Текст 5.

Special valve designs.

Valve noise is countered by special trim. Although noise can originate from vibration of valve parts, this is not the prime source if design is effective: cavitation and turbulence are more likely sources in this case. Analytical methods are available to predict valve noise, stated as db sound pressure level units at 3 ft distance, with accuracy of 5 db. Type of valve is a factor: a cage valve with four ports can be about 6 db valve, but may also carry more sound to a distant location. Best anti-noise efforts now cut 20 to 24 db off noise of an ordinary valve in a given service. This reduction in noise can be indispensable in bringing final value to the acceptable 80-90 db level.

Cavitation, besides causing noise, produces severe wear of trim and even valve body. Remedies are special trim and avoiding use of certain valve types. In some valves, flow direction makes a big difference. For example, an angle control valve may have much less cavitation if flow tends to open the plug rather than close it.

In cryogenics, valve design means more than merely lengthening the bonnet or top works to give room for insulation or to keep cold away from packing box.

Hydraulic and pneumatic control valves head.

Hydraulic pressure control valve assembly is pilot-operated, employs main poppet and ball-type valve. Excess line pressure depresses poppet, letting more oil flow back to tank. Setting at which this occurs depends on force pressing ball against its seat. Remote control by added ball type valve on an instrument panel is possible too.

Free flow of air or oil in desired direction (top) and adjustably restricted reverse flow (bottom) result with this valve. Sprig-loaded center “lift check” poppet opens to give free flow. Adjusting sleeve on outside of valve changes width of narrow annular return orifice. Valve has aluminum body, synthetic poppet, static O-ring seals.

Hydraulic and pneumatic valves follow the same principles of design and operation governing other valves. Why do these two valve classes look so different? For one reason, available pressures in hydraulics are high to keep work cylinders small or get better automatic control action. The small cylinders mean low flow rates, but the high pressures necessitate strong valve bodies. Also, high valve losses are not important, since the available pressure is high. Result may be heavy-wall forged body with small, sometimes tortuous passages. Some modern high-pressure steam and water valves resemble hydraulic valves in body-to-passage proportions. Spool valves, ball checks and lift checks are common. Some basic types are pressure-control, relief, check, sequencing, flow-control, two-way, three-way and four-way ones. Modern hydraulic valves are often bolted to O-ring-sealed subplates or to each other. Small diameter tubing is easily bent to accommodate subplate or valve.

Текст 6.

Actuators: developments for on-off service.

Actuators come into play when simple handwheels and hand levers no longer suffice to position valves. They range in complexity from operators-simple, manually-operated gear trains bolted to the top works of large seldom-opened valves to the sophisticated high-speed, high-power types that follow commands of automatic control systems.

Classifying of actuators is a big help in understanding this fast-moving field. One useful classification: on-off and modulating. On-off types, often simple and compact, merely open and close valves, with no intermediate positioning. Modulating actuators, which can set a valve at any desired stage, are more highly developed. Some on-off actuators can be converted to modulating service by addition of a positioner.

Another classification scheme divides actuators into linear or rotary. Ball, plug and butterfly valve are applications for the rotary actuator. Linear actuators can power all valves, using linkage when needed.

A third classification differentiates according to motive power and certain design basics. Diaphragm actuators and diaphragm-and-spring actuators are air-driven. Cylinder actuators are powered by air or oil. Electricity moves solenoid actuators, motor-driven gear trains and screws, pressurized oil for electro-hydraulic types. Then there are rotary-vane actuators which convert air pressure into arc motion 90 deg. or more.

Right now, on-off actuators are under intense development because of need for low-cost automation. Inventiveness and skill show in many of these, both rotary and linear.