Is it necessary to add a cut-off valve bypass for the control valve under this operating condition?
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This post was last edited by Rappy on 2017-4-8 09:49. Assume that a pneumatic control valve is used to regulate the flow rate of the cooling water in the reactor jacket, thereby controlling the temperature inside the reactor (for exothermic reactions). During design, we usually take into account the possibility of air supply failures; therefore, an FO mark is placed next to this control valve on the PID diagram, so that it opens fully in the event of a loss of air supply. Given that the reactor already has a high-temperature interlock to shut off feeding, and the control valve comes with a manual valve bypass, should another bypass valve be added in case the control valve fails to open, so that it can be activated by the reactor’s high-temperature interlock?
This post was last edited by Rappy on 2017-4-8 at 09:56. An additional question: when selecting a control valve, we choose between FO and FC based on the process requirements. If the medium is water, should we consider installing a bypass with safety interlocks to address potential failures of the control valve itself?
Choosing the correct FO/FC for the control valve does not resolve all safety issues. When the control valve gets stuck, it doesn’t matter if the air supply is functioning properly. In such a situation, the SIS system should activate, and the safety interlock system should have different levels of response: for example, when the temperature exceeds the limit, the cooling water flow should be increased to the maximum; when the temperature (or pressure) reaches its upper limit, the feed should be stopped, the discharge valve should be opened, and the vent valve should also be opened, etc., in order to prevent explosions.
The selection of FO\FC is based on ensuring the safety of the equipment in extreme situations that arise when external conditions change suddenly under normal operating conditions of the control valve – such as sudden power or gas outages in the production area or throughout the entire plant. Through proper design and selection, maximum safety is achieved in the event of severe explosion disasters. This design represented the maximum level of protection available prior to the existence of safety emergency shutdown systems such as SIS; of course, its safety level is much lower compared to modern SIS systems. Faults in control valves under normal operating conditions can be detected through the operator’s actions. If a problem is identified with a control valve, emergency measures can be taken by using the manual valves located on the pipeline connected to that valve; or, in some cases, the valve itself is equipped with a handwheel for operation. After that, the instrumentation maintenance staff is notified to handle the issue. In extreme situations such as sudden power or gas outages, all valves on site fail to function, and it is too late to operate the manual valves on site by operators. This is why the FOFC design requirement was introduced; of course, with the emergency shutdown system in place, safety is further enhanced.
Although production has become more automated these days and there are various chain systems in use, the safety of production still relies on human beings; therefore, the responsibilities of operators do not decrease as automation increases. In some companies in China, an overemphasis is placed on automatic control, which leads to a reduction in the number of operators on the front line under the pretext of saving labor costs. This approach ignores the safety aspects related to human factors. It is common for many enterprises to adopt a centralized control system, with control rooms located far away from the equipment areas – this practice is considered sophisticated. However, having too few assistant operators on site is certainly not a good thing.
In such cases, the appropriate material, internal structure, as well as diaphragm head or cylinder are selected based on the operating conditions. Another aspect is medium purification, which can significantly reduce the likelihood of sticking. These are just the personal opinions of a newcomer; please forgive any imperfections
After all, this kind of stuck situation represents a fault condition under abnormal conditions. If you insist on focusing on FO FC FL, please understand the relevant content more carefully. Don’t keep worrying about the valve getting stuck. If a valve gets stuck, there must be measures to deal with that situation; for example, as some people mentioned above, activating the relevant interlocks or protections.
What does it have to do with FOFC if all the control valves are faulty?
Normally, when installing control valves, a three-valve assembly is used, with a manual bypass; for emergency situations, cut-off valves are generally employed as the bypass. When using a valve with FO function, there is no need to worry about such issues under your operating conditions. FO stands for fail-open, which means that in the event of a failure – whether it’s a problem with the air supply, a power outage, or a fault with the actuator – the valve will still open (unless the valve is stuck). Thus, there is little need to concern oneself about the possibility of the valve not opening due to a failure.