Valve assembly is the last stage in the manufacturing process. Valve assembly is the process of combining the various parts and components of the valve together to make it a product according to the specified technical premise.
The whole machine is the most basic unit of valve assembly, and several parts constitute the components of the valve (such as valve cover, valve disc components, etc.). The assembly process of several parts forming a component is called component assembly, and the assembly process of several parts and components forming a valve is called general assembly. Assembly work has a great impact on product quality. Even if the design is accurate and the parts are qualified, if the assembly is improper, the valve will not meet the specified requirements and even cause sealing leakage. Therefore, special attention should be paid to the use of reasonable assembly methods to ensure the quality of the final product of the valve. The assembly process specified in the form of documents in production is called assembly process regulations.
Common assembly methods for valves
There are three common assembly methods for valves, namely complete replacement method, repair method and optional method.
1. Complete Interchange Method
When the valve is assembled by the complete interchange method, each part of the valve can be assembled without any trimming and selection, and the assembled product can meet the specified technical requirements. At this time, the valve parts must be processed in full accordance with the design requirements to meet the requirements of dimensional accuracy and form and position tolerance. The advantages of the complete interchange method are: the assembly work is simple and economical, the workers do not need a high level of skills, the production efficiency of the assembly process is high, and it is easy to organize the assembly line and organize specialized production. However, relatively speaking, when the complete replacement assembly is adopted, the processing accuracy of the parts is required to be higher. It is applicable to valves with relatively simple structures such as stop valves, check valves, ball valves, and valves of medium and small diameters.
2. Matching Method
The valve is assembled by the matching method, and the whole machine can be processed according to economic accuracy. During assembly, a certain size with adjustment and compensation functions is matched to achieve the specified assembly accuracy. The principle of the matching method is similar to the repair method, except that the way of changing the size of the compensation ring is different. The former is to change the size of the compensation ring by selecting accessories, and the latter is to change the size of the compensation ring by trimming accessories. For example: the top core and adjustment gasket of the double-disc wedge-type gate valve of the control valve model, the adjustment gasket between the two bodies of the split ball valve, etc., are to select special parts as compensation parts in the size chain related to the assembly accuracy, and adjust the thickness of the gasket to achieve the required assembly accuracy. In order to ensure that fixed compensation parts can be selected in different situations, a set of gaskets and sleeve compensation parts of hydraulic control valve models with different thicknesses and sizes need to be manufactured in advance for selection during assembly.
3. Repair method
The valve is assembled by the repair method, and the parts can be processed according to economic accuracy. During assembly, a certain size with adjustment and compensation functions is repaired to achieve the specified assembly target. For example, the gate and valve body of the wedge-type gate valve, because the processing cost to achieve the interchange requirements is too high, most manufacturers use the repair method process. That is, when the gate sealing surface is finally ground to control the opening size, the plate should be matched with the opening size of the valve body sealing surface to achieve the ultimate sealing requirements. Although this method adds the plate matching process, it greatly simplifies the dimensional accuracy requirements of the previous processing process. The plate matching process is operated by a dedicated person, and generally does not affect the production efficiency. Valve assembly process: The valve is generally assembled in a fixed site. The assembly and general assembly of the valve parts and components are carried out in the assembly workshop, and all the required parts and components are transported to the assembly site. Usually, the assembly of components and the general assembly are carried out by several groups of workers at the same time, which not only shortens the assembly cycle, but also facilitates the use of special assembly tools, and the requirements for the technical level of workers are relatively low.
Some foreign manufacturers or high-tech valves also use the mode of assembly suspension line or assembly turntable.
1. Preparation work before assembly
Before assembly, the valve parts need to remove the burrs caused by machining and the welding slag left by welding, and clean and cut the fillers and gaskets.
2. Cleaning of valve parts
As a valve installed to control the fluid pipeline, the inner cavity must be clean. In particular, for valves used in nuclear power, medicine, and food industries, the requirements for the cleanliness of the valve inner cavity are more stringent in order to ensure the purity of the medium and prevent medium contamination. Before assembly, the valve parts should be cleaned to remove the chips, residual lubricating oil, coolant and burrs, welding slag and other dirt on the parts. The valve is usually cleaned by spraying with alkaline water or hot water (kerosene can also be used for cleaning) or cleaning in an ultrasonic cleaning machine. After grinding and polishing, the parts need to be cleaned for the last time. The last cleaning is usually to brush the sealing surface with gasoline, then blow it dry with compressed air and wipe it with a cloth.
3. Preparation of packing and gaskets
Graphite packing is widely used because of its advantages such as corrosion resistance, good sealing and low friction coefficient. Packing and gaskets are used to prevent the medium from leaking through the valve stem and valve cover and the flange joint surface. These accessories must be prepared for cutting and use before the valve is assembled.
4. Assembly of valves
Valves are usually assembled in the order and method specified by the process with the valve body as the reference part. Before assembly, parts and components should be inspected to prevent undeburred and uncleaned parts from entering the final assembly. During the assembly process, parts should be handled with care to avoid bumps and scratches on processing personnel. Industrial butter should be applied to the moving parts of the valve (such as valve stems, bearings, etc.). The valve cover and the flange in the valve body are mostly connected by bolts. When tightening the bolts, they should be tightened evenly, interlaced, repeatedly, and evenly. Otherwise, the joint surface of the valve body and valve cover will leak due to uneven surrounding force. The wrench used for tightening should not be too long to prevent the pre-tightening force from being too large and affecting the bolt strength. For valves with strict requirements for pre-tightening force, a torque wrench should be used to tighten the bolts according to the specified torque requirements. After the final assembly is completed, the control mechanism should be rotated to check whether the movement of the valve opening and closing parts is flexible and whether there is any jamming. Whether the installation direction of the pressure reducing valve cover, bracket and other parts conforms to the requirements of the drawing, the valve can be tested only after all the inspections are passed.