What is a frangible bulb used for in fire detection systems?

A frangible bulb is specifically designed to detect temperature changes in fire detection systems, particularly in sprinklers and fire suppression systems. When the temperature around the bulb reaches a certain threshold, the liquid inside the bulb expands and causes the bulb to break. This breaking action initiates the release of water from the sprinkler system, effectively activating fire suppression when it detects heat.

The other options represent different functions that are not associated with a frangible bulb. For example, pressure relief is usually handled by pressure-relief valves in a system, while controlling water flow often involves different types of valves or electronic controls. The blocking of smoke is typically managed by smoke detectors, barriers, or ventilation systems, not by frangible bulbs. Thus, the primary purpose of a frangible bulb remains centered on detecting temperature changes, making it a crucial component in fire protection systems.