Sight Glass

Size: 1/2" - 8" (DN15 - DN200)

Pressure: Class 150 - 300 / PN 10-40

Standard: MSS SP-97, manufacturer-specific

Body Materials: A216 WCB, A351 CF8M, cast iron (GG25), ductile iron (GGG40), bronze/brass for small sizes

Glass: Borosilicate glass (tempered), optionally armored with steel protective shield. Glass disc mounted between body flanges with PTFE/EPDM gaskets on both sides.

Design: Flat glass window(s) in the body—single-window or dual-window (opposing sides for backlit visibility). Fluid passes through the straight-through bore; operator observes flow through the glass window. No moving parts inside the flow path.

Where You Need to See What's Happening

Process lines run inside closed pipes—you can't see the fluid. Whether it's flowing or stopped, clear or cloudy, the right color or off—instruments give you numbers, not pictures.

Sight glasses give you a visual window—flow confirmation, condition checking, gas detection, all through a glass panel in the body. Install where you need to confirm flow—pump discharge, control valve downstream, return lines where no-flow means trouble upstream. Install where you need to check condition—chemical process lines where color shows reaction progress, cooling returns where cloudiness means contamination, condensate where steam carryover shows as bubbles. Install where you need to detect gas—pump suction lines where air means cavitation risk, liquid distribution where gas pockets mean uneven delivery.

Borosilicate glass handles -50°C to +250°C for standard grades—covers most process applications. Specialized compositions go higher for hot oil and thermal fluid. The glass is tempered—resists thermal shock and normal handling impact. It doesn't resist a wrench dropped from six feet—but that's not the operating environment.

Single vs. Dual Window—Dark Fluids Need Backlight

Single-window: one glass panel—look through it and see the fluid. Works for clear fluids where ambient lighting provides enough visibility.

Dual-window: opposing panels on both sides—shine light through one, observe from the other. Backlit visibility through dark or opaque fluids. Essential for heavy oil, dark chemical streams, slurry lines—a single window on dark fluid shows a dark rectangle with zero information.

Don't skip dual windows on dark-fluid lines because they cost more—a single window on heavy oil is a sight glass that can't do its job. Dual costs 20—30% more and actually works—buy the one that works.

A: Standard borosilicate: -50°C to +250°C—specialized compositions go higher. Gasket material also limits the range—PTFE handles higher temps than EPDM. Select both glass and gasket for the process temperature, not just the glass.

A: Single—water is clear enough that ambient light provides sufficient visibility.

A: No fixed interval—replace when visibility is insufficient from scratching, etching, or obscuring. Clean water: years. Corrosive media or abrasive slurries: months—inspect during scheduled shutdowns. Inspect during scheduled shutdowns.

A: With caution—borosilicate handles the temperature, but steam means thermal cycling, condensate on the glass, thermal shock risk. Use PTFE or graphite gaskets rated for steam—consider armored glass for the containment benefit. Inspect more frequently than on liquid service.

A: Minimal—straight-through bore, no restrictions, no screen, no internal elements. Drop is essentially the same as straight pipe of the same length.