Observation window for hot alkaline and steam service: how fast the glass erodes, and when to fit a mica disc or FEP screen on the media side

01

Millimetres of borosilicate lost per month at pH 10 and 170–225 °C

Ebora Process Automation, which supplies sight glass fittings, writes that hardened borosilicate glass in a medium at 170 °C and pH 10 erodes by 0.2 mm a month, about 2.4 mm in a year, and gives 8.4 mm for the same glass at 200 °C. It says glass erodes faster above pH 7, because it resists acids well but alkalis less. [1]

A Schott patent on corrosion-resistant boiler sight glasses reports a harsher autoclave test: at 225 °C, 25 bar and pH 10, unprotected high-resistance borosilicate glass became 0.6 mm thinner in 70 hours, while a tin oxide (SnO₂) layer on the inner face prevented glass corrosion for at least those 70 hours. [5] Editorial calculation: 0.6 mm in 70 hours is about 0.0086 mm per hour, or roughly 6 mm over a 720-hour month if the rate held, some 30 times Ebora's 0.2 mm per month at 170 °C. The two figures come from different temperatures, pressures and test methods, so treat them as a range that shows how steeply loss rises with temperature, not as one curve.

Continental Trade, which sells mica shields and gauge glasses, describes the mechanism in steam boilers: mineral deposits crystallise on the glass and attack it, more so at higher temperature and pressure and depending on the pH of the water; the glass becomes tarnished and thinner, which reduces its strength and can lead to dangerous failure. It says losses for unprotected glass, depending on the pH and temperature of the boiler water, can reach millimetres per month. [3]

Give the window supplier the medium, its pH, the maximum temperature and pressure at the window, whether it is liquid, steam or both, and how long the window must stay in service between shutdowns.

02

Mica disc, FEP screen or tin oxide layer between the glass and the medium

ACI's PN 25 oval sight glass fitting, Type 337, shows how protection changes the rating of the same fitting. With borosilicate glass to DIN 7081 it is rated to 243 °C in saturated steam or hot water, to 280 °C where no technically significant glass attack occurs, and to 320 °C for transparent glass with mica protection, all at up to 25 bar(g). ACI offers an FEP protective screen or FEP coating for higher pH values and says mica lenses should be used with aggressive media or steam. [2]

Continental Trade says a mica shield should be mandatory where steam pressure exceeds 40 bar and temperature exceeds 200 °C. It supplies shields cut from natural muscovite mica in standard thickness bands from 0.10–0.15 mm up to 0.30–0.40 mm, grades mica by the visual categories of ASTM D351 (V-1 clear being the best), and warns that the layers must never be split, because the naturally bonded layers are what keep the shield tight. [3]

The Schott patent notes the limits of each protective route as they stood when it was filed: mica protects only within limits that depend on the quality of the deposit it came from, optical-grade mica had become scarce and expensive, and plastic layers on the glass were limited to about 200 °C and could peel off under vapour diffusion and strong flow at the window. [5]

State on the order which side of the glass faces the medium, whether a mica disc or FEP screen is to be fitted there, and whether the glass is transparent or reflex, since reflex glass cannot take a mica disc.

03

Borosilicate or aluminosilicate behind mica: 313 °C at 100 bar against 400 °C at 220 bar

For the modular viewing ports of high-pressure level indicators, in which each port holds a glass, a mica shield and a set of seals and springs in a heavy housing, Continental Trade lists borosilicate glass for saturated steam or alkaline media at up to 313 °C and 100 bar, and aluminosilicate glass at up to 400 °C and 220 bar, with a mica shield required in both cases. In non-aggressive media up to 36 °C it gives a permissible pressure of 340 bar for either glass. [4]

The same supplier states that the mica shield is not a permanent protection: continuous exposure to hot, high-pressure water also corrodes the mica, at a rate much lower than that of glass and higher at higher temperature. [4] Editorial note: these ratings belong to compact high-pressure ports, not to a large observation window on a vessel shell; a bigger glass in the same conditions is rated lower, so use them to see the gap between glass types, and take the actual rating from the window maker for the size you are buying.

04

White or opaque glass, six-month changes and keeping a spare disc

Continental Trade's instruction for steam-service viewing ports is to replace the mica, gasket and glass assembly immediately when the glass turns white or opaque, warning that failure to do so can end in the glass breaking and a violent discharge of high-velocity steam; it recommends a preventive schedule that replaces the assembly before the mica shield is dissolved. [4]

Ebora's rough guide, to be agreed with the window maker, is to replace a sight glass used under extreme conditions every six months and one in normal conditions every two years, and to order a spare glass with the fitting so that it is on the shelf when needed. [1] ACI adds that after the base frame is welded into the vessel the sealing face must be checked for warping and reworked if needed, that the specified bolt torques must be followed, and that the fitting's pressure rating does not cover the base flange, which has to be verified with the vessel under AD 2000 leaflet B9 or an equivalent code. [2]

Editorial calculation: at Ebora's 0.2 mm a month, a six-month change removes the disc after about 1.2 mm of loss; at the 2.4 mm a year it gives for the same conditions, a two-year interval would allow about 4.8 mm, which is why the shorter interval is attached to extreme service.

05

Medium and service data for a protected observation window

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

Medium: [liquor, boiler water or steam; pH; solids or deposits]
Conditions at the window: [maximum temperature; pressure; vacuum]
Window: [round or oval; glass size; transparent or reflex]
Glass: [borosilicate to DIN 7080/7081; aluminosilicate]
Protection: [mica disc; FEP screen or coating; none]
Fitting: [weld-in or weld-on base frame; frame material; gaskets]
Service: [planned replacement interval; spare glass and mica kits]

See manufacturer records Send an enquiry through CNFX