Only by fully understanding single seat and cage guided globe control valves can they be used correctly. This article helps you understand them better.
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Single Seat Globe Control Valve, the trim is interchangeable in the body. It's content to various technics liquid process by replacing the different trim assembled.
The single-seat globe control valve is the furthest common control valve type, a simple structure, is used for the strict sealed close situation. A single-seat globe control valve can be controlled the maximum flow rate by changing the size of the plug and seat ring. setting fluid characteristics by changing the curved surface. They are applicable in a wide scope of liquid services as well as steam services.
Among different types of valve bodies, single-seated valves have trim structures with the easiest collocation, for example, angle valves. As a result of high-pressure fluid is generally held the load to add the whole area of the bottom seat, made the single plug to be endured huge unbalanced force from fluid, relative to the control valve with balanced trim, the allowable pressure drops are much less for the single-seated valve on the process control.
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Cage Guided Control Valve, cage structure, straight stroke control valve. the trim is interchangeable in the body Its content to various technics liquid processes by replacing the different trim assembled.is designed focused on high-pressure drops, effectively reduce the effects of cavitation and the noise in the high-pressure drops services. favorable Interchangeability, be reached different flow characteristics by replacing different cage, to get much higher precision of control. Can be reduced noise, dispelled the effects of cavitation by replacing trim.
Cage Guided control valve size is in common use for 1-12, an actuator for standard diaphragm type. By metal seat, the suitable temperature range is -196 +538, and by soft seat, the suitable temperature range is -45 +200. It's content to most industrial process control.
The most excellent cage guided structure control valve is balance characteristic, for plug balance ports function, fluid pressure acted on static unbalance force of plug, it reduced unbalance force, then the valve can be configured actuator smaller than conventional single seat valve.
The most simple structure of cage guided control valve is twin-seat, the twin-seat structure is applicable to situations that requirement of leakage is not high. The cage-guided single-seat control valve has a sliding seal between plugs top with cage. by seals function, dispelling the leak that upriver fluid entered the low-pressure downriver system. So it have the tightly closed characteristic as conventional single seat valve, moreover have balance characteristic.
A more modern version of the globe valve design uses a piston-shaped plug inside a surrounding cage with ports cast or machined into it.
These cage-guided globe valves throttle flow by uncovering more or less of the port area in the surrounding cage as the plug moves up and down. The cage also serves to guide the plug so the stem need not be subjected to lateral forces as in a stem-guided valve design.
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A photograph of a cut-away control valve shows the appearance of the cage (in this case, with the plug in the fully closed position). Note the “T”-shaped ports in the cage, through which fluid flows as the plug moves up and out of the way:
An advantage of the cage-guided design is that the valve’s flowing characteristics may be easily altered just by replacing the cage with another having different size or shape of holes.
By contrast, stem-guided and port-guided globe valves are characterized by the shape of the plug, which requires further disassembly to replace than the cage in a cage-guided globe valve.
With most cage-guided valves all that is needed to replace the cage is to separate the bonnet from the rest of the valve body, at which point the cage may be lifted out of the body and swapped with another cage.
In order to change a globe valve’s plug, you must first separate the bonnet from the rest of the body and then de-couple the plug and plug stem from the actuator stem, being careful not to disturb the packing inside of the bonnet as you do so.
After replacing a plug, the “bench-set” of the valve must be re-adjusted to ensure proper seating pressure and stroke calibration.
Cage-guided globe valves are available with both balanced and unbalanced plugs. A balanced plug has one or more ports drilled from top to bottom, allowing fluid pressure to equalize on both sides of the plug. This helps minimize the forces acting on the plug which must be overcome by the actuator:
Unbalanced plugs generate a force equal to the product of the differential pressure across the plug and the plug’s area (F = PA), which may be quite substantial in some applications.
Balanced plugs do not generate this same force because they equalize the pressure on both sides of the plug, however, they exhibit the disadvantage of one more leak path when the valve is in the fully closed position (through the balancing ports, past the piston ring, and out the cage ports):
Thus, balanced and unbalanced cage-guided globe valves exhibit similar characteristics to double ported and single-ported stem- or port-guided globe valves, and for similar reasons.
Balanced cage guided valves are easy to position, just like double-ported stem-guided and port-guided globe valves.
However, balanced cage-guided valves tend to leak more when in the shut position due to a greater number of leak paths, much the same as with double-ported stem-guided and port-guided globe valves.
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