Gauge glasses in liquid level gauges on steam boilers are exposed to very high mechanical and chemical stresses. The interface between steam and water is continuously in motion: water evaporates, condensate forms. Above all, the condensate running down the gauge glasses leaves traces of abrasion.
Boiler water is always chemically treated water from which mineral substances have been extracted. The demineralised water tends to increase its mineral content by extracting it from the glass. This chemical attack on the glass is largely determined by pressure, temperature and pH-value of the boiler water.
With unprotected gauge glasses, steam pressures should not exceed 35 bar nor should it exceed the pH-value of 10, if an economic service life is required. This limiting pH-value applies for feed-water at about 20°C. It should be noted that the pH-value decreases with increasing temperature – 1.5 pH degrees at a temperature rise up to 300°C.
