Adjusting valve characteristics
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In our chemical production processes, control valves must not be used as shut-off valves, and shut-off valves must not be used as blind flanges. Why is that? This is based on the characteristics of valves; are there any differences between the sealing surfaces of control valves and shut-off valves in terms of shutting off flow and regulating flow?
There is generally some leakage when a control valve is closed.
Because when a control valve is closed, there is usually still some degree of leakage.
Control valves have a minimum leakage rate, and the same is true for isolation valves. To shut off a material pipeline, either a blind flange must be used or the short section must be removed.
Why is it necessary to leave a certain amount of leakage when closing a control valve? What’s there to fear if it’s turned off completely?
It is mainly for cost reasons; the main function of a valve is regulation, and it would be too costly to design it for shut-off purposes. To reduce torque and improve control accuracy, control valves typically have a relatively small valve stem, which results in high sensitivity but also leads to poorer sealing performance; hence, there is no need to include a control function. As valves, their seals are not very reliable, so they cannot be used as blind flanges.
The general requirement for control valves is a IV-grade seal, which results in a relatively high level of leakage. In contrast, cut-off valves are required to have a VI-grade seal, which is two grades higher. Therefore, control valves cannot generally be used as cut-off valves, as the leakage rate cannot be guaranteed. It’s not that they cannot be used as substitutes for cut-off valves at all; in some applications where the sealing requirements are not very strict, control valves can be used in this capacity. Generally, cost is a factor to consider – for example, under normal operating conditions, plug-type control valves are less expensive than ball valves. As for cut-off valves, they cannot be used as blind flanges; as mentioned earlier, the sealing performance of any valve is unreliable
Control valves are only used to regulate flow rate; they are not intended as stop valves
Ultimately, it’s a matter of money. Of course, you can choose control valves of grade VI or even with O-level leakage tolerance; however, such valves are very expensive. For example, in the case of straight-stroke valves, it is necessary to increase the thrust of the actuator, and the cost rises sharply as the diameter increases. It would be better to buy gate valves of the same diameter. It’s like trying to use an F1 car on urban roads – haha
The leakage amounts of the two are different; control valves allow a certain level of leakage, so they cannot be used for shut-off purposes