Sight Flow Indicator

A sight flow indicator is a short flanged spool with a transparent window that lets you see the fluid inside the pipe. It tells you at a glance whether the line is flowing, whether it is running full or part-empty, and whether the fluid has changed colour, gone cloudy or picked up solids — information no flow meter reading will give you.

The Double Window Sight Glass in metal-bodied construction. For corrosive service the lined equivalent is the PTFE Lined Sight Flow Indicator, and for thermoplastic lines the PVDF Sight Glass.

With windows on opposite sides you can backlight the flow — put a lamp at one window and look through the other — which makes a clear or pale fluid visible when a single window would show almost nothing. It also gives you a second viewing position when pipework or structure blocks the first, and a spare should one window become etched or fouled.

DN 25 to DN 200 (1″–8″) at ANSI 150# and ANSI 300#. Face-to-face and flange drilling follow the applicable standard so the unit drops into a flanged run without re-piping. Tell us the line size, flange standard, media and operating temperature and we will confirm the build.

Bodies and flanges in Carbon Steel (WCB), 304 and 316 stainless, CF8M, duplex, Hastelloy, or PTFE-lined where the media is corrosive. Windows are supplied in borosilicate or toughened glass depending on the pressure and thermal duty, with PTFE or elastomer gaskets selected for the media.

The window is a rated pressure-retaining component, selected and gasketed for the stated class rather than simply fitted into a housing. What breaks sight glasses in service is thermal shock and uneven bolt loading, not steady pressure — so avoid sudden hot-cold swings, tighten the cover bolts evenly to the specified torque, and replace a glass that has become etched or crazed rather than waiting for it to fail.

Where you actually need the information and can see it: pump discharge lines to confirm flow, downstream of a filter or strainer to spot carry-over, on dosing and transfer lines to confirm the batch is moving, and before a reactor inlet. Install it in a straight run away from immediate elbows so the flow is representative, and keep it out of positions exposed to mechanical knocks.

Yes — specify the PTFE lined version, where the body is lined the same way as our lined valve range and the window is sealed against a fluoropolymer face rather than bare metal. For strong acids and halogens on thermoplastic lines the PVDF sight glass is the alternative.

Very little. Keep the windows clean so the indication stays usable, inspect the glass at each shutdown for etching, scratches or crazing, and replace the gaskets whenever the cover is opened. A fouled or etched window is worth replacing early — a sight glass you cannot see through is worse than none, because it invites people to assume the line is empty.

MVS is a manufacturer — building at its own works since 1978, ISO 9001:2015 certified — and exports across several continents. The minimum order quantity is 20 pieces. Lined bodies, special alloys and non-standard flange drilling are built to order.