Which water flow switch cannot be used in dry sprinkler pipes?

Which water flow switch cannot be used in dry sprinkler pipes?

• A. Vane-type
• B. Pressure-type
• C. Magnetic-type
• D. Thermal-type

Answer: (A)

The concept being tested is how different water flow switches operate in dry sprinkler piping. In a dry system, the piping is charged with air, not water, until the dry-pipe valve releases water. A vane-type flow switch uses a paddle that must be driven by moving water to register a flow event. In an air-filled dry pipe, there isn’t a stable water column for the vane to be pushed by, so the switch is unreliable or may fail to activate when water actually begins to flow. That makes it unsuitable for dry-pipe systems. Other switch types don’t rely on a vane moving through a water-filled tube. A pressure-type switch detects the pressure drop that occurs as water starts to move through the line, which can happen in a dry-pipe context. A magnetic-type switch senses flow via a magnetic element carried by the water, triggering as water moves. A thermal-type switch responds to the cooling effect of flowing water, which also works when water begins to flow in a dry pipe. Because these operate correctly in an air-filled, dry system, they’re appropriate for dry-pipe applications, unlike the vane-type.

The concept being tested is how different water flow switches operate in dry sprinkler piping. In a dry system, the piping is charged with air, not water, until the dry-pipe valve releases water. A vane-type flow switch uses a paddle that must be driven by moving water to register a flow event. In an air-filled dry pipe, there isn’t a stable water column for the vane to be pushed by, so the switch is unreliable or may fail to activate when water actually begins to flow. That makes it unsuitable for dry-pipe systems.

Other switch types don’t rely on a vane moving through a water-filled tube. A pressure-type switch detects the pressure drop that occurs as water starts to move through the line, which can happen in a dry-pipe context. A magnetic-type switch senses flow via a magnetic element carried by the water, triggering as water moves. A thermal-type switch responds to the cooling effect of flowing water, which also works when water begins to flow in a dry pipe. Because these operate correctly in an air-filled, dry system, they’re appropriate for dry-pipe applications, unlike the vane-type.