Текст 2.

Valve anatomy.

 

Today's valves and actuators reflect willingness of both maker and user to "try new ideas and new materials, to borrow concepts and details from seemingly unrelated sectors of the art and, finally, to blend theory and experiment with the harsh lessons of field service. Wider use of TFE (tetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoro-propylene), EPT (ethylene propylene terpolymer) and other plastomers and elastomers is one example. Control by butterfly and plug valves is another; ingenious actuator mechanisms provide further examples.

Let's start our anatomy of valves with a rundown on types of the mechanism used to stop or reduce fluid flow. Globe, angle and gate valves with their long history are the basic types.

Globe and angle valves are preferred for manual throttling and give tight shut-off. Pressure loss is high. Plug-type disk or the even longer and thinner plug of the needle valve, reduces wear in highly throttled state. While many control valves are of the globe-type, the angle valve is becoming more popular; users find they may save fittings and, in larger sizes, produce a flow path with less erosion and pressure loss. Inclined seats and bonnets in a globe valve convert it to a Y-valve, which offers a far smoother flow path than the conventional globe or angle.

Gate valves are historically on-off valves for sectionalizing piping. The gate's «slide-past» principle can be adapted for modulating service, how­ever, and many valve types-have a spool or plug-like piece that rotates or slides past flow ports.

To press the gate valve's disk against one or both seats, most designs rely on a rigid one-piece tapered wedge. Variants include a two-piece disk, in which the parallel pieces are crammed apart by tapered surfaces as the disk approaches closure, and a one-piece wedge with two sealing faces able to flex independently because of suitable slotting. Gate valves have had metal seats and disks, sometimes of hardened material or faced with Stellite. Recently, however, a gate valve with TFE seat rings has appeared.

In these three time-proven types-globe, angle, gate-stem threads are generally needed for manual operation. Thread location is either inside, ex-posed to fluid, or outside, depending on service and user's wishes. Small valves usually have an inside thread in the bonnet or in the hollow disk of the gate valve. Larger gates are often out-side screw and yoke (OS & Y) type, with a threaded bushing at the yoke top to turn and move the threaded stem end. The stem rises, unlike that in a non-rising stem gate, whose disk has a threaded hole for the stem.

High-temperature and high-pressure service finds the globe, angle and gate getting top billing. Continuing developments in butterfly and ball valves are earning new applications. Sealing methods rely on elastomers and plastomers, materials that can handle up to 700 psi for water, oil and gas at ambient temperature.

 

Or saturated steam to about 150 psi. Temperature maximum for TFE or similar material is about 500 F, at reduced pressure.

These "pressure and temperature ranges, while not spectacular, nevertheless cover a large fraction of user on-off applications in process systems, heating and even utilities. Butterfly varieties, installed between flanges, are available in all sizes to about 200-in. diameters.

Ball valves with plastomer seat rings are most common in small sizes.

Plug valves of the lubricated type, with taper plug and grease grooves in body and plug, are used widely. Recent designs have incorporated TFE, either as seal rings on the turning plug or as stationary sleeves in the body.

The eccentric plug valve, relying on camming action to seat the plug face against the downstream opening, is in-creasing in variety and number. It is adaptable to modulation.

Butterfly, plug and ball valves all have in common the 90-degree stem turn between on and off positions. This rotation is easy to achieve with several types of compact actuator.

Other valve types appear on sub-sequent pages. Example: The diaphragm valve, which shuts off flow and prevents entry of fluid into the bonnet. The spool or cage-type valve is common in control valves, including hydraulic and pneumatic. This flow-control principle serves in valves up to 8 in. for controlling descaling water at pressures to 2500 psig.

Standardization of valve dimensions is underway, but not complete. Groups

such as the American National Standards Institute (ANSI) and the American Petroleum Institute (API) have promulgated some standards for dimensions and specifications, which some valves do not meet. Dimensional standards insure interchangeably among similar products of different makers.

Pressure standards separate a steam rating from a rating for water, oil and' gas at ambient temperature. The steam pressure rating is roughly half that for water, oil and gas but depends on material and temperature.

Valve repair costs are under closer scrutiny than ever. Users, learning the hard way, now total all their costs be-fore deciding how far to go in repairs. It may be that a small valve well-designed for complete and quick repair, will cost more in the long run than a new, "disposable" valve which may have the same initial lifetime but which can be scrapped when it leaks excessively. Large valves are a different story: Re-pair is usually advisable, either in the user's shop or at the maker's plant. Large valves repaired by the manufacturer are prized by many users as better than a new valve.