Discussion on issues related to steam flow measurement 7 L; Z* i2 C# Q$$$$ x w: f
In the heating industry, inaccurate measurement of steam flow is a common problem, the main reasons are as follows. * @3 H: X; H$$$$ s" F* @! [1 q0 O
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1.1 Superheated steam 2 ol _, L, I
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Steam is a relatively special medium. Generally speaking, steam refers to superheated steam. Superheated steam is a common power source and is often used to drive the turbine to rotate, which in turn drives the generator or centrifugal compressor. The superheated steam is obtained by heating the saturated steam. It contains no droplets or liquid mist and belongs to the actual gas. The temperature and pressure parameters of superheated steam are two independent parameters whose density should be determined by these two parameters. 2 R6 s6 R* \* V7 }" E
8 O3 K- ~! D* B/ P4 _ Superheated steam will change temperature due to changes in operating conditions (such as temperature and pressure) after long distances, especially in the case of low superheat. It is lowered to change from a superheated state to a saturated or supersaturated state, and converted into saturated steam or supersaturated steam with water droplets. The saturated steam is suddenly and greatly depressurized, and when the liquid adiabatically expands, it will also be converted into superheated steam, thus forming a vapor-liquid two-phase flow medium. $$$$ l6 m) _" ?. k+ m
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1.2 Saturated steam! V' y7 @3 U% a/ ?" \
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Steam that has not been heat treated is called saturated steam. It is a colorless, odorless, non-flammable and non-corrosive gas. Saturated steam has the following characteristics. " f7 x) X L9 r5 V# s" x: ^" [% A
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(1) There is a one-to-one correspondence between the temperature and pressure of saturated steam, and there is only one independent variable between the two. " z! ^+ W' y! p# A/ i7 p. ?6 n
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(2) The saturated steam is easily condensed. If there is heat loss during the transfer, droplets or liquid mist are formed in the steam, which causes a decrease in temperature and pressure. Steam containing droplets or liquid mist is called wet steam. Strictly speaking, saturated steam contains more or less two-phase fluids of droplets or liquid mist, so it cannot be described by the same gas state equation in different states. The content of droplets or liquid mist in saturated steam reflects the quality of the steam and is generally expressed by the dryness parameter. The dryness of steam refers to the percentage of dry steam per unit volume of saturated steam, expressed as "x". * n4 h5 h$$$$ R& ?. q
( m- [1 X3 S& G, ]6 z: |9 [ (3) Accurate metering of saturated steam flow is difficult because the dryness of saturated steam is difficult to guarantee. Generally, the flowmeter cannot accurately detect the flow of two-phase fluid, steam. Pressure fluctuations will cause changes in vapor density, and the flowmeter indication will produce additional errors. Therefore, in steam metering, it is necessary to try to maintain the dryness of the steam at the measurement point to meet the requirements, and if necessary, compensation measures should be taken to achieve accurate measurement. ' D5 c/ w7 H# ]- k
* i2 E0 ~) e3 `: o: t. f8 f( ? 2 Measurement Analysis $$$$ e6 r$$$$ _3 E @ o# z r
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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 insulation of the steam pipe and reducing the pressure loss of the steam 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.
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, o% L- j# [9 H; J1 ]+ F There are many types of flowmeters for detecting gas flow. They are most commonly used in speed and volumetric flowmeters. Their common feature is that they can only measure the volume under continuous conditions. The flow rate, and the volume flow rate is a function of the state. The volume flow rate under the working state cannot accurately represent the actual flow rate, and the engineering generally refers to the standard state volume flow rate or mass flow rate. 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 gaseous 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 factors for the state change of the steam are also complicated.
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5 @+ }7 f5 k) p y4 L8 x The density of superheated steam is determined by two parameters of temperature and pressure of steam, and the expression of density is different in different ranges of parameters, which cannot be expressed by the same formula. Therefore, a unified density calculation formula cannot be obtained, and only the temperature and pressure compensation formulas can be obtained by individual derivation. 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. ( C: V0 t1 s0 s. J6 ? G
6 z5 A1 ]" \1 U1 w Regardless of the flow rate used to detect the flow of saturated steam, pressure fluctuations must be taken in the case of steam pressure fluctuations. 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 set a negative error, otherwise a positive error will occur. The dry condition of the steam is In relation to the important conditions for accurate measurement of steam flow, an online steam dryness test instrument is currently being developed, and the dryness meter is applied to http://
Magnetic flap level gauge http://
Electromagnetic flowmeter http://
Annubar flowmeter http://
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