In the heating industry, inaccurate steam flow measurement is a common problem. The main reasons are as follows. 1 superheated steam
After long-distance transport of superheated steam, depending on operating conditions (such as temperature and pressure), especially when the superheat is not high, the superheated steam will be brought into saturation or supersaturation from an overheated state due to a decrease in the heat loss temperature. State, converted to saturated steam or supersaturated steam with water droplets. Saturated steam suddenly decompresses greatly. When the liquid expands adiabatically, it will also be saturated steam or supersaturated steam with water droplets. Saturated steam suddenly decompresses greatly. When the liquid expands adiabatically, it also turns into superheated steam. This forms a vapor-liquid two-phase flow medium.
2 Measurement analysis
Currently used to measure the steam flow, the measurement medium refers to single-phase superheated steam or saturated steam. For steam that is constantly changing in phase flow, there will certainly be inaccurate measurements. The solution to this problem is to maintain the superheat of the steam and minimize the water content of the steam, such as strengthening the insulation measures of the steam pipe, reducing the pressure loss of the steam, etc. to improve the measurement accuracy. However, these methods can not 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 media.
Many types of flowmeters are used to detect gas flow. Speed ​​and volume flowmeters are most commonly used. Their common feature is that they can only continuously measure the volumetric flow rate under operating conditions. Volumetric flow is a function of state. The volumetric flow rate does not represent the actual flow rate, and is generally expressed in terms of standard volume flow or mass flow. The standard state volume is 0°C, a volume of gas at 1 standard atmosphere or a volume of 20°C at 1 standard atmosphere. With mass flow as a unit of measure, it is currently not used much. When using a graduated gas flowmeter, the normal temperature and pressure of the selected gas are the design conditions, and the volumetric flow under the designed state is converted into the standard volume flow or mass flow. The conversion factor contains the gas density factor, and the working state of the gas medium deviates. In the design state, the flow indication will produce an error. In addition, changes in the composition, content, or temperature of the gaseous medium have an effect on the flow rate measurement. Therefore, the measurement of the steam flow rate requires compensating measures, and the compensating factors due to changes in the state of steam are also complicated.
The density of superheated steam is determined by two parameters of steam temperature and pressure, and in different ranges of parameters, the density expression form is not the same and can not be represented by the same formula, so a unified density calculation formula cannot be obtained. Derive the temperature and pressure compensation formula. When the temperature and pressure fluctuation range is large, in addition to temperature and pressure compensation, the gas expansion coefficient ε needs to be taken into consideration.
Regardless of the type of flow meter used to measure the flow rate of saturated steam, it is necessary to take pressure compensation measures when working under conditions where the steam pressure fluctuates. This is because the flow rate equation contains factors of steam density, and when the operating conditions are inconsistent with the design conditions, The reading will produce an error. The size of the error and the working pressure are related to the design pressure deviation. If P is set, P will have a negative error, otherwise a positive error will occur. The condition of dryness of steam is an important condition related to the ability to accurately measure the flow of steam. At present, the on-line steam dryness detection instrument is being developed. When the dryness meter is used in a steam flow metering and compensation system, the accuracy of measurement will be further improved.
At present, the following three measures should be taken:
(1) The steam piping must have good insulation measures to prevent heat loss.
(2) In the steam pipeline, it should be drained from section to section. A steam trap should be installed on the lowest part of the pipeline and on the pipeline in front of the meter to discharge the condensate in time.
(3) Boiler liquid level should be avoided during boiler operation to minimize the fluctuation of load.
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