Issue with the instrument installation method
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Question: The installation types for instruments are local (L), centralized (P), and local-centralized (LP). What do these terms mean respectively?
I understand: local means display or control at the site itself, centralized means that all displays or controls are sent to a central location (such as a control room) for display or control, and local-centralized is a combination of the two. Going to the automatic control section might help; perhaps some experts there can give you an accurate answer.
I have worked with control systems and operated DCS; my understanding on this is similar to that of the person above, but I am hoping for a clearer and more professional explanation. And how can these differences be represented on a PID diagram?
Based on their installation location, instruments can be divided into local instruments and instruments installed in the control room. Local instruments are those installed at the site, including instruments mounted on the local panel. Indoor instruments can be further divided into panel-mounted instruments and rack-mounted instruments. Panel-mounted instruments are those installed in front of the instrument panel, while rack-mounted instruments are those installed on the frame behind the instrument panel.
Based on their installation location, instruments can be classified as locally installed instruments, instruments installed on centralized dashboards, instruments installed on local dashboards, as well as instruments installed behind centralized dashboards or behind local dashboards. For details, refer to the instrument installation graphic symbols.
The installation methods for instruments include on-site installation, which refers to the primary instruments located at the site itself, such as bimetallic thermometers, pressure gauges, glass plate level gauges, or self-acting control valves, metal rotor flow meters, elliptical gear flow meters, etc. The on-site centralized installation you mentioned refers to a situation where the primary instruments at the site perform measurements while the data is sent to a DCS system for control; examples of such instruments include orifice flow meters, mass flow meters, pressure and level transmitters, thermocouples, thermal resistors, etc
On the PID diagram: 1. A circle indicates an instrument installed on-site; 2. A circle with a solid line inside it indicates an instrument installed in front of the control room dashboard; 3. A circle with a dashed line inside it indicates an instrument installed behind the control room dashboard; 4. A circle with two solid lines inside it indicates an instrument installed in front of the on-site dashboard; 5. A circle with two dashed lines inside it indicates an instrument installed behind the on-site dashboard.
\"Local\" (L) refers to on-site display, such as using pressure gauges or bimetallic thermometers at the location itself. \"Centralized\" (P) means that the data is transmitted via signal lines to the instrument panels in the central control room or to the DCS, where process parameter changes can be monitored through secondary instruments or the DCS operation station. \"Local centralized\" (LP) involves transmitting the data to instrument panels or cabinets located near a certain device on site or in a location where it is easy to monitor the process parameters
I believe that \"on-site\" refers to local indicator instruments such as pressure gauges and bimetallic thermometers; \"centralized\" refers to instrument types that are used for remote transmission so that data can be processed centrally in controllers, as well as for control interlocks; while \"on-site centralized\" refers to systems that provide on-site display, enable recording, allow remote transmission, and facilitate centralized data processing. For now, we are still using the instruments on-site; the second type of instrument has been phased out, while the third type is widely used, such as transmitters and level gauges. Generally, we employ on-site instruments in combination with remote-transmission instruments in the plant area