Sixteen questions about valves: which one posed a challenge for you?

Thread Content

I. What are the three main factors to consider when selecting an actuator? 1) The output of the actuator must be greater than the valve load and should be reasonably matched. 2) When checking the standard combination, it is necessary to consider whether the allowable pressure difference specified for the valve meets the process requirements. When there is a large pressure difference, the unbalanced force acting on the valve stem must be calculated. 3) It is necessary to consider whether the response speed of the actuator meets the requirements of the process operation, especially for electric actuators. II. What are the characteristics of electric actuators compared to pneumatic actuators, and what are the various types of output they offer? The electric drive source uses electricity, which is simple and convenient; it provides high thrust and torque as well as high stiffness. However, its structure is complex and its reliability is poor. They are more expensive than pneumatic ones in small and medium sizes. It is commonly used in situations where there is no air source or where strict explosion and fire prevention measures are not required. Electric actuators have three types of output forms: angular stroke, linear stroke, and multi-turn. III. Why is the shut-off pressure difference for angular travel valves relatively large? The cut-off pressure difference of angle-action valves is relatively high because the resultant force exerted by the medium on the valve core or valve plate generates a very small torque on the rotating shaft; as a result, such valves can withstand large pressure differences. Butterfly valves and ball valves are the most common angle-turn valves. IV. Which valves require flow direction selection? How to choose? Control 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 flowing open and flowing closed have their advantages and disadvantages. Valves of the flow-out type operate relatively stably, but they have poor self-cleaning capabilities and sealing performance, as well as a short lifespan ； 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-port quick-opening control valve adopts a flow-blocking type. V. Besides single- and double-seat valves and sleeve valves, what other valves have regulating functions? Diaphragm valves, butterfly valves, O-type ball valves (primarily used for shut-off), V-type ball valves (featuring a large modulation range and shearing action), and eccentric rotary valves are all valves with modulation capabilities. VI. Why is model selection more important than calculation? Compared to calculation, model selection is much more important and far 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; even the slightest carelessness can lead to an inappropriate choice. This not only results in waste of human, material, and financial resources but also leads to suboptimal performance, causing various issues related to use, such as reliability, lifespan, and operational quality. VII. Why cannot double-seal valves be used as shut-off valves? The advantage of the double-seat valve spool is its force-balancing structure, 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 purposes, the results are obviously poor; even with many improvements made to it (such as double-sealed sleeve valves), it is not a viable option. VIII. Why do two-seat valves tend to oscillate when operating at a small opening? For single-core valves, when the medium is of the flow-opening 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 valve core tends to cause vibration in the valve; this is why double-seat valves cannot be used at low opening degrees. IX. What are the characteristics of direct-acting single-seat control valves? In what situations is it applied? 1) The flow rate is low, as having only one valve element makes it easy to ensure sealing. The standard leakage rate is 0.01% of KV; with further design, it can be used as a shut-off valve. 2) The allowable pressure difference is small, as the thrust caused by unbalanced forces is large. For the DN100 valve, △P is only 120 KPa. 3) Low circulation capacity. The KV of DN100 is only 120. 　　It is often used in situations where the leakage rate is low and the pressure difference is not significant. X. What are the characteristics of direct-acting two-seat control valves? In what situations is it applied? 1) The allowable pressure difference is high, as it can compensate for many unbalanced forces. The ΔP for a DN100 valve is 280 KPa. 2) High throughput capacity. The KV for DN100 is 160. 3) The leakage volume is large; there are two reasons for this: the two valve cores cannot be sealed simultaneously. The standard discharge rate is 0.1% KV, which is 10 times that of a single-seat valve. 　　The straight-through two-way control valve is mainly used in applications with high pressure differences where strict leakage requirements do not apply. XI. 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. The throttling direction of a rotary valve is horizontal; the fluid flows in horizontally and exits horizontally, which makes it easy to carry away any impurities. Additionally, the flow path is simple, and there is little space for the fluid to settle, so rotary valves have good anti-clogging properties. XII. Under what circumstances is a valve positioner necessary? 1) Situations 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 ； 2) Cases where slow processes require an increased response speed of control valves. For example, control systems for parameters such as temperature, liquid level, and analysis. 3) Cases 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 greater than 1 MPa, or the inlet pressure P1 is greater than 10 MPa. 4) Cases where it is sometimes necessary to change between air-open and air-close operation modes for the proportional control system and control valves during operation. 5) Cases where it is necessary to change the flow characteristic of the control valve. 13. What are the seven steps to determine the size of a control valve? 1) Determine the calculated flow rate – Qmax, 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) ； 3) Calculate the flow coefficient — Use appropriate calculation formulas, charts, or software to determine the max and min values of KV ； 4) Selection of KV value – The initial diameter is determined by selecting the KV value from the selected product series that is closest to the maximum KV value ； 5) Opening degree verification – The valve opening degree shall be ≯90% at Qmax ； At Qmin, valve opening ≮10% ； 6) Verification of actual adjustable ratio – 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 expected? The sleeve valve made its debut in the 1960s and was widely used both domestically and abroad in the 1970s. In the petrochemical plants introduced in the 1980s, sleeve valves accounted for a significant proportion. At that time, many people believed that sleeve valves could replace single- and double-seat valves, thus becoming the second-generation 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? So, it can only be used together. 15. Why should hard seals be preferred for shut-off valves? Cut-off valves require as low a leakage rate as possible; soft-seal 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 standpoint of the dual criteria of low leakage and reliable sealing, soft-seal shutoff is inferior to hard-seal shutoff. 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 valve stem of linear travel control valves 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 to reduce backlash; however, the problem that arises from this is that a thin valve stem is prone to bending, and the packing also has a short lifespan. The best way to solve this problem is to use a travel valve stem, that is, a control valve of the angle-type. Its stem is 2 to 3 times thicker than that of a straight-stroke valve, and a 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.