Measurement, selection, and price of steam flow meters

Steam flow is generally measured using an orifice plate, with temperature and pressure compensation.

Flow meters can be broadly classified into several categories: (1) Differential pressure flow meters: 1. Pitot tube 2. Orifice plate 3. Venturi tube; (2) Rotameter; (3) Positive displacement flow meters: 1. Elliptical gear 2. Gear wheel 3. Scraper 4. Rotating piston; (4) Turbine flow meter; (5) Electromagnetic flow meter: Suitable for large pipe diameters, but expensive. Water supply must be stopped and pipes disconnected during installation. (6) Vortex flow meter; (7) Ultrasonic flow meter; (8) Mass flow meter; (9) Other flow meters. The measurement of steam flow has always been an important aspect in steam applications. The main purposes of using steam flow meters include: 1) Monitoring the efficiency of energy use ； 2) Improve the control of the manufacturing process ； 3) Measure steam consumption for internal or external billing. Currently, there are many misconceptions regarding steam measurement in China; many users assume that purchasing a high-quality flow meter will ensure accurate measurement results. The measurement of steam differs from that of other fluids such as water and air, and there are many factors that affect its accurate measurement in practical applications. As a result, it is common for a flow meter to pass its own calibration tests yet still appear to give inaccurate measurements in practice. The main factors affecting the accurate measurement of steam flow are as follows: 1) The actual steam flow is below the minimum flow rate that the flow meter can measure accurately (insufficient range ratio) ； 2) Insufficient straight pipe sections installed upstream and downstream of the flow meter (causing flow disturbances) ； 3) Incorrect compensation for steam density (imprecise temperature and pressure measurement) ； 4) Moisture content in steam (no dryness compensation) ； 5) Vibration and interference present at the site (vortex flow meter) ； 6) Differential pressure transmission error (differential pressure flow meter), etc. Let’s discuss in detail the practical issues that affect steam measurement: 1. Insufficient range ratio. The range ratio refers to the ratio of the maximum flow rate to the minimum flow rate that a flow meter can measure within the range where it can maintain a given level of accuracy and repeatability. But we must be careful when it comes to the range ratio, as it is based on the actual flow velocity; the maximum allowable velocity in steam systems is generally 35 m/s, and higher flow velocities can cause erosion and noise in the system. Moreover, the minimum flow velocity allowed by different flow meters varies; generally, vortex flow meters can measure a minimum steam flow velocity of 2.8 m/s. In cases where the range ratio is insufficient, it is necessary to use flow meters with a larger range ratio (the minimum allowable flow velocity for Gilflo ILVA flow meters is 0.6 m/s, with a maximum range ratio of up to 100:1), or to use multiple flow meters in parallel. 2. Insufficient straight pipe sections upstream and downstream: For traditional vortex flow meters or orifice plate flow meters, the required lengths of straight pipe sections before and after them are approximately 20D and 5D respectively. If there are insufficient straight sections upstream and downstream, the fluid will not develop adequately, resulting in vortices and distorted velocity distribution profiles. Distortions in the flow velocity profile are usually caused by local obstructions in the pipeline, such as valves, or bends, whereas vortices are generally induced by two or more spatial (three-dimensional) bends. Insufficient straight sections upstream and downstream can be adjusted by installing flow regulators. The simplest and most effective approach is to use flowmeters that require fewer specifications for the straight sections upstream and downstream, such as the Spiraax ILVA flowmeter (6D upstream, 3D downstream). 3. Incorrect compensation for steam density (imprecise temperature and pressure measurement). To accurately measure the mass flow rate of steam, it is necessary to take into account changes in steam pressure and temperature, that is, to compensate for the steam density. Different types of flowmeters are affected by changes in density in different ways. The signal output of a vortex flow meter is dependent only on the flow velocity, and not on the density, pressure, or temperature of the medium. In the case of a differential pressure flow meter, the mass flow rate depends on the geometric shape of the flow meter, the square root of the differential pressure, and the square root of the density. Therefore, vortex flowmeters are more affected by density changes than differential pressure flowmeters. a) Differences in compensation accuracy: The level of compensation accuracy that can be achieved through temperature compensation and pressure compensation depends not only on the accuracy of the temperature sensors and pressure transmitters, but also on the type of flow meter, the specific operating conditions of measurement, and the range selected for the pressure transmitter. Overall, temperature measurement has a significant impact on compensation accuracy. If temperature and pressure sensors of the same accuracy level are used, the density differences caused by temperature measurement errors will be greater than those caused by pressure measurement errors. For saturated steam at a pressure of 7 barg, temperature is measured using a Class A platinum resistance thermometer with an error margin of ±0.49°C; based on this, steam density tables are used to determine that the flow rate compensation uncertainty is ±0.56%R for differential pressure types and ±1.11%R for vortex flow meter types. If a pressure transmitter of class 0.2 is used for measurement, the error limit is ±2 kPa, with a flow compensation uncertainty of ±0.13%R for differential pressure types and ±0.25%R for vortex flow meter types. b) Factors affecting pressure measurement: In the measurement of steam pressure, the gravity of the condensed water inside the pressure tapping tube causes a certain difference between the pressure measured by the pressure transmitter and the actual steam pressure. If the pressure measurement error is not corrected, it will affect the calculation of steam density and lead to errors in flow measurement. Generally, for the aforementioned phenomenon, zero offset adjustment can be performed at the secondary meter (inside the flow computer), which is both simple and accurate. c) Factors affecting temperature measurement: Based on the actual use of flowmeters, temperature measurement errors are due not only to the inherent errors of the temperature sensing elements but also to improper installation. For example, insufficient insertion depth of the temperature-sensing platinum resistance, failure to fill the protective casing surrounding it with heat-conducting oil, incorrect lateral installation of the platinum resistance, and the absence of an insulation layer on the pipe where it is installed – all of these can result in lower temperature readings. For steam that requires pressure reduction, the flow meter should be installed before the pressure reducing valve; at this point the steam is not under reduced pressure, and the saturated steam compensation method can be used to ensure measurement accuracy. If the flow meter can only be installed behind the pressure reducing valve, the steam may overheat; therefore, temperature and pressure compensation must be carried out simultaneously to ensure accuracy. 4. Effect of steam dryness (saturated steam) At present, most of the flow meters used to measure steam flow are volume flow meters; the volume flow is measured first, and then the mass flow is calculated using the density of the steam, assuming that the steam is completely dry. However, the steam is not completely dry; if the effect of steam dryness is not taken into account, the resulting values will be lower than the actual flow rate. For differential pressure flow meters, the actual steam flow rate and the flow rate measured under the assumption of dry steam (steam dryness = 1) can be approximated using the following formula: Where: = mass flow rate at the actual dryness level, in kg/h = flow rate at dry saturated steam conditions, in kg/h; χ = steam dryness. Therefore, the secondary instrument of the flow meter (the flow computer) should have the capability to set the steam dryness level. However, it is also difficult to determine the steam dryness under actual operating conditions. If the quality of the steam at the inlet of the steam flow meter can be improved, the measurement accuracy of the steam flow meter can also be improved. Therefore, we recommend installing a steam-water separator upstream of the flow meter to improve measurement accuracy. 5. Pipeline vibration: Flowmeters such as vortex street flowmeters are sensitive to mechanical vibrations, and their measurement results can be easily affected by these vibrations; therefore, reliable support systems should be designed for the pipelines upstream and downstream of the flowmeter. If pipeline vibration is unavoidable, a differential pressure flow meter with strong interference resistance should be used, such as the Spillax ILVA flow meter. 6. Differential pressure transmission error (differential pressure flow meter) a) Zero drift: When the differential pressure transmitter is installed and put into use on site, it is often found that the zero output differs from that during the factory calibration. This deviation of the zero output is called static pressure error. The adjustment method involves applying the same static pressure to the positive and negative pressure chambers, opening one of the high/low pressure valves in the three-valve assembly while closing the other, and opening the balance valve. If it is suspected that the chambers have not yet been filled with the medium under test, the air (or liquid) accumulated in them can be removed through the drain valves on those chambers, after which the transmitter’s output can be checked again. b) Improper layout of pressure tapping pipes: The pressure tapping pipes should have a proper slope to allow bubbles that may form inside the pipes to rise to the main pipe more quickly, and to enable impurities and other contaminants in the pipes to sink to the drain valve more rapidly. The pressure lead pipes should be regularly inspected and maintained to ensure there are no leaks or blockages. The inner diameter of the pressure tap tube is related to the properties of the fluid being measured and the total length of the pressure tap tube; for steam systems, the inner diameter is generally around 10 mm. To avoid differences in fluid temperature within the positive and negative pressure transduction tubes, which can lead to density variations and thus transmission distortions, the positive and negative pressure transduction tubes should be arranged as closely as possible to each other. When used in outdoor areas or in extremely cold regions, the liquid in the pressure tap tube may freeze; therefore, heat tracing is required for insulation. However, it is necessary to avoid wrapping the heat tracing tubes directly around the pressure tap tube, as this can cause part of the fluid to vaporize and lead to false readings. Summary: To accurately measure steam flow, users must pay close attention to the following in practical applications: (1) Changes in steam flow rate; it is advisable to choose a flow meter with a large range ratio to avoid inaccurate measurements and flow rate \"loss\" at low flow levels. (2) Due to changes in the pressure and temperature of the steam system, to avoid non-measurable errors caused by fluctuations in pressure or temperature, the steam flow meter should have an automatic steam density compensation function. (3) The effect of saturated steam dryness: attention should be paid to dryness correction, while ensuring that high-quality steam is supplied to the flow meter. (4) Ensure proper measurement of steam flow; proper installation of the flow meter is crucial.

Steam measurement generally uses orifice plates, which are available in integrated and separate types; temperature and pressure compensation are also taken into account, and the cost of an orifice plate plus a differential transmitter is around 7000. The pressure transducer costs around 5,000; platinum resistors are inexpensive. The flow integrator costs about 2,000. It is possible to use one platinum resistor, or two 4-20mA signals.

These days, vortex flow meters are more commonly used for steam measurement. Compared to orifice plates, orifice plates have a relatively narrow range of measurement; typically, the ratio is only 1:4. This means that in order to achieve accurate measurements, the flow rate must remain within a specific range. Vortex flow meters, on the other hand, have a wider measurement range, usually around 1:20. When there are significant fluctuations in the flow rate, vortex flow meters are an ideal choice. In terms of cost, orifice plates are more expensive both at purchase and in terms of maintenance costs, and their performance is much worse when temperature and pressure compensation is required.

Thank you, which company’s products are you using?

We are using a vortex flowmeter

Have you ever used orifice plates with a wide range ratio? How was the experience? Please share your insights!

Vortex flow meters are generally used more often; they are easy to maintain, and when the measurement range is chosen appropriately, their accuracy is also higher than that of orifice plates. Following up on that question: everyone is talking about the temperature and pressure compensation formulas for vortex flowmeters. So, which formula is generally used for such compensation in vortex flowmeters?

I’ve learned it. Thank you for the explanation from above