Rising stem valves provide a stem that rises or falls as the valve opens or closes. This means that the actuation system converts rotational motion into linear movement of the valve stem when it is in operation. So, unlike its non-rising counterpart, the valve stem in this design indicates the position of the valve disc. In this article, we will review a rising stem gate valve, a rising stem ball valve, a rising stem globe valve, and compare the rising stem valve vs. its non-rising counterpart.
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Generally, gate valves serve to start or stop flow, rather than regulate it. As a result, the valve disc is completely removed from the flow path when in the open position, which minimizes pressure drop. But when in a completely closed position, there is no flow and no pressure drop.
This feature makes it the go-to option in isolation applications, especially in large-diameter and high-pressure lines. The rising stem gate valve has its thread external to the valve body. So, when the handwheel is rotated, it drives the nut within the bonnet, leading to the raising or lowering of the stem.
Thus, identifying the position of the valve is made easy as the rising stem is always up when open and down when closed. As such, a larger installation height is required for this valve design relative to the non-rising design. Another advantage of this design is the ease of lubrication because the threading is exposed.
Unlike the gate valve, the rising stem globe valve is suitable for regulating and also isolating flow. However, the pressure drop across this valve is more than in gate valves because of partial obstruction of the flow path when the valve is open. The configuration of globe valves can be for the valve disc to either open or close in the direction of fluid flow. As a result, the kinetic energy of the fluid aids the opening and or closing of the valve depending on the direction of flow.
The normally recommended flow direction is for the flow to enter and go underneath the disc and flow up and around the disc before exiting the valve. Globe valves come with a flow direction arrow on the body. This feature makes globe valves ideal for fail-open or fail-close valve applications. Also, it means that the direction of flow is an important consideration in sizing the actuator. Since globe valves are designed for the disc to push flush up against the seat when closed, there is no “wedging” torque/thrust effect to consider when actuating.
Because the entire pressure exerted on the valve disc is transmitted to the stem, the size of rising stem globe valves is typically limited to NPS 12. From the exterior, the rising stem globe valve looks like its gate valve counterpart but has a shorter stroke. Thus, it enables faster operation and makes it ideal for automation. Another advantage of the globe valve is that it comes in t-shape, y-shape, and angular (90 Degree) patterns, which is helpful when space is limited. The use of rising stem globe valves is common where regulation of flow is desired such as in cooling water systems, fuel oil systems, and turbine drains.
Similar to the gate valves, rising stem ball valves are used for on and off applications. Because their components are usually all-metal, they operate in a wide range of temperatures between -200℃ and 815℃. Their design is such that there is no abrasion between the ball and seat, nor seal rubbing. Both of these conditions promote the long-term integrity of the valve and minimize the torque requirement for opening and closing the valve.
Another feature that improves the longevity of these valves is that they are self-cleaning. When the valve is closed, the fluid flows along the entire sealing surface, cleaning impurities. And because the ball surface is hardened, it provides great seal performance even when in contact with severe media. Unlike the rising stems of gate and globe valves, ball valve stems rise and fall using stem guide grooves to slide along a rigid guide pin. This ensures proper alignment of the ball. There are two types of rising stem ball valves based on configuration.
Both rising and non-rising stem valves enjoy widespread adoption in industry. Choosing between the two requires a careful examination of the benefits of each type.
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The gate valve is a gate that opens and closes. The movement direction of the gate is perpendicular to the direction of the fluid. The gate valve can only be fully opened or fully closed, and cannot be adjusted or throttled.
Turn the handwheel to advance or retreat the threads of the handwheel and the valve stem, thereby raising or lowering the valve plate connected to the valve stem to achieve the opening and closing function.
The concealed stem gate valve is also called the rotary stem gate valve (also called the concealed stem wedge gate valve). The stem nut is set on the gate plate. The handwheel rotates to drive the stem to rotate, which lifts the gate plate. Usually there is a trapezoidal thread at the bottom of the stem. Through the thread at the bottom of the valve and the guide groove on the valve disc, the rotary motion is converted into linear motion, that is, the operating torque is converted into operating thrust.
When the valve is opened, when the gate lift height is equal to 1:1 times the valve diameter, the fluid passage is completely unblocked, but this position cannot be monitored during operation. In actual use, the top of the valve stem is used as a mark, that is, the position where it cannot be opened, as its fully open position. In order to consider the locking phenomenon caused by temperature changes, it is usually opened to the top position, and then retracted 1/2~1 turn as the fully open position of the valve. Therefore, the fully open position of the valve is determined by the position of the gate (i.e. the stroke).
The height of the gate valve always remains unchanged, so the installation space is small, and it is suitable for gate valves with large diameters or limited installation space. This structure should be equipped with an opening and closing indicator to indicate the opening and closing degree.
The valve stem thread is not only unable to be lubricated, but also directly exposed to medium erosion and is easily damaged.
1. The screw cannot be seen in the concealed stem valve , but the screw can be seen in the exposed stem valve.
2. The lifting screw of the concealed-stem flange gate valve only rotates without moving up or down. Only a rod is exposed, and its nut is fixed on the gate plate. The gate plate is lifted by rotating the screw, and there is no visible door frame. The lifting screw of the rising-stem flange gate valve is exposed, and the nut is close to the handwheel and is fixed (does not rotate or move axially). The gate plate is lifted by rotating the screw. The screw and the gate plate only have relative rotational motion but no relative axial displacement, and the appearance is a door-shaped bracket.
3. When the concealed stem valve is opened or closed, the steering wheel and the valve stem are connected together and relatively immobile. It is through the rotation of the valve stem at a fixed point that the valve disc is driven to lift up and lower the gate to complete the opening and closing. The rising stem valve is to raise or lower the valve disc through the threaded transmission of the valve stem and the steering wheel. To put it simply, the rising stem valve is a valve disc that moves up and down together with the valve stem, and the steering wheel is always at a fixed point.
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