Control valve, automatic, manual, cascade
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Under what circumstances should an adjustable valve be selected for automatic, manual, or cascade control (criteria for selection)? What are the advantages and disadvantages of each of these three adjustment methods?
Automatic adjustment is used when the system is operating stably, with a small adjustment range; this makes operation much easier for the staff. Manual adjustment is generally employed in case of system failures, or when starting up or shutting down the system – in such situations, the system switches from automatic to manual mode. Of course, some self-regulating valves cannot be operated automatically and can only be controlled manually. As for cascade control, it is used relatively rarely. For example, when water supply to a boiler is functioning properly, the valve opening and the liquid level can be controlled manually or automatically. If cascade control is to be employed, a flow controller is added to regulate the boiler’s liquid level; generally, manual or automatic control is sufficient, so cascade control is not often used
There are several more examples of cascade control: 1. One flow rate changes proportionally to another flow rate (ratio control). 2. Furnace temperature: There is a control valve on the fuel gas used to regulate the furnace temperature, but it adjusts only the fuel gas flow without adjusting the air supply. In some cases, the fuel gas flow may be set too high; in such situations, the excess fuel gas acts as a cooling medium. In these cases, a cascade control system in which the air flow rate changes according to the fuel gas flow rate can be employed. 3. Online sulfur analysis of the H2S/SO2 ratio (with an ideal value of 2:1) is used to feedback-control the air supply volume ahead.
Both automatic and manual transmissions need to be used in combination!