What component allows for controlled flow in a dry barrel hydrant?

The component that allows for controlled flow in a dry barrel hydrant is the drain valve. In the context of dry barrel hydrants, this valve serves the crucial function of allowing residual water in the hydrant body to drain out after the hydrant has been closed. This drainage prevents water from freezing inside the hydrant during cold weather, ensuring that the hydrant remains operational and free from ice damage.

When the hydrant is opened, water enters from the main supply and can flow through the outlet. Once the hydrant is shut off by the operator, the drain valve opens, allowing any remaining water in the hydrant's barrel to escape. This process helps maintain the functionality of the hydrant over time and minimizes maintenance issues related to freezing. Thus, the drain valve is essential for the controlled flow and preservation of the hydrant's integrity, making it the correct answer for this question.