Flow Switch | How to Select a Flow Switch as per Line Size | Flow Switch Size calculator.
Flow Switch | How to Select a Flow Switch as per Line Size | Flow Switch Size calculator.
A flow switch is an instrument used to detect the presence or absence of flow in a pipe or system. Its main purpose is to monitor, control, or protect equipment by triggering an alarm or switching a device ON/OFF when flow conditions change.
Definition: A flow switch is a device that detects the presence or absence of flow (liquid, gas, or air) in a pipe and changes a switch state to trigger alarms, control equipment, or protect systems. basically flow switch is a device that activates a switch when liquid, gas, or air starts flowing or stops flowing through a pipeline.
How it works
A flow switch has a sensing element (paddle, vane, thermal probe, ultrasonic sensor, etc.) and a switch mechanism (mechanical or electronic). When flow crosses a set threshold the switch changes state (ON ↔ OFF) and sends a signal to pumps, alarms, PLCs, or safety interlocks.
Common types
- Paddle / Vane Flow Switch — mechanical switch and paddle moves when flow passes, used in water & HVAC lines.
- Thermal Flow Switch — senses temperature change, no moving parts.
- Electronic / Ultrasonic Flow Switch — accurate, fast, solid-state.
- Pressure-differential Flow Switch — Measures pressure drop across an orifice or filter. senses ΔP across element or filter.
Applications
- Pump dry-run protection
- Cooling water monitoring for chillers & compressors
- Sprinkler / fire system flow detection
- Boiler & lubrication systems
- HVAC flow alarms and interlocks
Simple example
If a pump runs with no flow, the flow switch signals "NO FLOW" so the pump can be stopped automatically to prevent damage.
How to Select a Flow Switch as per Line Size
Flow switch selection depends mainly on the pipeline size, flow rate, and the type of fluid. Below is a simple and practical guide to choose the correct flow switch according to line size.
1️⃣ Check Pipeline Size
Select a flow switch model that supports your pipe diameter. Most manufacturers specify suitable sizes like ½”, 1”, 2”, 4”, etc.
- Small lines (½” to 1”) → Use thermal or electronic flow switch
- Medium lines (1” to 4”) → Use paddle/vane type
- Large lines (4” and above) → Use large paddle type or differential pressure flow switch
2️⃣ Determine Flow Velocity
The switch must activate within the allowed **minimum flow velocity**. Typical ranges:
- Water: 0.5 – 1.5 m/s
- Air: 5 – 10 m/s
- Oil: 0.3 – 1.0 m/s
If velocity is too low → choose a **thermal flow switch**.
3️⃣ Confirm Flow Rate
The flow switch must match the system's minimum and maximum flow rate. Example: If a 2” line has 20–100 LPM flow, check the switch’s LPM range.
4️⃣ Select the Appropriate Paddle Length (for paddle type)
Paddle switches require trimming the paddle according to pipe size:
- 1” pipe → short paddle
- 2–3” pipe → medium paddle
- 4–6” pipe → long paddle
5️⃣ Check Fluid Type
- Clean water → paddle/thermal
- Dirty or viscous fluid → thermal or DP flow switch
- Air/gas → thermal or electronic type
6️⃣ Ensure Correct Connection Type
- Threaded (½”–2” lines)
- Flanged (3”–12” lines)
7️⃣ Electrical Ratings (important)
- SPDT or DPDT contacts
- 24VDC / 230VAC switching
- Compatibility with PLC/DCS
💡 Quick Selection Example
Pipe Size: 3 inch Fluid: Water Flow: 40–120 LPM Best choice: Paddle vane type with medium paddle + SPDT contact.
Flow Switch Selection Table (as per Line Size)
| Line Size | Recommended Flow Switch Type | Typical Paddle Length | Application |
|---|---|---|---|
| ½” – 1” | Thermal / Electronic Flow Switch | Not Required | Low-flow lines, RO plant, small cooling lines |
| 1” – 2” | Paddle / Vane Type | Short Paddle | Water flow monitoring, pump protection |
| 2” – 4” | Paddle Type | Medium Paddle | Cooling towers, chillers, HVAC pipelines |
| 4” – 8” | Heavy-duty Paddle or DP Type | Long Paddle | Large industrial pipelines, process water |
| 8” and above | Differential Pressure (DP) Flow Switch | Not Applicable | High-flow industrial lines, firefighting mains |
Additional Selection Parameters
- Fluid Type: Water, Oil, Air, Chemical
- Minimum Flow Rate: Must match switch sensitivity
- Mounting: Threaded / Flanged
- Electrical Contact: SPDT or DPDT
- Operating Temperature & Pressure: Must suit process conditions
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