Help needed from a design newbie (ㄒoㄒ)

Pressure and feed interlocks; the instruments are of course on-site, while control is carried out in the control room, usually via DCS

PIAs marked with a box refer to instruments installed in the DCS control room; if they are SIS instruments, the legend used is different. Reference can be made to SH 3101-2017 – Legend for Petrochemical Process Diagrams. It appears that interlocking is involved, with interlocking between pressure and feed cut-off valves. The symbol ‘S’ indicates an alarm, while PIA stands for PIA. Interlock value: very high HH, high alarm H

1. There are two instruments installed on site here: one is a pressure transmitter PT, and the other is a shut-off valve XV (based on the symbols used, it is likely to be a pneumatic cylinder valve)； 2. PIA is a symbol enclosed in a box, indicating a pressure indication alarm that takes place within the central control system; a horizontal line in the middle shows that it is visible in the central control room (if an interlock signal is to be provided, it should be marked as PIAS). According to HG/T 20505-2014, this symbol enclosed in a box represents the function of a Basic Process Control System (BPCS); HG/T 20505-2000 (now obsolete) specified it directly as a DCS function. In short, the symbols with boxes are virtual gauges ； 3. The PIAS outputs interlock logic signals (electrical switch signals, i.e., DO) to the shut-off valve XV. If XV is a pneumatic cylinder valve, there should be a signal conversion between PIAS and XV (an electrical valve positioner or an electrical converter). Different types of signals are represented by different line styles.

There are legends explaining the process flow diagram

My experience: I haven’t checked the regulations; a PT is a pressure transmitter and it’s a gauge； XV is a automatic valve ； PIA control logic: P: Pressure ； I: Instruction ； A: Call the police ； PICAS: Guess

When looking at the PID diagram, it is necessary to consider the diagram on the home page as well; all the information is indicated there. 1. What you’re referring to here is actually a gauge. 2. The PT with a circle indicates that there is a pressure transmitter on-site; P stands for pressure, T stands for transmitter, and the circle denotes that it is installed on-site. The PIA with a circled box indicates the functions of this gauge – this pressure gauge has display and alarm functions in the DCS room. P represents the type of gauge (pressure), I represents display, and A represents alarm; the circled box indicates that this pressure gauge is located in the DCS control room. 3. PT and PIA are actually the same gauge, so there’s no need to distinguish which one is connected to the XV. To correct this: generally, if a gauge signal is connected to a control valve, then that gauge should have control functions. Therefore, in this case, PIA should be PICA, with the additional ‘C’ indicating that it has control functions.

A gauge, with different representations: PT for pressure transmitter, PIA for pressure indication and alarm in DCS

In the legend symbols, the letter C (Control) denotes control, specifically continuous adjustment. Logical control (i.e., interlocking) cannot be represented by C.

1. A remote pressure gauge and an instrument valve in an interlock circuit； A pressure sampling point on the reactor is transmitted to the DCS system via a transmitter PT ； 2. The letters inside the box represent the functions of the instrument; a horizontal line in the middle indicates that the display is available in the control room, and it is possible to set up digital displays for on-site presentation here ； 3. PT is merely a pressure transmitter; PT and PIA are considered to be the same as a remote pressure gauge. What XV receives is a pressure signal, and if there is no other signal source, it can be referred to as PV ； Additionally, PIA stands for pressure indication and alarm; it lacks upper and lower alarm limits, but has interlock signals. The instrument function should be denoted by S, that is, PIAS (it is assumed here that the square-shaped XV represents a switch valve without any regulating function)

This post was last edited by jia87314968 on 2023-1-14 08:58. We generally use PG to denote field instruments, PT for remote-controlled instruments, and PIA to represent instrument functions – where P stands for Pressure, I for Indication, C for Control, A for Alarm, S for Interlock, and Q for Accumulation. In a set of drawings, the symbols will be explained on the first page, or one can refer to the standard HGT 20505-2014 “Functional Indicators and Graphic Symbols for Process Measurement and Control Instruments”. Additionally, the function that is actually used for the instrument should be indicated accordingly; for example, if indication, alarm, and interlock functions are used, then it would be denoted as PIAS