What happens if a dry-barrel hydrant is opened only part way?

When a dry-barrel hydrant is opened only partially, the primary concern is that air can be introduced into the system, especially if the valve is not fully opened. This situation allows the potential for freezing to occur within the hydrant mechanism during cold weather. The correct answer reflects this potential issue with a dry-barrel hydrant when it is not fully opened, as introducing air into the system can lead to freezing conditions in the dry-barrel.

In dry climates or during winter conditions, water can remain trapped, and without sufficient flow of water to temper the mechanism, ice can form and disrupt operation. This is critical to understand because proper maintenance and operation of hydrant systems directly affect firefighting capabilities. Adequate water flow is essential for effective firefighting, and any obstruction or malfunction can have significant consequences in an emergency situation.

The other options describe scenarios that do not accurately reflect the behavior or risks associated with partially opened dry-barrel hydrants, emphasizing that understanding the mechanics behind hydrants and their operation is vital for effective firefighting strategy and safety.