Discussion and inquiries regarding logic interlock diagrams
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There are two questions related to logical interlocking; I’m hoping experts can offer some guidance! Logical description: There is an interlock between a shut-off valve EV101 and a fan C101. Requirements: 1: If the shut-off valve EV101 is closed, then the fan C101 also must be closed. Similarly, if fan C101 is turned off, then cut-off valve EV101 also closes. 2: If the shut-off valve EV101 is opened, then fan C101 also turns on. Similarly, if fan C101 is turned on, then cut-off valve EV101 also turns on. How should this logic interlock diagram be drawn? There are two pumps, P101A and P101B, which transport the same fluid through the same pipeline; these pumps do not have on-site control handles, but rather a remote control cabinet. Requirement: 1: Pumps A and B can be started simultaneously. 2: One of the two pumps must always be in operation; for example, if pump B stops, then pump A must continue to run and cannot stop. Conversely, if pump A stops, pump B must run. Logic Diagram.JPG (576.05 KB) 2009-3-2 13:11 Is this logic diagram correct? What is the logic behind that red line? It might not be appropriate to post this in this forum, but I’m posting it here for everyone to take a look and see if anyone can offer some guidance.
1. For the first question, it’s just a simple linkage between two outcomes; the interlock conditions can be used to control these two outcomes simultaneously； One thing to consider is whether the fan and cut-off valve have return signals; if so, these can be used to establish interlock conditions. 2. There seems to be something wrong with your diagram; the interlock mechanism for backing up between the two pumps is a common type of interlock that’s easy to identify. Normally, there should be a condition under which one pump starts, such as when pressure is low – but this isn’t apparent from your diagram.
First question: The cut-off valve and the fan are controlled by the same signal source, that is, one signal controls both devices simultaneously. Turn on and off together. Think of these two as a single device. Second question: Logically speaking, there should be no problem. There are two conditions for starting pump A; meeting either one will allow it to start: 1. Pump B is stopped while pump A is in automatic mode; 2. The start button for pump A is pressed. The start button is a normally open contact, while the stop button is a normally closed contact. (Not indicated in the figure) Both operations generate pulse signals. (Not indicated in the diagram) Then there’s the issue with the red wire; haha, the function of the red wire is for self-retention. After the pulse reset is initiated, maintain the continuation of the operation command. The P101A conductor operates when connected and stops when disconnected. This post was last edited by chmyh on 2009-3-3 10:12]
The two problems have no pressure or temperature as conditions for starting/stopping
1: The control of valves and fans is quite simple; an intermediate relay is used to control the contactors for each valve and fan respectively. 2: In the case of DCS control, a software element is used to control the contactors for pumps A/B, and the coils of A and B have an interlock function. According to the scanning sequence of the program, pump A will start automatically while pump B will be locked out; if pump A fails, pump B will start automatically
Could you upload the ladder diagram? That would make the discussion more intuitive