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Flow Meter Alarm Programming: Setting Up High and Low Flow Relay Output Thresholds
Quick Answer
High and low flow relay outputs trip when the measured flow rate crosses a set value. You enter the threshold in engineering units such as m3/h or kg/h. You also set hysteresis and delay time to stop relay chatter during start-up and stop conditions.
Most engineers skip the alarm relay setup during commissioning. In practice this is a mistake. A flow meter without alarm thresholds is just a display device. The relay output turns the meter into a control device. That relay can stop a pump, open a valve, or send a warning to a PLC. For B2B projects the high and low flow alarm thresholds are often specified by the process engineer before start-up.
Relay Output Basics for Flow Meter Panels
Silver Instruments flow meters come with two common relay output types. Mechanical relay outputs handle 250 V AC 3 A loads. Solid state relay outputs switch low voltage DC signals. You wire the high flow alarm relay to a control panel digital input or an external siren. The low flow alarm relay usually goes to the pump start circuit or a solenoid valve. Check the terminal diagram. The terminals are marked COM, NO, and NC. Set the logic to normally open or normally closed in the meter menu. Use separate 24 V DC power for the loop if the load is inductive. Add a flyback diode across DC coils.
Step by Step High and Low Flow Alarm Setup
The exact menu path depends on the flow meter model. On Silver Instruments electromagnetic flow meters you open the output menu and select Alarm 1. Then select High Flow or Low Flow. Enter the threshold in engineering units. For example a DN80 water line in a Vietnam food plant had a normal flow of 45 m3/h. The high alarm was set at 55 m3/h and the low alarm at 18 m3/h. The low alarm protected the feed pump from dry running.
After the threshold you set hysteresis. Hysteresis is the gap between the trip point and the reset point. A low flow alarm at 18 m3/h with 2 m3/h hysteresis trips at 18 m3/h and resets at 20 m3/h. Set the delay time from 2 to 10 seconds. This blocks false trips caused by air bubbles or pump start-up surges. So the relay stays quiet during normal flow.
In practice most sites set high flow alarm at 110 percent of normal maximum flow. Low flow alarm is often set at 30 to 40 percent of normal flow. But this changes with process. A batch reactor in Mexico may allow low flow down to 5 percent of full scale. A desalination plant high pressure feed line may not allow any low flow below 70 percent for more than 5 seconds.
For Coriolis mass flow meters you can set alarm thresholds in kg/h or t/h. Use the same approach. The display shows relay state as active or inactive. Some models have a simulation mode. Use simulation to test the relay wiring before you connect the process fluid.<
Recommended Silver Instruments Models
For conductive water and wastewater lines from DN15 to DN300 and conductivity above 5 µS/cm the Silver Instruments electromagnetic flow meter series works well. It accepts 4-20 mA HART, pulse, and two relay outputs. For fuel oil, chemicals, and viscous media from 0.5 cP to 500 cP use the Coriolis mass flow meter series. It handles high accuracy mass flow and density with two alarm relay outputs. For gas and compressed air the thermal mass flow meter series gives relay outputs tied to mass flow in kg/h or standard m3/h.
Field Cases From Australia and Southeast Asia
We have seen this on customer sites many times. A pump supplier in Western Australia set a low flow alarm at 22 m3/h on a DN125 electromagnetic flow meter. The bore water conductivity was around 1800 µS/cm. The relay stopped a bore water pump before the well level dropped too far. This was a simple setup. The only tricky part was the time delay. They started with 1 second and the relay chattered during water hammer. We changed the delay to 5 seconds and the problem disappeared.
Another case came from a paint manufacturer in Southeast Asia. They used a Coriolis mass flow meter on a resin batch line. The high flow alarm threshold was set at 12 kg/h above the normal dosing rate. The relay triggered a solenoid valve to stop the resin feed. That simple relay action saved one batch from overfeeding.
FAQ
What is the difference between a flow switch and a flow meter alarm relay? A flow switch only trips at one fixed point. A flow meter alarm relay gives you a programmable threshold plus hysteresis, delay, and display values.
Can I set both high and low flow alarms on one flow meter? Yes. Many Silver Instruments flow meters have two independent relay outputs. Use Relay 1 for high flow and Relay 2 for low flow.
What units should I use for the alarm threshold? Use the same engineering units as the process. For liquids use m3/h, L/min, or kg/h. For gas use standard m3/h or kg/h. The meter converts the signal internally.
How do I stop the relay from chattering at low flow? Increase hysteresis and time delay. Start with 2 percent hysteresis and a 5 second delay. Then fine tune from there.
What information should I send for a quotation? Send us your fluid, pipe size, flow range, relay output type, and power supply. Also state the high and low alarm set points if you have them.
Contact Silver Automation Instruments
Send your pressure in bar, temperature in °C, pipe size in DN, and flow range to get a specific quote for a flow meter alarm relay output model. Contact Silver Automation Instruments. Tel: +86-135-1213-1526. Whatsapp: 86-135-12131526. WeChat: +86 13512131526. Website: flow-meter.com.au
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