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<\/p>\nIt\u2019s very common to have a control valve as the final control element in industrial process plants. Depending on its function, the valve must control flow, temperature, pressure, level, or concentration.<\/p>\n
Considering that the control loop (composed of the measurement element, the control valve, and the controller) must control the process, this means that the process operates stably and it must behave according to the design, the control valve it shall be designed so that it can work within the entire operating range requested by the process. To do this, the process engineer must provide the operating limit values (min, normal and maximum) where the control valve shall be designed through the process datasheet.<\/p>\n
To determine the limit design data the valve sizing coefficient, that was introduced by Masoneilan in 1944, is commonly used and is defined considering the following formula:<\/p>\n
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Where the dimensioning coefficient (Cv) is a function of the flow rate (Q), the specific gravity (SG), and the valve pressure drop DP.<\/p>\n
Analyzing the previous equation to determine the minimum valve Cv (which will correspond approximately to 10% of its opening), the matching<\/u> process data should be used when: the flow is minimum, the lowest value of the specific gravity (normally at the highest coincident temperature) for the operation case at minimum flow rate and the highest pressure drop:<\/p>\n
Cv min<\/u>:<\/p>\n If there were other operating cases that could be evaluated, with the lowest specific gravity value in the process (with its coincident flow rate and pressure drop) it should also be evaluated for CV min.<\/p>\n Process variables are coincident when there is a possibility that these variables may be present simultaneously within an operating case. For example: 22 m3<\/sup>\/h of water is pumped at 5\u00ba C at 5 bar and a density of 1000 kg\/m3<\/sup>. The process data in the matching pump impulsion would be:<\/p>\n For the valve Cv maximum calculation (which will correspond approximately to 85% of its opening) it will be applied the same procedure, the matching<\/u> process data should be used when: the flow is maximum, the highest value of the specific gravity (normally at the lowest coincident temperature) for the operation case at maximum flow rate and the lowest pressure drop:<\/p>\n Cv max<\/u>:<\/p>\n\n
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