Some details to note when purchasing control valves

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When purchasing control valves, attention should be paid to certain specific details, as they are related to whether the valves can be installed and used properly, whether the accessories meet actual needs, and whether the contract can be fulfilled smoothly. 　　　　The technical annex to the contract shall include detailed explanations of all abbreviations (except those that are commonly used internationally). 　　　　The technical annex to the contract should specify the detailed requirements for any special treatments applied to control valves used in special applications. For example, when the process medium is oxygen, the manufacturer should be required to degrease the components on the valve that may come into contact with the medium. When the valve arrives at the installation site, the installer only needs to check the degreasing process; if it meets the requirements, the valve can be installed right away, which saves time and accelerates the project progress. 　　　　The valve openings for maximum, normal, and minimum flow rates should be appropriate. Generally speaking, the valve opening corresponding to the minimum flow rate should not be less than 15%, while the opening corresponding to the maximum flow rate should not exceed 90%. For normal flow rates, the ideal valve opening is between 60% and 80% (the optimal opening for butterfly valves can be slightly higher, but it still should not reach 100% at the maximum flow rate). 　　　　The units used for the process parameters in the contract attachments shall be those of the International System of Units. It should be specified whether the inlet and outlet pressures of the control valve are absolute pressures or gauge pressures; this should be stated either uniformly or separately for each pressure parameter. Generally, it is preferable to specify the pressures uniformly, while the other parameters should be indicated separately. 　　　　Attention should be paid to the noise issue. Noise causes environmental pollution, is harmful to human health and well-being; moreover, high noise levels along with significant vibrations affect the operational performance and service life of valves. Therefore, the noise level should be as low as possible; it generally should not exceed 80 dB. If it exceeds 85 dB, control valves equipped with noise-reduction features or other noise reduction measures should be considered. 　　　　Pay attention to the fail-safe position that the control valve should assume in the event of a failure in its air supply or power supply. The operating mode of the actuator in some control valves can be changed at the site of final installation, while that in others cannot; therefore, the contract’s technical annexes must specify the valve’s operating mode (air-open/air-close) and fail-safe position (fully open/closed/positioned). 　　　　Pay attention to the application scenarios of control valves. If explosion protection is required, the contract technical annex should specify the explosion protection type (intrinsically safe or flameproof) for accessories such as the electric/pneumatic valve positioner, solenoid valves, valve position transmitters, and valve position switches of the control valve. It should also indicate the control voltage level of the solenoid valves (24V DC/220V AC), whether the valve position switches are mechanical or proximity type, as well as the capacity of the switch contacts (including current value and voltage level). For the selection of solenoid valves, it is recommended to consider the entire system as well as the whole factory as a whole, and choose one or several reliable models from the same manufacturer; this will help reduce the need for spare parts. For the selection of electric/gas valve positioners, it is recommended to use digital valve positioners. It has the following advantages: (1) The digital valve positioner is based on a microprocessor, features two-way communication, and possesses strong diagnostic capabilities. Utilizing the diagnostic information, it is possible to carry out maintenance on control valves or implement scientific predictive maintenance, thereby preventing or reducing sudden failures. This helps to extend the period of continuous, safe operation of the factory, ensures that control valves remain in optimal working condition, and avoids unnecessary maintenance of valves based on arbitrary assumptions. 　　　　(2) If the digital valve positioner is ordered together with the control valve, the supplier is responsible for its assembly and commissioning, and there are generally no issues ； Moreover, it can generally directly replace the pneumatic or electrical valve positioner on standard valves. If the goal is merely to purchase digital valve positioners to replace conventional ones, it is recommended to seek technical advice first from manufacturers or agents of intelligent positioners. 　　　　(3) Digital valve positioners are simple to calibrate and easy to operate. A digital valve positioner equipped with its own keypad and LCD display, allowing for calibration operations to be carried out on the positioner itself ； Positioners without these auxiliary devices need to be operated using a hand controller. If the system permits, it is recommended that the handheld controller use HART protocol for communication. If the control system is a DCS and the manufacturer of both the control valve and the control system is the same, as long as the control system supports the communication protocol for digital valve positioners, it is possible to set parameters, calibrate, and perform other operations on the control valve directly from the DCS operator station. Compared to conventional valve positioners, digital valve positioners feature menu-based operation; they are highly functional, capable of automatically calibrating the valve stroke, offering good linearity and high precision, as well as enabling stable and optimized control of the valve’s response. If the input filter time constant is set, it is possible to slow down the response of the digital valve positioner, thereby improving the closed-loop control process; if this function is not required, the time constant can be set to 0. Other functions of digital valve positioners include: eliminating the small-stroke signal of control valves ； The input characteristics can be set using a set of tuning parameters, allowing for easy modification of the valve’s control characteristics ； The travel range of the control valve can be set arbitrarily within the 0–100% range ； Set valve stroke alarm and valve stroke deviation alarm ； Set valve travel limits, etc. 　　　　(4) The digital valve positioner features a modular design, requiring only few spare parts. It has strong resistance to vibration and electromagnetic interference (EMI), and there are no issues with zero-point or range drift ； Low gas consumption, high gas supply capacity ； It has low dynamic lag (usually less than 1%) and high positioning accuracy. 　　　　(5) In some cases, the use of digital valve positioners can save on instrument cables and safety barriers. An air pressure gauge must be installed on the air filter regulator itself as well as in all areas where gas pressure adjustment is required after the regulator, to facilitate the tuning and maintenance of the control valve’s gas circuit. 　　　　Select the appropriate end connection type for control valves and process pipelines (ring face connection/convex face connection/clamped connection). If matching flanges are provided, the valve supplier is required to supply gaskets between the valve and the matching flanges; the required flange material, pressure rating, and pipe designation for the flange connections shall be specified in the technical annex to the contract. If the wafer-type control valve itself has a single flange, the threading for the flange connection holes should be metric; if it is imperial, the valve supplier should be required to provide connection bolts. 　　　　If the control valve is equipped with a solenoid valve, it is recommended that the technical specifications for such solenoid valves in the contractual technical annex specify the manufacturer, brand, voltage rating, explosion protection type, material requirements, as well as the required operating time for opening and closing the control valve; however, it is not necessary to indicate the specific model of the solenoid valve. The specific model shall be selected by the valve supplier based on final calculations or factory testing, but it should be specified whether the supplier is allowed to use solenoid valves as pilot valves in the control air circuit. When the actuator of the control valve requires a large volume of air, using a solenoid valve as a pilot valve in combination with a pneumatic actuator with high flow capacity can help reduce costs by shortening the stroke time of the control valve. However, this also increases the number of components, as more pneumatic components mean more potential failure points; therefore, when considering such technical solutions, it is necessary to determine whether their use is acceptable. 　　　　It is recommended that both diaphragm actuators and piston actuators be equipped with air filters and pressure regulators, so that the actuators can withstand the pressure of the driving air supply. 　　　　Before shipment, the accessories equipped on the control valve shall be fully connected at the manufacturer’s site and tested to meet all the requirements specified in the contractual technical annexes. The control valve supplier should determine whether the connection pipes should be made of copper, PVC sheathing, or stainless steel, based on the application scenario of the valve; meanwhile, the diameter of the air pipeline connections should be determined according to the required stroke time of the valve. The diameter of the air connection tube for general control valves should not be less than 8×1 mm, as too small a diameter will result in a slow response time for the control valve. Of course, if the switching time of the control valve permits it, air connection tubes with a diameter of 6×1 mm can also be used. 　　　　The diameter of the control valve should, as much as possible, correspond to that specified in the design documents. If a change is necessary, it is necessary to inform the engineers from the process, piping, equipment, and related fields (if changes are not allowed, then the relevant professionals should discuss together a solution for making such changes) so that the appropriate design modifications can be carried out. 　　　　Attention should be paid to the corrosion of valves. If the control valves purchased suffer from issues such as cavitation, flashing, vaporization, or erosion, it is necessary to check whether appropriate measures have been taken to avoid these problems or to minimize the likelihood of their occurrence. The materials for the valve body, valve core, valve cage, and seat ring should be hard steel or alloy steel, or they should be treated by processes such as nitriding or provided with surface coatings (such as Stellite plating). Cavitation of control valves under various operating conditions must be absolutely avoided. 　　　　The technical annex to the contract must include requirements for specifying the external dimensions of the valves as well as the installation locations of accessories such as positioners, in order to determine whether there are any conflicts between the space required for installing the control valves and the installation positions indicated on the drawings. In the event of a conflict, it should first be determined whether it is possible to change the installation orientation of the control valve actuator or the position and orientation of the valve accessories. If the position of the valve cannot be changed, relevant professionals in areas such as process engineering, piping, equipment, and electrical systems should be notified so that they can modify the corresponding layout and routing designs as soon as possible. 　　　　The contracting party shall be required to provide a material analysis report for the key components of the control valve, a leakage test report for the control valve, a pressure testing report, as well as test reports or certificates of conformity, air circuit connection diagrams, and the procedures and methods used for pressure testing the control valve as well as those for testing its leakage rate, so that inspections and verifications can be carried out. 　　　　The tender document should require bidders to provide samples of the selected control valves (with detailed explanations of the specific model codes and abbreviations in the samples), as well as evidence of past applications. Technical contracts should require the contracting parties to provide a maintenance manual for the valve itself and its accessories, as well as the contact addresses, fax numbers, and telephone numbers of the accessory manufacturers and sellers. 　　　　The protection rating of the control valve accessories should not be lower than IP54/NEMA 4 (the protection rating of digital valve positioners is generally IP65/NEMA 4X), and the inlet and outlet ports of the junction box must be sealed. 　　　　The application tag number of the control valve should be indicated on the valve’s nameplate, or specified on a separate plate that is attached to the valve. Except in the case of double flow, the direction through which the medium is allowed to pass through the valve should be clearly indicated on the valve body. 　　　　Whether to install a handwheel should be determined based on the application scenario and criticality of the control valve. 　　　　The supplier of the control valve shall provide, upon request, the deadband parameters of the valve as well as the basic error parameters of the control valve for the electrical/pneumatic valve positioner. Generally, the deadband parameter of valves should not exceed 2%, and the basic error of the control valve in distribution/gas valve positioners should be less than ±10%; if a digital valve positioner is used, the basic error of the control valve should be less than ±1%. 　　　　Attention should be paid to the pressure requirements of the control valve for the air supply. Based on the actual conditions of the device, verify the requirements of the control valve actuator for instrument air pressure. If the instrument air pressure in the unit does not meet the requirements of the actuator, the valve supplier shall replace the actuator ； At the same time, high-quality instrument air should be provided, which must meet at least the following requirements: it should be clean and dry, free from any significantly corrosive impurities; the solid particles in the gas should be less than 0.1 g/m3, with particle diameters not exceeding 3 μm; and the oil content should be no more than 1×10‑6. According to statistics, cases where the locator fails to function properly due to dirty gas sources account for over 60% of all locator failures. 　　　　The designers at general design firms have already selected the materials for the valve core components (valve disc, seat, stem, cage, seat ring, packing, gasket). The material selected for the valve must be in line with the design requirements; if not, it is necessary to determine whether the alternative material is suitable. In case of doubts, technical clarification must be sought. 　　　　When the control valve arrives, the user should promptly inform the supplier of the time for unboxing, and request that the supplier send personnel to participate in the quality inspection. If the supplier refuses to send anyone for the inspection, the user should ask the supplier to send a fax or letter confirming this and accepting the results of the unboxing process conducted by the user. Inspection officers should record all inspection results in detail, without omitting any specifics. They should compare these inspection results with the technical attachments in the contract, note any discrepancies, and submit them together with the inspection report to the supplier in a timely manner, requesting that corrective action be taken promptly. Try to avoid delaying the time for unpacking and inspection; otherwise, it may affect claims and project progress. It should also be noted that parameters such as the tag number of the control valve, the model and series number of the valve body, actuator, and accessories must be recorded and kept on file for use when ordering replacement parts in the future. Because if these parameters are not recorded in a timely manner, after the control valve has been used for an extended period of time, the parameters on its nameplate will become blurred and difficult to read ； It is also possible to lose the nameplate, and some parameters are essential for purchasing spare parts. 　　　　When ordering the software required for digital positioners, simply specify the name of the software and indicate the latest version available at the time of delivery; it is not necessary to include the version number. 　　　　Ordering spare parts together with control valves can save costs; the quantity of spare parts to be purchased should be sufficient to meet the needs of normal production over a period of 1 to 2 years. If the service life of the components of the control valve is unclear, the supplier can be asked for recommendations, while also taking financial factors into consideration. 　　　　Avoid bundled bidding. When purchasing two or more different types of goods along with control valves, if a package contains both inexpensive and high-quality items as well as items with poor performance or extremely high costs, it is difficult to make a satisfactory choice when unpacking is not allowed. 　　　　It is recommended to link payments with the progress of delivery, product quality, and services, and to set aside a certain amount as a quality guarantee fund for assessing the quality of products and services provided by the supplier. 　　　　If the medium flowing through the control valve is in one-directional flow, the control valve must not be installed in reverse; otherwise, it will affect the valve’s flow capacity, flow characteristics, sealing performance, and operational stability. 　　　　At the time of signing the contract, it should be specified in the technical annexes that if the products supplied by the vendor fail to function properly or develop faults, the vendor must promptly adjust the products, address the existing issues, or replace them, and provide temporary substitutes. The vendor shall not delay addressing these problems for any reason, as this could disrupt the user’s normal production; otherwise, the vendor will be held accountable for poor service quality ； Once the problem is resolved, the costs incurred should be discussed and settled by both parties after the cause has been identified. 　　　　The port connections for the control valve accessories should be made from bottom to top, and not in reverse, to prevent water ingress from damaging the components. If the cable must be connected from top to bottom, it should be appropriately extended to ensure that there is a U-shaped bend before it enters the connection point, thereby preventing rainwater from entering the auxiliary instruments. If a conduit is used, a drainage outlet should be provided at the lower part of the U-shaped bend. When the cable is led out from the instrument cable tray, attention should be paid to the direction of the connection point of the conduit on the tray; the end of the conduit must not face upward ； The pipe ends should also be sealed to eliminate the possibility of water ingress. 　　　　The types of control valves, their materials, the methods of connection to pipelines, the selection of accessories, the preparation of tender documents, the evaluation of bids, the signing of technical contracts, installation, and maintenance are all part of a complex process; any flaw in any of these steps can affect the proper functioning of the control valves. Although there are various books, standards, and software available for reference by designers and technical personnel, it is generally not too difficult to carry out the work. However, from the process designers specifying the process parameters, preparing the tender documents for control valves, issuing the tenders, evaluating bids, addressing technical queries, preparing evaluation reports, approving those reports, conducting technical negotiations, and signing technical contracts, to the inspection upon delivery, installation, commissioning, and putting the system into operation, a considerable amount of time is required. Even the slightest carelessness during this process can lead to the omission of one or more steps, which may result in losses for the factory and affect the project schedule. This article merely summarizes some experiences and insights gained from the several purchases of control valves by Anhui Huaihua Group Co., Ltd. as well as during their use. It is recommended that sufficient time be allocated for purchasing control valves; whenever the technical requirements are met, preparations for tender documents, evaluation of bids, technical clarifications, and the signing of technical contracts should be carried out in advance, so as to ensure thorough and meticulous work and minimize technical errors. As for the progress of business contracts, it will be determined based on the project progress and funding situation.