Discussion on Replacing Orifice Flow Meters with Vortex Flow Meters

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Discussion on replacing orifice flow meters with vortex flow meters; the current technical levels of vortex flow meters and orifice flow meters. The basic structure of a vortex flow meter consists of a vortex generator, sensing elements, and a signal processing and amplification circuit. Research on vortex generators has reached a fairly advanced stage, with triangular-shaped generators being considered the most effective type. Sensing elements include thermistors, strain gauges, piezoelectric crystals, differential capacitors, and ultrasonic sensors. Many parts of the signal processing have been computerized. Vortex flowmeters are easy to install (can be installed directly on pipelines), have a small size, high interchangeability, and maintain high accuracy over long periods of operation; they can be used for measuring most liquids, gases, and vapors. Currently, sales of vortex flowmeters in the global market are increasing by about 30% each year. At present, the technical development of orifice plate flowmeters is still based on established empirical formulas. In 1980, the International Organization for Standardization merged the standards R541 and R781 into standard ISO5167 (1980). Due to its simple structure, low cost, and reliability, the orifice plate throttling device is suitable for measuring almost all types of media. The differential pressure transmitters used in conjunction with it have developed rapidly, compensating for the limitations of the orifice plate device itself. 2. Comprehensive performance evaluation of vortex flowmeters and orifice flowmeters. An orifice flowmeter consists of a throttling element, a pressure-taking device, and a differential pressure transmitter; in locations where freezing is a concern, heating measures are necessary for the pressure guiding tubes. A flow measurement circuit has around 20 static sealing points. The following problems arise during use: it is prone to freezing, clogging, and leakage, and the heating system can cause aging of the components in the differential pressure transmitter. In some cases, an isolating fluid must be used in the pressure guiding tubes. Due to heating issues or unstable process operations, the liquid levels in the positive and negative pressure guiding tubes often differ, resulting in a liquid column difference that leads to inaccurate flow readings. All of the above will cause changes in the flow coefficient and reduce measurement accuracy. By shortening the pressure guide tube and installing the differential pressure transmitter directly on the pipeline, there is still a dead zone for flow. The vortex flow meter has only 3 static sealing points, making it less prone to leakage; it has no dead zones in the flow path, does not require heating for insulation, and is not affected by factors such as fluid density, temperature, pressure, or viscosity. Its flow coefficient remains constant over time. However, when vortex street meters are used in environments with vibrations, it can lead to inaccurate flow measurement. Currently, vibration-resistant vortex flowmeters have been introduced to counter the impact of vibrations on inaccurate flow measurement. (1) Initial investment: An imported vortex flow meter costs approximately 20,000 yuan (for DN15–DN50 sizes), while a throttling device that includes a differential pressure transmitter, orifice plate and flanges, pressure guide tubes, valves, as well as an insulation or protection box costs 15,000 yuan. From a long-term perspective, it is still cost-effective to use vortex flow meters. (2) Installation costs: Vortex flowmeters are simple to install; it is only necessary to ensure that there are sufficient straight sections of pipe before and after the flowmeter. However, orifice plates require specific requirements regarding the straight sections, concentricity, pressure guiding tubes, transmitters, and insulation enclosures. As a result, the installation costs for these devices are several times higher than those of vortex flowmeters. (3) Maintenance costs: Vortex flowmeters generally do not experience failures, aside from the need for periodic calibration for accurate measurement, whereas orifice plates do. Tasks such as sealing leaks, regular cleaning, filling with isolation fluid, replacing pressure transmitters and valves, insulating the device, and cleaning the orifice plate require certain level of maintenance. For example, in 200 flow orifice plate measurement circuits that require insulation and heating, certain maintenance costs are needed every two years to upgrade the insulation and heating systems; this does not include the cost of replacing differential pressure transmitters or orifice plates. Calculations show that this amount is sufficient to purchase a certain number of imported vortex flow meters. (4) Operating costs: 1. Steam consumption cost – For 200 flow meter circuits that require insulation and heating, each heating point consumes 0.02 tons of steam per hour. Assuming an average of 4,300 hours per year, and with a steam cost of 40 yuan per ton, the annual steam consumption cost amounts to approximately 688,000 yuan; thus, each circuit incurs an annual cost of 0.344 million yuan. 2. Energy consumption cost: The pressure loss of vortical flow meters is smaller than that of orifice plates, at about 1/15 of that of orifice plates. Therefore, long-term operation results in lower energy consumption for pumps and fans. The orifice plate is 15 times that of a vortex street; when used for measuring gas or steam flow, due to its low density, the volumetric flow rate for a given pipe diameter is high, resulting in greater pressure losses and higher energy consumption. 3. Fines for pollution emissions: The fee for pollution control depends on the number of times waste is discharged; generally, it’s around 20 times per year. When the waste and materials emitted contaminate the atmospheric environment or when the wastewater exceeds regulatory standards, the environmental protection authorities impose fines as well. (5) Long-term operating accuracy: The design accuracy of the orifice plate system is 1.5%-2.5%. Since there is a non-linear relationship between differential pressure and flow rate, the error increases when the flow rate is below 30%, and this issue is particularly severe in the case of gases. On the other hand, long-term wear of the medium causes sharp edges to become blunter, which alters the flow coefficient and is also an important factor affecting accuracy. Due to the special structure of vortex streets, once the accuracy is determined in practice (around 0.5%-1% of the measured value), it remains almost constant. (6) Interchangeability: The same vortex flow meter can be used to measure the flow rate of gases, liquids, and steam. The electronic circuit boards and sensitive components are universal for flow meters of different diameters. Signal output typically takes three forms: pulse, analog, and digital signals. It can be switched using the switch on the circuit board, allowing the user to switch to another output signal at any time according to their needs. Changing the range is also very easy; for analog output, it is sufficient to adjust the input pulse frequency. The orifice plate is different: its orifice diameter is designed for a specific medium, it has poor interchangeability, and it is difficult to change the range; in such cases, the orifice plate must be recalculated. (7) Range ratio: Since the output frequency of vortex flowmeters is linearly related to flow rate, the range ratio for flow rate can reach 20:1–80:1. For example, Foxboro’s vortex flowmeters have a range ratio of 80:1 for measuring gases and vapors, and 40:1 for measuring liquids; this allows them to meet the requirements of high flow rate measurements while still maintaining accuracy. The differential pressure of a differential pressure flow meter is nonlinearly related to the flow rate; therefore, measurements are inaccurate at low flow rates, and the range ratio is only 3:1–5:1. If the range ratio is to be increased, it is necessary to find ways to improve the accuracy of the differential pressure transmitter. (8) Reliability: A more reliable vortex flow meter has now been developed, one that features two sets of electronic circuits and two sets of sensitive elements mounted on the same gauge body, with these elements being independent of each other. This makes it suitable for use in critical flow measurement applications, something that is difficult to achieve with differential pressure flow meters. (9) Energy-saving effect: If the steam consumption is 120 t/h, with a pressure of 3.9 Mpa and a temperature of 445°C, and if a perforated plate is used to measure the steam, this results in a pressure loss of 0.03–0.05 Mpa. Assuming a pressure loss of 0.03 Mpa, the electrical energy consumed by such a perforated plate is 82 kW. Over 300 days per year, this amounts to 590,000 kW/h of energy consumption; converted to standard coal, this is equivalent to 68.5 tons. If vortex flow meters are used, the energy consumption is only 1/15–1/20 of that of perforated plates. Using 1/20 as an estimate, the annual coal consumption is only 3.34 tons, which represents 3.33% of the energy consumption associated with perforated plates. (10) The impact of the flow characteristics of a one-element device on the control system: Since the output frequency of a vortex street is linearly related to flow rate, when it is combined with a control valve or regulator to form a control system, it acts as a lagging element with negligible time delay and time constant, and can thus be regarded as a proportional element. The characteristics of the generalized object depend entirely on other elements in the circuit, having little effect on the control system. This is not the case with orifice plates; since their output is linearly related to flow rate, the loop gain changes as flow rate varies. Although the flow characteristics of control valves can be used to compensate for the linear effects of the generalized object, the effect is not significant. Therefore, it is necessary to introduce a square root element. While the use of this element prevents the characteristics of the generalized object from changing with the operating point and improves the control quality, it results in faster responses and higher sensitivity at low flow rates, which can lead to increased control errors in the system. In summary, it is feasible to use vortex meters in place of orifice plates for many flow measurement applications; this approach saves money and effort, and it indeed brings numerous advantages to flow measurement.