If any of you have been around a boiler sight glass that explodes, I am sure the noise made you jump or else flinch as it sounds like a gunshot. Consider yourself fortunate if you were standing near the glass when it exploded and were not hit in the face or eyes by shards of glass.
Water treatment chemicals are frequently blamed for sight glass explosions when in fact, it is erosion at the base of the glass that is the root cause of the failures. My experience has been with round sight glasses being the most likely to fail. Flat sight glasses are much less prone to fail unless it is accidentally struck by a ladder or some other object.
Sight glass erosion occurs when condensation takes place in the piping connections above the glass. This results in a continuous stream of condensate running down into the glass. Since the liquid in the glass usually contains a high percentage of condensate, very little internal treatment chemicals are in direct contact with the glass itself.
You can identify erosion attack on round sight glasses as there is a thinning on the side near the bottom of the glass. Telltale cracks can sometimes be seen in this area prior to failure. If cracks are noticed, it is time to replace the glass. Flat sight glasses show this effect by thinning or complete removal of the vertical ribs on the glass. Some erosion of the the gage glass occurs when the water column and sight glass are given a blowdown.
To minimize erosion, round sight glasses should be replaced with the more expensive flat glasses. No matter which style of glass used, mica gaskets inserted between the glass and water will help protect the glass from erosion.
Now it is time for a story. The story illustrates the importance of having properly educated personnel who are responsible for maintenance and operation of steam boilers. I was in Illinois at a plant that I was able to sell because of a good relationship my company had with the plant manager. The account was sold with no buy in from the plant personnel as they were basically told that they were going to use our company for water treatment.
Things went well for the first few months and the maintenance manager was very helpful and followed all of our suggestions and recommendations. What I did not know was that he in fact had a deep dislike for being told by the plant manager to use us. When the plant manager left the company, the attitude of the maintenance manger suddenly changed. This became apparent when one of their fire tube boilers had been having sight glass failures on a regular basis (I was never told of this) and he blamed our chemicals when in fact the failures had occurred with the previous company as well. One day, the boiler operator asked me if it was safe to run a boiler without a sight glass. I asked him why he would ask that question. He told me that the sight glass had failed and when he reported it to the maintenance manager, he told him to run the boiler without one as he was never going to buy another sight glass! When I went to the see the maintenance manager, he got angry again and told me to mind my own business. I told him that his insurance inspector would red tag the boiler if he saw that it had a missing sight glass. He looked me in the eye and said, ” Well, we will just have to see about that.” I will let you draw your own conclusions about the total lack of safety and concern. I lost the account shortly after and was not disappointed to see this one go.