Is the linear control valve used for level setpoint regulation as specified in the HG standards obsolete?
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Recently, I reviewed the design documents for two foreign-funded projects, both carried out by large German companies. The more than 300 Globe control valves (of German brand) were all set to equal percentage values, including those used for level control. For this purpose, I specifically asked the Chinese staff from 2 valve suppliers; although they weren’t very familiar with the technical aspects, they both said they were unaware of these regulatory requirements in China. Moreover, all the globe valves they had manufactured before used equal percentage settings, including for the liquid level. But they don’t know why either, as the specific selection was done by the technical staff in Germany.
The selection of flow characteristics and process requirements affects the S value of the system; this value is not fixed. The smaller the S value, the more difficult it is to choose a control valve, and there will be more issues related to its use. Using a percentage-based approach can usually reduce the difficulties during the initial stage of system tuning.
At low opening degrees, equal percentage characteristic control valves make the control system more stable.
Equal percentage small valve positions can provide effective control……
Table 11.2.2 provides options for selecting control valve characteristics. One prerequisite is that if the linear valves chosen according to this table do not meet the requirements regarding the minimum and maximum Cv values, it is necessary to choose equal percentage valves instead of following this table.