oil pressure. The function of a check valve is to transmit oil pressure rapidly, and it allows flow in only one direction. This achieves the effect of quickly activating the pressure relief valve, preventing damage to equipment such as the compressor and even causing the system to shut down. But when the pressure is high, it can only be reduced slowly to prevent it from dropping too much at once. In simple terms, it serves to make the valve open quickly and close slowly (or vice versa; it’s not clear from your diagram since it’s just a screenshot)
The valve inside the small square is a needle valve + check valve