Some questions about pressure relief valves

Insights from the shanzhai industry; in simple terms, a pressure relief valve is a mechanism that achieves equilibrium between the spring force and the pressure behind the valve (the force acting on the valve element at the top and bottom, as well as the pressures on each end). However, this type of valve is suitable for pipelines with normal pressure reduction and constant flow. The drawback of a pressure relief valve is that once the pressure behind the valve exceeds its set value, the pressure will continue to rise slowly. This is particularly evident in pipelines with a large pressure difference. A self-acting pressure reducing valve incorporates a pressure conduit and a cylinder with a large force-bearing area on this basis, in order to control the pilot on the upstream side of the valve as well as the pilot on the downstream side according to requirements. This enables more accurate control of pressure, and overpressure behind the valve is minimal. So, within my control, I’ve changed them all to self-acting types. (For reference)

The pressure relief valve controls the pressure behind the valve, while the backpressure valve controls the pressure in front of the valve

To reduce the size of the boiler, steam boilers are typically designed to operate at high pressures within safe limits. The steam storage space of the same boiler can hold more mass of steam at high pressure; with the same evaporation area, the evaporation rate per unit area decreases at high pressure, which is conducive to producing high-quality steam. Operating at low pressure reduces the output and also causes the steam to contain water; therefore, boilers typically generate high-pressure steam. High-pressure steam has a high density; therefore, for pipes of the same diameter, the mass of high-pressure steam that can pass through is greater than that of low-pressure steam. Therefore, high-pressure steam is used in most steam transmission systems to reduce the size of the transmission pipes. This reduces the investment in pipes and pipe fittings, insulation materials, and supports. Under high pressure, the use of smaller pipes reduces the heat dissipation area and also lowers the pipe losses in steam transmission. Reduce the condensate pressure at the point of use to save energy. Reducing pressure can lower the temperature in the downstream pipes, decrease frictional losses, and at the same time reduce the loss of flash vapor when it is discharged from the steam trap to the condensate collection tank. It is worth noting that if the condensate water is continuously discharged, the energy loss when it is discharged at low pressure decreases due to contamination. Since the pressure and temperature of steam are related to each other, in some processes the temperature can be controlled by regulating the pressure. Such applications can be seen in sterilizers and high-pressure sterilization units; the surface temperature control in contact dryers used in papermaking and corrugation machines operates on the same principle. Pressure control is also the basis for heat exchanger temperature control. Under the same heat load, the volume of a heat exchanger operating with low-pressure steam is sometimes larger than that of one operating with high-pressure steam. Due to the lower design requirements, low-pressure heat exchangers may have a lower cost compared to high-pressure heat exchangers. The structure of the workshop determines that each piece of equipment has its maximum allowable working pressure (MAWP). If this pressure is lower than the maximum possible pressure of the supply steam, the steam must be depressurized to ensure that the pressure in the downstream systems does not exceed the maximum safe operating pressure. Many devices require steam at different pressures. A specific system flashes high-pressure condensate into low-pressure flash steam for use in other processes, thereby achieving energy savings. When the amount of vapor generated is insufficient, it is necessary to maintain a steady supply of low-pressure steam; in such cases, a pressure reducing valve is required to meet this need. Watt Energy Saving believes that pressure reduction is essentially a form of waste, as higher steam pressure leads to higher flue gas temperatures in the boiler, resulting in waste heat emissions and environmental pollution. In general, a higher steam pressure means that the steam at this point has a higher \"quality\". Sometimes, even to improve the \"quality\" of the steam, a small amount of high-pressure steam is used as power to increase the pressure of the steam (especially waste steam and exhausted steam) via a heat pump, thereby expanding its areas of use.