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How to Replace Sight-Glass Tubes on Boilers

The sight glass is a simple instrument, but it provides absolutely critical measurements of water levels for boiler operators. Should the sight glass fail or become damaged in some way, it can provide faulty readings to operators; ultimately, such problems can lead to boiler damage or even cause an explosion if the boiler-feed water level becomes too low. That is why you should be prepared to replace boiler sight glasses as needed. Below is more information on performing this routine but important task.

1. Make safety your personal responsibility and priority - Before you ever place a wrench on a nut, the first thing you need to do is prepare yourself to work on the boiler in a safe, no-nonsense manner. That means you should always wear the appropriate safety gear, including a pair of durable gloves that can protect your hands from burns and cuts as well as eye protection.

2. Idle the boiler - Once you have prepared for working safely, the next step is to idle the boiler, unless the boiler manufacturer approves sight-glass replacement during full operational status. If you need to idle the boiler, then follow established procedures for taking it offline before proceeding.

3. Isolate the sight-glass tube - After shutting down the boiler, if needed, you will need to isolate the sight-glass tube from the boiler feed water. Doing so requires you to shut off two valves, one which is located at the top and and other which is located at the bottom of the sight-glass tube.

4. Drain water from the sight-glass tube - When the sight glass has been isolated, the next step is removing any residual water from the tube. This involves locating a petcock beneath the sight glass and opening it to permit the water to drain. Be careful not to apply too much force to the petcock, or you risk breaking it from the valve stem. Allow all the water to drain from the sight tube into a bucket or other suitable container.

5. Loosen the nuts holding the tube in place - After draining all the water from the sight-glass tube, you will need to loosen the nuts holding the tube in the sight-glass assembly. You can use a pair of slip-joint pliers or an adjustable wrench to perform the loosening. Once the nuts are loosened, slide them away from the threads toward the center of the sight-glass tube and also pull the brass and rubber washers out of their seats.

6. Remove the sight-glass tube - Next, grasp the sight-glass tube near its bottom using firm, deliberate pressure and lift it gently out of the mounting. Be careful not to shatter the tube or spill any hazardous contents. In addition, slide the protector rods out of their location in order to gain better access to the tube.

7. Inspect the sight-glass tube - Once you have the old tube out of the sight-glass assembly, quickly examine the tube for signs of damage such as hairline cracks, discoloration, and other illnesses. If there are any flaws in the sight-glass tube, then it will need to be replaced.

8. Obtain a new sight-glass tube and attached hardware - After getting the glass tube out of the assembly, the next step is to obtain an exact replacement. Consult with the boiler manual guide to determine what specifications are needed, and order the new tube immediately. You may need to have the tube cut to a custom length to fit your boiler.

If the sight-glass tube appears to be merely dirty, then you can clean it using warm water and liquid dishwashing detergent. Using a bottle brush may also help you reach stubborn spots that aren't easily cleaned otherwise.

9. Install the new sight-glass tube - When you have either replaced or cleaned the deposits inside the sight-glass tube, you can reinstall the sight-glass tube assembly.

To begin reinstalling the tube, slide two nuts over the ends of the pipe, followed by brass washers and rubber grommets. Position the sight-glass tube so the bottom end is pushed down into the "socket." At that point, slip the rubber washer down the length of the tube as well as the brass washer on top of the rubber washer. When you pull the nuts down into position, tighten them by hand, and then apply a final tightening with the slip-joint pliers. Be sure not to provide too much force when tightening the nuts with tools, as they can quickly cause significant damage, including leaks.

After the "new" tubes are installed, the next step is to tighten the petcock and also reopen the attached drain valves. Restore operation to the boiler and test the sight glass for proper functioning.