How to draw interlocks in PID diagrams
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1. Figure 1 shows the PID for the sending tank; when the pressure inside the tank reaches the high alarm value, the inflation valve needs to be closed. In other words, there is a interlocking relationship between the pressure transmitter PT-5040-1, which is marked with a red line in the figure, and the valve KV5040-1E. The representation in this figure involves placing a diamond-shaped box after each of these two instruments (or valves), with the logical interlocking system number indicated inside the box (I think it should be written in two lines – letters on the top and the interlocking system number below). 2. Figure 2 shows the PID for the storage tank, with an interlocking connection established between the liquid level indicated in the figure and the feed valve. In this figure, an electric signal line is drawn directly (marked with a red line for easier identification). Can this signal line be omitted? And if the variable being measured and the control valve are not on the same PID diagram, how should they be represented? 3. Another question: In Figure 2, are the breathing valves arranged as one in use and one as a spare? The valves at the bottom are labeled CSC (sealed with a lead seal) and CSO (unsealed with a lead seal). The storage tank shown in the figure has a capacity of 1,000 m3; is it necessary to have a spare breathing valve for large storage tanks? I have seen storage tanks with a capacity of 15,000 liters that were equipped with three breathing valves, two of which were in use and one as a spare. Which specification requirement does this setting refer to?
1. Currently, different engineering design companies have varying rules regarding instrument numbering, but the symbols remain consistent: diamond symbols are used for logical interlocks, with ‘I’ or ‘IS’ usually written above the number and the actual number below it. This isn’t a big problem. 2. In Figure 2, the inlet valve is controlled via a level gauge; the signal lines cannot be removed, and it is these control schemes that PID takes into account. As for the areas on this page that are difficult to draw, broken lines can be used, with annotations later indicating which table they refer to and marking the instrument tag numbers. On another page, use arrows to indicate the table number and the page number where the table is located. 3. The diagram shows that the PSV is a safety valve. In our design, for the safety valves of the main equipment in the main process flow, one in use and one as backup are employed; some foreign processes use a single safety valve. You have three valves because the flow rate of one valve is insufficient; two valves are needed. If one valve were sufficient, no additional one would be installed. But as for the breather valve of the large storage tank, I’m not quite sure. The safety valve I’m referring to is a requirement within the process unit. There are no regulations regarding whether it should be in standby or not; or at least I haven’t seen any such regulations.
Thank you very much for your answer. I would like to ask you one more thing: in Figure 1, the pressure transmitter is used to control the valve at the gas inlet; it only controls whether the valve is open or closed (is this understanding correct?)； In Figure 2, the feed valve is controlled via a level gauge; by connecting signals in this way, it is possible to adjust the degree to which the valve opens, that is, its opening size can be varied. If only the opening or closing of the valve is controlled, can Figure 2 be drawn in a similar manner to Figure 1 (as shown in Figure 2.1 added here)? In this case, there is no need for signal wires to connect the level gauge to the inlet valve; instead, the level gauge transmits the data to the DCS, which then sends signals to control the inlet valve. In summary, it is to check whether the drawing methods for Figures 2.1 and 2.2 are both correct, and what the differences between them are.
If you hadn’t redrawn it, I wouldn’t have noticed; the original image had problems too. The cascading signal originates from PIA, and the cascading occurs after PIA. The arrows leading in a chain all point to the actuator. PIA should be PIAAS; S stands for chain. (Refer to the legend explanation in the design for this.) Typically, the PID diagram is created by the instrumentation team, who handle all aspects related to instrumentation. I’m not an expert in instrumentation, so there might be mistakes. But the drawings you made have quite obvious issues in terms of details. Valves that are typically operated in sequence use on-off valves; the terms may vary, but they all have two states: fully closed and fully open. At the top of the schematic is the drawing of the cylinder. A control valve that can be opened or closed to varying degrees, allowing for adjustment; typically, it has a certain range of adjustment possible. The top of the schematic is a semicircle. For the DCS with squares and circles, the letters on it must correspond to their functions: I stands for display, A for alarm, S for interlock, and C for control; these should match those shown in the PID diagram. In Figure 1 of yours, HH, H, L, and LL are all alarm settings. DCS and SIS interlocks are not the same system. I'm posting three pictures: one showing the schematic of the chain, one showing the schematic of the on/off valve, and one showing the schematic of the control valve. The cascade signals are measured separately, so in the first diagram, the signals from the cascade and control valves are not shown together.
I’m getting back to you – there’s an advanced mode, and in that mode there’s an option for uploading images
I’m also from a process engineering field, so I’m not very familiar with instrumentation-related topics. I’ve seen that HG/T 20505-2014 has been updated, and there are quite a few changes compared to HG/T 20505-2000. The explanations regarding the drawing methods for DCS, PLC, and SIS also differ slightly from those in the 2000 version. I generally understand the meanings of the letters and symbols used in instrumentation, but I’m not quite clear on the specific control schemes and principles involved
The national standard version can be taken a look at; it’s good to be aware of it, but I’ve never seen it. There’s not much to it when it comes to drawing instruments; you’ll understand it over time. It’s similar to learning CAD – it seems sophisticated at first when you haven’t used it, but once you use it regularly, you become proficient in it. If you really want to understand instruments, you can learn about the general methods for selecting them.
Thank you to the original poster for sharing; I’ve learned a lot from it