Valves

VALVES

Valves serve not only to regulate the flow of fluids but also to isolate piping or equipment for maintenance without interrupting other connected units. Valve design should keep pressure, temperature changes, & strain from connected piping from distortion or misaligning the sealing surfaces. The sealing should be of such material & design that the valve will remain tight over a reasonable service period. The principle types are illustrated below:

1) GATE VALVES:

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Fig: 12.3 gate valve

These are designed in two types. The wedge-shaped gate, ‘inclined seat’ type is most commonly used. The wedge gate is usually solid but may be flexible (partly cut into halves by a plane at right angles to the pipe), or split (completely cleft by such a plane). Flexible & split wedges minimize galling of the seat surfaces by distorting more easily to match angularly misaligned seats. In the double –disk, ‘parallel-seat’ type, an inclined plane device mounted between the disk converts stem force into axial force, pressing the disks against the seats, after the disks are positioned for closing. This gate assembly distorts automatically to match both angular misalignment of the seats & longitudinal shrinkage of the valve body on cooling. When shearing high velocity flows of dense fluids, the gate assembly shakes violently, & for this service the solid wedge r flexible wedge valves are preferred. Where valve operations are manual, small by-pass valves are installed in parallel with main valve may be used to eliminate the shake problem & tominimize manual effort in opening & closing the valves Gate valves may have non- rising stem, inside screw rising stem, or outside screw rising stems, listed in order of decreasing exposure of the stem threads to the fluid handled. Rising stem valves require more space, but the position of the steam visually indicates the position of the gate.

Gate valves are used to minimize pressure drop in...