Some questions regarding the wiring of the flow switch and its control
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This is a flow switch for the circulating water in the top condenser of a simple distillation column; it forms a safety interlock with the steam from the reboiler at the bottom of the column. Since no SIS is installed here, what does it mean that the shutdown interlock is handled via the DCS? Also, what is the purpose of setting the upper and lower limits for the flow switch? What kind of switch does it control? (Check the manual.) Our flow switch is a TFS1000 thermal conductivity flow switch.
1. What does it mean to implement cut-off interlocks on the DCS? Since the SIL (Safety Integrity Level) of such interlocks is not high, it is possible to implement them on the DCS; regulations state that interlocks with a SIL of 1 can be implemented on the DCS. 2. What is the purpose of setting upper and lower limits for flow switches? Thermal conductivity flow switches operate on a principle similar to that of thermal mass flow meters, allowing them to measure fluid flow. It is also possible to set limits for high and low flow rates, such that when the flow reaches these limits, the switch output changes (from on to off or vice versa). In this diagram, the flow switch is installed on the cooling water return line of the top condenser; it is presumed that when the flow rate is low (reaching the low set value of the flow switch), the heat source at the bottom is turned off.
Isn’t a flow switch used to measure the flow rate? Isn’t what’s displayed on-site the percentage of flow rate?