Analysis of control valves

Correctly select the end connection form of the control valve and the process pipeline (annular surface connection/convex surface connection/wafer type). If equipped with a counter flange, the valve supplier is required to provide a gasket between the valve and the counter flange, and the requirements, flange material, pressure grade and pipe number of the flange connection should be noted in the technical attachments of the contract. If the wafer-type control valve itself has a single flange, the thread of the flange connection hole should be metric. If the thread is imperial, the valve supplier should be required to provide connecting bolts.
If the control valve is equipped with a solenoid valve, it is recommended that the technical parameters of the solenoid valve in the technical attachments of the contract should indicate the manufacturer, brand, voltage level, explosion-proof form, material requirements and the opening and closing travel time requirements of the control valve, but there is no need to note Specify the specific model of the solenoid valve. The specific model should be selected by the valve supplier based on final calculations or factory testing. However, it should be noted whether the supplier is allowed to use the solenoid valve as a pilot valve in the control gas line. When the actuator of the control valve requires large air volume, in order to shorten the stroke 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 costs, but also increases the number of accessories, because the pneumatic valve The increase in circuit links means the increase in failure points. Therefore, when discussing technical accessories, it should be determined whether the air circuit control scheme is allowed to be adopted.
It is recommended that whether it is a diaphragm actuator or a piston actuator, it should be equipped with an air filter pressure reducer so that the actuator can be subject to the pressure of the driving air source.
Before shipment, the accessories equipped with the control valve should be completely connected at the production plant and tested to meet the requirements of the contract’s technical accessories. The control valve supplier should decide whether the connecting pipe is a steel pipe, PVC sheathed pipe or stainless steel pipe according to the application of the valve, and determine the diameter of the gas line connecting pipe according to the travel time requirements of the valve. Generally, the diameter of the air connection pipe of the control valve should not be less than 8×1mm, because too thin will cause the response speed of the control valve to be too slow. Of course, if the switching stroke time of the control valve allows, a pipe diameter of 6× can also be used. 1mm air connection tube.
The caliber size of the control valve should be consistent with the design document as much as possible. If it must be changed, it should be reported to the engineers of the process pipeline, equipment and related professions (if the change is not allowed, the relevant professionals should discuss and decide on the change plan together) for implementation. Corresponding design changes.
Attention should be paid to the corrosion problem of valves. If the purchased control valve has problems such as cavitation, flash evaporation, cavitation, and erosion, you should pay attention to 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 the valve body, valve core, valve cage, and seat ring should be made of hard steel or alloy stainles steel, or should be treated with nitriding or surface coating (such as Stellite coating). Cavitation in control valves should be absolutely avoided under all application conditions.
The technical attachments to the contract must include requirements to confirm the overall dimensions of the valve and the installation position of accessories such as positioners 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 at the installation position on the drawing. If there is a 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 in the process, piping, equipment, electrical and other fields should be notified to modify the corresponding layout and direction design as soon as possible.
The contractor should 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 Testing procedures and methods for testing control valve leakage for inspection and review.
The tender document should require the bidder to provide control valve selection samples (the specific model codes and abbreviations in the samples should be explained in detail) and application performance. The technical contract should require the contracting party to provide daily maintenance manuals for the valve body and its accessories, as well as the contact addresses, faxes, and phone numbers of the accessories agents and sellers.
The protection level of control valve accessories should not be lower than IP54/NEMA4 (the protection level of digital valve positioners is generally IP65/NEMA4X), and the inlet and outlet ports of the junction box should be sealed.
The application number of the control valve should be reflected in the valve nameplate, or separately marked on a plate and fixed to the valve. In addition to dual flow directions, the direction in which the medium is allowed to pass through the valve should be clearly marked on the valve body.
Whether to install a handwheel should be considered based on the application situation and criticality of the control valve.
The supplier of the control valve should 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 should not be greater than 2%. The basic error of the control valve of the distribution/gas valve positioner should be less than ±10%. If equipped with a digital valve positioner, the basic error of the control valve should be less than ±1%. .
Attention should be paid to the requirements of the control valve on the air source pressure. According to the actual situation of the device, check the requirements of the control valve actuator for instrument air pressure. If the instrument air pressure in the device cannot meet the requirements of the actuator, the valve supplier should replace the actuator. At the same time, high-quality instrument air should be provided, which should at least meet the following requirements: clean, dry, and non-obviously corrosive. Impurities, the solid particles contained in the gas are less than 0.1g/m3, the particle diameter is not more than 3μm, and the oil content is not more than 1×10 (-6 power). According to statistics, 90% of the cases where the positioner cannot work properly due to unclean gas source The failure rate of the entire positioner is more than 60%.
The designers of the general design unit have already selected the material of the valve body core parts (valve core, valve seat, valve stem, valve cage, seat ring, packing, gasket). The final valve material selection should 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 any doubt, technical clarification must be carried out.
When the control valve arrives, the supplier should be promptly notified of the unpacking time, and the supplier should be required to send personnel to participate in commodity inspection. If the supplier sends a fax or letter to confirm, and approves the user’s unpacking results. Commodity inspection personnel should record all commodity inspection results in detail without omitting any details, and should compare the commodity inspection results with the technical attachments of the contract, write down the deviations, and provide timely feedback to the supplier together with the commodity inspection report, and require timely processing. Try not to delay unpacking and inspection as much as possible, otherwise it may affect claims and project progress. It should also be noted that parameters such as the control valve’s position number, valve body, actuator and accessory model and serial number should be recorded and filed for future use when ordering accessories. Because if these parameters are not written down in time, after long-term use of the control valve, the parameters on the nameplate will be blurred and difficult to read, and the nameplate may also be lost. Some parameters are necessary for purchasing spare parts.
When ordering the software required for the digital locator, you only need to indicate the name of the software and the latest version at the time of delivery. The version number of the software does not need to be written.
Ordering spare parts and control valves together can save costs. The quantity of spare parts purchased should be based on the principle that it can meet the needs of normal production within 1 to 2 years. If you are unclear about the usage of components of the control valve, you can ask the supplier for recommendations and consider financial factors.
​ Avoid bundling tenders. Purchasing two or more different goods together with control valves. If there are both cheap and high-quality goods in one package and goods with poor performance or extremely expensive goods, it will be difficult to do so when unpacking is not allowed. The best of both worlds.
It is recommended to link payment with delivery progress, product quality, and services, and reserve a certain amount of warranty money 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 in the reverse direction, otherwise it will affect the valve’s flow capacity, flow characteristics, sealing performance and operational stability.
When signing a contract, one item should be specified in the technical appendix of the contract. When the products supplied by the supplier cannot be used normally or malfunction occurs, the supplier should debug the products provided in a timely manner, deal with existing problems, or replace them. , and provide temporary substitutes, and shall not delay the processing of the problem for any reason to affect the user’s normal production, otherwise the supplier will be held responsible for poor service quality; once the problem is solved, the costs incurred shall be determined after the cause is identified. Both parties discuss and resolve.
The port inlet of the control valve accessories should be connected from bottom to top and should not be connected in reverse to prevent water from damaging the parts. If the cable must be inserted from top to bottom, the cable should be lengthened appropriately to ensure a U-shaped bend before entering the wiring port. Prevent rainwater from entering the auxiliary instruments. If conduit is used, there should be a drain opening at the lower part of the U-shaped bend. When the cable is led out from the instrument cable bridge, attention should be paid to the direction of the interface of the threading pipe on the bridge. The pipe mouth must not be upward; the pipe mouth should be sealed to eliminate the possibility of water intrusion.
The type and material of the control valve, the connection method with the pipeline, the selection of accessories, the preparation of bidding documents, bid evaluation, signing of technical contracts, installation and maintenance are a tedious and complicated process. Loopholes in any link may affect the Use of control valves. Although there are various books, standards and software for reference by designers and user technicians, generally there will not be much difficulty. However, from process designers to proposing process parameters, preparing tender documents and issuing tender documents for control valves, , bid evaluation, technical clarification, formation of bid evaluation report, approval of bid evaluation report, technical negotiation, signing of technical contract, to unpacking and inspection, installation, debugging, and putting into use, generally it takes a long time, and a little carelessness during this period will If one or more items are ignored or missed, it may cause losses to the factory and affect the progress of the project. This article only summarizes some experiences and understandings during the several purchases and use of control valves at Anhui Huaihua Group Co., Ltd. It is recommended that sufficient time should be given for purchasing control valves. As long as the technical conditions are met, the bidding documents should be prepared in advance. In terms of preparation, bid evaluation, technical clarification, and signing of technical contracts, we must really do the work carefully and thoroughly to avoid technical mistakes. As for the progress of the business contract, it will be determined based on the project progress and funding situation.