I'm Lao Zhou, and I've been working in valve testing for over ten years. Today I want to talk about a phenomenon: many factories measure torque and only record the maximum value. They mark a "√" if it's acceptable and an "×" if it's unacceptable. While this seems simple and efficient, it actually misses a lot of information.
A true story
Last year, we helped a valve factory inspect a batch of ball valves. The factory inspection report showed that the torque of all valves was qualified. However, after installation, more than a dozen valves became stuck, and the on-site engineer reported that "the feel was not right."
We returned to the test bench and retested using the Zhongke Huigan system. The torque values were still within the acceptable range. However, when we checked the torque-angle curve, we discovered a problem: several valves showed a sharp peak when opened to around 45°. The absolute value of this peak was not exceeded, but its duration was very short, and it couldn't be detected by a regular torque wrench.
Upon disassembly and inspection, a small piece of welding slag was found adhering to the valve seat sealing surface. After cleaning, the curve returned to smooth after retesting. If only a single maximum value had been recorded initially, this problem would have been masked.
What can the torque curve tell you?
Peak torque at startup: If it is much greater than the operating torque, it indicates that the static friction is too high, which may be due to the seal being too tight or insufficient lubrication.
Fluctuations during operation: A smooth curve indicates a smooth valve body cavity and good fit. Frequent small fluctuations often indicate scratches, impurities, or misalignment of the sealing surface.
Closed-section rise: The slope of torque increase as the seal approaches full closure reflects whether the seal compression is appropriate. Too steep a rise indicates excessive tightness, while too flat a rise indicates a potentially inadequate seal.
Repeatability: The curves should largely overlap when the same valve is opened and closed multiple times. If the curve is significantly higher the second time than the first, it indicates that the sealing material is wearing or deformed.
Only recording the maximum value is equivalent to only looking at the exam score and not the answer sheet process.
You only know you passed the overall score, but you don't know which question you answered incorrectly. Similarly, you only know the torque is up to standard, but you don't know which stroke section is abnormal. And these abnormalities are precisely the early signs of potential malfunctions.
My suggestion
Factory inspection: In addition to recording the maximum torque, retain the torque-steering angle curve for each test. In case of customer complaints, the curve is the strongest evidence.
When selecting testing equipment, prioritize systems that can record the entire curve and automatically store the data. Manual wrenches and simple testing instruments should be phased out.
Develop the ability to read curves: Teach testers to analyze curve shapes, rather than just looking at traffic light-style "pass/fail" indicators.
Summarize
Torque is not an isolated number; it's the "electrocardiogram" of a valve's entire lifecycle, from closing to opening. Only by clearly observing the entire curve can you accurately determine whether the valve is truly healthy.
