1. Gauge pressure
Gauge pressure refers to the pressure value that is higher than the atmospheric pressure based on atmospheric pressure, and is usually measured by a pressure gauge. In thermal power plants, the pressure gauge on the steam pipe shows the pressure. For example, the gauge pressure of the main steam pipe might be shown as 9.8 MPa, which means that the steam pressure is 9.8 MPa higher than the local atmospheric pressure.
2. Absolute pressure
Absolute pressure is a pressure value based on an absolute vacuum, which is equal to the gauge pressure plus the local atmospheric pressure. Some thermal calculations and equipment design in thermal power plants require the use of absolute pressure. For example, the absolute pressure in the condenser, if the vacuum degree of the condenser is 95kPa, and the local atmospheric pressure is 101kPa, according to the absolute pressure = atmospheric pressure - vacuum, the absolute pressure is 6kPa.
3. Vacuum
Vacuum refers to the state below atmospheric pressure, and its magnitude is usually expressed in vacuum, which is the difference between atmospheric pressure and absolute pressure. The condenser of the thermal power plant is to maintain a high vacuum state to improve the work efficiency of the steam. For example, a 300MW condenser may have a vacuum level of more than 90 kPa during normal operation.
4. Negative pressure
Negative pressure is also essentially a state of pressure below atmospheric pressure, similar to the concept of vacuum, but more focused on describing the situation in which the pressure inside the system is lower than the outside atmospheric pressure. In the boiler induced air system of a thermal power plant, the induced draft fan keeps the flue at the tail of the boiler under negative pressure to prevent flue gas leakage. For example, the pressure in the flue at the tail of a boiler may be -500 Pa, i.e. 500 Pa below atmospheric pressure.
5. Back pressure
Back pressure refers to the pressure generated by the downstream to the upstream during the flow of the fluid. In the steam turbine of a thermal power plant, the exhaust pressure is a kind of back pressure. If the back pressure is too high, the expansion of steam in the steam turbine will not be sufficient, reducing the efficiency of the steam turbine. For example, when the exhaust back pressure of a steam turbine increases, the steam cannot be exhausted without sufficient work, resulting in a decrease in power generation.
6. Mutual relations
• Mathematical relationship: absolute pressure = gauge pressure + atmospheric pressure; Vacuum = atmospheric pressure - absolute pressure; The absolute value of negative pressure is numerically similar to that of vacuum, but the angle of expression is different; There is no direct fixed mathematical relationship between back pressure and several other parameters, but in the specific thermal system, the back pressure will affect the absolute pressure, gauge pressure, etc., such as the increase of the back pressure of the steam turbine, the absolute pressure and gauge pressure of the exhaust steam will also increase accordingly.
• Physical relationship: gauge pressure and absolute pressure are used to measure the amount of pressure within the system; Vacuum and negative pressure are descriptions of the state where the pressure of the system is lower than the atmospheric pressure, the vacuum is more emphasized, and the negative pressure is more focused on the direction of the pressure difference; The back pressure is mainly described from the perspective of the pressure effect of the downstream of the fluid flow on the upstream, and together with other parameters, it affects the operating state and energy conversion efficiency of the thermal system.
• At the outlet of the boiler superheater in a thermal power plant, the gauge pressure is displayed by a pressure gauge, and the operator monitors the steam pressure accordingly. In the overall analysis of the thermal system, it is necessary to convert the gauge pressure to absolute pressure.
• The condenser maintains a high vacuum through the pumping equipment, so that the exhaust steam of the steam turbine can condense quickly. If the vacuum level of the condenser decreases, the back pressure of the steam turbine will increase, resulting in a decrease in the efficiency of the steam turbine and an increase in power generation energy consumption.
• Maintain appropriate negative pressure in the furnace, if the negative pressure is too small, positive pressure may erupt outward; If the negative pressure is too large, a large amount of cold air will leak in, increase the heat loss of exhaust gas, and reduce the efficiency of the boiler.
In the actual operation of thermal power plants, gauge pressure is convenient for direct measurement and observation of equipment operating pressure; Absolute pressure is used for accurate thermodynamic calculations and analysis; Vacuum and negative pressure are essential to maintain the normal operation of condensers, boilers and other equipment; Back pressure is an important indicator to evaluate the performance of equipment such as steam turbines. An accurate understanding and grasp of these pressure concepts and their relationships is essential for the safe and efficient operation of thermal power plants.
