Are there any design specifications that specify the control accuracy of control valves?
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Are there any design specifications that specify the control accuracy of control valves? It seems that SH and HG do not provide any information regarding the regulation accuracy of the control valves, but self-acting control valves specify a range of 10% to 20%. In conventional engineering designs, the data sheets for control valves also do not indicate the regulation accuracy. Why is that?
The self-acting control valve specifies 10%~20% – which standard does this come from?
SH3005 Page 31
I just checked, and our company’s selection table for control valves doesn’t specify accuracy
The factory precision of our control valves is generally set at 1.5%, and this 1.5% is based on the stroke, as determined using a micrometer. In fact, this precision is related to the positioner as well as to the manufacturing of the valve. Since the Cv value of a control valve varies at different opening degrees, according to ANSI/ISA 75.11.01, generally speaking, a Cv value within ±10% of the value at a particular position is considered acceptable. For example, if the theoretical Cv value at 50% opening is 100, then an actual Cv value ranging from 90 to 110 is considered acceptable. From this perspective, discussing the precision of control valves doesn’t make much sense. If users need specific Cv value curves, calibration tests can be carried out, for which an additional fee must be paid to the manufacturer; this is how many critical valves in nuclear power plants are handled. Furthermore, if the measured Cv value curve matches well with the theoretical Cv value curve provided by the manufacturer, then it is definitely an excellent control valve manufacturer.
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SH/T 3521-2013 Technical Specifications for the Construction of Petrochemical Instrumentation Projects > 5 Calibration and Adjustment of Instruments > 5.8.8 The sensitivity test of control valves can be carried out using a dial indicator; the pressure applied to the diaphragm chamber is set at 10%, 50%, and 90% of the output/spring operating range, with the valve position remaining at the corresponding positions throughout these levels. By increasing or decreasing the signal pressure, the pressure change at which the valve stem begins to move is determined – this value shall not exceed 1.5% of the signal range, while for control valves equipped with valve positioners, this value shall not exceed 0.3%. The required range for this specification is 0.3%-1.5%, and deviations of more than 2% in actual operations are quite possible. It is recommended to install valve position feedback for most critical control valves, so that the control room can monitor the deviation values in real time and carry out predictive maintenance to prevent issues from arising.
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