Today, we’re turning our attention to a critical component of many industries – steam flow meters. These unsung heroes play a pivotal role in ensuring the efficient and safe transportation of steam within pipelines.
Steam, with its unique characteristics and applications in various sectors like energy generation, manufacturing, and HVAC systems, demands specialized flow measurement solutions. In this article, we’ll explore the fascinating world of flow meters designed expressly for steam pipelines.
Steam Flow Meter
Steam flow meters cannot be evaluated like other items of energy-saving equipment or energy-saving schemes. The steam flow meter is an essential piece of equipment for effective steam housekeeping. It provides the knowledge of steam usage and cost, vital to an efficiently operated plant or building. The main benefits of using steam are factors like Plant efficiency which adjusts actual conditions to ideal conditions, energy efficiency by using steam meter output to optimize energy use, and steam control, which determines the correct pressure and temperature needed. Control valve adjustment, steam custody transfer, and many more.
Steam measurement presents far more challenges than liquid flow measurement. Most steam flow meters assess the velocity or volumetric flow of the steam, and unless done correctly, the physical qualities of steam will impede the ability to precisely measure and define a mass flow rate.
Steam, a compressible fluid, means a reduction in pressure results in a reduction in density. Temperature and pressure in steam pipes are constantly changing. Changes in the system’s dynamics, control system operation, and instrument calibration can lead in considerable distinctions between actual pressure/temperature and a meter’s design parameters. Accurate steam flow measurement generally requires measuring the fluid’s temperature, pressure, and flow. This data is sent to a flow computer or electronic device (either internal or external to the flow meter electronics), and the flow rate is rectified (or compensated) based on actual fluid conditions.
The temperatures associated with measuring steam flow are frequently fairly high. These temperatures can have an impact on the accuracy and lifespan of measuring electronics. Steam users know that some flow manufacturers use close-tolerance moving parts that can be affected by moisture or impurities in the steam. Improperly designed or installed components can leak steam systems and impact plant safety. Poor-quality steam’s erosive nature can destroy steam flow sensor devices, resulting in errors and/or device failure.
The challenges of measuring steam can be simplified by measuring the condensed steam or condensate. Measuring condensate (i.e., high-temperature hot water) is an accepted practice, often less expensive and more reliable than steam measuring. Inaccuracies in condensate measurement are caused by unaccounted-for system steam losses, depending on the application. These losses are frequently difficult to detect and quantify, affecting condensate measurement precision.
Types of Flow Meters
DP flow transmitters and primary elements account for over half of the flow meters sold for steam flow applications. DP flow transmitters sold for steam flow have many advantages and disadvantages as those sold to measure liquid and gas flow. Pressure drop, interference with the flow stream, and wear over time are all disadvantages. DP flow transmitters also have limited rangeability and rely on the square-root method for calculating flow.
On the one hand, DP flow transmitters offer some compelling steam flow measurement advantages. They are inexpensive and simple to install. DP flow is also the most researched and well-understood form of flow measurement. And multivariable DP flow transmitters can monitor more than one process variable, such as differential pressure, process pressure, and temperature, allowing mass flow to be measured.
The principal element employed determines the efficacy of a DP flow transmitter for steam flow. Flow nozzles are well suited for steam flow because they can handle the high temperatures and pressures accompanying steam flows. Steam flow is commonly measured using orifice plates. All the primary elements, including Pitot tubes, Venturi tubes, and wedge elements, can be used to measure steam flow.
Vortex flow meters have some advantages over other new-technology flow-meters when measuring gas and steam flow. Coriolis meters are still being used to monitor gas flow, and the use of Coriolis meters to measure steam flow is just getting started. While ultrasonic meters have been used for many years to measure gas flow, steam flow is a new application. Magnetic flow meters are not capable of measuring gas or steam flow. Multivariable differential-pressure flow meters can measure liquid, gas, and steam. Due to the existence of a primary element, most multivariable DP flow meters have significantly higher pressure drop than vortex meters.
Steam is the most difficult fluid to measure. This is due to the high pressure and temperature of steam and because the measurement parameters vary with the type of steam. Wet steam, saturated steam, and superheated steam are the three basic forms of steam. Steam is frequently measured in industrial facilities and power plants. Vortex meters, in addition to its capacity to withstand high process temperatures and pressures, have a wide rangeability, allowing them to detect steam flow at varied velocities. In process and power plants, steam is often measured from a boiler.
Variable-area meters, commonly known as rotameters, have a limited utility in steam applications. One issue is that many do not have an output signal and must be read manually, however some businesses have created variable-area meters that do. Most process and power plants today are highly automated, making a flow meter that must be read manually obsolete.
Another reason for the limited use of variable-area flow meters is low accuracy. Many variable-area meters have accuracy levels in the +/-5 to +/-10 percent range, which is generally unsuitable for measurements in process and power plants. If end-users want to go to a low-cost meter, they will be likelier to try a DP flow meter than a variable-area meter. Variable-area meters do have the advantage of being low cost, though, so they are a good fit for noncritical measurements where high accuracy is not a requirement.
Turbine flow meters are widely used for liquid and gas applications but have very limited use for steam flow measurement. One significant difficulty is the influence of condensation on measurement accuracy. When condensation occurs in a flowing stream, the fluid becomes a mixture of steam and water, which creates a two-phase flow. The condensate might damage the blades and impair measurement precision.
Turbine flow meters have an advantage since they perform better at low flow rates than vortex flow meters. They also have a favorable turndown ratio. The most favorable condition for steam flow measurement by turbine meters is the measurement of dry steam. However, because changes so readily influence steam in temperature and pressure, steam flow conditions can change very quickly. Steam is most stable at the boiler, although cold places in the line might cause condensation.
Coriolis flow meters are considered an emerging technique for measuring steam flow. They are like turbine meters, having problems dealing with condensation in steam. When steam condenses, water droplets form a two-phase flow, which is a mixture of steam and water. The presence of water interferes with the flow measurement’s accuracy.
Dry steam flow meters can be used to measure flow. However, because steam is easily affected by temperature and pressure changes, steam-flow conditions can change quickly. Before Coriolis meters to be a major element of the solution for steam flow measurement, various technological hurdles must be overcome by Coriolis vendors.
Ultrasonic flow meters have many advantages over both traditional and new-technology flow meters. They are minimally invasive, have high accuracy, cause little-to-no pressure drop, and have no moving parts. However, some technical limitations make steam flow a difficult application for ultrasonic flow meters.
Clamp-on flow meters have one limitation of ultrasonic flow meters. The speed with which an ultrasonic wave travels through metal may differ from the speed of the wave through steam. This can interfere with the accurate calculation of the flow rate of the steam. In addition, the exact thickness of the pipe is only sometimes known, either because there is buildup or deposits on the pipe wall or simply because this is an unknown variable. This problem with pipe thickness affects all clamp-on flow meters, whether they measure liquid, gas, or steam.
The transducers in spoolpiece ultrasonic flow meters might become overheated due to the temperature of the steam. If this occurs, the transducers may be ruined. While some ultrasonic flow meter suppliers have successfully found a way to deal with this issue, it remains an important technical challenge.
Orifice Flow Meter
Historically, one of the most frequent flow meters used to monitor steam flow was the orifice flow meter. The orifice flow meter for steam functions identically for natural gas flow. Orifice flow flow meters are frequently used in steam measurement to monitor boiler steam production, amounts of steam provided to a process or tenant, or in mass balance activities for efficiency calculation or trends.
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