Discussion on some problems of control valve procu

2022-08-10
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Discussion on several problems of control valve procurement and type selection

correctly select the end connection form of control valve and process pipeline (annular connection/convex connection/wafer type). If equipped with a pair of flanges, the valve supplier is required to provide the valve and (3) the stress diagram of Jinan assaying universal experimental machine: carry out the stress analysis according to a certain sequence Generally, gravity is analyzed first; Then the gasket between the companion flanges around the object for 1 week shall be indicated in the technical annex of the contract with the requirements, flange material, pressure grade and pipe number of the flange connecting pipe. If the wafer type control valve itself has a single flange, the screw thread of the flange connection hole should be metric. If the screw thread is British, the valve supplier should be required to provide connecting bolts

if the control valve is equipped with a solenoid valve, it is suggested that the technical parameters of the solenoid valve in the technical annex of the contract should indicate the manufacturer, brand, voltage level, explosion-proof form and material. The confidence requirements of expanding and strengthening the tensile test machine industry and the travel time requirements of the control valve for opening and closing must be firm, but it is not necessary to indicate the specific model of the solenoid valve. The specific model should be selected by the valve supplier according to the final calculation or factory test, However, it should be noted whether the supplier is allowed to use the solenoid valve as the pilot valve in the control air circuit. When the actuator of the control valve requires atmospheric volume, in order to shorten the travel time of the control valve, the solenoid valve is used as a pilot valve in combination with a pneumatic relay with large flow capacity, which can reduce the cost, but also increase the number of accessories, because the increase of air circuit links means the increase of fault points, so it is necessary to determine whether the air circuit control scheme is allowed when discussing the technical accessories

it is recommended that both diaphragm actuators and piston actuators should be equipped with air filter pressure reducer, so that the actuator can be under the pressure of the driving air source

before delivery, the accessories equipped with the control valve shall be completely connected in the manufacturer and tested to meet the requirements of the technical annexes of the contract. The supplier of control valve shall decide whether the connecting pipe is steel pipe, PVC protective sleeve or stainless steel pipe according to the application of the valve, and determine the diameter of the connecting pipe of the gas circuit according to the travel time requirements of the valve. Generally, the diameter of the connecting pipe of the gas circuit of the control valve shall not be less than 8 × 1mm, because too fine will cause the response speed of the control valve to be too slow. Of course, if the switching travel time of the control valve allows, the pipe diameter of 6 can also be used × 1mm air connection pipe

the diameter and size of the control valve should be consistent with the design documents as far as possible. If it must be changed, it should be reported to the engineers of the process pipeline, equipment and related disciplines (if the change is not allowed, the relevant professionals should discuss and decide the change plan together), so as to change the performance indicators of the corresponding plastic parts, such as heat and oxygen aging resistance, light aging resistance, scratch resistance, white mark resistance, tackiness resistance, etc

attention should be paid to the corrosion of valves. If the purchased control valve has cavitation, flash, cavitation, scouring and other problems, it should be noted whether corresponding treatment measures have been taken to avoid these problems or reduce the possibility of these phenomena as much as possible. The material of valve body, spool, cage and stay ring shall be hard steel or alloy steel, or nitriding or surface coating (such as Stellite plating). Cavitation phenomenon of control valve under various application conditions shall be absolutely avoided

the technical annex of the contract must have the requirements to confirm the overall dimension of the valve and the installation position of the positioner and other accessories, so as to determine whether there is a conflict between the installation space of the control valve, the installation orientation of the accessories and the available space of the installation position on the drawing. In case of conflict, first confirm whether the installation position of the control valve actuator or the installation position and orientation of the valve accessories can be changed. If the position of the valve cannot be changed, relevant professionals such as process, piping, equipment, electrical and so on should be notified to modify the corresponding layout and direction design as soon as possible

the Contractor shall be required to provide the material analysis report of the key components of the control valve, the control rubber tensile testing machine, the tensile testing machine used to test the tensile properties of materials, the leakage test report of many valves, the pressure test report, the formation of the test report or certificate of conformity, the air circuit connection diagram, the pressure test procedures and methods of the control valve used, and the test procedures and methods of testing the leakage of the control valve, for inspection and review

in the bidding document, the bidder shall be required to provide model selection samples of control valves (specify the specific model codes and abbreviations in the samples) and application performance. In the technical contract, the Contractor shall be required to provide the daily maintenance manual of the valve body and its accessories, and the contact address, fax and number of the accessory agent and seller

the protection grade of control valve accessories should not be lower than ip54/nema4 (the protection grade of digital valve positioner is generally ip65/nema4x), and the inlet and outlet ports of the junction box should be sealed

the application tag number of the control valve should be reflected in the nameplate of the valve, or separately indicated on a brand and fixed on the valve. Except for double flow, the direction of medium allowed by the valve shall be clearly marked on the valve body

whether to set the handwheel should be considered according to the application and criticality of the control valve

the supplier of the control valve shall be required to provide the dead zone parameters of the valve and the basic error parameters of the control valve of the power distribution/gas valve positioner. Generally, the dead zone parameter of the valve shall not be greater than 2%, and the basic error of the control valve of the power distribution/gas valve positioner shall be less than ± 10%. If the digital valve positioner is equipped, the basic error of the control valve shall be less than ± 1%

pay attention to the requirements of the control valve on the air supply pressure. Check the requirements of the control valve actuator on the instrument air pressure according to the actual situation of the device. If the instrument air pressure in the device cannot meet the requirements of the actuator, the valve supplier should replace the actuator and provide high-quality instrument air, which should at least meet the following requirements: clean, dry, free of obvious corrosive impurities, solid particles in the gas less than 0.1g/m3, and particle diameter no more than 3 μ m. Oil content is not greater than 1 × 10 (-6th power). According to statistics, the situation that the positioner cannot work normally due to unclean air source accounts for more than 60% of the failure rate of the whole positioner

the designers of general design units have selected the materials of valve body core parts (valve core, valve seat, valve stem, cage, seat ring, packing, gasket). The final valve material selection shall be consistent with the design requirements. If it is inconsistent, the material change must be confirmed

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