Low pressure packing should never be used to pack which type of steam valves?

The correct answer is based on the principle that low pressure packing is not suitable for high pressure applications due to the risk of failure under stress. High pressure steam valves are designed to operate at high pressures and any packing used must be able to withstand the mechanical forces involved. Low pressure packing materials may not provide the necessary strength and sealing capability to maintain proper function under the high pressure conditions that these valves face.

Using low pressure packing in high pressure situations can lead to leaks, potential system failures, and safety hazards. High pressure valves require packing that is specifically rated for the pressures they will encounter, ensuring safe and effective operation. This choice recognizes the importance of using the correct materials that match the operating conditions of the system.

The other options are not as critical in the context of using low pressure packing; low temperature valves and standard pressure valves may safely operate with lower specification packing as they do not face the same risk of mechanical failure due to pressure.