![]() This is especially problematic when the stem is made of a different material than the body. ![]() Even if a double-ported valve is prepared in a shop for the best shut-off possible 835, it may not completely shut off when installed due to dimensional changes caused by process fluid heating or cooling the valve stem and body. With two plugs needing to come to simultaneous rest on two seats to achieve a fluid-tight seal, there is precious little room for error or dimensional instability. While double-ported globe valves certainly enjoy the advantage of easier actuation compared to their single-ported cousins, they also suffer from a distinct disadvantage: the near impossibility of tight shut-off. Double-ported, port-guided control valves also exist, with two sets of port-guided plugs and seats throttling fluid flow. This particular double-ported globe valve happens to be stem-guided, with bushings guiding the upper stem and also a lower stem (on the bottom side of the valve body). The following photograph shows a disassembled Fisher “A-body” double-ported globe valve, with the double plug plainly visible on the right: the stem position is less affected by pressure drop across the valve). This makes for a control valve that is easier to actuate (i.e. If the plug areas are approximately equal, then the forces will likewise be approximately equal and therefore nearly cancel. In a double-ported globe valve, there will be two opposed force vectors, one generated at the upper plug and another generated at the lower plug. In a single-ported globe valve, there will only be one force generated by the process pressure. The purpose of a double-ported globe valve is to minimize the force applied to the stem by process fluid pressure across the plugs:ĭifferential pressure of the process fluid (\(P_1 - P_2\)) across a valve plug will generate a force parallel to the stem as described by the formula \(F = PA\), with \(A\) being the plug’s effective area presented for the pressure to act upon. These are called double-ported globe valves. Some globe valves use a pair of plugs (on the same stem) and a matching pair of seats to throttle fluid flow. This means that the stem may be made smaller in diameter than if the valve trim were stem-guided, minimizing sliding friction and improving control behavior.Ī photograph showing a small port-guided globe valve plug appears in the following photograph: Thus, the seat ring acts as a guide for the plug to keep the centerlines of the plug and seat always aligned, minimizing guiding stresses that would otherwise be placed on the stem. Yet another variation on the globe valve design is the port-guided valve, where the plug has an unusual shape, projecting into the seat. A set of three photographs shows a needle valve in the fully-closed, mid-open, and fully-open positions (left-to-right): Needle valves are very common as manually-actuated valves used to control low flow rates of air or oil. For this reason, this particular style of globe valve is called a stem-guided globe valve.Ī variation on the stem-guided globe valve design is the needle valve, where the plug is extremely small in diameter and usually fits well into the seat hole rather than merely sitting on top of it. The left-hand photo shows the valve body in the fully closed position, while the middle photo shows the valve half-open, and the right-hand photo shows the valve fully open:Īs you can see from these photographs, the valve plug is guided by the stem to maintain alignment with the centerline of the seat. A photograph of a small (2 inch) globe valve body appears here:Ī set of three photographs showing a cut-away Masoneilan model 21000 globe valve body illustrates just how the moving plug and stationary seat work together to throttle flow in a direct-acting globe valve. ![]() The globe valve design is one of the most popular sliding-stem valve designs used in throttling service. Fluid flows through a hole in the center of the seat, and is more or less restricted by the plug’s proximity to that hole. Globe valves restrict the flow of fluid by altering the distance between a movable plug and a stationary seat (in some cases, a pair of plugs and matching seats). Of course, a reverse-acting valve body would behave just the opposite: opening up as the stem is pushed in and closing off as the stem is drawn out. Conversely, a direct-acting valve shuts off (closes) when the stem is pushed into the body. Most sliding-stem control valves are direct acting, which means the valve opens up wider as the stem is drawn out of the body. Some examples of sliding-stem valve body designs are shown here: A sliding-stem valve body is one where the moving parts slide with a linear motion. ![]()
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