Things to be aware of when measuring steam Posted by : admin / Posted on : Aug 05， 2019

First we need to understand the characteristics of steam in order to make accurate measurements. Steam is also known as "water vapor." The various steams are classified according to pressure and temperature: saturated steam, superheated steam. Steam plays a very important role in industrial production. The main purpose is heating/humidification; generating power; as a drive, power generation by a power plant is also generated by heating water to generate steam.
Superheated steam: If saturated steam is continuously heated, its temperature will rise and exceed the saturation temperature at that pressure. This steam exceeding the saturation temperature is called superheated steam. Superheated steam has its own application areas, such as turbines used in generator sets, which push the motor through nozzles to the motor. However, superheated steam is rarely used in the heat transfer process of industrial processes because the superheated steam must be cooled to the saturation temperature before the condensation releases the evaporating enthalpy. Obviously, the superheated steam is cooled to the saturation temperature compared to the evaporating enthalpy of the saturated steam. The amount of heat released is small, which can degrade the performance of the process equipment.
At present, there are many kinds of steam flowmeters, such as vortex flowmeters, orifice flowmeters, volumetric flowmeters, and crossflow flowmeters. The higher the accuracy, the higher the requirements for external conditions, and the higher the pressure and flow. Need to be stable or constant, at least for a small range of fluctuations. Vortex flowmeter has become one of the most mainstream flowmeters for measuring steam flow. It is characterized by low pressure loss, large measuring range and high precision. It is almost free of fluid density, pressure, temperature and viscosity when measuring volumetric flow. The influence of parameters. No moving mechanical parts, so high reliability and low maintenance. Instrument parameters can be stable for a long time. The vortex flowmeter uses a piezoelectric stress sensor with high reliability and can operate in the operating temperature range of -20 °C to +250 °C. There are analog standard signals and digital pulse signal outputs, which are easy to use with digital systems such as computers. It is a relatively advanced and ideal measuring instrument.
The vortex flowmeters are evaluated for their measurement performance under single-phase flow conditions. There is currently no single-phase flowmeter for measuring the system variation of two-phase flow. However, the two-phase flow exists objectively, and its influence on flow measurement is unavoidable. Despite the difficulties, people are still trying to study the mechanism of its influence on flow measurement, and take corresponding countermeasures to improve the accuracy of flow measurement.
Vortex flowmeters are difficult to measure steam flow: during long-distance transport, dry steam will partially condense due to heat loss, resulting in reduced steam dryness and wet steam. The gas-liquid two-phase flow structure in the horizontal pipe is related to the gas-liquid two-phase volume ratio and the flow velocity. In the steam pipe, since the volume ratio of the condensed water in the wet steam is small, the drain pipe drawn from the bottom of the horizontal pipe is made. , can receive a good hydrophobic effect. When the flow rate is particularly high, it will also behave as a circular flow, that is, there is a liquid film on the pipe wall, and the central part of the pipe is a gas core with droplets. Due to the influence of gravity during horizontal flow, the lower liquid film is higher than the upper pipe. The thickness of the wall, in the vertical ascending pipeline, the basic structure of the gas-liquid two-phase flow has a fine bubble flow structure, a elastic flow structure, a massive flow structure, a ring flow structure with fibers and a ring flow structure.
However, since the volume of condensed water in the wet steam is relatively small, the flow of excess water in the ascending pipe often appears as an annular flow structure, but when the amount of water is particularly large, it also appears as a ring with fibers. Flow structure. Among them, the fibrous fluid is actually a condensed water. Flow in a vertically descending pipe: In a vertically descending pipe, the structure of the gas-liquid two-phase flow is similar to that of a vertically ascending flow, but differs not only in the opposite flow direction but also in the case of the same average flow velocity. The flow rate of the liquid in the vertical drop pipe is much faster than the flow rate of the liquid in the vertical rise pipe.
Vortex flowmeters measure steam. Accurately arrange the steam traps accurately: People have long discovered that when the steam has more water, the vortex flowmeter will have a "leakage pulse" phenomenon, that is, in the case of a stable steam flow rate, the vortex flow It should have a stable pulse output proportional to the flow rate. But sometimes it is found that the output pulse of the meter is inexplicably less. The distribution of the output pulse recorded on the two-dimensional coordinates can also be clearly seen. The pulse that should be approximately evenly distributed is one less at a place. Pulses, in severe cases, are a lot less pulses, and in the worst case, there are no pulses at all. This may be related to the formation of a vortex column by impinging on a large volume of droplets having a non-uniform distribution on the vortex generator.
The rational placement of the hydrophobic point is critical to adequate hydrophobicity. The wet steam is a two-phase fluid in terms of properties, but in the case of low humidity, it can be treated as a single-phase fluid and measured with a general flow meter. Severe steam with water will cause a “leakage pulse” phenomenon in the vortex flowmeter. In more serious cases, the vortex flowmeter will have no pulse output at all. For the case where the steam severely affects the normal measurement of the vortex flowmeter, the usual treatment method is to fully hydrophobic at the appropriate position upstream of the vortex flowmeter. Reasonably arrange the steam trap to make the dryness of the wet steam as high as possible, and ensure the normal measurement of the vortex flowmeter.