Saturated steam metering system

 Untreated steam is called saturated steam. He is a colorless, tasteless, non-flammable and non-corrosive gas. The content of droplets or mist in saturated steam reflects the mass of steam, which is usually expressed by the parameter of dryness. The dryness of steam is the percentage of saturated steam per unit volume.

The saturated steam measurement system has the following characteristics:

①There is only one independent variable between temperature and pressure of saturated steam.

②The saturated steam is easy to condense, and the loss of heat during transmission will lead to the reduction of temperature and pressure.Strictly speaking, saturated steam is more or less a two-phase fluid containing droplet or mist, so the same equation of gas state cannot be used to describe different states.

③It is difficult to accurately measure the saturated steam flow, because the dryness of the saturated steam is difficult to be guaranteed, and the general flow meter cannot accurately detect the flow rate of the two-phase fluid. The fluctuation of steam pressure will cause the change of steam density, and the indicator value of the flow meter will generate additional error.Therefore, in the measurement of steam, we must try to maintain the dryness of the steam at the measuring point to meet the requirements, and should take compensation measures when necessary to achieve accurate measurement.

●Measurement analysis

At present, the flow meter is used to measure the steam flow. The measuring medium refers to the single phase superheated steam or saturated steam. In the case of steam, where the phase flow changes frequently, there must be an inaccurate measurement. The solution to this problem is to maintain the superheat of steam, to reduce the moisture content of steam as far as possible, for example, to strengthen the insulation measures of steam pipes and to reduce the pressure loss of steam so as to improve the accuracy of steam measurement. However, these methods can not completely solve the problem of inaccuracy in steam flow measurement. The fundamental solution to this problem is to develop a flow meter that can be used to measure two-phase flow media.

When the working state of steam deviates from the design state, the flow value will generate errors.The measurement of steam flow also has an influence on the flow measurement. Therefore, the measurement of steam flow needs to take compensation measures, and the compensation factor is relatively complicated due to the change of steam state.The density of superheated steam is determined by the temperature and pressure of steam, and the density expression is different in different range of parameters and cannot be expressed by the same general formula. Therefore, the unified density calculation formula cannot be obtained, and only the temperature and pressure compensation formula can be obtained by individual derivation.In the case of large fluctuation of temperature and pressure, the compensation of gas expansion coefficient should be considered in addition to temperature and pressure compensation.

Whatever Flowmeter is used to detect the flow rate of saturated steam, it is necessary to take pressure compensation measures to work under the condition of steam pressure fluctuation, because in the flow equation there is a factor of steam density. When the working conditions are not consistent with the technical conditions, there will be errors in reading, and the magnitude of error and the working pressure will be related to the magnitude of the deviation of the design pressure. There will be a negative error when the working conditions are different from the technical conditions, otherwise there will be positive errors. The dryness condition of steam is an important condition for accurately measuring steam flow rate. At present, an on-line steam dryness measuring instrument is being developed, which is applied to steam Flowmeter and compensation system. Step by step to improve the accuracy of the measurement.

Three measures should be taken at present:

(1)Piping for conveying steam must be well insulated to prevent heat loss.

(2)In the steam pipe road to be section by section of steam, in the lowest point of the pipe and the instrument before the pipe should be set steam traps, timely discharge condensed water.

(3)The boiler operation should avoid the phenomenon of excessive liquid level in the drum and reduce the large fluctuation of the load as far as possible.

Selection comparison of saturated steam flow meter:

At present, there are more than 60 kinds of industrial flow meters, mainly including vortex flowmeter, differential pressure flowmeter (orifice plate, equal-speed tube, curved pipe) flowmeter, split rotor flowmeter, ox pa flowmeter, float flowmeter and so on.In history, there has not been a flow meter applicable to any fluid, any range, any  flow state and any use conditions. Any flow meter has its specific applicability and limitations.If the flow meter is not properly selected, the flow measurement must be inaccurate.But flow measurement is a complex technology, and there are many different kinds of flow meters, even for a certain application, choose a suitable flow meter becomes a highly technical work, need careful and thorough consideration before making a final choice and weigh many factors related to the measurement problem, therefore, the correct selection of instrument is the instrument to normal use the key to practical application, too many failures are caused by the instrument type selection is not reasonable, detailed understanding of the site application conditions and medium parameters, selecting proper pressure, temperature, protective, explosion-proof grade and material, structure,To ensure the instrument can operate in the best condition.

Steam metering should take into account five main factors when selecting flow meter: measuring method, performance requirement and instrument specification, fluid characteristic, environmental condition, economic condition (purchase cost, installation cost, operation cost, check cost, etc.) Maintenance costs) In our practical work, the orifice plate (nozzle) Flowmeter and vortex Flowmeter are mostly used in the central heating work of production in industrial and mining enterprises. The comparison between these two Flowmeters is given here as an example.

●Differential pressure flowmeter

A differential pressure Flowmeter is a flow meter that is calculated by a flow detector installed in a pipeline (I. e., a differential pressure device for short), a known fluid condition and the geometry of a primary instrument and a pipe, and a differential  pressure device is used to calculate the  flow meter. The lead pressure tube and differential pressure gauge are composed of three parts. This kind of differential pressure Flowmeter, represented by orifice plate Flowmeter, has a long history of application, high degree of standardization, very common application, and can be used without real flow calibration. The standardization, serialization and generalizability of differential pressure display instruments are high, and the theoretical accuracy is high. Wide range of application and strong adaptability, low initial investment cost. However, through practical application, it is found that the orifice plate Flowmeter is also Shortcomings:

①In the application, many factors (design parameters are inconsistent with operating conditions, length of upstream straight pipe is insufficient, hole plate and pipe are different centers, hole plate A is polluted, sharp Angle wear, etc.) have A great influence on the measurement accuracy, which increases the measurement error and reduces the accuracy.In particular, the differential pressure transmitter shall be calibrated frequently to ensure the accuracy of zero and the three-way valve shall be checked frequently to prevent the measurement misalignment caused by blockage.

②The installation works are large, troublesome and demanding, and the workload of frequent maintenance and cleaning is large.

③The operation of differential pressure transmitter is required, which increases the maintenance workload. In addition, the pressure pipe should be laid, and the heat insulation of the pressure pipe should be carried out in winter. The pipe should not be installed outdoors.

④The flow range ratio is 1: 3～ 1: 4, the range is low, the measurement of small flow is difficult, and the flow range is narrow.

⑤High pressure loss, non - linear scale, high running cost.

LGL series orifice plate flowmeter +P23 series differential pressure transmitter +P21 series high temperature pressure transmitter +XSR Chinese type flow integrator + armored thermal resistance(not used for integrator, for reference)

LGV series V cone flowmeter +P23 series differential pressure transmitter +P21 series high temperature pressure transmitter +XSFT Chinese type flow integrator + armored thermal resistance(not used for integrator, for reference)

●Vortex Flowmeter

Vortex Flowmeter is a new type of Flowmeter developed successfully on the basis of Carmen Vortex principle. During the period of rapid development of Vortex Flowmeter from 1970 to 80's, many types of Vortex Flowmeter have been developed and put on the market in large quantities. During the peak period of development in China, there were dozens of vortex Flowmeters. It should be said that Vortex Flowmeters are still a developing product, but because they have the advantage that other Flowmeters cannot be obtained, the proportion of using Vortex Flowmeters has increased substantially. It has been widely used in various fields and will dominate the flow meter in the future. It is an ideal substitute for orifice Flowmeter. It has the following characteristics:

①The structure is simple and solid, the measuring part is not movable, the long-term operation is very reliable.

②Low maintenance and easy to maintain, low installation cost.

③Output and flow proportional to the pulse signal, zero drift, high accuracy, and easy to network with the computer.

④Wide range of applications, applicable to various gas, steam and liquid flow measurement.

⑤Flow measurement range is wide, range ratio up to 1:10.

⑥Low pressure loss, low operation cost, more energy-saving significance.

⑦In a certain range of Reynolds numbers, the frequency of the output signal is not affected by the physical properties of the fluid and the variation of the composition. The instrument coefficient is only related to the shape and size of the vortex generator, and there is no need to compensate for the measurement of the volume flow of the fluid. There is no need to calibrate the coefficient of the instrument after changing the fittings.

But the Flowmeter also has some limitations

①Vortex flowmeter is a kind of velocity flowmeter. The stability of vortex separation is affected by the flow rate. Therefore, it has certain requirements on the straight pipe section, generally 10D before and 5D after.

②When measuring liquid, the upper flow velocity is limited by pressure loss and cavitation phenomenon, usually (0.5 ~ 8) m / s.

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

④The stress vortex flowmeter is sensitive to vibration, so it is necessary to install flow timing in the pipeline with large vibration, and the pipeline should have certain shock absorption measures.

⑤The stress-type vortex Flowmeter uses piezoelectric crystal as the detection sensor, so it is limited by temperature, which is (-40 ~ 300) ℃, and the measuring temperature is not more than 400 ℃. Otherwise, the probe will easily lose its accuracy due to aging.

⑥Anti-electromagnetic interference and radio-frequency interference are poor.

⑦The measurement is affected when the fluid medium is two-phase flow or pulsating flow.

LUGB series vortex flow meter (below 250 ℃) +P21 series high temperature pressure transmitter +XSR Chinese type flow integrator + armored thermal resistance (for reference, not for calculation)