The meaning and differences of HS in instrument flow diagrams
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This post was last edited by lcl3423 on 2022-3-19 at 15:41. What is the difference between HS in the green circle in Image-01 and the function in Image-02, and what do they each represent?
According to HG/T 20505 - 2014, the initial letter H denotes manual (remote) operation, while the subsequent letter S indicates the output function, which refers to the switch. The circle with a box represents the basic process control system (mostly DCS), and the thin solid line in the middle indicates visibility. In other words, a switch signal is manually sent via DCS.
May I ask, in this HS, one is connected to the interlock symbol I while the other is connected to the motor – what are the differences in their meaning and functions?
Along with interlocking, there is also a need to implement the interlocking logic; the other one is the signal that directly controls the start and stop of the pump
Connected to the interlock symbol I indicates operation with other logical signals to jointly generate a control logic I； Being connected to the motor means direct control of the motor.
It is recommended to take a look at the logic diagrams for the two circuits; once you do that, it will be clear what HS stands for and what the differences are. I don’t understand why some people insist on trying to figure out the logic by relying solely on PID values, guessing based on those symbols. Wouldn’t it be better to simply examine the logic diagrams directly?
HS being connected to the interlock refers to the double contacts of the emergency interlock button on the control panel, which are connected to the SIS； What should be connected to the HS and the motor is the on-site manual operation switch.
A circle with a box indicates that it is within the DCS, not at the field site.
What is the difference between a diamond frame with a square and a circle with a square symbol?
A diamond frame with a square indicates the safety interlock functions in the central control room, while a circle with a square symbol represents the BPCS functions in the central control room. Generally, the safety interlock function is a SIS function, while the BPCS function is a DCS function.