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1. What are the three main factors to consider when selecting an actuator? ①The output of the actuator must be greater than the load on the valve, with a proper match. ②When checking the standard combination, it is necessary to consider whether the permissible pressure difference specified for the valve meets the process requirements. When there is a large pressure difference, it is necessary to calculate the unbalanced force acting on the valve core. ③It is necessary to consider whether the response speed of the actuator meets the requirements of the process operation, especially for electric actuators. 2. What are the characteristics of electric actuators compared to pneumatic actuators, and what are the various types of outputs they offer? Electric drive sources use electricity, which is simple and convenient; they offer high thrust and torque as well as high stiffness. But it has a complex structure and poor reliability. It is more expensive than pneumatic versions in small and medium sizes. It is commonly used in situations where there is no air supply or where strict explosion-proof and fire-resistant measures are not required. Electric actuators come in three types of output: angular stroke, linear stroke, and multi-turn. 3. Why do angle-type valves have a higher shut-off pressure difference? The cut-off pressure difference of angle-type valves is relatively high, as the resultant force exerted by the medium on the valve core or valve plate generates a very small torque on the rotating shaft; therefore, it can withstand a large pressure difference. Butterfly valves and ball valves are the most common angle-of-rotation valves. Control valves yunrun.com.cn/product/list_88.html 4. Which valves require selection of flow direction? How to choose? Regulating valves with a single seal, such as single-seat valves, high-pressure valves, and single-seal sleeve valves without balance holes, require flow direction selection. Both flow open and flow closed have their advantages and disadvantages. Flow-open type valves operate relatively stably, but have poor self-cleaning capabilities and sealing performance, as well as a short service life ； Valves of the flow-blocking type have a long service life, good self-cleaning properties and sealing performance, but their stability is poor when the valve stem diameter is smaller than that of the valve core. Single-seat valves, low-flow valves, and single-seal sleeve valves are usually selected with flow-through operation; flow-blocking operation can be chosen when there is severe erosion or a need for self-cleaning. The two-position quick-opening control valve adopts a flow-blocking type. 5. What other valves with regulating functions exist besides single-seat, double-seat, and sleeve valves? Diaphragm valves, butterfly valves, O-ring ball valves (primarily used for shut-off), V-ball valves (with a high adjustment ratio and shear action), and eccentric rotary valves are all valves with adjustment capabilities. 6. Why is valve selection considered more important than calculation? Compared to calculation, selection is much more important and much more complex. Because the calculation is merely a simple formula-based process; what matters is not the accuracy of the formula itself, but whether the given process parameters are accurate. The process of selection involves many factors; a slight mistake can lead to an inappropriate choice. This not only results in wasted resources in terms of labor, materials, and funds but also leads to suboptimal performance, causing various issues related to use, such as reliability, lifespan, and operational quality. 7. Why cannot double-seal valves be used as shut-off valves? The advantage of the double-seat valve spool is its force-balanced design, which allows for high pressure differences; however, its major drawback is that the two sealing surfaces cannot make good contact at the same time, resulting in significant leakage. If it is artificially and forcibly used for cutting off flows, the results are obviously poor; even with many improvements made to it (such as double-sealed sleeve valves), it is not a viable option. 8. Why do two-seat valves tend to oscillate when operating at a small opening? For single-core systems, when the dielectric is of the flow-open type, the valve exhibits good stability ； When the medium is flow-closed, the valve has poor stability. A two-seat valve has two spools; the lower spool is in the flow-blocking position, while the upper spool is in the flow-allowing position. Thus, when operating at low opening degrees, the flow-blocking type valve core tends to cause vibration in the valve, and this is why double-seat valves cannot be used at low opening degrees. 9. What are the characteristics of direct-acting single-seat control valves? In what situations is it applied? ①The leakage rate is low, as there is only one valve core, making it easy to ensure sealing. The standard discharge rate is 0.01% KV; it can be further designed as a shut-off valve. ②The allowable pressure difference is low due to the large thrust from unbalanced forces. The △P for a DN100 valve is only 120 KPa. ③Low circulation capacity. The KV for DN100 is only 120. It is often used in situations with low leakage rates and moderate pressure differences. 10. What are the characteristics of a direct-acting two-seat control valve? In what situations is it applied? ①The allowable pressure difference is large, as it can counteract many unbalanced forces. The ΔP for a DN100 valve is 280 KPa. ②It has high circulation capacity. The KV for DN100 is 160. ③The leakage is severe for two reasons: the valve cores cannot seal simultaneously. The standard discharge rate is 0.1% KV, which is 10 times that of a single-seat valve. Direct-through two-seat control valves are mainly used in applications with high pressure differences where strict leakage requirements do not apply. 11. Why do straight-stroke control valves have poor anti-clogging performance, while angle-stroke valves have good anti-clogging performance? In a straight-stroke valve, the valve core performs vertical throttling, while the medium flows in and out horizontally; as a result, the flow channels within the valve chamber have to make turns, making the flow path of the valve quite complex (with a shape similar to an inverted “S”). As a result, there are many dead zones that provide space for the medium to settle, which over time leads to blockages. For quarter-turn valves, the direction of throttling is horizontal; the medium flows in and out horizontally. This helps to carry away any impurities present in the medium. Additionally, the flow path is simple, leaving little room for sediment to accumulate. Therefore, quarter-turn valves exhibit excellent anti-clogging performance. 12. Under what circumstances is a valve positioner needed? ①Applications with high friction and the need for precise positioning. For example, high-temperature and low-temperature control valves or control valves using flexible graphite packing ； ②Situations where slow processes require an increased response speed of control valves. For example, control systems for parameters such as temperature, liquid level, and analysis. ③Applications where it is necessary to increase the output force and cutting force of the actuator. For example, single-seat valves with DN≥25, and double-seat valves with DN＞100. In cases where the pressure drop ΔP across the valve is >1 MPa or the inlet pressure P1 is >10 MPa. ④In split-control systems and control valves, it is sometimes necessary to change between air-open and air-close operation modes during use. ⑥Situations where it is necessary to change the flow characteristics of the control valve. 13. What are the seven steps to determine the diameter of a control valve? ①Determine the calculated flow rates – Qmax and Qmin. ② Determine the calculated pressure difference – select the resistance ratio S value based on the characteristics of the system, and then determine the pressure difference (when the valve is fully open) ； ③Calculate the flow coefficient – use appropriate formula charts or software to determine the max and min values of KV ； ④KV value selection – The preliminary caliber is determined by selecting the KV value from the selected product series that is closest to the maximum KV value ； ⑤Opening degree verification – When Qmax is required, the valve opening degree should be ≯90% ； At Qmin, valve opening ≮10% ； ⑥Actual adjustable ratio verification – generally, it should be ≮10 ； R_actual > R_required ⑦ Determine the diameter – if it fails, select a different KV value and verify again. 14. Why hasn’t the sleeve valve succeeded in replacing single- and double-seat valves as hoped? Sleeve valves were introduced in the 1960s, and became widely used at home and abroad in the 1970s. In the petrochemical plants installed in the 1980s, sleeve valves accounted for a large proportion. At that time, many people believed that sleeve valves could replace single- and double-seat valves and become the second generation of valve products. These days, that is not the case; single-seat valves, double-seat valves, and sleeve valves are all used equally. This is because the sleeve valve only improves the throttling mechanism, stability, and maintainability compared to single-seat valves; however, its weight, clogging resistance, and leakage performance are on par with those of single-seat and double-seat valves. How can it then replace them? Therefore, they can only be used jointly. 15. Why should hard-sealed valves be preferred as much as possible for shut-off valves? Cut-off valves require as low a leakage rate as possible; soft-sealed valves have the lowest leakage rate, which means they provide good cut-off performance, but they are not wear-resistant and have poor reliability. From the dual criteria of low leakage and reliable sealing, soft-seal cutoff is inferior to hard-seal cutoff. Such as fully functional ultra-lightweight control valves, which are sealed and protected by wear-resistant alloys, offering high reliability with a leakage rate of 10–7; they are already capable of meeting the requirements of shut-off valves. 16. Why is the stem of a straight-stroke control valve relatively thin? It involves a simple mechanical principle: sliding friction is high while rolling friction is low. In a straight-stroke valve, as the valve stem moves up and down, the packing becomes slightly compressed, which causes it to wrap tightly around the valve stem and results in a larger backset. To this end, the valve stem is designed to be very thin, and PTFE packing with a low friction coefficient is commonly used as packing to minimize backlash. However, this leads to the problem that a thin valve stem is prone to bending, and the lifespan of the packing is also shortened. The best way to solve this problem is to use a pilot-operated valve stem, that is, a control valve of the angle-type. Its stem is 2 to 3 times thicker than that of a linear-type valve stem, and graphite packing with a long service life is used; the stem has good stiffness, the packing lasts longer, which results in lower friction torque and less hysteresis.