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When the time comes for you to replace the level gauge sight glass in your boiler, there are some important safety steps you should follow. Not only do you need the proper equipment, but you also need to know which level gauge valves to shut off first and then in what order you should turn them on again. 

In this article, we’ll guide you through the list of supplies you’ll need to install your new level gauge sight glass, as well as a step-by-step process for completing the installation safely.


How Do I Know If The Sight Glass Needs To Be Replaced?

There are three telltale signs to let you know that your level gauge sight glass needs to be replaced.

The first is if it’s leaking, which can happen overtime as water and steam slowly erode the glass away, or if the glass tube gets knocked at any point in time. 

The second sign is if the surface of the glass is cloudy or dirty. If you can’t see the water levels in the gauge anymore, then the sight glass is no longer doing its job and it’s time to install a new one.

Finally, the third sign is if you notice the glass is thinning. This is a prelude to the tube cracking or leaking.


What Do I Need To Install A Sight Glass?

Before you get ready to replace your sight glass, here are a few supplies you should have on hand.

  1. An adjustable wrench
  2. Heat resistant gloves
  3. Eye goggles
  4. Red line gauge glass (made out of a borosilicate tube)
  5. 2 Zyton gaskets
  6. 2 level gauge washers 

Your sight glass setup should already be equipped with 2 brass nuts. Check them when you take them off to make sure they aren’t corroded. If they are, it’s wise to replace these parts, as well.


How Do I Install A Sight Glass?

Now that you have your supplies at hand, it’s time to start the process of replacing your level gauge sight glass.


Step 1: Drain The Gauge And Shut Off The Level Gauge Valves

The first step to safely replacing a sight glass is to drain away the water from inside it and shut off the steam and water valves. These precautions will prevent boiling water from spraying onto you from either of the valve connections or the leak point. 

First, drain the glass to depressurize the glass. Next, close the water valve and finally the steam valve. After that, you can remove the sight glass guard rods and move on to the next step.


Step 2: Remove The Sight Glass

After you’ve removed the sight glass guard rods, use an adjustable wrench to loosen the brass nuts affixing the glass to the valves. 

Next, gently toggle the glass up to reach the rubber gasket at the bottom of the tube and scoot it into the middle of the tube so that it’s out of the way. At this point, you should be able to pull down on the borosilicate tube and free it from the top valve.


Step 3: Measure And Cut A New Zight Red Line Gauge Glass

The third step is to measure your new Zight red line gauge glass so you can cut it to fit in the valves.

The easiest way to do this is to take your borosilicate tube, put one end up into the top valve connection, and make a mark on the tube within about a 1/8 of an inch of the bottom valve. This way, you’ll have enough room to place the glass back in the valves and also toggle it back out the next time it needs to be replaced.

Once you have your mark, use a glass scoring tool and score a thin line around the borosilicate tube. Then, in one quick movement, snap the tube at the line. Now you have a freshly cut red line gauge glass.

Make sure your tube fits by placing one end all the way up into the top connection and gently letting the other end slide into place at the bottom. If both ends fit, then the sight glass is the correct length.


Step 4: Slide Nuts, Washers, And Zyton Gaskets Onto New Sight Glass

In this step, you need to slide the brass nuts, brass washers, and Zyton gaskets onto either end of the new sight glass. On one end, start with the nut, then a washer, then a gasket. Repeat on the other end. Slide all these parts into the middle of the sight glass for now.


Step 5: Install Sight Glass, Slide Gaskets Into Place, And Tighten Nuts

Next, install the sight glass and then slide each gasket into place. Make sure that the gaskets sit securely at either end of the tube so that the metal of the valves and the metal of the washers and nuts aren’t scraping against one another. In that scenario, you run the risk of cracking the nut, washer, or valve when you tighten them together.

Once your gaskets are in position, tighten each nut until they start to become snug. Then, move your sight glass up and down to make sure that the bottom end of the tube is not sitting on the metal of the valve. 

Finally, you can finish tightening the nuts. You can use your hands during this part (properly gloved), as you only need to make it tight enough to make a seal.


Step 6: Turn Valves Back On, Close Drain, And Verify Your Sight Glass Is Working

The last step begins with opening the valves partway. The first one you should open partway is the steam valve, so it can preheat the glass. Then, close the drain, and finally, open the water valve.

After a few seconds have passed and you verify that there are no leaks, open the steam and water valves all the way. The water level in your sight glass should stabilize. You may now put the guard rods back in.

At this point, your sight glass replacement installation is complete!



For all your sight glass replacement needs, including a borosilicate glass tube, Zyton gaskets, and level gauge valves, visit the Zight online store.