Proper selection and use of steam flow meters

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I. Types of steam flow meters There are many types of flow meters, but relatively few are designed for measuring steam flow. Currently, the main instruments used for measuring steam flow include vortex flow meters, differential pressure flow meters (orifice plates, venturi tubes, elbow tubes), split-rotor flow meters, Anemometer-type flow meters, and float-type flow meters. There are over 60 different types of flow meters for industrial use. This is because, throughout history, there has never been a single flow meter that can be used with any fluid, across any range, under any flow conditions, or in any application scenario. Each type of flow meter has its own specific areas of application as well as its limitations. If the flow meter is not selected properly, the flow measurement will definitely be inaccurate. Moreover, flow measurement is a complex technique, and there are a wide variety of flow meters available. Even for a specific application, selecting an appropriate flow meter becomes a highly technical task that requires careful and thorough consideration of many factors related to measurement issues before making a final decision. There are 5 main factors in selecting a flow meter: the properties of the fluid being measured, the production process conditions (temperature, pressure, etc.), installation requirements, maintenance needs, and the characteristics of the flow meter itself. For steam metering, the same 5 factors need to be considered as well. The most commonly used meters for measuring steam flow in the market are vortex flow meters, orifice plate differential pressure flow meters (commonly used for large diameters), and split-vane flow meters (mainly mechanical; electronic digital models are also available nowadays). The following mainly introduces the features of these three types of steam flow meters, as well as their installation and operation requirements. II. Introduction to the characteristics of various flow meters 1. Vortex flow meter: The vortex flow meter is a new type of flow meter developed based on the Karman vortex street principle. Thanks to advantages that other flow meters do not possess, it has seen rapid development since the 1970s. It is reported that in developed countries such as Japan, Europe, and the United States, the use of vortex flowmeters has increased significantly; they are now widely used in various fields and are set to play a dominant role in flow meters in the future. They represent an ideal alternative to orifice flowmeters. It has the following features: ① It has a simple and robust structure with no moving parts, ensuring high reliability over long-term operation ； ②It is very easy to maintain, and the installation cost is low ； ③The sensor does not come into direct contact with the medium, offering stable performance and a long service life ； ④It outputs pulse signals proportional to the flow rate, with no zero drift, high precision, and is easy to connect to a computer ； ⑤It has a wide measurement range, with a range ratio of up to 1:10⑥. It experiences low pressure loss, resulting in lower operating costs and greater energy savings ； ⑦Within a certain range of Reynolds numbers, the frequency of the output signal is not affected by the physical properties or composition of the fluid; the instrument coefficient depends only on the shape and size of the vortex generator. No compensation is required to measure the volumetric flow rate of the fluid, and there is no need to recalibrate the instrument’s coefficients after replacing the components ； ⑧Wide range of applications; capable of measuring flow rates for both gases and liquids ； ⑨The calibration cycle is 2-4 years. At the same time, vortex flowmeters also have the following limitations: ① Vortex flowmeters are velocity-type flowmeters, and the stability of vortex separation is affected by the flow velocity; therefore, they require a certain length of straight pipe section, typically 10D in front and 5D behind ； ②When measuring liquids, the upper flow velocity is limited by pressure losses and cavitation, and is generally 0.5-8 m/s ； ③When measuring gases, the upper flow velocity is limited by changes in the compressibility of the medium, while the lower flow velocity is restricted by the Reynolds number and the sensitivity of the sensor; for steam, it is 8–25 m/s ； ④Stress-type vortex flowmeters are sensitive to vibrations; therefore, when installing such flowmeters in pipes with high vibration levels, it is necessary to implement certain vibration-damping measures in the pipes ； ⑤Stress-type vortex flowmeters use piezoelectric crystals as detection sensors, so they are limited by temperature, typically ranging from -40 to +300°C. Figure 1: Integrated vortex flow meter with temperature and pressure compensation. 2. Differential pressure orifice flow meter. The orifice flow meter, also known as a differential pressure flow meter, consists of a primary sensing element (throttle element) and secondary components (differential pressure transmitter and flow display unit); it is widely used for measuring the flow rate of gases, steam, and liquids. It features a simple structure, easy maintenance, stable performance, and reliable operation. ▲The structure of the throttling device is easy to replicate; it is simple and robust, offers stable and reliable performance, has a long service life, and is inexpensive. ▲Orifice plate calculations adhere to international standards and are easy to manufacture. ▲ They have a wide range of applications: they can be used to measure all types of single-phase flows, as well as some mixed-phase flows. ▲The standard throttle device can be put into use without the need for actual flow calibration. ▲The integrated orifice plate is easier to install; it requires no pressure lead pipes, and can be connected directly to differential pressure transmitters and pressure transmitters. Requirements for the pipeline when installing orifice plate flow meters: 1. When installing an orifice plate flow meter, a measuring pipe section should be provided, with straight pipe sections of at least 10DN in front and 5DN behind to improve measurement accuracy. 2. If valves need to be installed before and after the orifice plate flow meter, gate valves are the best choice, and they should be kept fully open during operation ； The control valve should be located in the pipeline 5DN downstream. 3. The inner diameter of the pressure introduction pipeline is related to the pipeline length and the degree of contamination of the medium; typically, pipes with an inner diameter of 8–12 mm are used for lengths up to 45 meters. 4. When measuring liquid flow rate, the horizontal sections of the pressure tap pipes should be at the same level. If a throttling element is installed in a vertical pipe and the pressure tapping tubes are spaced at a certain distance from each other (in the vertical direction), this affects the zero point of the differential pressure transmitter, and it must be corrected through \"zero point shift\". 5. The pressure tapping pipelines must be supported by sturdy brackets; the two pipelines for taking pressure readings should be placed as close to each other as possible, and away from heat sources or sources of vibration. When measuring steam flow, they should be wrapped with insulating material, and if necessary (for example, when the temperature is below 0°C), heating pipes should be used to prevent freezing. When measuring dirty flow, an isolator or settler should be installed. 6. The pressure guiding pipeline must always maintain a single-phase fluid state. When the fluid under test is a gas, the entire pressure conduit (including the pressure chamber of the differential pressure gauge) is filled with the gas phase ； When the fluid under test is a liquid, the pressure transduction pipeline must be entirely filled with the liquid phase; there must be no bubbles at all. To this end, a drain valve should be installed at the lowest point of the pressure tapping line, or an exhaust valve at the highest point; special attention must be paid to this when installing or servicing differential pressure transmitters. Figure 2: Orifice plate differential pressure flow meter. 3. Vane-type steam flow meter. The vane-type steam flow meter is a specialized instrument for measuring the cumulative mass flow rate of steam, and it is available in both mechanical and intelligent versions ； It provides an accurate reading of the mass of steam flowing through the meter, via on-site display on an indicator dial (mechanical type) or an LCD screen (smart type); it can also output pulse signals, and features either manual pressure compensation or automatic temperature compensation. Due to its simple structure, moderate price, and wide measurement range, this flow meter is widely used in industries such as petroleum, chemicals, textiles, and heating, where steam flow measurement is required. As the steam to be measured flows through the flow meter body, it is throttled by an orifice plate. Under the action of the pressure difference before and after, a portion of the steam passes through nozzles to enter the diverter tube; the jet generated thereby drives the vanes at the upper end of the rotor shaft to rotate. At the lower end of the rotor shaft, there are two damping blades that rotate within a damping chamber filled with damping fluid, allowing the vanes to fully absorb the kinetic energy of the jet. When the rotational torque of the rotor is in balance with the damping torque, the rotational speed of the rotor is proportional to the mass flow rate of the steam flowing through the flow meter. Through adjustments and scaling, the actual cumulative steam flow rate is reflected on the indicator dial. Figure 3: Shunt rotor flow meter. 3. Precautions to be taken during the use of steam flow meters: 1. The density compensation for steam must be done in a scientific and accurate manner. 　　To accurately measure the mass flow rate of steam, it is necessary to take into account changes in steam pressure and temperature, and compensate for the steam density using a flow integrator. The platinum resistance used for measuring steam temperature must be installed in accordance with relevant standards: the temperature-sensing platinum resistor should be placed at the center of the pipe, located 5 times the pipe diameter downstream of the flow meter, and the pipe where the resistor is installed should be insulated to ensure accurate temperature readings. When measuring vapor pressure, it is important to note that if a pressure tap is used, zero point adjustment must be performed (since the gravity of the condensate in the pressure tap can cause a difference between the pressure measured by the pressure transmitter and the actual pressure, leading to errors in density compensation); this adjustment can also be made within the flow integrator. The pressure transmitter is installed 4 times the pipe diameter downstream of the steam flow meter; the valves and gaskets in front of the pressure transmitter must be in good condition and unobstructed to ensure accurate measurement of steam pressure. If compensation is achieved by setting pressure and temperature, the values set should be as close as possible to the actual values; otherwise, the error will be large, and this method is generally not recommended. 　　It is essential to correctly set the operating mode of the steam flow meter in the flow integrator, as this is crucial for the accurate calculation of steam costs. In applications where it is difficult to determine the steam condition accurately, it is recommended to use an intelligent flow integrator, combined with platinum resistors and pressure transmitters for temperature and pressure compensation, so as to achieve the most accurate measurement of the steam mass flow rate. 　　2. Proper installation of straight pipe sections upstream and downstream of the steam flow meter. For traditional vortex flow meters, the required lengths of straight pipe sections ahead of and behind the meter are 20 times the pipe diameter and 5 times the pipe diameter respectively (this is a technical requirement ensuring that there are no obstacles such as valves in front of the flow meter) ； A straight pipe section must be added in the presence of obstacles; see the manufacturer’s instructions for details). If the straight sections upstream and downstream are insufficient, it will result in inadequate development of steam flow within the pipeline, causing distortions in the velocity distribution profile. Users can adjust the flow velocity distribution in the pipeline by installing a flow straightener or adding a straight pipe section before the steam flow meter, thereby ensuring that the fluid at the steam flow meter is in a fully developed state. For some large-diameter steam flowmeters, it is more important to meet the installation requirements for the straight pipe sections upstream and downstream. 　　3. The range ratio of a steam flow meter should be reasonable. The range ratio refers to the ratio between the maximum and minimum flows that a flow meter can measure while still maintaining a given level of accuracy. Users should select a flow meter based on their actual usage volume; theoretically, the range of the steam flow meter under consideration should fully cover the user’s usage range. Using the steam flow meter beyond the upper flow limit or below the lower flow limit will result in severe inaccuracies in its measurements. For example, for a vortex flow meter with an actual average flow rate of 5 t/h, a vortex flow meter with a diameter of 150 mm is generally suitable; however, when the flow rate drops to 0.3 t/h or exceeds 15 t/h, the meter suffers from serious measurement inaccuracies. 　　4. Vibration and electromagnetic interference present at the site should be avoided. The vortex flow meters, which are most commonly used in steam measurement, are sensitive to mechanical vibration due to their design principles; as a result, their measurement results can be affected by such vibrations. It is necessary to provide reliable support for the pipe sections before and after the steam flow meter, as well as install vibration-damping components. If pipe vibration is inevitable, it is advisable to use steam flow meters with stronger resistance to interference, such as gas ultrasonic flow meters, intelligent vortex flow meters, or differential pressure flow meters. If there is vibration interference at the location of the steam flow meter, it will generate low-frequency pulse signals that affect the in-use vortex-type steam flow meter. The steam flow meter then transmits these pulses as flow signals to the flow integrator, resulting in an accumulation of flow volume; as a result, some vortex-type steam flow meters continue to show a certain amount of accumulated value even when no steam is flowing for a period of time. This is why the flow meter keeps counting even when no steam is used; therefore, steam users must, even when they are not using steam, record the initial reading together with the steam supply party, in order to prevent the steam flow meter from counting unnecessarily. 　　5. It is important to conduct regular inspections in accordance with the law. The Metrology Law and GB17167-2006 \"General Rules for the Provision and Management of Energy Measurement Instruments in Energy-Using Units\" specify that measurement instruments and energy measurement instruments subject to mandatory inspection must be checked regularly; any instrument that fails to meet the required standards after such inspection shall not be used. Steam flow meters must be calibrated regularly, as this is a prerequisite for ensuring accurate measurement of steam flow. Therefore, end-users must send the steam flow meters they use to the local authorized metrology institutions every year for calibration. If the steam flow meter passes the calibration tests but still appears to give inaccurate readings in actual use, the user should look for solutions by checking aspects such as whether the meter is installed correctly, its appropriate selection, the settings related to its operating conditions, external interference, and any changes in the actual conditions of the steam.