Superheated steam is a relatively special medium, and the steam generally refers to superheated steam. Superheated steam is a common power source that is often used to drive the rotation of steam turbines, which in turn drives generators or centrifugal compressors to work. Superheated steam is obtained by heating saturated steam, which contains no droplets or mist, and is an actual gas. The temperature and pressure parameters of superheated steam are two independent parameters, and their density should be determined by these two parameters. It can be found by the superheated steam density meter. After long-distance transportation, with the change of working conditions (such as temperature, pressure, etc.), especially when the superheat is not high, the superheated steam will change from the superheated state to the saturated or supersaturated state due to the decrease in heat loss temperature, and transform into saturated steam or supersaturated steam with water droplets. Saturated steam suddenly greatly reduces pressure, and when the liquid expands with adiabatic expansion, it will also transform into superheated steam, so that a vapor-liquid two-phase flow medium is formed.
Schematic diagram of installation of superheated steam metering system
Analysis of measurements
At present, flow meters are used to measure steam flow, and the measurement medium refers to single-phase superheated steam or saturated steam. For steam with frequently varying phase flows, there will certainly be inaccurate measurements. The solution to this problem is to maintain the superheat of steam and minimize the moisture content of steam, such as strengthening the insulation measures of steam pipelines and reducing the pressure loss of steam, so as to improve the accuracy of steam measurement. However, these methods cannot completely solve the problem of inaccurate steam flow measurement, and the fundamental solution to this problem is to develop a flow meter that can measure the two-phase flow medium.
When the working state of steam deviates from the design state, the flow rate will cause errors. It also has an impact on flow measurement, so the measurement of steam flow needs to take compensation measures, and the compensation factors due to the state change of steam are also more complicated. The density of superheated steam is determined by the temperature and pressure parameters of steam, and within different parameters, the expression form of density is also different, and it cannot be expressed by the same general formula, so a unified density calculation formula cannot be obtained, and the temperature and pressure compensation formula can only be derived individually. In the case of large temperature and pressure fluctuations, in addition to temperature and pressure compensation, it is also necessary to consider the compensation of the gas expansion coefficient.
(1) The pipeline for transporting steam must have good insulation measures to prevent heat loss.
(2) On the steam pipeline, the water should be trapped section by section, and a trap should be set up at the lowest point of the pipeline and the pipe in front of the instrument to discharge the condensate in time.
(3) The boiler should avoid the phenomenon of excessive drum liquid level in boiler operation to minimize large fluctuations in load.
Comparison of the selection of superheated steam flow meters At present, there are more than 60 types of industrial flow meters, mainly includingVortex flow meter, differential pressure type (orifice flowmeter, equal velocity pipe, bent pipe). V-cone flow meter, shunt rotary flow meter, Aniuba flow meter, float flow meter, etc. There is no flow meter that is applicable to any fluid, any range, any flow state, and any use condition in history. If the flow meter is not chosen properly, the flow rate will definitely not be measured. But flow measurement is a complex technology, and with so many different types of flow meters, choosing the right flow meter for a given application becomes a very technical task that requires careful and in-depth consideration and weighing of many factors related to measurement issues before making a final choice. Therefore, the correct selection of the instrument is the key to the normal use of the instrument, in practical application, too many faults are caused by the unreasonable selection of the instrument, learn more about the working conditions and medium parameters of the field application, choose the appropriate pressure, temperature, protection, explosion-proof grade and material, structure to ensure that the instrument can run in the best state. Steam metering should consider five main factors when choosing a flow meter: measurement method, performance requirements and instrument specifications, fluid characteristics to be measured, environmental conditions, and economic conditions (purchase cost, installation cost, operating cost, calibration cost, maintenance cost). In our actual work, whether it is used for production in industrial and mining enterprises, the most commonly used are orifice (nozzle) flow meters, V-cone flow meters and vortex flow meters, and here we take these two flow meters as examples to compare:
Differential pressure flow meter
The differential pressure flowmeter is a meter that calculates the flow rate by calculating the differential pressure generated by the flow detection device (i.e., the differential pressure generator, referred to as the primary meter) installed in the pipeline, the known fluid conditions and the geometric dimensions of the primary meter and the pipeline, and consists of three parts: differential pressure device, pressure pipe and differential pressure gauge. This differential pressure flowmeter represented by orifice flowmeter has a long history of application, high degree of standardization, and is very widely used, but does not require the standardization of the differential pressure display instrument, as well as the high degree of serialization and generalization, high theoretical accuracy, wide range of applications, strong adaptability, and low initial investment cost. However, after practical application, it is found that the orifice flow meter also has shortcomings:
(1) Many factors in the application (design parameters are inconsistent with working condition parameters, insufficient length of upstream straight pipe sections, uncentric orifice plate and pipe, contamination of the A side of the orifice plate, wear at sharp angles, etc.) have a great impact on its measurement accuracy, increasing its measurement error and reducing accuracy. In particular, the differential pressure transmitter should be checked frequently to ensure the accuracy of the zero point, and the tee valve should be checked frequently to prevent measurement errors caused by blockage.
(2) The installation project is large, troublesome, and demanding, and requires frequent maintenance and dismantling and cleaning.
(3) It is necessary to use a differential pressure transmitter, which increases the workload of maintenance, and the pressure guide pipe needs to be laid outside, and the pressure guide pipe needs to be insulated in winter, and it cannot be installed outdoors.
(4) The flow range ratio is 1∶3~1∶4 and the range is low, which is difficult to measure for small flow rates and the flow range is narrow.
(5) The pressure loss is large, the scale is nonlinear, and the operating cost is high.
LGK Series Orifice Plate + P31 Differential Pressure Transmitter + P21 Pressure Transmitter + XSR22 Intelligent Dual Compensation Flow Totalizer + Temperature Sensor (for Accumulation Calculation)
LGK Series V-Cone + P31 Differential Pressure Transmitter + P21 Pressure Transmitter + XSR22 Intelligent Dual Compensation Flow Totalizer + Temperature Sensor (for Accumulation Calculator)
Vortex flow meter
The vortex flowmeter is a new type of flowmeter successfully developed based on the principle of Carmen vortex street. The 70s and 80s were a period of rapid development of vortex flow meters, and many types of vortex flow meters with fluid resistance and detection were developed and put on the market in large quantities. In our country's vortex street at the peak of development, it has reached dozens, it should be said that the vortex flowmeter is still a developing product, but because it has the advantages of other flowmeters that cannot be obtained, the proportion of the use of vortex flowmeter has increased significantly, has been widely used in various fields, will dominate in the future flow meter, is an ideal replacement product for orifice flowmeter. It has the following characteristics:
(1) The structure is simple and firm, there are no moving parts in the measuring part, and the long-term operation is very reliable.
(2) The maintenance amount is small, the maintenance is very convenient, and the installation cost is low.
(3) The output pulse signal is directly proportional to the flow rate, no zero drift, high accuracy, and convenient networking with computers.
(4) It has a wide range of applications and is suitable for flow measurement of various gases, vapors and liquids.
(5) The flow measurement range is wide, and the range ratio can reach 1∶10.
(6) Small pressure loss, low operating cost, more energy-saving significance.
(7) Within a certain range of Reynolds, the output signal frequency is not affected by the physical properties and components of the fluid, and the instrument coefficient is only related to the shape and size of the vortex generator.
However, the flow meter also has certain limitations:
(1) Vortex flowmeter is a kind of speed flowmeter, the stability of vortex separation is affected by the flow velocity, so it has certain requirements for the straight pipe section, generally the first 10D, the back 5D.
(2) When measuring liquids, the upper flow velocity is limited by pressure loss and cavitation, which is generally (0.5-8) m/s.
(3) When measuring gas, the upper flow rate is limited by the change of medium compressibility, the lower limit flow rate is limited by the Reynolds number and sensor sensitivity, and the steam is (8-25) m/s.
(4) The stress vortex flowmeter is more sensitive to vibration, so when installing a flowmeter in a pipeline with large vibration, the pipeline should have certain shock absorption measures.
(5) The stress vortex flowmeter uses piezoelectric crystal as the detection sensor, so it is limited by temperature, generally (-40-+300) °C, and the measurement temperature is not higher than 400 °C, otherwise the probe is very easy to be inaccurate due to aging.
(6) Poor anti-electromagnetic interference and radio frequency interference.
(7) When the fluid medium is two-phase flow or pulsating flow, it has an impact on the measurement.
LUGB Series Vortex Flow Meter (below 350 degrees) + P21 High Temperature Pressure Transmitter + XSR22 Intelligent Dual Compensation Flow Accumulator + Temperature Sensor (for accumulation)
