Interrelationships between pressure types

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Pressure is a crucial parameter. It not only helps us monitor the operating status of the equipment, but it is also directly related to the efficiency of the entire thermal system. Depending on the reference standard, pressure can be divided into several types: gauge pressure, absolute pressure, vacuum, negative pressure, and back pressure. 01 Gauge Pressure Gauge pressure refers to the pressure value relative to atmospheric pressure, and is usually measured directly by a pressure gauge. The pressure of equipment in a factory is often referenced with atmospheric pressure as the zero point; in other words, when the gauge pressure is zero, it means that the pressure inside the equipment is equal to atmospheric pressure. Application example: For instance, in steam pipes, the pressure shown on a pressure gauge is usually gauge pressure. If the gauge pressure in the main steam pipe is 9.8 MPa, this means that the pressure of the steam is 9.8 MPa higher than the local atmospheric pressure. Gauge pressure is commonly used for daily monitoring to help operators understand the operating condition of the equipment in real time. 02 Absolute Pressure: Gauge pressure refers to the pressure value relative to atmospheric pressure, and is usually measured directly by a pressure gauge. The pressure of equipment in a factory is often referenced with atmospheric pressure as the zero point; in other words, when the gauge pressure is zero, it means that the pressure inside the equipment is equal to atmospheric pressure. Application example: Taking the condenser as an example, if the vacuum level of the condenser is 95 kPa and the local atmospheric pressure is 101 kPa, then the absolute pressure inside the condenser is 6 kPa. When performing thermodynamic calculations, absolute pressure is more important than gauge pressure, as it is directly related to the thermodynamic state of the steam and energy transfer. 03 Vacuum A vacuum usually refers to a state with pressure lower than atmospheric pressure, and it is typically measured by the degree of vacuum, which is the difference between atmospheric pressure and absolute pressure. The condenser needs to maintain a high vacuum level, which helps improve the efficiency of steam condensation and thus allows for the recovery of more heat. Application example: For a 300 MW unit, the vacuum level can reach over 90 kPa when the condenser is operating normally. At this point, the absolute pressure of the condenser is very low, which promotes rapid condensation of steam and thereby improves thermal efficiency. 04 Negative Pressure Negative pressure refers to a condition where the pressure is lower than atmospheric pressure. Although it is similar to vacuum level, it focuses more on describing the pressure difference within a system. Negative pressure is often used to describe a situation where the pressure inside a device is lower than the external atmospheric pressure. Application example: In the boiler exhaust system, negative pressure is used to prevent smoke leakage. The exhaust fan maintains a negative pressure in the boiler’s rear flue, thereby preventing smoke from escaping and ensuring the safety of the equipment as well as optimal combustion efficiency. For example, the negative pressure in the boiler’s exhaust flue may be -500 Pa, meaning that the pressure inside the flue is 500 Pa lower than atmospheric pressure. 05 Back Pressure: Back pressure is the reverse pressure exerted by the downstream area on the upstream area as fluid flows through pipes or equipment. In the turbines of factories, the back pressure of the exhaust steam serves this purpose. Excess back pressure can affect the expansion process of steam, thereby reducing the efficiency of the turbine. Application example: If the back pressure of the turbine exhaust is too high, steam cannot expand sufficiently within the turbine, resulting in a decrease in power generation. Therefore, maintaining an appropriate back pressure is crucial for ensuring the efficient operation of the turbine. 06 Interrelationships between pressure types Pressure parameters such as gauge pressure, absolute pressure, vacuum, negative pressure, and back pressure are interconnected and influence one another during operation. ● Mathematical relationship: Absolute pressure = Gauge pressure + Atmospheric pressure