Compared with other liquid flow measurement, the measurement of steam flow is one of the most challenging measurement solutions. Most steam flow meters measure the speed or volume flow of steam. Unless carefully done, the physical properties of steam will impair the ability to measure and precisely define mass flow.

For steam, energy is mainly contained in the latent heat and to a lesser extent in the sensible heat of the fluid. When the steam condenses into water, the latent heat energy is released. If the temperature of the condensate decreases further, additional sensible heat energy can be released. In steam measurement, the energy content of steam is a function of steam mass, temperature, and pressure. Even after the steam releases its potential energy, the thermal condensate retains considerable thermal energy, which may or may not be recovered (and used) in a constructive manner. The energy manager should be familiar with the entire steam cycle, including steam supply and condensate recovery.

Compared with other liquid flow measurement, the measurement of steam flow is one of the most challenging measurement solutions. Most steam flow meters measure the speed or volume flow of steam. Unless carefully done, the physical properties of steam will impair the ability to measure and precisely define mass flow.

Vapor is a compressible fluid; therefore, a decrease in pressure causes a decrease in density. The temperature and pressure in the steam pipes are dynamic. Changes in system dynamics, control system operation, and instrument calibration can cause considerable differences between actual pressure / temperature and instrument design parameters. Accurate steam flow measurement usually requires measuring the temperature, pressure, and flow of the fluid. This information is transmitted to an electronic device or a flow computer (inside or outside the flow meter electronics) and the flow is corrected (or compensated) based on actual fluid conditions.

The temperature associated with steam flow measurement is usually high. These temperatures affect the accuracy and lifetime of the measurement electronics. Some measurement techniques use tight-tolerance moving parts that may be affected by moisture or impurities in the steam. Improperly designed or installed components can cause leaks in the steam system and affect plant safety. The corrosive nature of poor quality steam can damage the steam flow sensing element and cause inaccuracies and / or equipment failure.

The challenge of measuring steam can simplify measuring condensing steam or condensate. The measurement of condensate (ie, hot water) is a generally accepted practice and is generally cheaper and more reliable than steam measurement. Depending on the application, the inherent inaccuracies in condensate measurement are due to unaccounted system steam losses. These losses are often difficult to find and quantify, affecting the accuracy of condensate measurement.

The volume measurement methods used in steam measurement can be divided into two operating designs:

Different pressure

Speed ​​measurement technology.

differential

For steam, three differential pressure flow meters are highlighted: orifice plate flow meters, Annubar flow meters and spring-loaded variable area flow meters. All differential pressure meters rely on the velocity-pressure relationship of the flowing fluid to operate.

Differential Pressure-Orifice Flowmeter

Historically, orifice flow meters are one of the most commonly used flow meters for measuring steam flow. Orifice flow meters for steam have the same function as natural gas flow. For steam measurement, orifice flow meters are commonly used to monitor boiler steam production, the amount of steam delivered to a process or tenant, or for mass balancing activities for efficiency calculations or trends.

Differential pressure-Annubar flowmeter

Anuba flow meters (a variant of a simple pitot tube) also take advantage of the velocity-pressure relationship of the flowing fluid. The device that causes the pressure change is a pipe inserted into the steam stream.

Differential Pressure-Spring Type Variable Section Flowmeter

Spring-loaded variable area flow meters are a variant of rotor flow meters. There are alternative configurations, but in general, flow is used for spring-mounted floats or plugs. The float can be shaped to give a linear relationship between pressure differential and flow rate. Another variation of the spring-loaded variable area flowmeter is a direct online variable area flowmeter that uses a strain gauge sensor on the spring instead of a differential pressure sensor.

speed

The two main types of velocity flow meters used for steam flow, turbines, and vortex shedding all sense some flow characteristics that are proportional to the velocity of the fluid.

Speed-Turbine Flow Meter

The multi-blade impeller-like device is located in the fluid flow in the turbine flow meter and is horizontal to the fluid flow. As the fluid passes through the turbine blades, the impeller rotates at a speed that is related to the speed of the fluid. Blade speed can be sensed through a variety of technologies, including magnetic pickups, mechanical gears, and photovoltaic cells. The pulse generated by the blade rotation is proportional to the velocity of the fluid and therefore proportional to the flow velocity.

Speed-Vortex Flowmeter

Vortex flowmeters sense flow disturbances around a stationary body (called a fluid blocker) located in the middle of a fluid flow. When the fluid flows around the obstructing fluid, eddies or vortices are generated downstream; the frequency of these eddies is proportional to the velocity of the fluid.