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Boiler Sight Glass Erosion

Problem
We have a boiler that has needed its Borosilicate round sight glass changed out 3x within the last year. The glass has been degraded on the top above where the rubber gasket seals it to the sight glass valve.

We are just wondering if you can provide me a chemical perspective as to why this particular boiler might be having this problem more so than any of the others that we work on.

Will high pH cause sight glass wear?

Boiler plants all experience gauge glass erosion to some degree. During normal operation of the boiler, condensation forms in the piping above the gauge glass, sending a continuous stream of condensate down into it. Because the gauge glass is always full of condensate, the internal boiler treatment has little effect on the erosion that takes place.

Solution
Chemical treatment of boiler feed water to reduce steel corrosion will produce an alkalinity of the water at pH values between 10 and 11, sometimes higher, leading to further increases in the rate of wear of the glass.

Fortunately, the water in contact with the gauge glass, being furnished largely by condensate through the upper connection to the boiler, will be less alkaline than that in the boiler. This condensate, by flowing over the glass, dissolves minute quantities of silica. These small quantities of silica in solution inhibit the attack of the boiler water in the glass to a considerable extent.

The fresh condensate entering the gauge will often attack the glass in upper areas, more than in the lower part of the gauge, where the temperature is lower and where the degree of saturation of silica is greater. This effect is particularly noticeable in the case of tubular gauge glasses.

Erosion above the water level is caused by the velocity which results from condensate that has rapidly formed at the top of the glass, replacing boiler water in the lower portion of the glass. Erosion at the bottom of a round gauge glass is caused by the high velocity which occurs when the glass is blown down or used for sampling purposes.

There are generally two types of boiler column gauge glasses round and flat. Erosion of a round gauge glass thins the side, usually at the bottom of the glass. Erosion of a flat gauge glass thins or completely removes the vertical ribs, which are designed to permit an improved view of the water level.

Recommendations to reduce erosion:
  1. If the plant is using round glasses, change to flat ones. These have a greater cross-sectional area, which reduces velocity and related erosion.
  2. A mica gasket or shield should be inserted between the water and the glass. If flat glasses are already installed, place a mica shield between the glass and the water. Mica is a very insoluble, impervious material that will protect the glass and not interfere with the optical properties.
  3. Always use Borosilicate glass (Pyrex).
  4. Limit sampling or blowdown from gauge glasses without mica shields.
Other factors that can impact the degradation of the site glass are:
  • The temperature drop between the boiler and the gauge column.
  • Boiler operation · Sight glass design of flanges, bolts, gaskets and glass.
  • The type of metal, kind of glass (Borosilicate glass resists thermal stress better then common soda lime glass), and gasket material must meet design criteria.
  • Improper installation can also have an impact
  • GAUGE GLASS REMOVED FROM SERVICE SHOULD NOT BE REUSED.
Boiler Sight Glass Erosion