Issues with instrument selection
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Why is a level switch needed? If a level gauge is used to create interlocks in the DCS, can’t that achieve the same effect as a level switch? Is it because it’s easier to integrate level switches and similar devices into an SIS system?
It depends on the importance of the switch interlock.
In SIS, transmitters are used in place of switches for interlocking, while switches are still widely used in DCS. After all, installation and maintenance are relatively easy; certain types of switches do not require consideration of equipment openings and are not sensitive to operating conditions.
So, a chemical dosing pump is equipped with a differential pressure transmitter that serves as a level gauge, and there’s also a level switch for added safety
Generally, it is the level gauge that provides indication, while the level switch is used for alarm purposes. It is also possible that, from a safety perspective, the level switch is used for interlocking.
There is no strict distinction; either can be used. The only thing is that devices with interlock and control functions need to be separated – in other words, a device equipped with a control valve should not also have an interlock function
Generally, switches are cheaper than analog sensors; installing a switch in places where continuous level monitoring is not necessary not only saves costs but also ensures reliability.
The signal from a level gauge is usually a continuous analog value; in a DCS, converting it into a digital signal corresponding to a certain liquid level requires calculations and conversions before it can be used. Although this eliminates the need for hardware dedicated to level switching, the software processing involved, along with the execution of that digital signal, still requires a series of hardware components. By using a level switch directly, a range of data conversion operations and the hardware needed to handle digital signal processing are eliminated, as level switches have a certain capacity to handle loads. There are simple and reliable options available; they’re better than complicated choices.