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1: For the two valves located at the high-pressure section of the hydrocracking unit, should thin-film pneumatic control valves, hard-sealed ball valves, or pneumatic double-parallel gate valves be used? It seems that the UP process package specifies control valves; thin-film valves may not be able to shut tightly. The design approaches for ball valves and gate valves vary among different design institutes – Luoyang Institute tends to prefer gate valves, while SEI uses both types. 2: As for the pressure relief valves at the high-pressure section of diesel hydrorefining units, it seems that gate valves are more commonly used. I’m not entirely sure about this. The advantage of ball valves is that they are suitable for situations where frequent opening and closing is required, at low pressure levels, and in cases where small size is important; under such conditions, ball valves can open and close quickly, generally meeting the requirements for interlock systems. However, in applications involving high pressure, high pressure drops, and large sizes, there are numerous safety hazards. The sealing surface of a ball valve has a large area (usually 3–4 times that of a gate valve’s sealing surface); when the medium contains particles, dust, or other contaminants, or when the valve is not in use for an extended period, the sealing surface is prone to damage and leakage. The surface of the ball may also become uneven due to scaling or rust, which increases the resistance to movement and makes it difficult to open the valve. The initial fast operating speed of such valves can deteriorate to a slow one, and this phenomenon is common when the valve is not used for a long time. For the self-protecting interlock valves used in high-pressure hydrogen environments and devices subject to sulfur corrosion, they are either open or closed during normal production operations and generally do not need to operate. With proper operation, unless an emergency occurs, they may not need to be activated even once throughout a production cycle. At this point, it is necessary to take into account the possibility of failures arising from wear, scaling, corrosion, etc., as well as issues such as the valve not being able to close or open properly in emergency situations. In high-temperature and high-pressure environments, due to the thermal expansion and contraction properties of metals as well as problems related to manufacturing precision, some ball valves that were initially quite flexible may no longer function as smoothly as they did during testing, and the time required to open and close them increases. However, such delays are not acceptable in industrial operations (the process requirements specify that the valve must close within 1 second to prevent backflow of gas that could damage pump equipment). For parallel plate gate valves, their structural design ensures that the valve stem can expand and contract freely due to the free ends at both sides, which allows for thermal expansion caused by temperature increases. The stem of the ball valve is stuck at both ends, which makes it prone to twisting and deformation. When it contracts due to temperature drops, its valve plates are designed to be elastic (an elastic structure exists between the two valve plates), allowing them to fully adapt to the changes in the contraction of the valve body ； In a ball valve, the entire ball is in tight contact with the valve seat to achieve sealing; changes in temperature can cause the valve seat to lock the ball, preventing it from opening (or closing) and resulting in sticking. If we consider only these two factors, and taking into account the characteristics of valves, then pneumatic ball valves with angular motion are used for switching valves in low-pressure applications where interlocking is required, while pneumatic double-parallel gate valves with linear motion are used for switching valves in high-pressure applications where interlocking is necessary. But is this really the correct solution?