Control valve inspection test – is it effective?

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Adjustable valve inspection test 1: The adjustable valve is one of the most widely used instruments in the petrochemical industry; it is installed on process pipelines. The control valve responds to external input signals and moves its valve stem to the corresponding position in proportion to those signals. By changing the gap between the valve core and the valve seat, it achieves flow control, thereby regulating parameters such as pressure, temperature, and liquid level in the system. In accordance with the **Standard of the People’s Republic of China ‘Code for Construction and Acceptance of Automated Instrumentation Projects’ GB50093-2002**, Article 11.1.1 stipulates that \"before installation and use, instruments shall be inspected, calibrated, and tested to ensure that they meet the requirements specified in the design documents as well as the technical specifications outlined in the product’s technical documentation.\" Article 11.1.8 states that \"the conditions, items, and methods for calibrating and testing instruments shall comply with the provisions of the product’s technical documentation and the requirements of the design documents.\" The performance of control valves is directly related to the proper progress of plant commissioning and production; testing control valves is an important method for evaluating their performance parameters. Sometimes, different testing conclusions arise due to ambiguous parameters or inconsistent standards. The procedures, methods, and contents of the inspection should comply with the provisions of relevant standards; for items not specified in the standards, extensions should be made depending on the type of valve. 2 Comparison of inspection items between domestic and international standards As the introduction of equipment and technologies accelerates, some commonly used technical parameters in China can sometimes lead to confusion with those abroad. The specifications define the inspection items for regulating valves, including pressure testing of the valve body, seat sealing test, leakage check of the diaphragm head (cylinder), as well as stroke and full-stroke time. 2.1 Valve body pressure test The valve body pressure test is used to evaluate the pressure resistance of the valve body, including checking for any sand holes in the cast structure, ensuring that the mechanical connections are secure, and detecting any deformation. The test is carried out by specialized departments using dedicated equipment. The medium used for the test is clean water; the pressure is increased to 1.5 times the nominal pressure with the valves fully open, and a pass result is achieved if there are no visible leaks within the specified time. 2.2 Seat Sealing Test The seat sealing test is conducted to check the tightness between the valve seat and the valve stem. The structural design of the control valve determines the sealing level between its valve core and seat. Seal inspection involves checking the leakage rate at the valve seat when the control valve is completely closed; in other words, under specified experimental conditions, a test fluid is passed through a valve that is in a closed state to measure the amount of leakage ； The medium used is selected according to the test procedure; generally, clean water is used, while air is used for the shut-off valve. Table 1: List of typical strength test pressures for valves used domestically and internationally. Domestic sectors vs. International sectors: Nominal pressure (MPa), Test pressure (MPa), Test time (min). Nominal pressure (MPa), Test pressure (MPa), Test time (min): PG1.6 – 2.5 – 3150 – 210; PG4.0 – 6.0 – 3300 – 510; PG6.4 – 10 – 3600 – 1010. To understand the method for calculating leakage in control valves, it is necessary to be clear about the flow coefficient, as well as the differences in its definition and representation both domestically and internationally, and the relationship between them. In China, the symbols for the flow coefficient of control valves are C and Kv; these are used to express this value in either the engineering unit system (MKS) or the International System of Units (SI). Specifically, with the control valve fully open: C represents the volume in cubic meters of water at a temperature of 5–40°C that flows through the valve within 1 hour under a pressure drop of 1 kg/cm2. Kv—The volume in m3 of water at a temperature of 5~40°C that flows through the control valve within 1 hour under a pressure drop of 105 Pa. Abroad, imperial units are generally used, with the symbol Cv being employed; again, under conditions of the valve being fully open, Cv represents the number of US gallons per minute that flow through the control valve for water at a temperature of 60°F (15.61°C) under a pressure drop of 1 lb/in2 (7 kPa). The relationship among the three is: Cv=1.17C and Kv=1.01C. Additionally, Fisher uses Cg and Cs to represent the flow coefficients for gases and vapors, respectively. A comparison of the leakage classification standards for control valves as specified in the petrochemical industry standard SH3521-1999 and the American standard ANSI B16.104-1976 is shown in Table 2: Comparison of Leakage Classification Standards for Control Valves. Leakage level, test medium, test procedure, maximum leakage rate at the valve seat (L/h): SH3521-1999, ANSI B16.104; SH3521-1999, ANSI B16.104; SH3521-1999, ANSI B16.104; SH3521-1999, ANSI B16.104. Class II: water or gas, water or air; AA: 0.5% C, 0.5% Cv. Class III: water or gas, water or air; BA: 0.1% C, 0.1% Cv. Class IV: water or gas, water or air; AA: 0.01% C, 0.01% Cv. Class V: water; BB: 1.8×10-7×D×ΔP (where D is the valve seat diameter in mm, and ΔP is the pressure difference across the valve in kPa); 5×10-12 m3/min. Class VI: gas, air or nitrogen; AC: see table below. Here, C and Cv are calculated based on the \"nominal capacity\"; they differ from the flow coefficients and capacity values discussed earlier. They represent the flow rate when the control valve is fully open, under a certain pressure difference across it, and this flow rate is not proportional to that pressure difference. The simplified formula for calculating the leakage rate of control valves is as follows: (1) When the test medium is room temperature water, Q = Cv × (∆p)¹/² × 1.4245 × allowable leakage rate. (2) When the test medium is air, if ∆P ≥ P₁/₂, then Q = Cv × P₁ × 2.424456 × allowable leakage rate; otherwise, Q = Cv × ∆P × (P₁ + P₂)¹/² × 2.79445 × allowable leakage rate. (3) When the test medium is nitrogen, if ∆P ≥ P₁/₂, then Q = Cv × P₁ × 2.446166 × allowable leakage rate; otherwise,