Sight glass assembly

01

Choose the glass type and fitting standard

Metaglas, a sight glass maker and MAXOS sales representative, describes toughened borosilicate safety sight glasses to DIN 7080, used where vessel processes must be viewed under pressure, thermal and chemical stress, and long sight glasses to DIN 7081 in reflex and transparent forms. Its DIN 7080 page gives minimum surface compressive stress of 40 N/mm² for non-special tempered glass and 100 N/mm² for specially tempered glass, and refers to a catalogue for dimensions, temperature and chemical resistance. [1]

Metaglas describes its metal-fused sight glasses as glass fused into a steel ring, where the different expansion of glass and metal puts the glass under compressive stress, and lists circular sight glass fittings to DIN 28120 or similar and clamp-connection sight glasses to DIN 32676. [2]

In its comparison, Metaglas states that toughened glass needs faces flat and smooth within 0.07 mm to avoid uneven torquing, that re-bolting creates residual stress that can lead to failure, and that surface damage or erosion can cause sudden failure; it says its metal-fused glass tolerates uneven torquing and surface damage and remains leak-tight. These are the maker's own claims. [2]

Editorial note: name the glass standard, fitting standard and the maker's pressure and temperature rating for the exact size, and ask for the installation and torque procedure in writing. [1] [2]

Give the supplier the vessel or pipe connection (flange standard, clamp size, weld-on), nominal size, design pressure and temperature, medium and cleaning chemicals, glass type and standard, gasket material, body material and surface finish, shape (round or obround), lights and wiper or spray nozzles, documentation (pressure equipment, material certificates) and quantity.

02

Set shape, bolting, cleaning and lighting

LJ Star's guidance says the choice of sight glass shape should follow what the process needs to show. It says round sight glasses, including fused constructions, withstand higher pressure and are the more suitable shape above 150 psig, while obround windows give a wider view for watching flow, level and blending. [3]

The same guidance notes that every sight glass collects residue — condensation, splashing, powders and crystallisation — and that cleaning nozzles positioned along an obround window can reach its full length including the corners. [3]

Editorial calculation: 150 psig is about 10.3 bar gauge (150 × 0.06895). [3]

03

Flange, float ball, tank, turbine-type and 3-way sight glasses on offer

NTGD lists flange sight glasses alongside float ball, tank, turbine-type and 3-way sight glasses and sight glass valves.

Ask any supplier for the glass and fitting standards, rated pressure and temperature for the size quoted, and the bolting procedure. [1] [2]

04

Check the pressure, temperature and bolt torque of a DIN 28120 glass

Alfa Laval's leaflet for its bolted sight glass to DIN 28120 describes a base flange welded into or onto the vessel wall, gaskets, a borosilicate glass disc to DIN 7080, a cover flange, nuts and studs. It rates the assembly at a maximum product pressure of 145 psi (10 bar) down to full vacuum, and at -14 °F to +284 °F with EPDM gaskets, rising to 536 °F with Klingersil gaskets. The leaflet states that the pressure resistance of the disc depends on the care with which the bolts and nuts are tightened, working on diametrically opposed pairs, and that its tabled torque values have to be strictly respected; its text gives the torques in Nm while the table header says lbf ft, so confirm the unit with the maker. [4]

The leaflet lists AISI 316L base flanges and AISI 321 cover flanges with 3.1B/AD-W2 material certificates, a 2.2 certificate for the glass disc, and studs to DIN 938/934 in A2-70. It asks buyers to state the DN size and the gasket material on the product side. [4]

Editorial calculation: -14 °F is about -26 °C, 284 °F is 140 °C and 536 °F is 280 °C, using (°F − 32) × 5/9. The 145 psi rating sits just below the 150 psig level above which LJ Star favours a round glass. [4] [3]

05

Glass disc, fused steel ring, fitting body, gaskets and bolting, shape, lights and nozzles

Gaskets and bolting are the row tied to failure: Metaglas says re-bolting toughened glass leaves residual stress, and Alfa Laval says the disc's pressure resistance depends on careful tightening in opposed pairs. [2] [4]

06

DN, rated pressure and temperature, glass type and thickness, body and seal materials for a sight glass order

For one DIN 28120 design, Alfa Laval's leaflet fills four of these rows: 10 bar down to full vacuum, -26 to 140 °C with EPDM gaskets, 316L base flanges and disc thicknesses for DN 100 to 150; surface roughness, leakage rate and weight have no published value in the sources. [4]

07

Send the connection, duty and glass data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Connection: [flange std; clamp; weld-on; DN]
Duty: [pressure; temperature; medium; CIP chemicals]
Glass: [DIN 7080 toughened; metal-fused]
Fitting: [DIN 28120; DIN 32676; other]
Shape: [round; obround]
Accessories: [light; spray nozzle; wiper]
Documents: [pressure rating; material certs]
Quantity: [pieces]

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