What is the difference between single control and shared control?
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In HG-20505, the annotations for the graphic symbols of instrument equipment and functions are as follows (partial): I would like to ask what it means to add a horizontal line inside the circle shown above? This style of drawing is rarely seen in P&IDs.
It generally refers to field instruments, such as PG
It’s clearly written on the right: those without a line represent the site, while those with a line represent the control room
In the earlier standards, it was explicitly specified as “a single instrument,” while the current standards extend this to “a single instrument or function.” A single instrument refers to a physical instrument that actually exists; one enclosed in a frame indicates a virtual instrument or function. Since DCS/SIS are now widely used for process control and interlock functions, it is the virtual \"instruments\" within DCS/SIS that carry out various functions. Some smaller projects may still use conventional instruments, so the standards also include provisions for such instruments. By sharing is meant processing multiple signals to achieve certain functions, or the ability to reuse such functions across multiple instances. In other words, multiple parameters or functions are shared.
This post was last edited by Elder Gàngzǐmó on 2021-3-24 at 12:46. Thank you, expert. So this symbol refers to instruments with display functions that are installed in the control room, and signals are transmitted through on-site transmitters and similar devices, is that correct?
A circle without an outer frame represents a conventional instrument; if there is a thin solid line in the middle, it indicates that it is installed on the dashboard of the control room (visible). On-site transmitter signal transmission indicates that it is one transmitter. If installed on-site (such as pressure transmitters, flow transmitters, level transmitters), there are no lines inside the circle. If installed in the control room (such as a temperature transmitter installed inside the control panel), the circle is dashed, indicating that it is installed behind the dashboard (invisible).
I still don’t quite understand. Could you please explain again? For example, in the case of a reactor, how is temperature measurement carried out – is the instrument installed on the dashboard in the control room, or shouldn’t the sensing element be located on-site?
For the temperature display in the reaction kettle, if it is to be shown in the control room, detection elements (temperature sensors such as thermocouples) are installed on-site, and the signals are sent to the control room, where display instruments (such as automatic potentiometers) are installed. The graphic of the on-site detection element is a circle without a solid line in the center, while the instrument display inside the controller shows a circle with a solid line in the middle. If a temperature transmitter is used, there are three scenarios: 1. Integrated temperature transmitter, installed on-site, without a solid line circle in between ； 2. On-site temperature sensing element, on-site temperature transmitter; two circles without solid lines in between ； 3. On-site temperature sensing element, temperature transmitter in the control room; one with a circle without a solid line in the middle, and one with a circle containing a dashed line in the middle. If DCS is used, the on-site part remains the same; the indications in the control room are virtual instruments within the DCS, represented by circles with a box around them and a thin solid line in the middle.