Steam is widely used in industrial production, and it is the source of energy in many processes. The measurement and process control of steam are very important. Steam has its own particularity, the temperature and pressure are very large, and the requirements for the measuring parts of the flowmeter are relatively high. In the case of frequent flow changes and a large range of changes, in order to meet its measurement accuracy, the flowmeter is required to have a larger turndown ratio. The following is a simple comparative analysis of several flow meters commonly used for steam measurement.
1. Orifice flowmeter
1. Measuring principle: differential pressure principle
2. Advantages: (1) The orifice steam flowmeter is the earliest product of ZUI in the flowmeter. It is characterized by a wide range of measurement media and a standardized product. (2) The price is cheap. (3) Precision.
3. Limitations: (1) The long-term measurement accuracy of the orifice plate is low. The throttle of the orifice plate has been scoured and worn by the fluid for a long time, and the diameter-to-value ratio changes, which cannot guarantee the long-term measurement accuracy. (2) The orifice plate has a large blocking area and a large pressure loss, which will cause more energy waste. (3) The range ratio of the orifice plate is small, generally 3:1, and the measurement accuracy cannot be guaranteed under the working conditions with large changes in the flow range. (4) Orifice plate installation adopts
Pipeline installation, inconvenient installation, high installation cost, and difficult maintenance.
Two, vortex flowmeter
1. Measurement principle: The vortex flowmeter uses the principle of fluid oscillation to measure the flow. When the fluid passes through the thirsty flow transmitter in the pipeline, two rows of vortexes proportional to the flow rate are alternately generated after the vortex generator of the triangular column. , The release frequency of the vortex is proportional to the flow velocity, and the frequency signal is converted into the corresponding flow value through the converter.
2. Advantages: (1) The frequency generated by the vortex street is only a function of the flow rate, and has nothing to do with the density and viscosity of the medium. (2) The range is relatively wide up to 10:1. (3) The pressure loss is small, about 1/3~1/2 of the orifice plate. (4) High precision, the flow signal is converted from a high-precision frequency signal. Within a certain Reynolds number range, the output frequency signal is not affected by fluid physical properties (density, viscosity) and composition. (5) With throttling differential pressure
Compared with the flowmeter, there is no need for pressure guiding tube and three-valve group, etc., which reduces leakage, blockage and freezing (6) The price is moderate.
Limitations: (1) The stability of the vortex separation is affected by the distortion of the flow velocity distribution and the swirling flow. It is necessary to configure a long enough straight pipe section or install a flow regulator (rectifier) according to the different forms of flow blocking parts on the upstream side. (2) It is not suitable to be used in strong vibration places for pipeline mechanical vibration than MIN. (3) The instrument lacks theoretical research and practical experience in pulsating flow and mixed phase flow. (4) The frequency signal detection of Kejie is done by the electrical converter, and the temperature and pressure resistance level is limited, so it is not suitable for the measurement of high temperature and high pressure steam.
1. Measuring principle: Combining the advantages of two different differential pressure technologies of Venturi and Venturi tube, it is a very unique flow sensor.
2. Advantages: (1) High accuracy, up to 0.5% of soil, good repeatability, up to 0.05% (2) Range ratio as high as 65:1, which has obvious advantages for working conditions with large range changes. (3) *The pressure loss is relatively small. (4) There is no requirement for straight pipe section. (5) For small flow and low flow rate, increase the flow rate through the "Venturi tube" to achieve better measurement. "Venturi tube" also has a rectifying function, which makes long-term operation stable. (6) There is no electrical device in the measurement part, and the temperature and pressure resistance is high, which is suitable for high temperature and high pressure working conditions.
3. Limitations: (1) The price is relatively expensive, and the small diameter DN200 is about RMB 60,000.
Four, average velocity tube flowmeter
1. Measuring principle: differential pressure principle and radial velocity average principle.
2. Advantages: (1) The range ratio is 10:1, which is suitable for working conditions with large range changes. (2) Measurement accuracy ±1%, repeatability ±0.1%. (3) High pressure and high pressure resistance, suitable for high temperature and high pressure steam measurement. (4) The pressure loss is small, about 1/4~1/2 of the orifice plate. (5) No need for pipeline installation, simple installation and convenient maintenance. (6) The price is moderate
3. Limitations: (1) For frequent start-up and shutdown conditions, there is still a lack of theoretical research and practical experience in the miscible flow.
Five, variable area flowmeter
1. Measuring principle: the principle of differential pressure, according to the flow change, automatically control the flow of the throttling element, and ensure the stable output of the differential pressure signal.
2. Advantages: (1) Contains the advantages of traditional differential pressure flowmeter technology such as mature technology and high measurement accuracy. (2) The turndown ratio is as high as 100:1. When the scale flow rate, the common flow rate, and the small flow rate of ZUI are very different, the accuracy of the measurement under each flow rate can be guaranteed. (3) Rely on its own throttling element over-flux regulator to adjust the throttling element over-flux, especially in the case of small flow, to achieve signal amplification and improve measurement accuracy. No manual adjustment is required.