Problems in Manufacturing Marine Valves

1. An introduction to marine valves

A marine valve is a device used to control the fluid pressure, flow and flow direction in the ship pipeline in order to meet the environmental conditions of the ship. It is currently widely used in various systems of the ship. The unique working environment of ships makes marine valves different from ordinary valves, forming an independent system. The differences between marine valves and ordinary valves are reflected in the aspects of shapes, weights and materials. Every ship requires thousands of different valves. On ships, gate valves, globe valves, butterfly valves, ball valves, check valves, safety valves, flow control valves and other valves are widely used. Marine valves are mainly made of cast iron, alloy steel, copper alloy, aluminum alloy, titanium alloy and stainless steel. The applicable media includes fresh water, sea water, air, steam, fuel oil and lubricating oil. The maximum operating pressure can reach 600MPa, the highest working temperature 650°C (main bodies) and1200°C for refractory lining; the minimum working temperature can reach 196°C, and the maximum nominal diameter can reach 5350mm. In general, marine valves are indispensable on ships, and the operating environment is relatively harsh. The performance of marine valves directly affects the safety of the system and overall performance of ships. High requirements are put forward for the quality and reliability of marine valves due to the harsh operating environment of marine valves. Generally speaking, marine valves must have anti-vibration, anti-corrosion and other properties. Non-standard customization, light weights, miniaturization, and easy disassembly are also important features for marine valves. In addition, the manufacture of civil marine valves must comply with the relevant standards of the international standard ISO 9002. There should be quality certificates from various classification societies. Therefore, it is difficult to manufacture marine valves; the cost remains high, and the quality often varies from factory to factory.
 
2. The status of quality management of marine valves
Since the founding of China, China's marine valve industry has absorbed the Soviet Union's marine valve technology after years of development, and has been self-reliant, actively developing marine valve related technologies, and the technical level has been significantly improved. The specifications, types and related marine standards are relatively complete, which meet the development of the shipbuilding industry. Compared with international first-class manufacturers, the gaps of domestic manufacturers in the quality management of marine valves are mainly in the following aspects:
 
2.1 Raw materials
The quality of raw materials directly determines the ultimate reliability of marine valves. When marine valves are used, they are often affected by the harsh external environment, salt spray, mold, shock and swing. Therefore, the quality control of raw materials must be strict. In the field of ships, there are many non-standard customized valves, and there are not many types of valves which are mass-produced. The procurement amount of materials is small, but the requirements for materials are high. These factors all pose challenges to the procurement and inspection of materials. The casting materials purchased by domestic valve factories often have many casting defects, and the batch numbers are mixed. The parts that are prone to casting defects are not analyzed, paid attention to and checked, and the traceability of the products is poor. The heat number management of forgings is chaotic; the detection and analysis of chemical components are not paid attention to, and the quality of forgings is poor.
 
2.2 Processes
The process quality control is not strictly carried out in accordance with the quality assurance system, which is not combined with actual production. The processes often include machining, assembly, welding, heat treatment, non-destructive testing, test process and other processes. There are certain difficulties with the processes due to many interdisciplinary subjects and many details required additional attention. The processes lack feasibility and accuracy in guiding manufacturing.
 
2.3 Assembly
The preparations before assembly are insufficient. Cleaning and drying of parts are inadequate. Chamfers and burrs are not completely removed. Geometric tolerances are not strictly inspected. The assembly is not carried out according to the standard operating procedures, and there are patchwork and violent assembly; the lubricating oil is not smeared or insufficiently smeared when installing the shaft sleeve and O-ring, which leads to leakages when the final product is assembled and tested, and affects the quality of the product and time limit for a project.
 
2.4 Skills and technology
The skills of technical personnel are not the same, so the assembled marine valves are different. The skills training for employees is enough; the overall technical reserve level is low, and there is a phenomenon that one person has multiple jobs. Usually, one person needs to be engaged in assembly and testing.
 
3. Quality management and control measures in the manufacturing process
The quality of marine valves reflects the management level, process and technical level of the factory, and is an important embodiment of the overall industrial level. With the development of the market, domestic valve manufacturers pay more and more attention to the quality management of valves, so the manufacturing capacity has been improved to a certain extent. However, there is still a huge gap between the quality management level of the international first-class manufacturers.