Replacing a Boiler Sight Glass

| Edward McKellar

The Boilers Gauge Glass

The boilers gauge glass or sight glass as some may refer to it is an essential boiler room component. Boiler gauge glasses give operators a visual indication of the boiler’s water level along with an understanding of what’s occurring inside the boiler, or how the boiler is responding to certain changes within the system. The sight glass of a boiler is many times integral to what’s known as the water column and has the primary low-water cut-off installed. The low-water cutoff is what many would hail as the most important of all safety controls within a mechanical room. 

A boilers sight glass should be changed at set intervals based on the operating conditions of the boiler. A nice clean fresh looking sight glass tube replaced annually at a minimum is among best practices simply to ensure that a safe and perfectly functioning glass is installed. Gauge glasses can become etched and have weak spots that are hard to notice until it has failed. Etching in a gauge glass can lead to the tube shattering resulting and an unsafe operating condition.

Sight glasses can sometimes become cloudy or stained through operations making the visual indication of the water level difficult or impossible for operators. Sound water treatment along with daily blowdowns are the key to getting the optimum life from an installed gauge glass.

How to replace a boiler’s gauge glass.

It’s always wise to begin with a safety brief, that being said, we will be cutting glass and installing it into a live boiler. So always be mindful of the conditions that present any risk for personnel or property damage. Work gloves along with proper eye protection are minimum required personal protective equipment.

Start by first ensuring that you have the proper spare parts on hand needed for the replacement. With a tubular style sight glass this should include appropriately sized boiler gauge glass tubing, the rubber gaskets along with brass rings all in the appropriate size. Along with the right parts, you’ll need to have a tape measure and a fine tip permanent marker, an adjustable wrench, and a glass tube cutter. Having a small piece of plumber’s cloth is a good idea in the event your cut ends have any small burrs or protrusions.

Isolate the gauge glass by closing the upper and lower gauge glass valves. Many water columns have a drain cock on the lower valve assembly to then drain the sight glasses water. Once drained the proper seating of both valves can be verified as the gauge glass should remain empty.
Loosen the brass hardware from both the upper and lower valve assembly. Once free, gently remove the tubing from the water column. It may be tempting to simply use the old sight glass as a template, however, the last replacement could have been improperly measured so It’s best to measure and ensure the glass is properly sized for the installation.

Gauge glass valves are designed with a stop inside that will not allow the tubing to go any further into the valve. While the gauge glass is removed make note of this stop and its distance. A small gauge or ruler may be helpful to ensure an accurate measurement. Measure the distance from the face of the upper valve assembly to the lower valve assembly. Add the measurement from one of the valve’s faces to the stop casting to the face-to-face measurement. This is the length that your gauge glass should be cut to.

Measure the distance from a factory edge of the glass tubing to your desired length. Make a mark to indicate the measurement on the glass to be cut. Now roll the tubing over 180° and take a second measurement again marking the glass. Not only is this ensuring you measured accurately, but it’s also giving a second point of reference for the scoring or cutting that will be occurring.

Cutting the Glass 

Now that the measurement is taken it’s time to score the glass and make our cut on the boilers gauge glass. A couple of the common methods are the chain-style cutters and a simple spring-loaded wheel cutter. Regardless of the method what’s most important is keeping the line to be scored straight and making a flush cut on the end.

The way that these tools function is to simply weaken the glass in a continuous run. After it’s scored a simple bend should deliver a nice clean break with very little concern of surface loss, fragmentation, or cracking.

After cutting the glass any minor imperfections can be smoothed out with the use of some plumbers cloth. The tube can then be installed and the brass hardware tightened. After opening the valves slowly to allow a steady rise in temperature and pressure. Monitor the tubing for leaks and snug up the brass hardware. If the leak persists, isolate valves and inspect the installation and repeat if necessary.

If you’re looking for any boiler gauge glass parts and accessories or cutting devices, contact Power Mechanical Inc. today. We have thousands of in-stock boiler parts and most are available for immediate shipment.  You can also contact us for boiler rentals, purchase a boiler and boiler training.


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