Текст 3. Check valves, bonnets, stem seals.

Check valves, bonnets, stem seals.

 

The check valve accomplishes its end by several different valve types. The lift check, somewhat resembling the globe valve, has a disk that moves perpendicularly to seat. The ball check is another common design. While ball weight is often enough to give quick seating, a spring is used at times particularly in hydraulic and pneumatic control valves.

Swing checks, with end-hung flaps or balanced titling disks resembling butterfly valves, see service at all pressures and temperatures. Flow resistance is less through swing checks than through lift and ball checks. The familiar boiler stop-check valve is basically lift check. This valve is bonnet holds a threaded stem that can drive the disk down to its seated position, giving positive isolation of a boiler foe maintenance safety or shutdown.

Most valves have removable closures at top. Some- at bottom for assembly, inspection, repair. Exceptions are those ball and butterfly valves having end access. Union bonnet gets preference for small valves, while bolted bonnet is common in large valves. Since the bonnet is a static seal, removed infrequently, final seal welding is sometimes found on large high-pressure steel units easily ground off for disassembly maintenance.

A sliding or rotating seal prevents fluid escape past moving stem of a valve. Some valves have no external stem. Examples: check valves, solenoid actuated valves and many hydraulic and pneumatic valves. For these three classes, reversed fluid flow, electromagnetic force, and pressurized internal pistons supply actuation.

Packing boxes, with lanterns in some cases are most common for sliding stems. Asbestos and TFE are usual materials. With packing boxes, over-tightening is most common operating fault. In control valves, where sliding load on stem should be low, packing is sometimes spring-loaded. Aim here is to prevent excessively tight packing. The bellows seal, perfect when unbroken, becomes long and unwieldy for high rise disks. It is still common, however, for critical leak applications. Valve stems for 90-deg. Rotation often incorporate O-ring seals.

 

Текст 4.

 

Control valve innovations cover all types.

 

Plug, butterfly, ball and sliding-gate control valves do not have a major unbalance problem yet need strong actuators to overcome friction and some fluid-caused unbalance.

Range ability- the ratio of maximum to minimum flow at which valve characteristic is still evident- is another valued attribute in a control valve. A single valve with high rangу ability can handle a wide range of flows that would otherwise need a large and a small valve, each having lower range ability. The best modern control valves have rangeability of nearly 100 to 1 under favorable conditions, although in many cases the value is far less: 50 to 1 for V-port plugs and 25 to 1 for contoured plugs are reasonable.

Cavitation can be a serious problem. Here, the bubbles, formed as pressure decreases, collapse when pressure later recovers. The collapse induces high local metal stresses, causing pitting. Ball and butterfly control valves are two high recovery types, prone to cavitation. A globe valve is usually low recovery and not prone to cavitation.

High side-loads on valve plugs and stems are resisted by guides in the form of cages or bushings. Latter support stems above and below cages and seats. Buffeting is another potential hazard since fluid flow is never entirely smooth, tending to move plug. When the plug moves, the control system may overcorrect or lag behind, thus perpetuate or even amplify the original motion. Balanced plug valve is most susceptible to low frequency buffeting.

As you study the modulating-control valves, note that most valve types are represented. Bodies, cast or forged, range from cast iron to stainless. Quick change of trim is practical by bolting together split bodies or relying on gaskets and flat joints instead of time-consuming threads, which are sometimes leakage paths.