What should you avoid when admitting steam to a steam piping system?

When admitting steam to a steam piping system, it is crucial to avoid having water in the piping. This is because the presence of water can lead to water hammer, which is a destructive phenomenon that occurs when steam rapidly condenses and creates a shockwave, causing pressure surges. This can damage pipes, fittings, and other components of the system. Steam should only enter the piping system when it is adequately dry, ensuring efficient thermal transfer and preventing potential hazards associated with liquid water presence.

The other factors noted in the choices can also impact system performance and safety, but they do not pose the same immediate risk of mechanical failure as water does within steam piping. For instance, excess steam pressure could lead to operational inefficiencies or strain on components, while using an incorrect valve might cause improper flow regulation. Allowing air in the system can reduce efficiency and cause corrosion but does not have the same potential for immediate destructive impact as water does.