BOILER SIGHT GLASS FAILURE

CAER PresentationMarch 27, 2014Jim Riley

Acknowledgement: Bill Gamba, Joint Health and Safety Committee Co-Chair for investigation contributions

PRESENTATION OVERVIEW• Description of a 600# boiler sight glass failure • Damage mechanism associated with the failure• Progressive degradation over time of mica and glass• ITPM (inspection, testing, and preventive maintenance) tasks to

prevent failures

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600# Boiler sight glass failure• Split level gauge with two sight

glasses/housings• Normal level in upper sight glass• Glass thinned to the point that it

fractured and failed• Small steam release. No process

materials were released. No unit upset occurred

• Sight glass was isolated using chain wheels located at grade to close the block valves.

• Redundant split level sight glasses at other end of the boiler and level float chamber/transmitter provided water level indication while this sight glass was serviced.

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Damage mechanisms• Alumina silicate high temperature

glass is a silica lattice with alkali oxides as fluxing agents. It corrodes under two conditions:– Leaching (acid effect) where

hydronium ions (H3O +) are exchanged for the alkalis in the glass

– Etching (alkali effect) by hydroxyl ions (OH-) acting upon the anions Na+, K+, and B+++ followed by dissolution.

• Ruby mica shields occur as monoclinic crystals which resist acids and alkalis. It degrades due to the layered pattern.– Condensing drops at the steam

vapor/BFW level range– River patterns in the ruby mica

layers attacked

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PROGRESSIVE DEGRADATION OF MICA/GLASS

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ITPM TASKS TO PREVENT FAILURES• Boiler operator surveillance specifically looking for ‘milky

appearance’. Train operators on what to look for.• Redundant visual inspections by boiler permit inspectors at

annual and 6-month external permit inspections.• Block in the sight glass if the ‘milky appearance’ due to

etching/leaching of the glass becomes apparent. Do not leave the sight glass in service to further degrade.

• Service the sight glass by replacing both the mica shields and the glass when they are ‘milky appearance’.

• Service the sight glass by replacing both the mica shields and the glass when the internal boiler inspection is done based on permit variance period allowance.

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Any questions that you may have?

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