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Quick Answer: A water flow sensor switch with an integrated dry-run protection relay stops a pump when flow drops below a safe setpoint. This protects seals, bearings, and pump housings in water transfer, reverse osmosis, and cooling loops. Silver Automation Instruments supplies ready-to-wire sensor switch and relay sets for DN15 to DN100 lines.
A water flow sensor switch detects actual flow through a pipe. It does not infer flow from pressure or motor current. The sensor side can be thermal, paddle, or inline rotor type. When flow falls below the setpoint, the switch contact changes state. That contact feeds a safety relay or a pump contactor.
Most engineers choose a thermal flow sensor switch for clean water below DN25. A paddle flow switch works better for larger lines and higher flow up to 150 L/min. Silver Instruments stocks both types for common DN sizes.
Pump dry-run failure is common in remote pumping stations. A blocked strainer, closed valve, or loss of suction drops flow to zero while the motor keeps running. The seal heats up in seconds. The bearing loses lubrication. In practice, a 2.2 kW centrifugal pump can fail within 20 minutes under dry running.
We have seen this on customer sites many times. A water treatment contractor in Saudi Arabia lost two booster pumps in one month because a float valve stuck shut. The motor ran dry overnight. After adding a Silver Instruments flow sensor switch with a dry-run relay, the pump tripped in 5 seconds on the next low-flow event. Because a dry pump fails fast, the relay must trip within seconds.
The sensor switch sends a dry contact or PNP signal when flow drops under the setpoint. The safety relay receives that signal and opens the pump contactor. A time delay filters false trips from flow surges or pipe fill cycles. Typical delay range is 0.5 to 10 seconds.
Some Silver Instruments relay modules include a manual reset button. Others auto-reset after flow returns. For sewage or slurry pumps, we recommend manual reset. It forces a site operator to check the suction line before restart.
You need six data points before you select a water flow sensor switch and relay set.
Pipe size in DN. We cover DN15, DN20, DN25, DN40, DN50, DN80, and DN100.
Normal flow and minimum safe flow in L/min or m³/h.
Media type. Clean water, seawater, chemical, or wastewater.
Pressure and temperature. Standard sensors handle 10 bar and 0 to 70 °C. High temperature versions handle up to 100 °C.
Power supply. 24 VDC or 230 VAC are standard.
Contact rating. Silver Instruments relays are rated 5 A at 250 VAC.
For a DN25 clean water booster pump running 30 to 60 L/min, a common pairing is the Silver Instruments WFS-DN25 thermal flow sensor switch and the PDR-1 dry-run relay. Set the trip point at 12 L/min and the delay at 3 seconds. This avoids nuisance trips during pump start.
Water booster skids in Indonesia and the Philippines use flow sensor switches on pump discharge. A local pump skid builder in Surabaya fitted 14 booster sets with WFS-DN25 sensors and PDR-1 relays last year. The added cost per skid was under 4 percent. No callbacks for dry-run seal damage were reported in the first six months.
Reverse osmosis systems need reliable feed flow confirmation. If the cartridge filter clogs, the high pressure pump can run dry and destroy the pump head. A flow sensor switch in the feed line before the pump gives early trip protection.
Cooling water loops in food and beverage plants also benefit. A dairy in New Zealand used one WFS-DN40 paddle switch on a glycol water return line. It cut a 15 kW circulation pump when return flow dropped under 40 L/min. That protected the heat exchanger and pump seals.
Seawater desalination plants in the Middle East use dry-run relays on dosing pumps and transfer pumps. For seawater, choose a sensor with titanium or PVC wetted parts. Do not use brass or stainless steel that will corrode in high chloride water.
Mount the flow sensor switch in a straight pipe section. Keep 10 diameters upstream and 5 diameters downstream. This gives stable flow reading and fewer false trips.
Install the sensor in the vertical or horizontal pipe with the probe fully immersed. Air pockets can cause false low flow. Do not mount the sensor at the top of a horizontal pipe if air can collect there.
For paddle type switches, make sure the paddle length matches pipe ID. Cut the paddle as needed. A paddle too long can block the pipe and a paddle too short will not trip at the right flow.
The WFS-DN15 thermal flow sensor switch suits low flow from 0.5 to 10 L/min. It works well for chemical dosing and small cooling circuits. The WFS-DN25 covers 1 to 60 L/min. The WFS-DN40 and WFS-DN50 paddle switches cover 20 to 150 L/min. The PDR-1 relay module operates on 24 VDC or 230 VAC and offers adjustable delay and manual or auto reset.
For hazardous areas, ask about ATEX Zone 1 rated flow switches. We can supply sensors with flameproof housings and relay barriers for gas environments.
Send us your pipe size in DN, normal flow range in L/min, media type, pressure in bar, temperature in °C, and power supply. We will recommend a sensor switch and relay pairing with price and delivery time.
Contact Silver Automation Instruments today. Tel: +86-25-68650347. Whatsapp: +86-25-52155837. WeChat: +86 15365082610.
Can one flow sensor switch protect a pump directly without a relay?
Yes, if the switch contact rating is high enough. But most flow switches have a low contact rating. A relay module gives a 5 A or 10 A output for the pump contactor and adds adjustable delay.
What flow setpoint should I use for a booster pump?
Set the trip point at about 30 to 40 percent of normal flow. For a pump running at 50 L/min, set the trip point at 15 to 20 L/min. This reduces false trips during startup or small demand changes.
Does the flow sensor switch work with dirty water?
Thermal sensors can foul in sticky or dirty media. For wastewater or slurry, use a paddle type or a pressure based dry-run device. Silver Instruments can supply paddle switches with stainless steel bodies for dirty water.
What pipe sizes do you cover?
We cover DN15 to DN100 as standard. Larger pipe sizes up to DN200 are possible with custom paddle length on request.
Can the relay reset automatically after flow returns?
Yes. The PDR-1 relay has an auto reset mode and a manual reset mode. Auto reset works for unstaffed pump stations. Manual reset is better for sewage or slurry lines where the cause of low flow should be checked.
