Installing a Low-Water Cutoff
The Watts No. 89 low-water cutoff illustrated in Figure 15-24 is suitable for low-pressure steam heating boilers with a maximum operating steam pressure of 15 psi. Although installation is relatively simple,
the instructions should be carefully followed. This is a boiler safety device, and it must operate properly or the boiler can be damaged.
Before installing the Watts low-water cutoff, the gauge glass and cocks must be removed from the boiler. When you have done this, proceed as follows:
1. Install the 1⁄2-in tee (with long and short nipples) on the float chamber by inserting the short nipple in the float chamber tapping in the end opposite the switch. Screw the tee tight with the long nipple pointing in the correct direction, depend- ing on the location of the boiler (Figure 15-26).
2. Insert the long nipple into the lower glass gauge tapping on the boiler.
3. Swing the entire float chamber until the nipple is made up tight. Line up the cutoff so that the top of the switch box is level (Figure 15-27).
4. Screw the nipple in the tee carrying the tubing connector the upper gauge glass tapping and pull up tight. Install a compression coupling in the top float chamber tapping (Figure 15-28).
5. Hold the tube bend in position and mark the tube on a level with the top of the top of the hex on the compression coupling. Cut off the tube at this mark (Figure 15-29).
6. Slide the ring and nut over the end of the connector tube.
7. Slide the end of the tube into the compression coupling in the float chamber, and tighten both couplings.
8. Install a drain valve in the bottom float chamber tapping (Figure 15-30).
9. Reinstall the gauge cocks in the end of the tees and replace the gauge glass.
Fusible Plugs
A fusible plug is used on some boilers as a protection against dangerous low-water conditions. Examples of fusible plugs are shown in Figure 15-31. They are generally made of bronze and filled with pure tin. When the temperature (and pressure) in the boiler builds up to about 450°F (the approximate melting point of pure tin), the tin core melts and relieves the pressure within the boiler.
Fusible plugs must comply with ASME standards and are avail- able in several sizes for use on boilers having a steam pressure of less than 250 psi.