b. Valves. Because of the large number of valves used in systems, a mechanic
must know how to maintain and repair them. All valves should be checked regularly
for leaks. Leaks may be caused by leaking washers or leaking bonnets.
(1) Globe valves.
(a) Washers. To repair a globe or compression type valve (fig 44) with
a rubber or fiber disk, first shut off the water at the main and drain the line.
Using a suitable wrench, such as a crescent-type adjustable wrench or a monkey
wrench, remove the valve bonnet and stem and inspect the disk for wear. If the
disk shows wear, unscrew the disk retaining nut and replace the disk with another
of the correct size. Inspect the seat of the valve for pitting. If pitting is
severe, replace the whole valve, but when the pitting is minor, the valve will
often function perfectly with a new disk. Several manufacturers make valves with
replaceable brass valve seats. To repair such a valve, follow the manufacturer's
recommendations.
Figure 44.
Details of globe valve.
(b) Bonnets. Before replacing the stem and bonnet, slide the bonnet up
toward the handle on the stem to expose the bonnet packing. If the valve is
leaking through the bonnet or if the packing appears worn, wrap a few strands of
lamp wicking dipped in pipe joint compound around the stem and then slide the
bonnet down over the packing. Screw the bonnet and stem assembly back into the
valve body.
(2) Gate valves. Maintenance of the gate valve is similar to that of the
globe valve except that the only part of the valve which needs attention is the
bonnet packing. This is replaced in the same way as on the globe valve.
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