Switch cut-off valve from automatic to manual operation
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This post was last edited by Er Dongwen Dake on 2017-9-13 at 11:32. The hydrogenation unit has several isolation valves and control valves, all of which are equipped with handwheels; however, the handwheels are hardly ever used (the operation is always automatic, with no switch to manual mode). Now many newcomers don’t know how to switch from automatic to manual mode. Could everyone share some information on the automatic and manual switching operations for common control valves and shut-off valves in such systems? It would be great if control pneumatic circuit diagrams could be included as well. Thank you all. I usually go to the installation site to check the instruments as well; it’s a good opportunity for everyone to exchange ideas. Below are the pictures of a stripping tower bottom cut-off valve that I am currently shooting.
Most of these handwheels are not needed, as control valves come equipped with auxiliary lines; when a control valve fails, manual valves are used for adjustment. As for the shut-off valves, they are basically interlock valves. These interlock valves remain inactive under normal conditions; once they operate, it triggers an interlock mechanism, which means the system stops operating. In emergency situations, if the shut-off valve fails, the process control system does not try to use the handwheel to fix the issue – instead, people go to the site to close the manual valves on the pipelines. In short, the setting of this handwheel is essentially pointless; in order to ensure that the on-site instrumentation meets the design requirements for safety, valve manufacturers implement various safety measures depending on the safety conditions on site, such as remote control, local control, electric control, pneumatic control, manual control, and so on. . . Otherwise, this valve does not meet the specifications and cannot be sold. Of course, this handwheel isn’t useless either; at least when dealing with faults in these valves, it can be used to turn them, thereby helping to remove any stuck valve cores or valve stems as much as possible. As for the operation by process engineers, this is rare in plants of this type, because in such plants it is the responsibility of instrument technicians to handle these valves; hence process engineers do not get involved. The operation of the valve actuators and the use of hand wheels are also carried out by instrument technicians. Of course, situations vary from plant to plant. To prevent accidental operation, these handwheels are generally equipped with locking mechanisms such as limit pins, switching knobs, and changeover levers.
What Brother Sikong said upstairs is quite comprehensive and accurate; indeed, in reality, the actual use of hand wheels is practically non-existent.
Yes, although valve failure falls within the scope of instrument operation. It is important to understand how to switch valves from automatic to manual mode, and vice versa. The purpose of this is to prevent any issues with a valve from going unaddressed in a timely manner (either in terms of opening or closing it); otherwise, the arrival of a technician could result in delays in handling the issue, leading to losses in the production process. As an operator, it is also useful to have a basic understanding of various valves, especially the steps for switching cut-off valves between automatic and manual mode.
The interlocked isolation valve does not have a bypass line, and manual mechanisms are not permitted per the specifications. It is designed so that production is not affected by valve failures.