Steam is the most widely used heat-carrying working fluid and an important secondary energy source, which is widely used in electric power, smelting, steel, automobiles, textile printing and dyeing, chemical fiber, food, medicine and other fields. As an important energy source in various industries, the accurate measurement of steam flow is of great significance to saving energy and improving the economic benefits of enterprises.
Steam is divided into saturated steam and superheated steam affected by temperature and pressure changes, and its flow rate is greatly affected by steam density, so temperature and pressure compensation is required when measuring steam flow (saturated steam can compensate for temperature or pressure alone).
At present, the flow meters used for steam flow measurement mainly include vortex flow meters and differential pressure flow meters. Due to the large pressure loss, large maintenance and low accuracy level of differential pressure flowmeters, the advantages of measuring steam flow with vortex flowmeters are obvious.
Saturated steam metering system
Steam that has not been heat-treated is called saturated steam. It is a colorless, odorless, non-flammable and non-corrosive gas. The content of droplets or liquid mist in saturated steam reflects the quality of steam, which is generally expressed by the parameter dryness. The dryness of steam refers to the percentage of dry steam in a unit volume of saturated steam.
Saturated steam metering system has the following characteristics:
(1) The temperature and pressure of saturated steam correspond to each other, and there is only one independent variable between the two.
(2) Saturated steam is easy to condense, and if there is heat loss during the transmission process, it will lead to a decrease in temperature and pressure. Strictly speaking, saturated steam contains more or less two-phase fluids of droplets or mist, so different states cannot be described by the same equation of gas state.
(3) It is difficult to accurately measure the flow rate of saturated steam, because the dryness of saturated steam is difficult to guarantee, and the general flow meter can not accurately detect the flow rate of duplex fluid. Therefore, in steam metering, it is necessary to try to maintain the dryness of the steam at the measurement point to meet the requirements, and if necessary, compensation measures should be taken to achieve accurate measurement.
Schematic diagram of the installation of the saturated steam metering system
Saturated steam metering system installation diagram 1 (pressure and temperature double compensation)
Saturated steam metering system installation diagram 1 (pressure compensation)
Vortex flow meter + temperature sensor + pressure sensor On-site installation drawing
Superheated steam metering system
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
Installation diagram of superheated steam metering system 1 (orifice flow meter + temperature and pressure compensation type)
Installation diagram of superheated steam metering system 1 (high-temperature vortex flowmeter + temperature and pressure compensation type)
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.
无论采用何种流量计检测饱和蒸汽的流量，在蒸汽压力波动的条件下工作，必须采取压力补偿措施，这是因为在流量方程中，都含有蒸汽密度的因素，工作条件与设计条件不一致时，读数会产生误差，误差的大小和工作压力与设计压力偏差的大小有关，P实>P设将出现负误差，否则将出现正误差。蒸汽的干度条件是关系到能否准确计量蒸汽流量的重要条件，目前正在研制在线蒸汽干度检测仪表，待干度仪表应用于蒸汽流量计量与补偿系统，必将进一步提高计量的准确性。目前应采取以下三项措施：
(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 Saturated Steam Flow Meters
At present, there are more than 60 types of industrial flow meters, mainly including vortex flow meters, differential pressure (orifice plates, average velocity pipes, bends) flow meters, shunt rotor flow meters, Aniu Ba flow meters, float flow meters, 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 central heating in industrial and mining enterprises, the most used are orifice (nozzle) flowmeters and vortex flowmeters, here are two types of flowmeters 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 loosener 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 Flow Meter + P31 Series Differential Pressure Transmitter + P21 Series High Temperature Pressure Transmitter + XSR22 Chinese Flow Accumulator + Armored Thermal Resistor (Not for Accumulation, for Reference)
LGV Series V-Cone Flowmeter + P31 Series Differential Pressure Transmitter + P21 Series High Temperature Pressure Transmitter + XSR22 Chinese Flow Totalizer + Armored Thermal Resistor (Not for Accumulation, for Reference)
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, this 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-+350) °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 250 degrees) + P21 Series High Temperature Pressure Transmitter + XSRSS Chinese Type Flow Totalizer + Armored Thermal Resistor (for reference, not for accumulation)
