Regarding the installation of shut-off valves upstream and downstream of the control valve without providing a bypass
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Hello, colleagues. Recently I came across a process package designed by foreigners, and I noticed that isolation valves were installed upstream and downstream of the control valve, but no bypass was provided. For example, in a atmospheric-pressure storage tank that uses nitrogen sealing lines and segmented control, control valves are installed on both the nitrogen inlet and outlet pipelines, with shut-off valves placed upstream and downstream of these control valves. However, there is no bypass pipeline. During my studies, I was taught that shut-off valves should be installed before and after control valves, along with a bypass pipeline, in case the control valves fail. But in this case, shut-off valves are present yet no bypass pipeline is available – what is the purpose of such an arrangement?
Perhaps the pipelines can be disconnected during the production process without affecting it, or there is maintenance time when the pipelines are used intermittently. The knowledge in books is universal; in practice, specific issues need to be analyzed on a case-by-case basis. For example, in pipelines for materials that tend to crystallize, no bypass is provided ； If a bypass is installed, crystallization occurs during normal production, and the bypass thus serves no purpose.
It is recommended to consider its necessity from the perspective of the entire system process; this is not absolute. For the top-blown converters in steel mills, bypasses are not required for supplying oxygen and nitrogen.
One of our technical directors once said that it’s not necessary to set up a bypass; people with extensive experience in factory work know this
For the staged control of nitrogen seal pipelines, a bypass cannot be installed. If the nitrogen seal valve fails, once the bypass is activated (or if there is a leak in the bypass), it may cause the storage tank to remain under excessive pressure, posing a safety risk ； On the other hand, a bypass is usually installed so that it can be activated when the control valve needs repair, thereby ensuring continuous production. If the nitrogen seal valve of the storage tank needs repair, it can still function normally even in the absence of a bypass; therefore, it is safer not to install a bypass.
One type is, as they say, easy to crystallize; using a bypass medium will prevent normal operation. The other type has strict requirements regarding flow rate – if the control valve fails, the process stops, and it’s not allowed to continue production by using a bypass without controlling the flow rate
Not all control valves need to be equipped with bypasses； It is primarily determined by a comprehensive consideration of on-site safety and process conditions, as well as cost, of course. Therefore, if a quick repair does not affect site safety and the operational conditions, there is no need to install it. Thank you!
It might be a matter of standards, but I think foreign equipment is more reliable.
You just need to consider that for process pipelines and piping production, it’s necessary to determine whether production can proceed without bypasses. If it’s possible, then bypasses aren’t needed, as many isolation valves are operated by being fully open or fully closed during normal production; if they need to be closed, the production process must be stopped