Steam can be divided into saturated steam and superheated steam. In the flow calculation device, the mass flow rate can be obtained based on the temperature and pressure of the superheated steam and the look-up table density. However, when the superheated steam is transported over a long distance, or because the heat insulation measures of the pipeline are improper, the heat loss temperature tends to decrease from the heat loss. The overheated state enters a critical saturation state, and even part of the steam condensation changes into a water droplet, and becomes a wet saturated steam (supersaturated steam).
In the heating industry, the inaccuracy of steam flow meter measurement is a common problem. The main reasons are analyzed as follows.
1. Measurement analysis: At present, the measured steam flow is used, and 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 accuracy of the measurement. 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.
2. After long-distance transportation, superheated steam will change from overheated state to saturation due to changes in working conditions (such as temperature and pressure), especially if the superheat is not high. In an oversaturated state, it turns into saturated steam or supersaturated steam with water droplets. Saturated steam suddenly decompresses greatly, and when liquid adiabatically expands, 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.
Many types of flowmeters are used to detect gas flow. The speed and volumetric flowmeters are widely 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. Therefore, the measurement of steam flow needs to take compensating measures, and the compensating factors due to the change of steam state are also more complicated.
The density of superheated steam is determined by the two parameters of steam temperature and pressure, and in the different ranges of the parameters, the density expression form is not the same and can not be represented by the same formula, so the unified density calculation formula can not be obtained, and can only be individual 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 all factors of steam density, and when the operating conditions are inconsistent with the design conditions, The reading will produce errors. The size of the error and the working pressure are related to the design pressure deviation.
At present, the following three measures should be taken:
(1) The steam piping must have good insulation measures to prevent heat loss.
(2) The boiler liquid level should be avoided during boiler operation to minimize the fluctuation of the load.
(3) In the steam pipeline, it should be drained from section to section. A steam trap should be installed on the low point of the pipeline and the pipeline in front of the instrument to discharge the condensate in time.
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