I have questions regarding control and connectivity; could some expert please explain?
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This post was last edited by lnikuaix on 2022-9-16 at 11:29. I would like to ask what constitutes control and what constitutes interlocking in a process I have recently been working on a project that involves a new liquid ring vacuum pump. It is necessary to add an instrument circuit in the inlet pipeline of the liquid ring vacuum pump, one that monitors pressure levels in order to control the start and stop of the pump – the pump should start when the pressure exceeds a certain level and stop when it falls below that level. I would like to know whether this instrument circuit serves for control (C) or interlock (S)
In the previous flowchart, C was used, but for this project it has been determined that S is the appropriate value. What falls under control and what falls under interlocking – which ones are considered control and which ones interlocking?
Controls include PID control, distributed control, fuzzy control, simple control, cascade control, complex control, etc.; these are generally used for the controlled parameters to achieve closed-loop regulation. Interlock systems, on the other hand, trigger subsequent actions automatically once a certain condition is met, such as starting or stopping pumps or opening or closing valves.
Interlock is a prerequisite for control (it determines when control should be applied); in this case, stopping the pump when pressure exceeds a certain level is a suitable approach for interlock management. If the pump is variable-frequency, control is achieved by adjusting the frequency using pressure until the frequency reaches 0 and the pump stops.
This post was last edited by feng*aosa on 2022-9-16 at 16:38. In process flows, interlocking and control are sometimes organized according to management or equipment categories (for the sake of clarity in the diagrams), so there’s no need to worry too much about it. Generally: When a control signal is sent based on a threshold value without checking the result of the process execution, it is called interlocking ； Control in which, after a control signal is issued based on a threshold value, the control signal is changed according to another threshold value or the status of the original threshold value, is known as (on-off) control ； Adjusting the control signal continuously based on the process condition is called regulation. Sometimes, for ease of management: everything carried out by the DCS is classified as control, and interlocks are also considered part of control ； Anything performed by a SIS or PLC is classified as interlocking, and the control should also be interlocked. There are also cases where anything controlled by logic is considered a chain. .
As defined in the company’s management guidelines, process interlock refers to an instrument function that, when a certain process parameter (such as pressure, temperature, level, flow rate, etc.) exceeds the safe operating range, uses the operating status of actuators or devices to bring that parameter or the process back to a safe state. Equipment interlock refers to the instrument function that enables automatic protection of equipment from damage when parameters such as the vibration, displacement, speed, and bearing temperature of the equipment (units, pumps, motors, etc.), as well as lubricant pressure, liquid level, dry gas seal pressure, and flow rate, exceed the allowed limits. I think it should be S here, which means interlocking.
Analog signals use C, digital signals use S
Original or broad interpretation: Control: the activity of managing or exerting control over something. The act of managing or controlling something. Chivalrous interpretation: Control, also expressed as regulating; interlock. Control (C:Control) – in the past, it was more accurately described as regulating, that is, adjustments made to maintain certain parameters constant. Therefore, the continuous adjustment of parameters such as temperature, pressure, liquid level, flow rate, and physical properties is referred to as control (C:Control). Interlock: To connect together (parts of a system, for example) so that the movement or operation of individual parts affects each other. Connecting various components of a system together so that the actions of one component influence those of the others. The original purpose of interlocking is to intervene in a process when certain conditions are met; it belongs to logical control. As stated by the original poster, continuous adjustments made to maintain certain parameters constant are denoted by C, while interventions that are carried out due to certain conditions are denoted by S. It should be noted that according to the explanations in HG/T 20505-2014, S (Switch) refers to a switch – that is, when a certain condition is met, a switch signal is generated, and this signal triggers the interlock system to take action.
My personal understanding is that interlocks have only two options: either they are on or off (0 or 100). Control is used to adjust the level; generally, this range is 0-100, but it’s not possible to completely close a valve. Control serves the purpose of production, while interlocks ensure safety by allowing the valves to be closed in case of an accident, thereby reducing the impact of such accidents.