Cage Guided Globe Control Valve
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Cage guided valves are widely used because of their many advantages. Let's learn more about them together.
Sleeve adjustment valve in the fluid from the sleeve outward flow, known as the center to the outside flow direction. Conversely, the flow is called external to the center. However, it is usually used to flow from the lower part of the sleeve and out through the opening of the cage. There are 3, 4, or 6 throttle openings symmetrically located in the sleeve. The shape of the throttle openings is related to the desired flow characteristics. The flow characteristics of the control valve can therefore be easily changed by changing the sleeve (shape of the throttle opening).
The valve seat is pressed against the valve body by the valve cover and is not threaded. Easy to install and maintain.
By replacing different sleeves, different flow coefficients and different flow characteristics can be obtained.
To reduce control valve noise, sleeve control valves are available as a type with noise-reducing inner parts. It has a sleeve and spool with a number of small holes. The small holes are used to increase the resistance. The velocity head is converted into kinetic energy to reduce noise. Typically, this type of sleeve control valve can reduce noise by more than 10dB.
Therefore, they are widely used in applications where noise reduction is required. To reduce control valve noise, multi-stage pressure reduction methods can also be used. The total pressure drop at both ends of these noise-reducing control valves is distributed to all levels so that all levels do not cause the fluid to flash and cavitate.
This reduces the noise of the control valve and can weaken and prevent scouring and wear caused by flashing and cavitation. The bottom of the spool of the sleeve control valve is flat, so that if cavitation occurs, the impact from the rupture of the bubble does not act on the spool, but is absorbed by the medium itself. Therefore, the impact of cavitation of the sleeve adjustment valve is small, and the service life is long.
As a result of the sleeve and the spool between the graphite piston ring seal. After the long-term operation, the wear of the seal ring makes the leakage of the sleeve adjustment valve larger than that of the single-seated valve.
The reverse sleeve control valve is particularly suitable for applications where the valve internals require frequent inspection and maintenance, as the spool can be removed from below.
Usually, there are two types of sleeve adjustment valves, one is a balanced valve, the other is an unbalanced valve. The balancing valve has a balancing hole on the spool so that the unbalance force on the spool is greatly reduced. At the same time, it has a damping effect, which is beneficial to the stable operation of the control valve. Therefore, this type of control valve is often used in applications with high differential pressure and low noise requirements.
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.