The issue of selecting flow ratio
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There are two types of media, and each type has three flow meters. The process requirements dictate that an interlock shutdown should occur when the flow ratio of these two types of media changes. All three flow meters are analog; how should this be handled during DCS configuration? 1. Take the middle value of each medium for comparison. 2. Is it to take the average of the three values for each medium for comparison? If an average is taken, it seems to me that any issue with a single flow meter could lead to a shutdown. 3. Three ratios are calculated on a one-to-one basis, and then a two-out-of-three interlock is applied. But how should the one-to-one correspondence be established? Moreover, in a one-to-one correspondence, if there is a problem with one of the flow meters, the ratio will exceed the allowed limits, and this will be considered an interlock condition, which in turn makes things less safe.
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Two-out-of-three voting: in the event that one path fails, immediate action is taken to address the issue with the instruments on site. The purpose of this two-out-of-three system is to ensure safety at the site while also improving operability
First, select three values and then compute the ratio before participating in chaining; any other value is unsafe as it can lead to incorrect chaining.
This post was last edited by liu530014416 on 2012-11-1 21:04: How much for 6 flow meters! 4 are enough. This interlock may not be activated after startup.
6 flow meters, how much is that! 4 are enough.
The approach is as follows: 1. First, perform a three-out-of-two voting for analog signals using three flowmeters in the same medium. (There are several ways to handle the three-zone two-level control for analog signals: using the minimum value, the median value, or the maximum value, etc.; you can check previous posts for more details.) 2. Ratio control is applied to the two-out-of-three results of the two media. Additionally, regarding whether you should use 6 flow meters, I think you can calculate the SIL level, because as far as I understand it, the purpose of using a three-out-of-two configuration is also to increase the SIL level of the circuit. Based on the calculation results, determine whether 6 flow meters are truly necessary. For reference!
I think this type of ratio-based interlock shutdown is not feasible; for example, when the flow rates of both media are very low, but the ratio remains normal, no shutdown will occur. Would it be better if we added the condition of minimum flow?