Factors to consider when purchasing valves

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　1. General Requirements 1.1 The specifications and categories of valves shall meet the requirements specified in the pipeline design documents. 1.2 The model of the valve shall indicate the requirements of the relevant national standard number. If it is a corporate standard, relevant specifications for the model should be indicated. 1.3 The operating pressure of the valve must be ≥ the operating pressure of the pipeline; provided that it does not affect the price, the pressure that the valve can withstand should be higher than the actual operating pressure of the pipeline ； Either side of the valve when it is closed should be able to withstand 1.1 times the working pressure of the valve without leaking ； With the valve open, the valve body shall be able to withstand a pressure twice that of the valve’s operating pressure. 1.4 The valve manufacturing standards shall specify the corresponding national standard numbers; if it is a corporate standard, the corporate document shall be attached to the purchase contract. 　　2. Valve specifications 　　2.1 The material of the valve body should be ductile iron, with the grade specified along with the actual physical and chemical test data of the iron. 2.2 Valve stem material: stainless steel valve stems (2CR13) are preferred; for large-diameter valves, valve stems with a stainless steel core should also be used. 2.3 The material of the nut is cast aluminum brass or cast aluminum bronze, with hardness and strength both greater than those of the valve stem. 2.4 The material of the valve stem bushing shall have a hardness and strength that are not greater than those of the valve stem, and it shall not cause electrochemical corrosion with the valve stem and valve body when exposed to water. 2.5 Material of the sealing surface ① Different valve types result in varying sealing methods and material requirements ； ②For ordinary wedge-type gate valves, the material of the copper ring, its fixing method, and the grinding method should all be specified ； ③Physical, chemical, and hygiene test data of the rubber-lined material used in soft-seal gate valves ； ④The butterfly valve should indicate the material of the sealing surface on the valve body and the material of the sealing surface on the butterfly disc ； Their physicochemical test data, particularly the hygiene requirements for rubber, aging resistance, and wear resistance ； Nitrile rubber and EPDM are typically used; the use of recycled rubber is strictly prohibited. 2.6 Valve stem packing ① Since the valves in piping systems are usually not operated frequently, it is required that the packing remain intact for several years, without deteriorating, thus maintaining its sealing properties over time ； ②The valve shaft packing should also maintain good sealing performance under frequent opening and closing ； ③Given the above requirements, the valve shaft packing is designed to remain unchanged for life or for over a decade ； ④If the filler needs to be replaced, the valve design should take into account measures for replacement under water pressure. 　　3. Transmission gearbox 3.1 The material of the gearbox, as well as the requirements for internal and external corrosion protection, are in line with those applicable to valve bodies. 　　3.2 The enclosure shall have sealing measures, and it must be able to withstand immersion under a 3-meter water column pressure after assembly. 　　3.3 For the opening and closing limit devices on the box, their adjustment nuts should be located either inside the box or outside it, but special tools are required to operate them. 　　3.4 The transmission structure is reasonably designed; it allows the valve shaft to rotate only during opening and closing, preventing it from moving up and down. The fit between the transmission components is appropriate, so no separation or slippage occurs when operating under load. 　　3.5 The transmission housing and the valve shaft seal must not be connected to form a leak-free unit; otherwise, reliable measures to prevent leakage must be in place. 　　3.6 There should be no debris inside the box, and the gear engagement areas must be protected with lubricant. 　　4. Valve operating mechanism 　　4.1 The direction in which the valve should be closed during operation is always clockwise. 　　4.2 Due to the valves in the pipeline network, which are often operated manually, the number of rotations should not be excessive; even for large-diameter valves, it should remain between 200 and 600 rotations. 　　4.3 To facilitate operation by a single person, under normal piping conditions, the maximum opening and closing torque should be 240 N-m. 　　4.4 The operating end of the valve should be square-shaped with standardized dimensions, and it should face the ground so that it can be operated directly from the ground. Valves with wheels are not suitable for underground piping systems. 　　4.5 Display panel for indicating the degree of valve opening/closing ① The scale lines indicating the degree of valve opening/closing should be cast on the gearbox cover or on the housing of the display panel after direction change; they must all face downward. The scale lines are coated with fluorescent powder to ensure visibility ； ②In cases where proper management is ensured, the material for the dial needle can be stainless steel plate; otherwise, painted steel plate should be used. Aluminum sheet must not be used for this purpose ； ③The dial needle is eye-catching and firmly fixed; once the opening and closing adjustment is accurate, it should be locked with rivets. 　　4.6 If the valve is buried deep and the distance between the operating mechanism and the display panel from the ground is ≥1.5 m, an extension rod should be provided and fixed securely, so that it can be observed and operated from the ground. In other words, the operation of opening and closing valves in the pipeline network should not be carried out by working down in the well. 5. Performance testing of valves 　5.1 When valves of a certain specification are manufactured in bulk, an authoritative institution should be commissioned to conduct tests on the following performance aspects: ① The opening and closing torque of the valve under working pressure ； ②Number of consecutive opening and closing cycles under working pressure that can ensure a tight seal of the valve ； ③Detection of the flow resistance coefficient of valves under pipeline water conveyance conditions. 　　5.2 Before leaving the factory, the valves shall undergo the following tests: ① With the valve in the open position, the valve body shall be subjected to an internal pressure test equal to twice the working pressure of the valve ； ②With the valve closed, each side is subjected to 1.1 times the working pressure of the valve, with no leakage ； However, for butterfly valves with metal seals, the leakage rate is also no greater than the relevant requirements. 　　6. Internal and external anti-corrosion treatment of valves 　6.1 On the inside and outside of the valve body (including the gearbox), sandblasting should be carried out first to remove rust, followed by electrostatic spraying of non-toxic powdered epoxy resin in a thickness of at least 0.3 mm. When it is difficult to use electrostatic spraying with non-toxic epoxy resin for extra-large valves, similar non-toxic epoxy paint should be applied by brushing or spraying. 　　6.2 The interior of the valve body and all parts of the valve disc must be fully protected against corrosion; they should not rust when immersed in water, and no electrochemical corrosion should occur between the different metals ； Firstly, the smooth surface reduces water resistance. 　　6.3 For the sanitary requirements of epoxy resin or paint used for anti-corrosion inside the valve body, test reports from relevant authoritative agencies shall be provided. The chemical and physical properties must also meet the relevant requirements. 　　7. Packaging and transportation of valves 　　7.1 Light blocking plates should be installed on both sides of the valves to secure them in place. 　　7.2 Valves of medium and small diameter should be tied with straw ropes and transported in containers. 　　7.3 Large-diameter valves also come with a simple wooden frame solid packaging to prevent damage during transportation. 　　8. Manufacturer’s instructions for the valve 　　Since a valve is a type of equipment, the manufacturer’s instructions should include the following relevant data: valve specifications ； Model ； Work stress ； Manufacturing standards ； Valve body material ； Valve stem material ； Sealing material ； Valve stem packing material ； Material of valve stem bushing ； Internal and external anti-corrosion materials ； Operation start direction ； RPM ； Opening and closing torque under working pressure ； Name of the manufacturing plant ； Date of manufacture ； Factory serial number ； Weight ； Bore diameter, number of holes, and center hole spacing of the connecting flange ； Indicate the control dimensions of length, width, and height in a graphical manner ； Valve flow resistance coefficient ； Effective opening and closing cycles ； Data related to factory testing of valves, as well as precautions for installation and maintenance.