What does HL OVERRIDE on the PID diagram mean? How are high/low selection and override control specifically implemented?
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I am at the linear polyethylene plant and see HL OVERRIDE on the PID diagram. The specific process is as follows: the process gas needs to be condensed in condenser E-5060. There is a control valve, TV-50204, on the refrigerant inlet line; this valve is controlled by two controllers – one being the liquid level controller for the condenser and the other being the temperature controller located on the process gas outlet line from the condenser. Both controllers have H and L signals, and this control valve is of the FC type. I would like to know how it is controlled How are high/low selection and override control connected in series?
OVERRIDE control, also known as selection control, involves sending two control parameters to a high (low) selector; based on the set conditions, the high (low) selector sends the appropriate control parameter signal to the regulator, thus forming a closed-loop control system for normal operation.
I have both high selectors and low selectors here; I don’t know how to control them Could you explain it in detail?
This post was last edited by backhamm1982 on 2016-1-11 at 14:03. The high selector selects the one with higher controller parameters as the valve adjustment command.
From a process perspective, it is possible to determine whether to use high-selection or low-selection. 1. This is a kettle-type heat exchanger; there are coils inside for cooling the material, while the refrigerant, which is in a gas-liquid two-phase state, flows along the shell side. The refrigerant may be propylene or ethylene. 2. When the heat exchange load is at the normal operating level, the liquid level in the tank is sufficient to submerge the coils (submersion = cooling of the material); above this liquid level lies the space where the refrigerant evaporates, and it is only through evaporation that sufficient cooling capacity can be generated. 3. When the liquid level is below the high-alarm level, the TIC controls the inlet valve of the refrigerant (the FIC cascade control is not relevant in this case). However, when the heat exchange required by the process increases, the control valve automatically opens wider to increase the flow rate. Yet it’s possible that the increased flow rate is not fast enough to facilitate evaporation, resulting in an increase in the liquid level (the heat exchange process is much slower than the flow rate process). This reduces the available space for evaporation, failing to achieve the goal of increasing cooling capacity – a situation that only makes things worse. 4. At this point, OVERRIDE’s LIC steps in and takes control of the valve’s opening degree. Based on Analysis 3, at this point LIC must close the valve to some extent in order to restore the evaporation space and increase the cooling capacity to stabilize the process. 5. In conclusion, when the valve is of the FC type, it is a low selector (the LIC forces the valve to close) ； When the valve is in FO mode, it is a high selector (LIC also forces the valve to close). Turning it down is a requirement of the manufacturing process; it was decided by the boss. As for the low-level alarm, I don’t think it will override the valve. Because: when the heat exchange load decreases, the TIC automatically reduces the valve opening, allowing the liquid level to drop until the heat exchange tubes are exposed above the surface of the liquid; once they are exposed, no heat exchange occurs. The lower the load, the more tubes become exposed. Therefore, there is no need for the LIC to override the control system, as doing so would definitely raise the liquid level and increase the heat exchange load, which is not what the process requirements dictate. The heat exchanger design takes into account the heat transfer load based on the condition that all the heat transfer tubes are completely submerged. @jiaguoyun