The specific volume of superheated steam is determined by the temperature and pressure of steam two parameters, and in different parameter ranges, the expression form of the specific volume is not the same, can not be expressed in the same general formula, so you can not get a unified formula for calculating the specific volume, can only be derived individually to find the temperature and pressure compensation metric. In the temperature, pressure fluctuations in the range of occasions, in addition to temperature, pressure compensation, but also need to consider the compensation of superheated steam volume expansion coefficient ε.

1 Uncertainty in Superheated Steam Flow Measurement

Superheated steam is obtained by heating and warming saturated steam. The temperature and pressure of superheated steam are two independent parameters, and other state parameters are determined by these two parameters. Superheated steam after delivery, with the change in working conditions (such as temperature, pressure), especially in the case of superheat is not high, because of heat loss temperature reduction from the superheated state into the saturated or supersaturated state, transformed into saturated steam or supersaturated steam with water droplets. Saturated steam suddenly and substantially depressurized, part of the droplets in the adiabatic expansion will also be transformed into superheated vortex steam flow meter, which forms a vapor-liquid two-phase flow medium, the general flow meter can not accurately detect the flow of two-phase fluid, resulting in flow measurement errors.

2 Uncertainty of Saturated Steam Flow Measurement

The temperature and pressure of saturated steam correspond to each other, and there is only one independent parameter between the two. Saturated steam is prone to condensation, and if there is any heat loss during transmission, liquid droplets or liquid mist will be formed in the steam, which will result in a decrease in temperature and pressure. Steam containing liquid droplets or liquid mist is called wet steam. Strictly speaking, saturated steam contains more or less liquid droplets or liquid mist, so the different states cannot be described by the same gas state equation. The amount of liquid droplets or liquid mist in saturated steam reflects the quality of the steam, which is generally expressed by the parameter dryness. The dryness of steam is the percentage of dry steam in a unit volume of saturated steam.

Accurate measurement of saturated steam flow is more difficult, the general flowmeter can not accurately detect the flow of two-phase fluid, steam pressure fluctuations will cause changes in the specific volume of steam, the flow measurement value will produce additional errors. Therefore, in the measurement of saturated steam, must try to maintain a stable steam break at the measurement point, if necessary, should also take compensatory measures.

3 Measurement Error Analysis

The current use of flow meters to measure steam flow, the measurement medium are referred to single-phase flow of superheated steam or saturated steam. For steam with frequent changes in phase flow, there are problems of inaccurate measurement. In response, it is necessary to maintain the superheat of steam and minimize the water content of steam, such as strengthening insulation measures for steam piping and reducing the pressure loss of steam, in order to improve the accuracy of measurement. However, these methods do not completely solve the problem of accurate steam flow measurement, the fundamental solution to this problem is the development of a two-phase flow of the medium can be measured flow meter.

Used to detect the flow of gas flow meters of many kinds, to the velocity type and more common than the volume flow meter applications, their common feature is to determine the specific volume flow of gas, and the specific volume flow Gv is a function of the state, the working state of the gas than the volume flow does not exactly reflect the actual flow. In this regard, engineering is generally expressed in terms of standard state specific volume flow value or mass flow rate. The use of scale gas flowmeter, selected gas normal temperature, pressure for the design conditions, the design state of the specific volume flow rate converted to standard specific volume flow rate or mass flow rate, the conversion factor contains the gas specific volume factors, when the gas working state deviates from the set juice state, specific volume flow rate measurement value will produce errors. In addition, changes in the zen content of the gas, content or temperature, all have an impact on the flow measurement. Therefore, the measurement of steam flow meter is more likely to require compensation measures, and the compensation factors are complicated by changes in the state of the steam.