How to choose control valve

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How to choose control valve and regulating valve

correctly select the end connection form between control valve and process pipe (annular surface connection/convex surface connection/wafer type). If a counter flange is provided, the valve supplier is required to provide the gasket between the valve and the companion flange, and the requirements, flange material, pressure grade and pipe number of the flange connecting pipe shall be indicated in the technical annex of the contract. If the wafer type control valve itself has a single flange, the screw thread of the flange connection hole shall be metric. If the screw thread is British, the valve supplier shall 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, material requirements and the travel time requirements for the opening and closing of the control valve, 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 air 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, 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 accessories of the contract. The control valve supplier shall decide whether the connecting pipe is steel pipe, PVC protective sleeve or stainless steel pipe according to the application occasions of the valve, and determine the diameter of the gas circuit connecting pipe according to the travel time requirements of the valve. The pipe diameter of the gas circuit connecting pipe of the general 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 circuit connecting pipe

the diameter and size of the control valve shall be consistent with the design documents as far as possible. If it must be changed, it shall be reported to the engineers of the process pipeline, equipment and related disciplines (if the change is not allowed, the relevant professionals shall discuss and decide the change scheme together) to carry out the corresponding design change

attention shall be paid to the corrosion of valves. If the purchased control valve has cavitation, flash, cavitation, scouring and other problems, it shall 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, valve core, valve cage and stay ring shall be hard steel or alloy steel, or nitriding or surface coating (such as Stellite plating). Cavitation 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 any conflict between the installation space of the control valve, the installation orientation of accessories and the available space of the installation position on the drawing. In case of any conflict, first confirm whether the installation position of the control valve actuator or the installation position and position of the valve accessories can be changed. If the position of the valve cannot be changed, inform the relevant professionals of process, piping, equipment, electrical and so on 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 valve leakage test report, the pressure test report, the test report or certificate of conformity, the gas circuit connection diagram, the control valve pressure test procedures and methods used, and the test procedures and methods for testing the control valve leakage 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 shall 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 shall be sealed

the applied shear test of control valve is divided into single shear test and two-way shear test. The tag number shall 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 hand wheel shall 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. Therefore, the measurement and control system of the friction and wear testing machine has developed rapidly. Generally, the dead zone parameter of the valve should not be greater than 2%, and the basic error of the control valve of the power distribution/gas valve positioner should be less than ± 10%. If the digital valve positioner is equipped, the basic error of the control valve should be less than ± 1%

pay attention to the requirements of the control valve on the air supply pressure. Check the requirements of control valve actuator for 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 shall replace the actuator and provide high-quality instrument air, which shall 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 not greater than 1 × 10 (-6th power). According to statistics, the failure of positioner 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, valve cage, seat ring, packing and gasket). The final valve material selection shall be consistent with the design requirements. If it is inconsistent, it must be confirmed whether the material is suitable after the change. If there is doubt, it must be subject to technical clarification

when the control valve arrives, the supplier shall be informed of the unpacking time in time, and the supplier shall be required to send personnel to participate in the commodity inspection. If the supplier sends a fax or a letter for confirmation, and the unpacking results of the user shall be recognized. The commodity inspection personnel shall record all the commodity inspection results in detail without omitting any details. In addition, they shall compare the commodity inspection results with the technical annexes of the contract, record the deviation, and timely feed back to the supplier together with the commodity inspection report, and require timely handling. The time of unpacking and commodity inspection shall not be delayed as far as possible, otherwise claims and project progress may be affected. It should also be noted that the tag number of the control valve, the model and serial number of the valve body, actuator and accessories should be recorded and put on record for future ordering of accessories. If these parameters are not recorded in time, the parameters on the nameplate of the control valve will be blurred and difficult to identify after long-term use, and the nameplate may also be lost. Some parameters are necessary for purchasing spare parts

when ordering the software required by the digital positioner, it is only necessary to specify the name of the software and the latest version at the time of delivery. It is imperative to explore the new development path of plastic granulator technology. The version number of the software can be omitted

ordering spare parts and control valves together can save costs. The quantity of purchased spare parts shall meet the needs of normal production within 1~2 years. If it is not clear about the use of soft rubber and other parts on the surface between the control valve and the sample, the supplier can be asked to recommend them, and the financial factor should be considered

do not bind bidding. Two or more different goods are purchased together with the control valve. If a package contains both low-cost and high-quality goods and goods with poor performance or extremely expensive goods, it is difficult to make a complete choice when unpacking is not allowed

it is suggested to link payment with delivery progress, product quality and service, and reserve a certain quality guarantee deposit for the assessment of product quality and service quality provided by the supplier

if the medium passing through the control valve is one-way flow, the control valve must not be installed reversely, otherwise the flow capacity, flow characteristics, sealing performance and operation stability of the valve will be affected

when signing the contract, the technical annex of the contract shall specify that when the products supplied by the supplier can not be used normally or have faults, the supplier shall timely debug the products provided, deal with the existing problems, or replace them, and provide temporary supplies. It is not allowed to delay the treatment of the problems for any reason to affect the normal production of users, otherwise the supplier will be investigated for poor service quality; Once the problem is solved, both parties shall discuss and solve the expenses incurred after finding out the causes

the port incoming line of the control valve accessories shall be connected from bottom to top and shall not be reversed to prevent water from damaging the parts. If the cable must be connected from top to bottom, the cable shall be lengthened appropriately to ensure that there is a U-bend before entering the wiring port. Prevent rainwater from entering the auxiliary instrument. If a conduit is used, a drain shall be left at the lower part of the U-bend. When the cable is led out from the instrument cable tray, pay attention to the interface direction of the conduit on the tray, and the nozzle must not be upward; The pipe orifice shall be sealed to eliminate the possibility of water inflow

the type, material, connection mode with pipeline, selection of accessories, preparation of bidding documents, bid evaluation, signing of technical contract, installation and maintenance of control valve are complicated processes. Any loophole in any link may affect the use of control valve. Although there are various books, standards and software for the reference of designers and users' technicians, there is generally no great difficulty. However, from the process designers proposing process parameters, preparing the bidding documents for control valves, issuing bids, bid evaluation, technical clarification, forming bid evaluation reports, approving bid evaluation reports, technical negotiation, signing technical contracts, to unpacking inspection, installation, commissioning and putting into use, Generally, it will take quite a long time, during which one or more items will be neglected or omitted due to a slight carelessness, which may cause losses to the plant and affect the project progress. This paper only summarizes some experiences and understandings in the process of purchasing and using control valves in Anhui Huaihua Group Co., Ltd. for several times. It is suggested that there should be sufficient time for purchasing control valves. As long as the technical conditions are met, the preparation of bidding documents, bid evaluation, technical clarification, signing of technical contracts and other work should be carried out in advance, so as to really do the work in detail and avoid technical mistakes. As for the progress of the business contract, it shall be determined according to the project progress and funds

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