【2026 Flow Measurement】Selection of Steam Flow Meters – Saturated Steam and Superheated Steam
Thread Content
Steam is one of the most common fluids in the industrial sector, but many people are not clear about the difference between \"saturated steam\" and \"superheated steam\", which can lead to confusion when making selections – using a gauge designed for measuring saturated steam to measure superheated steam results in wildly inaccurate readings.
I. Saturated steam and superheated steam: Saturated steam: At a certain pressure, when water is heated to the point of boiling and begins to vaporize, the temperature of the steam equals the saturation temperature at that pressure. At this point, the liquid and gas are in dynamic equilibrium: evaporation and condensation occur simultaneously, and there is a fixed one-to-one relationship between the temperature and pressure of the vapor. For example: at a pressure of 0.10 MPa (1 atmosphere), the saturation temperature is 99.09℃ ； At a pressure of 4.05 MPa, the saturation temperature is 249.18°C.
Superheated steam: When saturated steam is further heated, and its temperature exceeds the saturation temperature at that pressure, it becomes superheated steam. The temperature above this value is called superheat. The temperature and pressure of the superheated steam are no longer fixed; they increase as heat is added.
II. From the perspective of heat usage: Why should it not be used indiscriminately? Saturated steam is the “best choice” for heat exchange equipment. Steam heat exchange relies primarily on the latent heat of condensation, which occurs when vapor turns into liquid – 1 kilogram of saturated steam at 100°C, when condensed into water at 100°C, releases approximately 539 calories of heat, with no change in temperature. Due to its high heat transfer coefficient and large latent heat, saturated steam requires a smaller heat exchange area for the same heat demand, resulting in lower equipment investment.
Superheated steam is much more troublesome. Superheated steam must first be cooled to the saturation temperature, releasing very little sensible heat (only about 0.44 calories per 1°C drop), before it can release latent heat through condensation. This process has a low heat transfer coefficient, low production efficiency, and requires a larger heat exchange area. Experience data shows that for every 2°C increase in superheat, the heat exchanger requires a 1% increase in its heat exchange area.
Therefore, in the vast majority of heat application scenarios, saturated steam is a more economical and efficient choice. Superheated steam is generally used in special applications such as driving turbines in power plants.
III. Main selection options for steam metering Steam flow meters mainly include vortex flow meters, differential pressure flow meters (orifice plates), elbow flow meters, and other types.
1. Vortex flow meter – the “workhorse” for steam measurement. The vortex flow meter is currently the most common choice for measuring industrial steam, and it is also one of the preferred options for steam flow measurement. Core advantage: No moving parts: simple and robust structure, high reliability, and low maintenance requirements. Wide range ratio: up to 1:10 or more. Low pressure loss: low operating costs. Wide range of applications: suitable for gases, liquids, and vapors. A vortex flow meter calibrated with air was used for actual steam flow testing, with the comparison error being able to be kept within 2.5%.
Limitations to note: Requirements exist for the straight pipe sections: generally 10D in front and 5D at the back. It is sensitive to pipeline vibration; sections with high vibration levels require the installation of damping measures. Temperature limit: Piezoelectric sensors generally operate in the range of -40°C to +300°C. When measuring steam, temperature and pressure compensation must be performed to convert the volumetric flow rate under operating conditions into a mass flow rate.
2. Differential pressure flow meter (orifice plate flow meter) – a classic and proven solution. Orifice plate flow meters have the longest history, are supported by international standards, offer high theoretical accuracy, and are widely used in steam measurement. Advantages: Simple structure, high versatility, long service life, easy signal transmission over long distances, and accuracy of ±0.5% to ±1%. It also has many disadvantages: high pressure loss (resulting in high electricity consumption during long-term operation), a narrow range ratio (about 3:1), complicated installation and maintenance, and increased errors due to the wear of the sharp edges on the orifice plate.