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1. Introduction to the valve

1.1 Nominal diameter and nominal pressure

The two important indicators of the valve are the nominal diameter and the nominal pressure. The nominal diameter: "DN+number" is used to indicate the valve diameter, but it is not the actual measured value of the valve diameter. According to the national standard, the actual measured value is greater than or equal to 95% of the nominal diameter. Generally the valve adopts the metric system, and the American standard valve is the British system; and the nominal pressure: "PN+number" is used to indicate the pressure. The nominal pressure is not equal to the actual measured value of the valve. It is the maximum working pressure allowed by the valve at the reference temperature. Should be "closed" to this value.

1.2 Valve characteristics

The characteristics of the valve are mainly manifested in the use and structure. The use characteristics include the type, structure, and material of the valve. The range and performance of the valve with different use characteristics are different; and the structural characteristics of the valve mainly include the valve The size, connection method, and structure of the valve stem, etc., valves of different structures require different maintenance and repair methods, which need to be selected according to the actual situation.

1.3 Performance Index

The performance indicators of the valve mainly include three aspects: flow coefficient, cavitation coefficient and sealing performance. First, the flow coefficient is an indicator of the flow capacity of the valve. The larger the flow coefficient, the smaller the pressure loss when the medium passes through the valve; secondly, the cavitation coefficient: needs to be measured according to the working conditions; the final sealing performance is the medium of the valve seal An indicator of the size of the leakage is divided into internal leakage and external leakage. Internal leakage refers to "not tightly closed", and external leakage refers to the flow from inside the valve to the outside of the valve.

1.4 Casting defects

Due to the casting process used in the production of valves, some defects such as blisters, pores, porosity, slag holes, etc. hole defects; cold and hot cracks defects; structural shape and size defects are unavoidable.

1.5 Production standard

Valve production is divided into two types: national standard and enterprise standard. The valve name must be marked with the national standard number or the relevant description of the enterprise standard. The valve manual must indicate: specification and model, working pressure, manufacturing standard, stem material, sealing material, packing material, fluid flow direction, manufacturer name, factory date, factory number, precautions for installation and maintenance, etc.

2. Analysis of the pros and cons of pipeline valves

Different working conditions, different environments, different media and different use requirements have given birth to various types of valves. Take several commonly used valves as examples to briefly analyze their advantages and disadvantages.

2.1 Gate valve

The working principle of 　　 gate valve is mainly to close and drain the fluid through the pressure valve. It has the advantages of small fluid pressure loss, unrestricted fluid flow, simple structure, sturdiness and durability, and mature manufacturing technology. However, the disadvantages are large volume, bulky opening height, large installation space, easy scratches on the sealing surface, easy fall off of the gate and valve stem, and high-speed liquid flow will cause the gate to vibrate under partial opening conditions. Suitable for piping systems that are not frequently opened and closed.

2.2 Globe valve

The working principle of the globe valve is mainly that the disc moves along the center of the valve seat to open and close the valve. The advantages are high temperature resistance, wear resistance, better manufacturing technology, and easy maintenance. The disadvantage is that the fluid pressure loss is large and the opening speed is slow. Only allow the medium to flow in one direction, and the sealing performance is slightly poor. Scope of application: Suitable for heating and steam systems. In pipeline systems, cut-off valves are generally used to cut off the flow.

2.3 Butterfly valve

The working principle of 　　 butterfly valve is that the disc is a disc, and the valve stem reciprocates within the range of 0°～90° to realize the opening and closing of the valve. The advantages are simple structure, small size, light weight, simple operation and low fluid pressure loss. The disadvantage is that the flow adjustment range is not large, the sealing performance is poor, and the valve plate is greatly eroded by the medium. Scope of application: Suitable for small space and large diameter pipeline systems.

3. Valve installation and use

3.1 Installation of check valve

Check valves are divided into two types: lift type and swing type. When DN<50mm, the lift type should be used; when the diameter DN≥50mm, the swing type should be used. Lift check valves can only be installed on horizontal pipelines, and swing check valves can be installed on horizontal or vertical pipelines. When installing a lift check valve, ensure that its valve flap is vertical for flexible lifting; when installing a swing check valve, ensure that its pin shaft is horizontal for flexible swing. The resistance of the lift check valve is larger than that of the swing check valve; a slow-close check valve should be used at the outlet of large-diameter pipelines and water pumps, and a ball valve or gate valve should be installed on the upper part of the check valve to facilitate maintenance; There is no need to install check valves for pipe sections equipped with pipe backflow preventers.

3.2 Installation of pressure reducing valve

The installation of a pressure reducing valve generally requires an additional bypass pipe. However, when the water supply system needs to add a pressure reducing valve in a partition, it must not be equipped with a bypass pipe, but two sets of pressure reducing valves with the same configuration, one for use and one for preparation. The pressure reducing valve should be installed upright on a horizontal pipe and cannot be tilted.

3.3 Control valve

Sometimes in order to adjust the pressure, flow and other parameters of the control fluid, it is necessary to install an adjustment mechanism (control valve). The principle is to change the flow area between the valve disc and the valve seat to achieve the purpose of adjustment. The control methods generally include electric or pneumatic Type, hydraulic type, etc. Commonly used control valves include electric control valves, pressure reducing valves, pneumatic control valves, and pressure regulating valves.

3.4 Principles of valve selection and use

Stop valves are generally used in water supply pipe networks with DN≤50mm, and air-conditioning pipes should be used as little as possible. Gate valves are usually used in situations where the gate does not need to be opened and closed frequently, and the gate is kept fully open or fully closed for a long time. The gate valve cannot be used to adjust the flow rate, and is generally not used in pipelines that transport mud and other media. When the gate valve has a small opening, the closing speed must be slow, otherwise water hammer will occur. In general, the handle type (D71X, D41X) is adopted when the diameter of the butterfly valve is less than 150mm, and the worm gear type (D371X, D341X) is adopted when the diameter of the butterfly valve is >150mm. When the butterfly valve is used to adjust the flow, it is not suitable for long-term use within 15° of opening, which will cause vibration and cavitation. Selection of other functional valves. ① Close tighter valves: stop valves, plunger valves, ball valves; ② valves that control flow: stop valves, cock valves, ball valves; ③ valves that change direction: plug valves, ball valves; ④ fluids with suspended particles Valves: ball valves and stopcock valves; ⑤The most convenient valves for fully open and fully closed: ball valves, gate valves; ⑥Suitable valves for places with small installation space: butterfly valves, ball valves; ⑦Suitable valves for pump inlet and outlet: butterfly valves; ⑧ points , Valves used between the water collectors: differential pressure bypass valve; ⑨ Directional valves: stop valves, throttle valves, pressure reducing valves, check valves; ⑩Valves used in variable flow systems in heating engineering: Balance valve.

3.5 Valve installation

The inner wall paint of the valve of the water supply project must be non-toxic, and the valve used in the sewage project should be corrosion resistant and not easy to be blocked. The quality of the valve installation directly affects the use effect. If installed improperly, the performance of the valve may decrease, so attention must be paid.

First, before installing the valve, you must read the valve manual carefully; distinguish the direction and position of the valve.

Second, when the valve is hoisted, the rope should be tied to the flange connection between the valve cover and the valve body, and cannot be tied to the handwheel or valve stem.

Third, when the valve is connected to the pipe counterpart, it is not allowed to force the valve to be matched. The flange bolts should be tightened diagonally in stages, and finally tightened in turn.

Fourth, don’t have too many turns during the whole installation of the valve.

Fifth, the installation of the valve part must ensure the stability of the hemp wire, and the paint along the hemp wire is used to reduce the accumulation of rust, which will affect the safety of the valve.

Sixth, during the installation of steam-type pipelines, the fixed valve bracket installed must be set in the correct position and proper installation and use rules to ensure that the thermal expansion and contraction of the pipeline will not cause the valve to burst.