Valve characteristics


Once the chemical process engineer performs heat and material balance (hand calculation, spreadsheet, process simulation software etc), results must be shared with other disciplines in order that engineering documentation for plant design can be done.

As a chemical process engineer in plant design, you may be responsible for the process datasheet for control valves only. However, the process datasheet alone is not enough for the purchase of the correct control valve for the project. A specification datasheet is needed and filled by a chemical process engineer or instrumentation engineer depending on Company´s organization.

A control valve specification datasheet will not be completed without the information on required flow characteristics chosen by the process/instrumentation engineer.

The flow characteristics of the control valve is the relationship between the flow rate through the valve and the valve travel as the travel is varied from 0 to 100%.

To choose the correct control valve flow characteristics, ideally, control valve inherent flow characteristics would be one that would result in a linear installed characteristic and a uniform installed gain. Although a dynamic analysis of the control loop is recommended for selection, there are some rules of thumb that can be used for many applications.

The quick-opening flow characteristic provides for maximum change in flow rate at low valve travel with a fairly linear relationship. The valve is often used for on/off applications because the full Cv is achieved with a short stroke.

In the linear, the relationship between opening and flow is almost a straight line. A given change in opening results in an equally large
change in the flow. The linear-valve plug is commonly specified for liquid level control and for certain flow control applications requiring constant gain.

In the equal-percentage flow characteristic, equal increments of valve travel produce equal-percentage changes in the existing flow. Valves with an equal-percentage flow characteristic are generally used for pressure control applications.

Where detailed process knowledge is lacking, as a rule of thumb, use equal-percentage characteristics at 70% opening for the valve sizing.

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