10 common problems and solutions for control valves

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Control valves are used to regulate process parameters such as fluid flow rate, pressure, temperature, and liquid level in the field of industrial automation process control. Based on the control signals from the automation system, they automatically adjust the opening degree of the valve, thereby enabling the regulation of fluid flow rate, pressure, temperature, and liquid level. Control valves are used to regulate process parameters such as fluid flow rate, pressure, temperature, and liquid level in the field of industrial automation process control. Based on the control signals from the automation system, they automatically adjust the opening degree of the valve, thereby enabling the regulation of fluid flow rate, pressure, temperature, and liquid level. I. Why does it tend to oscillate when operating at a small opening with dual valve seats? For single-core systems, when the dielectric is of the flow-open type, the valve exhibits good stability ； When the medium is of the flow-closed type, the valve has poor stability. A double-seat valve has two spools: the lower spool is in a flow-blocking position while the upper spool is in a flow-allowing position. As a result, when the valve operates at a low opening degree, the flow-blocking spool can easily cause vibration in the valve; this is why the upper seat cannot be used for operation at low opening degrees. II. Why cannot double-seal valves be used as shut-off valves? The advantage of valves with dual seat inserts is their force-balanced structure, which allows for high pressure differences; however, their major drawback is that the two sealing surfaces cannot make good contact at the same time, resulting in significant leakage. If such valves are forced to be used for shut-off purposes, the results will obviously be poor. Even with various improvements made to them (such as double-seal sleeve valves), it remains unsuitable to use them in this capacity. III. Why do straight-stroke control valves have poor anti-clogging performance, while angle-stroke valves have good anti-clogging performance? In straight-stroke valves, the valve element performs throttling in a vertical direction, while the fluid flows in and out horizontally; as a result, the flow paths within the valve chamber must make turns, making the flow path quite complex (resembling an inverted S shape). This creates many dead zones that provide space for sediment to accumulate, leading to clogging over time. In angle-stroke valves, throttling occurs in a horizontal direction – the fluid flows in horizontally and exits horizontally as well – which helps to carry away any impurities. Additionally, the flow path is simpler, with less space for sediment to accumulate, which is why angle-stroke valves have good anti-clogging performance. IV. Why is the stem of a straight-stroke control valve relatively thin? It is based on a simple mechanical principle: sliding friction is high while rolling friction is low. In a straight-stroke valve, as the stem moves up and down, the packing becomes slightly compressed, which causes it to wrap tightly around the stem and result in a large backlash. Therefore, the stem is designed to be very thin. Additionally, PTFE packing with a low coefficient of friction is often used to reduce backlash. However, this leads to the problem that a thin stem is prone to bending, and the lifespan of the packing is also shortened. The solution to this problem is to use a rotary valve stem, that is, a control valve of the angle-type. Its stem is 2 to 3 times thicker than that of a linear-stroke valve, and asbestos 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. V. Why do angle-type valves have a high shut-off pressure difference? The high shut-off pressure difference of angle-type valves is due to the fact that 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 high pressure differences. VI. Why do rubber-lined butterfly valves and fluorine-lined diaphragm valves have a short service life when used with desalinated water? Desalinated water contains low concentrations of acids or bases, which are highly corrosive to rubber components. The corrosion of rubber manifests itself as swelling, aging, and reduced strength; as a result, these valves do not perform well. The underlying reason for this is the fact that rubber is not resistant to corrosion. The back-gasket diaphragm valve was improved to a fluorine-lined diaphragm valve with better corrosion resistance, but the diaphragm of this fluorine-lined diaphragm valve cannot withstand bending up and down and gets damaged, thus reducing the valve’s service life. The current solution is to use special ball valves for water treatment, which can last 5 to 8 years. VII. Why didn’t sleeve valves succeed in replacing single- and double-seat valves? Introduced in the 1960s, sleeve valves were widely used at home and abroad in the 1970s, and they accounted for a large proportion in the petrochemical plants built in the 1980s. At that time, many people believed that sleeve valves could replace single- and double-seat valves and become the next generation of valve products. Up to now, 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, resistance to clogging, and leakage characteristics are the same as those of single-seat and double-seat valves. How can it then replace them? Therefore, they can only be used together. VIII. Why should hard-sealed valves be preferred for cut-off valves? Cut-off valves require as low a leakage rate as possible; while soft-sealed valves offer good cut-off performance, they are not wear-resistant and have poor reliability. Based on the dual criteria of low leakage volume and reliable sealing, soft-seal cut-off valves are not as good as hard-seal cut-off valves; full-function ultra-lightweight control valves, for example, have sealing surfaces protected by wear-resistant alloys, offering high reliability and low leakage rates, thus meeting the requirements of cut-off valves. IX. Why is selection more important than calculation? Compared to calculation, selection is far more important and complex, as calculation is merely a simple formula-based process; what matters here is not the accuracy of the formula itself, but rather whether the given process parameters are accurate. There are many aspects to consider in design; even the slightest mistake can lead to inappropriate selection, resulting not only in waste of human, material, and financial resources but also in suboptimal performance, with various usage issues arising such as reliability, lifespan, and operational quality. X. Why are piston actuators being used more and more in pneumatic valves? For pneumatic valves, the piston actuator can make full use of the air supply pressure, resulting in a smaller size compared to diaphragm types and greater thrust; moreover, the O-rings in the piston are more reliable than those in diaphragm actuators, which is why they are being used more and more often. 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