A: I will begin with some clarification of terminology. Globe control valves are usually grouped into two major categories. Cage guided and stem (or top) guided valves. Although cage guided valves are sometimes unbalanced, I will limit this discussion to the more common balanced design since that is what the question is about. In the stem guided valves, the entire process pressure is acting upward on the valve plug (and opposing the actuator). If the bottom of the plug in a small, unbalanced stem guided valve like the one pictured in Figure 1 has a surface area of one square inch, and the pressure the valve has to shut off against is 100 pounds per square inch, the upward force that the actuator would need to overcome would be 100 pounds. In that case, there would probably be no problem. But as the valve size increases (and thus the surface area of the bottom of the plug) and the process shut-off pressure increases, the actuator
size requirement can increase considerably, sometimes to the point that there is no suitable actuator. The stem guided valve gets its name from the fact that the plug is held in place (guided) by a guide bushing, most often in the valve bonnet. The problem is often solved by using a balanced cage guided valve like the one pictured on the left-hand side of Figure 1. Unlike the stem guided valve where the process pressure is pushing upward on the surface area of the bottom of the plug, the process flow (and pressure) is entering from the side and simply pressing against the side of the plug rather than the bottom of the plug, as seen in Figure 2. There are balancing holes drilled between the top of the plug and the inside of the plug so any pressure that is pushing up on the plug is also pushing down on the plug.
What the questioner has overlooked is that “balanced” cage guided globe valves are not perfectly balanced but have a small unbalance effect. Figure 2 illustrates the reason for this unbalance. On the right is a drawing of a cage guided valve’s plug, cage, seat and piston ring. On the left is a cross sectional drawing of the left-hand portion of the plug, cage, seat and piston ring. The tolerance between the cage ID and the plug OD has been exaggerated in the drawing. When the valve is closed, the process pressure comes through the triangular windows on the plug and pushes upward on the piston ring and on the portion of the plug that is not contacting the seat due to different angles on the plug and the seat. The process pressure force acting on these small areas is the only force trying to open the valve. There cannot be any difference in pressure between the top of the plug and the bottom of the hollow portion of the plug because of the balancing holes that are drilled through the top of the plug. Because of these small unbalanced areas, the answer to the question is that a test pressure greater than the value quoted in the FCI 70.2 Standard can influence the result of the leak test of a “balanced” globe valve.
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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.
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