Company Profile

Flow Monitors and Sight Flow Indicators: Visual Line Inspection and Process Safety Switches

Quick Answer: A sight flow indicator gives operators a direct visual check of flow direction, color, and air bubbles. A flow monitor sends a switch or 4-20 mA signal when flow falls below a set point. Silver Automation Instruments supplies both from DN15 to DN100 with 316L, PTFE lined, and ATEX Zone 1 options for water, chemical, food, and oil and gas lines.

What Flow Monitors and Sight Flow Indicators Do

A sight flow indicator is an inline window. It lets a plant operator see liquid movement without opening the pipe. Body materials include 304, 316L, PVC, and cast steel. The glass is usually borosilicate or tempered soda lime. For dark liquids or low flow rates, a rotor or flapper element helps show movement. A flow monitor works differently. It has a paddle, thermal sensor, or ultrasonic sensor that detects flow speed. When flow drops below a set point, the unit changes a relay contact. That contact can trigger an alarm, stop a pump, or send a signal to a PLC.

Most engineers buy both devices for the same critical line. The sight flow indicator gives human confirmation. The flow monitor gives automatic machine protection. If a cooling water line stops, the flow monitor cuts the pump before a heat exchanger overheats. A sight glass on the same line lets the operator see the condition during startup and flushing.

Visual Line Inspection in the Field

Visual line inspection matters in plants where operators still walk the floor. A sight flow indicator with a drip tube helps on cooling water lines. A full view model helps on edible oil transfer lines. A PTFE lined model works for hydrochloric acid dosing. In a wastewater plant in Vietnam, operators use sight flow indicators to check polymer dosing at low flow. The line is DN25 PVC. The indicator body is PVC with EPDM seals and a polycarbonate shield. Flow range is 30 to 300 L/h. That covers the dosing pump output.

For high temperature lines above 100 °C, we supply stainless steel bodies with borosilicate glass and graphite gaskets. For steam tracing, the sight glass rating must match the line pressure and temperature. We have seen many sites where a sight glass fails because the gasket material did not match the solvent. This is why we ask for the full chemical name and temperature before quoting.

Process Safety Switches and Control Wiring

Process safety switches are used for pump protection, compressor cooling, and burner fuel line proving. A common model is the Silver Instruments SI-FM-20 paddle flow switch. It has a 316L paddle, a DN15 to DN100 process connection, and a micro switch rated 5 A at 250 V AC. The set point is adjustable from 0.5 to 8 m/s for liquids. For gas lines, we use a thermal flow switch with no moving parts.

Wiring is straightforward. The micro switch has SPDT contacts. You connect the common and normally open wires to the PLC digital input. For 4-20 mA output, use a flow monitor with HART. That output can feed a local display or a control room. In ATEX Zone 1 areas, specify the explosion proof enclosure and intrinsically safe relay barrier. We supply certificates with each batch.

For total flow and a safety output on one line, pair an electromagnetic flow meter with a flow monitor. The flow meter gives 4-20 mA HART output and totalizer data. The flow monitor gives a relay signal if flow falls below a set point.

Materials, Sizes, and Operating Limits

Standard sizes run from DN15 to DN100. Flanged connections follow ASME B16.5 or EN 1092-1. Threaded connections follow NPT or BSPT. Pressure rating is PN16 as standard, with PN40 on request. Temperature depends on the glass and seal. Borosilicate glass with Viton seals handles 0 to 120 °C. PTFE lined bodies handle strong acids but reduce the full view window size.

For food and beverage lines, we supply