The stem of a rising stem valve rises or lowers as the valve opens or closes. This means that when the valve is turned on, the actuation system turns rotational motion into linear movement of the valve stem. In this design, the valve stem shows the position of the valve disc, as opposed to its non-rising cousin. This article will look at rising stem gate valves, rising stem ball valves, and rising stem globe valves, as well as compare rising stem valves to their non-rising counterparts.
Rising Stem Gate Valve
Gate valves, in general, function to initiate or stop flow rather than to regulate it. As a result, when the valve is open, the valve disc is totally removed from the flow path, minimizing pressure drop. However, there is no flow and no pressure drop in a completely closed position.
Because of this property, it is the preferred choice for isolation applications, particularly in large-diameter and high-pressure lines. The thread on the rising stem gate valve is located outside of the valve body. So, when the handwheel is rotated, it drives the nut within the bonnet, raising or lowering the stem.
Thus, identifying the valve’s position is easy as the rising stem is always up when open and down when closed. As such, a more significant installation height is required for this valve design relative to the non-rising design. Another advantage of this design is that the threading is easily lubricated because it is exposed.
Rising Stem Globe Valve
Unlike the gate valve, the rising stem globe valve is suitable for regulating and 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 opens. Globe valves can be configured so that the valve disc opens or closes in the direction of fluid flow. As a result, depending on the direction of flow, the fluid’s kinetic energy aids in the opening and/or closing of the valve.
The flow path that is typically recommended is for the flow to enter and move underneath the disc, then flow up and around the disc before exiting the valve. The body of a globe valve has a flow direction arrow. Globe valves are appropriate for fail-open or fail-close valve applications because of this property. It also implies that the flow direction is an important factor to consider while sizing the actuator. There is no “wedging” torque/thrust effect to consider when actuating globe valves because the disc is designed to push flush up against the seat when closed.
Because the full valve disc pressure is transmitted to the stem, the size of rising stem globe valves is normally limited to NPS 12. The rising stem globe valve appears similar to its gate valve counterpart but has a shorter stroke. As a result, it allows for speedier operation and is perfect 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 there is limited space. Rising stem globe valves are used every day where flow regulation is desired, such as in cooling water systems, fuel oil systems, and turbine drains.
Rising Stem Ball Valve
Rising stem ball valves, like gate valves, are used for on and off applications. Because their components are typically all-metal, they can function at temperatures ranging from -200°C to 815°C. Their design prevents abrasion between the ball and the seat, as well as seal rubbing. Both of these factors increase the valve’s long-term integrity and reduce the torque required to open and close the valve.
Self-cleaning is another characteristic that extends the life of these valves. When the valve is closed, fluid flows along the entire sealing surface, washing away contaminants. Furthermore, because the ball surface has been toughened, it delivers excellent seal performance even when in contact with harsh substances. Unlike the rising stems of gate and globe valves, ball valve stems rise and fall by sliding over a stiff guide pin via stem guide grooves. This ensures that the ball is properly aligned. Rising stem ball valves are classified into two varieties based on their configuration.
Rising Stem vs. Non-rising Stem Valve
Stem Movement:
Rising Stem Valve: In a rising stem valve, the stem moves upward or downward as the valve is opened or closed. This movement provides a visual indication of the valve’s position, making it easier to determine whether it is open or closed.
Non-Rising Stem Valve: In contrast, the stem of a non-rising stem valve does not move vertically as the valve opens or closes. Instead, the internal mechanism of the valve is designed to move the disc or gate, allowing for control of the flow without any visible change in the stem’s position.
Space Considerations:
Rising Stem Valve: These valves typically require more vertical space due to the upward movement of the stem. This can be a crucial factor in installations where space constraints are a concern.
Non-Rising Stem Valve: Non-rising stem valves are often preferred in applications where vertical space is limited because the stem does not extend beyond the valve’s dimensions.
Applications:
Rising Stem Valve: Commonly used in applications where a visual indication of the valve position is essential, such as in fire protection systems or when the valve is manually operated.
Non-Rising Stem Valve: Often chosen in scenarios where space is restricted or when a more compact design is preferred. These valves are commonly found in underground pipelines.
Maintenance and Accessibility:
Rising Stem Valve: The external movement of the stem allows for easy visual inspection and maintenance. This can be advantageous when quick assessments of valve status are necessary.
Non-Rising Stem Valve: The internal movement of the stem makes it less susceptible to damage or contamination from external factors, providing a certain level of protection and potentially reducing maintenance needs.
Sealing Mechanism:
Rising Stem Valve: Typically employs a threaded stem design that moves through a threaded bushing, ensuring a seal between the stem and the valve body.
Non-Rising Stem Valve: Often utilizes a flexible wedge or other sealing mechanisms that do not rely on the stem’s movement for effective sealing.
Appearance:
Rising Stem Valves: It is simple to tell if the valve is open or closed from a distance. This is useful for operators of complex pipe installations.
Non-Rising Stem Valves: Its appearance does not tell if it is in an open or closed position. So, an indicator is used to verify this.
Position of stem nut and threading:
Rising Stem Valves: The stem nut, which rotates to elevate or lower the stem, is housed within the bonnet or bracket. The stem threading is the same. Because they are external, they are simple to maintain.
Non-Rising Stem Valves: The stem nut is concealed within the valve body, so it is in contact with the fluid. Therefore, it will corrode and erode, etc.
Mode of Transmission:
Rising Stem Valves: When the handwheel rotates, the stem nut also rotates in place as the stem rises or falls.
Non-Rising Stem Valves: The handwheel and the stem are connected, so there is no relative motion between them.
Final Thoughts
Understanding these differences allows engineers and operators to make informed decisions based on specific application requirements, spatial constraints, and maintenance considerations. Each type of valve has its advantages, and the choice between a rising stem and a non-rising stem depends on the unique demands of the system in which they are employed.
