A question from a netizen on HaiChuan regarding the differences between imperial and metric piping systems

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Most of those upstairs have got it right. Many beginners get confused by the metric and imperial systems right from the start. In fact, for most users, it would be ideal if there were only one standard in the world. The purpose of ISO was precisely to unify these standards. As for why it’s so difficult to find a common standard in reality, a typical example is that most countries have their own languages, and often one country has more than one language. No one is willing to give up their own language – you need to understand this on your own. The United States and Germany, which emerged after the Industrial Revolution, were both industrial powers and remained rivals for a long time; it’s natural that they each followed their own paths. When China chose the former Soviet Union, it was essentially choosing the European system (the former Soviet Union adopted Germany’s metric system). But with the collapse of the Soviet Union in the early 1990s and the deterioration of Sino-Soviet relations, China began to introduce large-scale ethylene production facilities and moved closer to the United States. As a result, the petrochemical industry naturally adopted the imperial system in terms of standards. To this day, many standards in the petrochemical industry are based on American standards – this is simply the trend of the times. That’s why the mechanical industry still primarily uses the metric system, while the petrochemical industry has chosen to follow the imperial system. As for the differences between metric and imperial systems, they are directly reflected in variations in dimensions, units, and standards. Taking flanges as an example, the world currently has both the European system (based on metric units) and the American system (based on imperial units). Although there are similarities between them, they are not interchangeable. Different flanges mean that the pipes, fittings, valves, etc., connected to them will also differ. The American system is designed for larger pipe diameters; to address this issue, various series have been developed, such as Series A and B, Series I and II, or what are commonly referred to as metric (meter-based) and imperial systems. Components from the same series can generally be used together in terms of connection dimensions and sealing surfaces, but components from different series usually cannot be mixed. As mentioned earlier, when deciding whether to use metric or imperial units for a new project, design firms don’t simply follow their own habits. Projects involving foreign investment generally opt for the imperial system, as it offers convenience in many aspects. Other factors such as the ease and cost of purchasing materials and equipment also need to be taken into consideration. In the petrochemical industry, the imperial system remains the standard due to the need for international compatibility. Many old specifications are being phased out, but the metric system is still used to facilitate maintenance of older installations. It is hoped that one day there will be fewer classifications in the standards, and that our own specifications will form a cohesive system, rather than the current chaotic and even contradictory situation