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This post was last edited by henglinkou on 2021-8-23 at 20:34. Post No. 55 in the series “Lao K talks about various aspects of the petrochemical design industry”: Pressure representation in engineering design. Engineering technicians are familiar with pressure, but they may not always know how to represent it in the most appropriate way; improper representation of pressure can lead to confusion or errors. The representations of pressure include absolute pressure, gauge pressure, and vacuum level. Absolute pressure is the true pressure; gauge pressure equals absolute pressure minus atmospheric pressure, while vacuum level equals atmospheric pressure minus absolute pressure. How should these three methods be used appropriately in engineering design? Gauge pressure is independent of atmospheric pressure, while absolute pressure and vacuum degree are both related to atmospheric pressure. Atmospheric pressure is related to altitude, temperature, and humidity. The relationship between atmospheric pressure and altitude is such that as altitude increases, atmospheric pressure decreases. Within a range of 3000 meters, for every 12 meters increase in altitude, atmospheric pressure drops by 1 mmHg, which is approximately 133 Pa ； Relationship between atmospheric pressure and temperature: The higher the temperature, the lower the pressure ； Relationship between atmospheric pressure and humidity: The higher the humidity, the lower the pressure. Therefore, the atmospheric pressure varies in different altitude areas; the atmospheric pressure in the inland regions is around 101.3 KPaA, while it is around 90 KPaA in the Tibetan Plateau region ； The atmospheric pressure in the same area also fluctuates. In engineering design, gauge pressure is often used as a given, or it is thought that the way it is expressed doesn’t matter. The following situations warrant attention: 1. When precise control of pressure is required, it is recommended to use absolute pressure. For example, if the operating pressure of a reactor needs to be controlled at around 500 KPaA with high precision, using gauge pressure would not be suitable in such cases. If the first project is constructed on the mainland, it is acceptable to use a gauge pressure of 400 KPaG; there is no issue with this. However, if a similar project is built in Qinghai, people often fail to notice and still use 400 KPaG to indicate the reaction pressure, which leads to problems. 2. For vacuum vessels, especially those with high and precise requirements for vacuum level, it is recommended to use gauge pressure for representation; the pressure gauge installed on such vessels should be a gauge pressure gauge that displays the gauge pressure directly. Expressing it in gauge pressure can lead to confusion and ambiguity. For example, if the absolute operating pressure of a vacuum vessel is 10 KPaA, and the project is located in Jiangsu where the atmospheric pressure is 101.3 KPaA, then the gauge pressure of this vacuum vessel is 10 – 101.3 = -91.3 KPaG. The degree of vacuum is 101.3 – 10 = 91.3 KPa ； If the project is located in a place in Qinghai where the atmospheric pressure is 90 KPaA, the gauge pressure of this vacuum vessel is 10 – 90 = -80 KPaG, and the degree of vacuum is 90 – 10 = 80 KPa. 3. In some manuals and specifications, or during certain calculations, pressure ratios may occur; these manuals and specifications sometimes use gauge pressure directly for such ratios, without clearly indicating whether it is a gauge-pressure ratio or an absolute-pressure ratio. If both P1 and P2 are gauge pressures, then the gauge pressure ratio is P2/P1, while the absolute pressure ratio is (P2 + atmospheric pressure)/(P1 + atmospheric pressure); obviously, these two ratios are different. It is incorrect to use gauge pressure ratios to calculate pressure ratios; all pressure ratios should be absolute pressure ratios. Tip: You can enter “Lao K’s Weekly Updates – Talks on Matters in the Petrochemical Design Industry” into the website’s search engine to view previously posted articles