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How can we know whether a valve meets the application requirements?
Whether a valve’s performance meets the application requirements can ultimately only be confirmed through actual application. However, before that, by testing, understanding the application requirements, and then conducting tests on the valve that are as close as possible to the requirements, we can gain knowledge of the valve’s applicability.
The function of a valve depends on its structure, while the performance of a valve must be determined through testing!
Testing of valves can be divided into research and development testing and production testing.
The excellent performance of a valve is achieved through comprehensive research and development testing, while the stable manufacturing quality of a valve is confirmed through careful production testing.
Research and development testing is comprehensive testing of sample characteristics.
The valve research and development process: After receiving a research and development task, the designer analyzes the requirements (task specification), performs calculations and conceptual design, and produces drawings. After the sample is trial-manufactured according to the design drawings, comprehensive performance testing must be conducted. The test results are compared with the requirements, and the design is improved based on the gaps, followed by re-manufacturing and re-testing. This process is repeated, and requirements can also be supplemented and modified during this period, until the sample performance meets the requirements. Testing is an indispensable link in this process.
Research and development testing is conducted not only during the research and development process, but also at the end, to make design approval and finalization assessment of component design and manufacturing process. The test pieces must use materials and processes from series production, so that the test results can represent the level of series products.
The following tests generally need to be conducted:
1) Steady-state performance: such as pressure difference – flow characteristics, regulation and control performance, sealing (leakage), noise, etc.
2) Transient performance: such as step response of relief valves and electro-proportional valves, starting surge of flow valves, working range of electromagnetic directional valves, etc.
3) Pressure resistance performance: Hydraulic systems often experience short-term overpressure, requiring pressure resistance testing with static pressure at 125% of the allowable pressure. After testing, a performance test must be conducted again. Only when performance remains basically unchanged can it be demonstrated that the valve can withstand overpressure without aftereffects.
4) Burst test: Gradually pressurize the component until bursting. The burst pressure must be 3 to 4 times higher than the rated pressure to be qualified.
Component bursting means that the stress exceeds the tensile strength limit of the material.
The elastic limit of metal materials commonly used in hydraulic valves is not less than 60% of the tensile strength limit.
If the tensile strength limit is more than 4 times (3 times) the rated pressure, the elastic limit is more than 2.4 times (1.8 times) the rated pressure. When the instantaneous pressure peak is within this range, the component can return to its original shape, and the repeat performance test after pressure resistance testing can be omitted.
Burst testing is suitable for cylindrical or valve block hollow components, but has little significance for solid components (such as valve spools and armatures).
Durability testing is also called life testing or fatigue testing. Even with theoretical calculation methods, actual testing is still ultimately needed for verification. Long-term operation close to actual working conditions is commonly used to determine the long-term reliability of the valve.
The following points should be noted:
1) Predict possible damage: Influencing factors include raw material quality, surface performance of valve spool and valve body, working lubrication conditions, electromagnetic coil performance, etc.; the initial state of mating moving pairs, seal ring grooves, springs, etc. must be recorded and periodically inspected and compared during testing.
2) Conduct burst pressure testing first: It takes less time than durability testing and can quickly identify weak points.
3) Simplify the test system: Under conventional working conditions, hydraulic valve conditions are mild, and a simple test system can reduce costs and losses.
4) Intensify test conditions: Shorten test time through high pressure, large flow, addition of contaminants, etc., but test conditions differ from actual working conditions, requiring analysis of results combined with experience.
5) Test important components separately: Such as springs and electromagnetic coils, such testing has simple working conditions and low cost.
6) Infer through analogy: Valves with similar materials, processes, dimensions, and stress have similar durability; good test records must be kept.
7) The purpose of testing is to find weak points: Durability testing by high-level manufacturers is to discover component damage locations, thereby improving design, materials, or processes, not merely to prove product excellence.
If a valve has a specific application, special requirements of the application (such as drastic load changes, special hydraulic fluids) must be investigated and targeted testing conducted.
Create similar environments to assess valve performance, such as high and low temperature, corrosion resistance (salt spray), vibration, shock, explosion-proof testing, etc.
Valves with excellent performance come from repeated testing and improvement. Relying solely on drawings or measurements is insufficient to manufacture good valves. Testing can help understand the working method of valves, compare product gaps, and eliminate problems.
Standards have been formulated to summarize experience and facilitate communication:
International standards generally only recommend testing methods and do not specify performance indicators; Chinese national standards mostly adopt or modify international standards.
In Chinese machinery industry standards (JB), type testing corresponds to research and development testing, and factory testing corresponds to production testing.
Industry testing standards for some valves:
Standards generally include technical terminology, test conditions, test items and methods, test accuracy, etc., but it should be noted:
1) Standards are formulated by people and may have shortcomings. Technological development will bring new situations, so they can be selected according to actual conditions and need not be blindly implemented.
2) Chinese hydraulic testing standards are recommendatory. Differences in hydraulic fluid, temperature, and other parameters among different enterprises may affect result comparability; performance indicators in JB standards are minimum requirements, and enterprise standards should be higher.
3) World-class large companies basically do not implement Chinese test standards, and high-end products in China often do not implement them either. Enterprises need to formulate higher standards to manufacture high-end products.
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