[Time-Limited Quick Answer Contest, Round 3] 001 (Completed) (1 member won +40 wealth points)

The characteristic curve of a valve determines its regulating performance. For a globe valve, if flow rate changes within the range of 95% to 100% are considered meaningless, then a flow rate variation across the entire range can be achieved already when the valve opening is between 0% and 5%. Such a valve cannot be used for balancing adjustments in hydraulic systems. Since the theoretical characteristic curve of a valve is determined under maximum pressure differences, in actual operating conditions, as long as the valve’s authority factor is not 1, there is a large pressure difference across the valve at low opening degrees, while this pressure difference is smaller at high opening degrees. As a result, the value of dG/dC increases at low opening degrees and decreases at high opening degrees, causing the actual operating curve of the valve to shift in the direction of faster opening. The smaller the valve’s authority factor, the greater this shift. For valves with linear characteristics, such shifts in performance lead to a reduction in the effective range of opening degrees available for regulation; therefore, it is better to use a characteristic curve that follows a lower chord arc, such as an equal percentage characteristic. For valves with equal percentage characteristic curves, the actual operating curve may approach a linear characteristic when the valve authority is between 0.3 and 0.5. 　　Moreover, when the valve is opened to a small degree, the flow velocity through the valve is too high, resulting in intense turbulent vortices behind the valve. The pressure in these vortical areas is very low; when this pressure falls below the saturated pressure corresponding to the water temperature, vaporization occurs, leading to steam hammering: severe noise, vibration of the valve and pipes, and damage to the valve, pipes, and pipe supports. To prevent such accidents, it is necessary first, in the design of the valve flow channel, to ensure that the valve plug and seat create a narrow throttling passage at low opening degrees, thereby restricting the formation of intense turbulent vortices ； Secondly, when using valves for control, try to increase their valve coefficient as much as possible to avoid operating the valves at low opening degrees. Additionally, whenever pressure conditions are not a concern, it is advisable to install the balance valve on the return water pipe with lower water temperature.

Stop valve: The closing element of a stop valve is a plug-shaped valve disc, whose sealing surface is either flat or conical; the valve disc moves in a straight line along the centerline of the fluid. The movement pattern of the valve stem can be of the lift-type (where the valve stem moves up and down while the handwheel does not move), or of the lift-rotation type (where both the handwheel and the valve stem rotate and move up and down, with the nut located on the valve body). Stop valves are only suitable for full open and full closed positions; they cannot be used for regulation or throttling. Globe valves are classified as forcibly sealing valves; therefore, when the valve is closed, pressure must be applied to the valve disc to ensure that the sealing surface does not leak. When the medium enters the valve from below the valve disc, the resistance that the operating force must overcome consists of the friction between the valve stem and the packing, as well as the thrust generated by the pressure of the medium. The force required to close the valve is greater than that needed to open it; therefore, the diameter of the valve stem must be large, otherwise the valve stem may bend. In recent years, since the advent of self-sealing valves, the flow direction of the medium in globe valves has changed to enter the valve chamber from above the valve disc. As a result, under the pressure of the medium, the force required to close the valve is low, while the force required to open it is high; consequently, the diameter of the valve stem can be reduced accordingly. When the globe valve is opened, the flow rate reaches its maximum when the opening height of the valve disc is 25% to 30% of the nominal diameter, indicating that the valve has reached its fully open position. Therefore, the fully open position of the globe valve should be determined by the travel of the valve disc. This post was last edited by hgshy on 2009-4-13 12:27]

Participation: The closing element of a globe valve is a plug-shaped disc, whose sealing surface is flat or conical; the disc moves in a straight line along the centerline of the fluid. Globe valves are classified as forcibly sealing valves; therefore, when the valve is closed, pressure must be applied to the valve disc to ensure that the sealing surface does not leak. When the globe valve is opened, the flow rate reaches its maximum when the opening height of the valve disc is 25% to 30% of the nominal diameter, indicating that the valve has reached its fully open position. Therefore, the fully open position of the globe valve should be determined by the travel of the valve disc. Globe valves have a simple structure, making them easy to manufacture and maintain. The working stroke is short, and the opening and closing time is brief. It has good sealing performance, low friction between the sealing surfaces, and a long service life. However, the fluid resistance is high, requiring significant force to open and close it. Not suitable for media with particles, high viscosity, or tendency to coking. The adjustment performance is poor. Stop valves are only suitable for full open and full closed positions; they are generally not intended for regulation or throttling. Flow regulation is generally done using control valves.

Flow control involves adjusting the valve based on the measurement data from the flow meter. In practical operation, it’s not possible to carry out such calculations; moreover, each valve has its own characteristics regarding the degree to which it can be opened, and even valves of the same model vary from one another, so there’s no one-size-fits-all approach. Stop valve: It uses a valve disc mounted below the valve stem, in combination with a seat inside the valve body, to control the opening and closing of the valve. The structure diagram is as posted by the original poster. Features of globe valves: they are easy to manufacture and maintain, and allow for relatively precise control of flow rate. Chemical plants make extensive use of pipes for transporting various materials. As the fluid changes direction when passing through a stop valve, the resistance is high, making it unsuitable for materials with high viscosity. Since the liquid flows from top to bottom through the valve seat. Solids tend to accumulate on the valve seat, affecting its tightness; therefore it is also not suitable for use with suspensions and crystalline liquids ； The sealing surfaces of the valve core in globe valves come in soft and hard types. The soft surface features a flat valve seat for the disc spool, providing good sealing properties and facilitating the replacement and protection of the valve seat; however, it is not wear-resistant, unsuitable for throttling purposes, and its operating temperature is limited by the material used. Hard-surface valves typically use hard conical or spherical valve cores paired with conical seat surfaces to achieve linear sealing; they exhibit good corrosion and erosion resistance as well as excellent control properties, making them particularly suitable for throttling, which is why they are also known as throttle valves. This post was last edited by zx9527007 on 2009-4-12 23:33.]

Verification of the adjustable ratio of control valves: It is generally required that the actual adjustable ratio be no less than 10. The adjustable ratio of a control valve is the ratio of the maximum flow rate to the minimum flow rate that can be controlled by the valve; it is also known as the adjustable range. If denoted by R, then R = Qmax/Qmin. It should be noted that the minimum flow rate is different from the leakage rate. The minimum flow rate refers to the lower limit of the adjustable flow rate, and it is generally 2-4% of the maximum flow rate ； The leakage rate is the amount that leaks when the valve is fully closed, and it is only 0.1–0.01% of the maximum flow rate. In other words, the ideal regulation ratio is equal to the ratio of the maximum flow capacity to the minimum flow capacity; it reflects the degree of regulation capability of the control valve and is determined by the structural design. In actual operation, control valves are always connected in series with the piping system or in parallel with bypass valves. As the resistance of the piping system changes or as the degree to which the bypass valve is opened varies, the adjustable range of the control valve also changes accordingly; this adjustable range is referred to as the actual adjustable range. Since the C value of the valve has been set to be greater than the calculated Cmax value when selecting the valve diameter, and especially the restrictions on the maximum and minimum opening angles during use, the adjustable ratio decreases; generally, the R value is only around 10. In addition, it is also affected by the distortion of the working flow characteristics, which reduces the adjustable ratio. When the control valve selected cannot meet the regulation requirements for both the maximum and minimum flow rates in the process, in addition to increasing the system pressure, two control valves can be used for split-range control to satisfy the requirement for a Adjustable range.

Gate valve: A valve in which the closing element (valve disc) moves along the center line of the valve seat. Based on this manner of movement of the valve disc, the change in the valve seat opening is directly proportional to the stroke of the valve disc. Due to the relatively short opening and closing stroke of the valve stem in this type of valve, as well as its highly reliable shut-off function, and because the change in the valve seat orifice is proportional to the stroke of the valve disc, it is highly suitable for regulating flow rate. Therefore, this type of valve is very suitable for cutting off, regulating, and throttling. Based on the characteristics of the process medium, an appropriate flow characteristic is selected. The flow characteristic of a globe valve represents the relationship between the relative flow rate of the medium passing through the control valve and its relative displacement (the degree of opening of the control valve). Generally, by changing the cross-sectional area through which the valve core and seat allow flow, it is possible to control the flow rate. But in reality, due to various factors, such as changes in the cross-sectional area, there are also changes in the pressure difference before and after the valve, and these changes in pressure difference in turn cause changes in flow rate. Stop valves are always used in conjunction with pipes, equipment, etc. Due to differences in the system piping layout, the presence of piping resistance causes variations in the pressure drop across the control valve. The desired operational characteristics must be selected based on the system’s features, and then the appropriate ideal characteristics should be chosen taking into account the piping conditions. Generally, at the maximum flow rate, the opening degree of the stop valve should be around 90%. If this opening degree is too low, it indicates that the control valve chosen is too large; it operates at a low opening degree frequently, which results in reduced regulation performance and economic waste. The minimum opening should be no less than 10%; otherwise, the valve spool and seat will suffer severe erosion by the fluid, leading to a deterioration in their performance or even failure. Due to different flow characteristics, the relationship between the valve’s relative opening degree and the relative flow rate also varies. In practical use, globe valves are always connected in series with the pipeline system or in parallel with bypass valves; as the resistance of the pipeline system changes or the degree to which the bypass valve is open changes, the adjustable range of the globe valve also changes accordingly. Normally, globe valves are only suitable for full open and full closed positions, and are used for rough flow regulation when frequent adjustment of the flow rate is not required.

The globe valve, also known as a stop valve, is one of the most widely used types of valves. It is popular because it experiences low friction between its sealing surfaces during operation, is durable, requires little clearance for full opening, is easy to manufacture, and convenient to maintain. It is suitable not only for medium and low pressures but also for high pressures. Its closing principle relies on the pressure from the valve stem to bring the sealing surface of the valve disc into close contact with the sealing surface of the valve seat, thereby preventing the flow of the medium. A stop valve allows fluid to flow in only one direction, and it has a specific orientation when installed. Its structure results in a length greater than that of gate valves, and it presents high fluid resistance; moreover, its sealing reliability is not strong during long-term operation. Stop valves are divided into three categories: straight-through, right-angle, and parallel flow diagonal stop valves. A globe valve operates by rotating the handwheel, which causes the valve stem to rise and fall and thus moves the valve disc in a straight line parallel to the flow direction of the medium, thereby achieving opening and closing. When the handwheel is rotated clockwise, the working medium enters through the channel beneath the valve disc; at this time, the sealing force applied must be equal to or greater than the sum of the required pressure on the sealing surface and the upward force exerted by the medium. Stop valves can only be used in a fully open position; customization allows for regulation and throttling. 　　When the globe valve is opened, the flow rate reaches its maximum when the opening height of the valve disc is 25% to 30% of the nominal diameter, indicating that the valve has reached its fully open position. Therefore, the fully open position of the globe valve should be determined by the travel of the valve disc. This post was last edited by tuubt on 2009-4-13 10:57]

In fact, using a globe valve to control flow is not a professional method of control; globe valves have low control precision and are only suitable for applications that do not require precise control； Secondly, the control speed is slow. In terms of regulation, it is primarily done by adjusting the opening degree (the size of the gap) between the valve core and the fluid passage of the valve. I personally do not recommend using globe valves for flow control!

I believe that stop valves are generally only suitable for full open and full closed positions, and not for regulation or throttling. The erosion of the valve body is quite severe. In actual production, there are also cases where the opening is appropriately reduced to control pressure and flow. Each valve has its own flow characteristic curve; theoretically, the flow rate can be determined by calculating the cross-sectional area based on the valve opening degree, just as in the PID control used for control valves. Control valves are generally used only for rough adjustment, regulating parameters such as pressure, temperature, and flow rate to meet production requirements

Stop valves should not be used for regulating flow; throttle valves should be used instead for that purpose. There is no opening indication on the stop valve or throttle valve, and the flow rate is proportional to the square of the flow area; therefore, it is not possible to accurately determine the degree of valve opening during on-site operation. The control of on-site flow rate generally relies on on-site flow meters to adjust the opening degree, or it is controlled based on practical operational experience.

During the stage when the stop valve begins to close from its fully open position, as the valve disc descends, a pressure difference is created in front of and behind the valve disc to prevent it from moving further down, and this resistance increases rapidly as the valve disc continues to descend. When the stop valve is fully closed, the pressure difference across the valve disc equals the operating pressure of the medium, and this is when the resistance is greatest. Combined with the forced sealing force, the operating force at the moment the globe valve closes increases rapidly. During the valve opening process, the thrust generated by the medium pressure or the pressure difference before and behind the valve disc both contribute to opening the valve. It should be noted that the torque at the moment of opening the valve may exceed that at the moment of closing it, as a greater static friction force has to be overcome at that time. When a globe valve is opened, the flow rate reaches its maximum when the opening height of the valve disc is 25% to 30% of the valve’s nominal diameter; this indicates that the globe valve has reached its fully open position. Therefore, the fully open position of a globe valve should be determined by the stroke of the valve disc. The behavior of a globe valve when it is closed, and when it is reopened after being fully closed, is similar to that of a gate valve with forced sealing; therefore, the closing position of a globe valve should be determined by increasing the operating torque to a specified value.