What to do if there is internal leakage in the shut-off valve on the bypass of the on-site control valve
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First of all, I would like to state that this problem has not occurred in production; it is something I imagined out of thin air. Taking the factory where I work as an example: Nitrogen at 7 kg/cm2G is sent throughout the facility after its pressure is reduced to 3.5 kg/cm2G by a self-acting pressure reducing valve. There is a stop valve on the maintenance bypass of this pressure reducing valve. What concerns me is that, during normal operation, leakage from this stop valve could cause the pressure in the pipes downstream of the pressure reducing valve to rise to 7 kg/cm2G. In that case, would it be necessary to shut down the entire factory in order to replace the stop valve? Are there similar concerns regarding the shut-off valves on the bypass lines of the control valves on the process pipelines?
It’s possible. But the internal leakage can’t be that severe, right? It can’t have leaked all of a sudden, either.
Is it also possible to imagine installing two series-connected shut-off valves? Even if one of them leaks, there’s still the other one, so the probability of both leaking at the same time should be very low
I believe a slight internal leakage in the bypass valve will not affect the production operation of the subsequent stages!
Compared to other valves, globe valves present relatively higher resistance； For pipes with a small diameter, leakage can be disregarded; for pipes with a large diameter, the flow direction can be reversed, using a high-in-low-out configuration.
Since it is imaginary, a configuration with two valves in series can be used during design, as the likelihood of both valves failing at the same time is low. Additionally, on the process pipelines, there is usually only one shut-off valve in the bypass circuit of the control valve; even if internal leakage occurs, the amount of leakage will not be significant, and the opening degree of the control valve can be reduced to address it.
The design of chemical processing units should take into account the level of risk probability and the severity of potential hazards. Internal leaks are possible, but what is the likelihood of them occurring and how severe could the consequences be? It’s not possible to install double valves next to all control valves; it’s similar to buying insurance – when there is a risk, one must decide what amount of cost is acceptable in order to avoid that risk.
Any valve manufactured domestically has the potential for internal leakage; it depends on luck.
Hehe, that’s a great awareness. I think adding a few more in parallel should work.
My opinion is as follows: 1. A shut-off valve on the maintenance bypass of the pressure relief valve is sufficient; this valve is not operated frequently, so internal leakage is almost impossible to occur； 2. Under no circumstances should another shut-off valve be added; it’s unnecessary ； 3. Pressures of 7 kg/cm2G/3.5 kg/cm2G are not considered high; if higher pressures are required, an overpressure protection device (such as a safety valve or relief valve) or an alarm device can be installed.
This post was last edited by Kuzhou on 2010-2-1 at 16:14. Valves are subjected to leak tests and pressure tests before installation, so internal leakage can be avoided from the start of installation. Internal leakage can only occur during use. The bypass valves are basically not used, so the likelihood of internal leakage due to frequent operation of the valves is very low. Therefore, the safety factor regarding leaks from bypass valves is high enough, and there is no need to worry about it. On the other hand, the occurrence of leakage is a gradual process; usually, there is a small amount of internal leakage at first, which then gradually increases to larger amounts of internal leakage. At low flow rates, such flow rates are not sufficient to affect the pressures before and after the pressure regulator. When the amount of internal leakage increases enough to impact the pressure behind the valve, it becomes apparent during system operation, and there is ample time to replace the valve. Replacing the valve is a relatively simple process that generally does not cause significant delays. Moreover, if a certain process step is very sensitive to the supply and pressure of nitrogen, I believe it would be appropriate to install a buffer tank in front of that device. If a buffer tank is not necessary, it can be assumed that the device is not that sensitive to nitrogen supply, so a few minutes of delay should not be a problem. Therefore, there is no need to worry about the internal leakage issue with this valve. Furthermore, I believe it is not advisable to connect two valves in series in the bypass circuit. First, based on the above two points, there is no need to worry about internal leakage in the stop valve; such a setup would only increase costs and complicate the piping system. Moreover, what is the purpose of installing bypass valves? It is intended to ensure that the medium can pass temporarily through a bypass in the event of a failure in the control valve, thereby maintaining the process conditions and avoiding unnecessary shutdowns; however, having two valves would undoubtedly increase the complexity of operations and prolong the time required for them. And those who are involved in installation know that every additional connection or weld point creates another potential point for leakage. In summary, I believe there is no need to install two stop valves.