Figure 4-14.Gate valve.
Gate valves are not designed for and cannot be used
for throttling. Most gate valves have a wedge-shaped
gate, but some have a gate of uniform thickness. The
gate is connected to the valve stem and is positioned
by rotating the handwheel. The port is the full size of
the pipe and extends through the valve.
Some types of gate valves have a rising stem,
and a glance at the valve will tell whether it is open
or closed. In the type of valve with the nonrising
stem, the stem revolves in the bonnet and the gate is
raised or lowered by the threads on the internal end
of the stem. On this type of valve, a pointer is usually
in-stalled to indicate the open or closed positions.
Gate valves operate properly with either face on
the inlet side, thus simplifying installation. Case or
forged steel valves have disks and seats made of
nickel-copper alloy, chromium steel, or a steel treated
with a hard facing material. Valve stems are made of
corrosion resistant steel. Handwheels are made of
fabricated steel, brass, or aluminum. Except for
malleable iron or aluminum handwheels, bronze gate
valves are made entirely of bronze.
It is a good practice to put a gate valve back
together the same way it came apart. Although
the valve operates with either face on the inlet
side, after installation and use in a specific flow
pattern, one side of the valve may wear a little
differently from the other. To ensure a tight fit
and smooth operation, put it back the same
way it came out.
Globe valves (fig. 4-15) are so called because of
the globular shape of their bodies. It must be noted
Figure 4-15. Globe valve.