Figure 6-3.—Three-way ball valve.GATE VALVESGate valves are used when a straight-line flowof fluid and minimum flow restriction are needed.Gate valves are so-named because the part thateither stops or allows flow through the valveacts somewhat like a gate. The gate is usuallywedge-shaped. When the valve is wide open thegate is fully drawn up into the valve bonnet. Thisleaves an opening for flow through the valve thesame size as the pipe in which the valve is installed(fig. 6-4). Therefore, there is little pressure dropor flow restriction through the valve.Gate valves are not suitable for throttlingpurposes. The control of flow is difficult becauseof the valve’s design, and the flow of fluidslapping against a partially open gate cancause extensive damage to the valve. Except asspecifically authorized, gate valves should not beused for throttling.Gate valves are classified as either rising-stemor nonrising-stem valves. The nonrising-stemvalve is shown in figure 6-4. The stem is threadedinto the gate. As the handwheel on the stem isrotated, the gate travels up or down the stem onthe threads while the stem remains verticallystationary. This type of valve will almost alwayshave a pointer indicator threaded onto the upperend of the stem to indicate the position of the gate.Valves with rising stems (fig. 6-5) are usedwhen it is important to know by immediateinspection whether the valve is open or closed andwhen the threads (stem and gate) exposed to thefluid could become damaged by fluid contami-nants. In this valve, the stem rises out of the valvewhen the valve is opened.GLOBE VALVESGlobe valves are probably the most commonvalves in existence. The globe valve gets its nameFigure 6-4.—Operation of a gate valve.6-3Figure 6-5.—Rising stem gate valve.
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