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One of the faults of piston-type pressure relief valves is that the valve does not open. The reasons are: ① The control channel is blocked by debris (the channel is narrow and prone to blockage) ； ②The piston is stuck in the highest position due to rust ； Although it is under stress at the top, it cannot move downward and thus the main passage cannot be opened. Therefore, a filter must be installed in front of the piston pressure relief valve ； For newly installed or long-idle pressure relief valves, it is essential to disassemble them for inspection and cleaning. The second fault is that the valve is in direct passage mode, failing to provide pressure reduction. The reasons are: ① The piston gets stuck at a certain position (not at the highest position) ； ②The main valve stem gets stuck at a certain position in the cylinder disc guide hole (not at the sealing position) ； ③The spring below the main valve disc is broken or failed ； ④The pulse valve stem gets stuck at a certain position within the valve seat hole (not at the sealing position), causing the piston to be under pressure at all times ； ⑤Debris is stuck or there are scratches between the two sealing surfaces of the main valve disc and the main valve seat ； ⑥The diaphragm fails due to fatigue or damage. For pressure relief valves, it is particularly important to emphasize regular inspection. If dirt or rust is found, it should be removed promptly ； If the piston rings are damaged, they should be replaced ； If the spring fails, it needs to be replaced with a functional one ； If the sealing surfaces do not fit tightly, they should be ground again ； If the diaphragm is not functioning properly, it needs to be replaced ； Upon use, if it is confirmed that the valve handle is too thick, it should be sanded. The third fault is that the pressure behind the valve cannot be adjusted. In addition to the various reasons mentioned above, there are also: ① Failure of the adjustment spring ； ②The cap seam is leaking and cannot maintain pressure. This also requires timely inspection and handling to prevent it. Furthermore, there is another phenomenon, namely fluctuating pressure pulses downstream of the valve, which are extremely unstable. This is due to a large difference between the input medium and the output medium; a valve with an appropriate valve diameter should be selected again. Another reason for unstable pressure behind the valve is the improper selection of the adjustment spring ； It is a pressure reducing valve with a specified pressure; there are several types of adjustment springs available. For example, for a specified pressure of 16 kilograms per square centimeter, the adjustment springs can be of 1–3 kilograms per square centimeter, 2–8 kilograms per square centimeter, or 7–11 kilograms per square centimeter. If the pressure required after the valve is only 1–3 kilograms per square centimeter, but the spring used is one with a setting of 7–11 kilograms per square centimeter, then the pressure after the valve cannot be adjusted accurately. 2. One of the faults of spring-type safety valves is leakage at the sealing surface. The reasons are: ① Debris is trapped between the sealing surfaces ； ②The sealing surface is damaged. Such failures need to be prevented through regular maintenance. The second flaw is low sensitivity. The reasons are: ① Spring fatigue ； ②The spring was used improperly. The spring is fatigued and should undoubtedly be replaced. Improper use of the spring is due to the user’s failure to pay attention to a spring-type safety valve with a specified pressure; it has several pressure ranges, and each range corresponds to a specific spring. For a safety valve with a nominal pressure of 16 kilograms per square centimeter, if it is used in a pressure range of 2.5–4 kilograms per square centimeter and equipped with a spring rated for 10–16 kilograms per square centimeter, it can still open, but its response is inconsistent and not very sensitive. 6.2.3 Check valves or non-return valves: Common faults include: ① Broken valve disc ； ②Medium backflow. The reason for the valve disc breaking is that the pressures of the fluid on both sides of the check valve are in equilibrium, resulting in a back-and-forth force; this causes the valve disc to strike against the valve seat repeatedly, and valve discs made of brittle materials such as cast iron or brass get broken as a result. The preventive measure is to use check valves with valve discs made of ductile materials. The reasons for medium backflow are: ① Damage to the sealing surface ； ②Impurities get trapped in it. Repairing the sealing surface and cleaning away impurities can prevent backflow. The above description of common faults and preventive methods is only intended to provide guidance; in actual use, other faults may arise. To proactively and flexibly prevent valve failures, the most fundamental thing is to be familiar with its structure, materials, and operating principles.