In the heating industry, inaccurate measurement of steam flow is a common problem, the main reasons are as follows.

1.1 Superheated steam

Steam is a relatively special medium. Generally speaking, steam refers to superheated steam. Superheated steam is a common power source, which is often used to drive the turbine to rotate, which in turn drives the generator or centrifugal compressor to work. Superheated steam is obtained by heating the saturated steam. It contains no droplets or liquid mist and belongs to the actual gas. The temperature and pressure parameters of superheated steam are two independent parameters whose density should be determined by these two parameters.

Superheated steam, after long-distance transportation, changes with operating conditions (such as temperature, pressure), especially in the case of low superheat, it will change from overheating to saturation or supersaturation due to the heat loss temperature. State, converted to saturated steam or supersaturated steam with water droplets. The saturated steam is suddenly and greatly decompressed, and when the liquid adiabatically expands, it will also be converted into superheated steam, thus forming a vapor-liquid two-phase flow medium.

1.2 saturated steam

Steam that has not been heat treated is called saturated steam. It is a colorless, odorless, non-flammable and non-corrosive gas. Saturated steam has the following characteristics.

(1) There is a one-to-one correspondence between the temperature and pressure of saturated steam, and there is only one independent variable between them.

(2) Saturated steam is easily condensed, and if there is heat loss during the transfer, droplets or liquid mist are formed in the steam, which causes a decrease in temperature and pressure. Steam containing droplets or liquid mist is called wet steam. Strictly speaking, saturated steam contains more or less two-phase fluids of droplets or liquid mist, so it cannot be described by the same gas state equation in different states. The content of droplets or liquid mist in saturated steam reflects the quality of the steam and is generally expressed by the dryness parameter. The dryness of steam refers to the percentage of dry steam per unit volume of saturated steam, expressed as "x".

(3) Accurate measurement of saturated steam flow is difficult because the dryness of saturated steam is difficult to guarantee. Generally, the flowmeter cannot accurately detect the flow of two-phase fluid. The fluctuation of steam pressure will cause the change of vapor density, and the flowmeter shows additional error. . Therefore, in steam metering, it is necessary to try to maintain the dryness of the steam at the measuring point to meet the requirements, and if necessary, compensation measures should be taken to achieve accurate measurement.

Flow meters are currently used to measure steam flow, and the measurement medium refers to single-phase superheated steam or saturated steam. For steam with a constantly changing phase flow, there will definitely be problems with inaccurate measurements. The solution to this problem is to maintain the superheat of the steam and minimize the moisture content of the steam, such as strengthening the steam pipe insulation measures, reducing the pressure loss of the steam, etc., in order to improve the accuracy of the measurement. However, these methods cannot completely solve the problem of inaccurate steam flow measurement. The fundamental solution to this problem is to develop a flow meter that can measure two-phase flow medium.

There are many types of flowmeters for detecting gas flow. The most common applications are speed and volumetric flowmeters. Their common feature is that they can only measure the volumetric flow under continuous conditions, and the volumetric flow is a function of the state. The volumetric flow rate does not accurately represent the actual flow rate, and is generally expressed in terms of standard state volume flow or mass flow. The standard state volume is 0 ° C, a gas volume at 1 standard atmosphere or a volume at 20 ° C, 1 standard atmosphere. In the case of mass flow as a unit of measurement, there are currently not many applications. When a graduated gas flowmeter is used, the normal temperature and pressure of the selected gas are designed as conditions, and the volumetric flow rate in the designed state is converted into a standard volumetric flow rate or a mass flow rate, and the conversion coefficient contains a factor of gas density, when the working state of the gas medium deviates. In the design state, the flow indication will produce an error. In addition, the composition, content or temperature of the gas medium also affects the flow measurement. Therefore, the measurement of the steam flow requires compensation measures, and the compensation factor for the state change of the steam is also complicated.

The density of superheated steam is determined by two parameters of temperature and pressure of steam. In the different ranges of parameters, the expression of density is different and cannot be expressed by the same formula. Therefore, a unified density calculation formula cannot be obtained. Derivation to obtain the temperature and pressure compensation formula. In the case where the temperature and pressure fluctuation range are large, in addition to temperature and pressure compensation, compensation for the gas expansion coefficient ε needs to be considered.

Regardless of the flow rate of the saturated steam used to detect the flow of saturated steam, pressure compensation must be taken when the steam pressure fluctuates. This is because the flow rate equation contains the factor of steam density. When the working conditions are inconsistent with the design conditions, The reading will produce an error. The magnitude of the error and the working pressure are related to the deviation of the design pressure. P = P will cause a negative error, otherwise a positive error will occur. The dry condition of steam is an important condition related to the accurate measurement of steam flow. At present, an online steam dryness detecting instrument is being developed. The dry meter is applied to the steam flow metering and compensation system, which will further improve the accuracy of the metering. The following three measures should be taken at present:

(1) The piping for conveying steam must have good insulation measures to prevent heat loss.

(2) The steam pipeline should be drained one by one. At the lowest point of the pipeline and the pipeline in front of the instrument, a steam trap should be installed to discharge the condensed water in time.

(3) In the operation of the boiler, the phenomenon that the steam drum liquid level is too high should be avoided, and the large fluctuation of the load should be minimized.

For steam metering, five main factors should be considered when selecting a flow meter: the characteristics of the fluid being tested, the conditions of the production process, the installation conditions, the maintenance requirements, and the characteristics of the flow meter. Here, we will focus on the characteristics of flow meters, installation conditions, maintenance requirements, and several issues that should be noted when selecting flow meters. At present, the instruments for measuring steam flow mainly include vortex flowmeter, differential pressure type (orifice plate, constant velocity pipe, elbow) flowmeter, split rotor flowmeter, annubar flowmeter, float flowmeter, etc. The vortex flowmeter, the orifice flowmeter and the elbow flowmeter are taken as an example for illustration.

3.1 vortex flowmeter

The vortex flowmeter is a new type of flowmeter developed based on the principle of Karman vortex street. Since it has the advantages that other flowmeters cannot have, it has developed rapidly since the 1970s. According to reports, the proportion of vortex flowmeters used in developed countries such as Japan, Europe and the United States has risen sharply. It has been widely used in various fields and will dominate the flow meter in the future. It is an ideal substitute for orifice flowmeters. It has the following characteristics:

1 The structure is simple and firm, no moving parts, and it is very reliable for long-term operation;

2 Maintenance is very convenient and the installation cost is low;

3 The sensor does not directly contact the medium, and the performance is stable and the service life is long;

4 Output pulse signal proportional to the flow, no zero drift, high precision, and easy to network with the computer;

5 measuring range is wide, the range ratio can reach 1:10;

6 The pressure loss is small, the operating cost is low, and it is more energy-saving;

7 Within a certain Reynolds number range, the output signal frequency is not affected by fluid physical properties and composition changes. The meter factor is only related to the shape and size of the vortex generator. The volumetric flow rate of the measured fluid does not need to be compensated. There is no need to recalibrate after changing the fittings. The coefficient of the meter;

8 Wide range of applications, gas and liquid flow can be measured;

9 The verification period is 2 to 4 years.

However, the flowmeter also has certain limitations:

1 Vortex flowmeter is a speed flowmeter. The stability of vortex separation is affected by the flow rate, so it has certain requirements for straight pipe sections, generally the first 10D and the last 5D;

2 When measuring liquid, the upper limit flow rate is limited by pressure loss and cavitation, generally 0.5~8m/s;

3 When measuring gas, the upper limit flow rate is limited by the change of compressibility of the medium, and the lower limit flow rate is limited by Reynolds number and sensor sensitivity, and the steam is 8~25m/s;

4 Stress type vortex flowmeter is sensitive to vibration, so when installing the flowmeter in a pipe with large vibration, the pipeline must have certain damping measures;

5 Stress type vortex flowmeter uses piezoelectric crystal as the detection sensor, so it is limited by temperature, generally -40 ~ +300 °C.

3.2 Differential pressure flowmeter

The differential pressure flowmeter represented by the orifice flowmeter has a long history of application, has international standards, high theoretical precision, and is widely used. However, after decades of application, it has been found that orifice flowmeters are also inadequate:

1 Many factors in the application (the design parameters are inconsistent with the working parameters, the upstream straight pipe section is insufficient, the orifice plate and the pipe are not concentric, the orifice A surface is contaminated, the acute angle wear, etc.) has a very large influence on the measurement accuracy, making it measure The error increases;

2 Installation is more troublesome, and the workload of maintenance and disassembly is large;

3 It needs to be equipped with differential pressure transmitter, which increases the workload of maintenance. It also needs to install pressure guiding tube, and it needs to keep the pressure guiding tube warm in winter, and it can not be installed outdoors;

4 The flow range ratio is 1:3, which has great limitations;

5 If the installation is not correct, steam leakage is likely to occur;

6 The pressure loss is large and the running cost is high.

3.3 elbow flowmeter

The elbow flowmeter is actually a 90 degree standard elbow with no flow sensor that is simpler than its construction. With the development of the machining industry and the continuous improvement of industry standardization and standardized management, the standard mechanism used as the elbow sensor is more cost-effective. It is characterized by:

1 Simple structure and low price.

2 The elbow flowmeter sensor is resistant to wear and is not sensitive to minor wear.

3 The installation is simple, and the direct welding method can be used for installation, so that the trouble of running on the spot is completely solved.

4 It has strong adaptability, wide measuring range and strict requirements for straight pipe sections. As long as the flow rate of the pipe can be measured by orifice plate, vortex street and constant velocity tube flowmeter, it can be measured by the elbow flowmeter, and it is resistant to high temperature, high pressure, impact, vibration, moisture and moisture. In terms of dust, etc., the elbow flowmeter is far superior to other flowmeters.

5 The elbow flowmeter has a turndown ratio of 1:10. For steam, its applicable range is 0-70m/s, which can better meet the requirements of steam flow measurement.

6 Elbow flowmeters Due to their special measurement principle, the requirements for straight pipe sections are not strict in practical applications. Generally, only the first 5D and the last 2D are required, which is far lower than the requirements of other flow measuring devices.

7 The elbow flowmeter has high precision and good reproducibility. The measurement accuracy can reach 1.14%, and the reproducibility accuracy can reach 0.2%. After one installation, it is no longer necessary to disassemble and disassemble. Therefore, the installation accuracy can be optimized. Guarantee.

8 The most prominent feature of the elbow flowmeter is that there are no additional throttles or inserts, which can greatly reduce the power consumption of the fluid transported in the pipeline and save energy, especially for those measuring large systems, large diameters and low pressure heads. The benefits are even more obvious.

For example: In order to maintain the normal operation of an orifice flowmeter installed in a heating pipeline with thousands of tons of flow per hour, a heating season requires about tens of thousands of kilowatt-hours, equivalent to tens of thousands of yuan. Here, only the pressure loss of the orifice flowmeter is considered to be several thousand Pa, which is far beyond this value in actual operation. Even with the pressure loss of these thousands of kPa, the additional operating costs caused by it can not be ignored. The data given in Table 1 is the current consumption of the main pipeline of the heat pipe network in different pressure losses, the additional power consumption of the circulating pump caused by the orifice flowmeter, the electricity consumption cost, the equivalent standard coal quantity, and the coal purchase cost. The operating data of the orifice plate flowmeter in a heating season, in which the operating days are 120 days, the electricity price is 0.35 yuan/degree, and the standard coal price is 200 yuan/t.

A medium-sized heat network with a flow rate of 4000m3/h, when the pressure loss of the orifice plate is 30 kPa, only one orifice flowmeter consumes 96,000 kWh of electricity, the operating cost is 31,200 yuan, and the flow rate is 10000 m3/h. The large-scale heating network has an additional power consumption of 240,000 kWh and an operating cost of 78,000 yuan. The additional resistance loss measured by the elbow flowmeter is much smaller. If the elbow flowmeter is used instead of the orifice flowmeter for metering, the operating cost can be greatly reduced, and considerable economic benefits can be obtained.

In summary, the selection of steam flow meters is very important. Accurate measurement of steam flow is a problem that the production department needs and is generally concerned about. With the development of the economy, the call for higher measurement levels is getting higher and higher. Therefore, we should do some meticulous technical work for the actual situation of production, and effectively explore a successful road to steam flow measurement.

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