【Abstract】 What factors we should consider for steam flow meter selection?Which type of flow meter can be used to measure steam？This paper introduces the application status of steam flow meters and the problems in steam flow metering and how to upgrade equipment.
The client company is a production enterprise mainly engaged in zinc smelting and valuable metal recycling in the northwest. The company has a set of flue gas acid production system. In order to fully recycle and recycle the flue gas waste heat generated by zinc concentrate smelting, it is equipped with one. The waste heat boiler, the waste heat boiler, uses the heat of the high-temperature flue gas supplied from the boiling roaster to produce steam. The boiler produces saturated steam or superheated steam for heat production to achieve waste heat utilization.
Flow sensor types:
The flow sensor can be divided into a water flow sensor, a plug-in flow sensor, a vane air flow sensor, a vortex flow sensor, a Karman swirl air flow sensor, a hot wire air flow sensor, and the like. Among them, the water flow sensor is mainly composed of a copper valve body, a water flow rotor assembly, a steady flow component and a Hall element. It is mainly installed at the inlet end of the water heater to measure the influent flow. When the water flows through the rotor assembly, the magnetic rotor rotates and the rotational speed changes linearly with the flow rate. In this product, the output pulse signal is fed back to the controller by the Hall element. The controller determines the magnitude of the water flow, adjusts the current of the proportional valve, and controls the gas volume through the proportional valve to prevent the gas water heater from appearing during use. The phenomenon of warm summer and cool winter.
The flowmeter is further divided into a differential pressure flowmeter, a rotameter, a throttle flowmeter, a slit flowmeter, a volumetric flowmeter, an electromagnetic flowmeter, an ultrasonic flowmeter, and the like. Classified by media: liquid flow meter types and gas flow meter types.
The gas flow meter types:
A gas flow meter is a meter that meters the flow of gas. Installed in the pipeline to record the amount of gas flowing through. Can measure gas, air, nitrogen, acetylene, phosgene, hydrogen, natural gas, nitrogen, liquefied petroleum gas, hydrogen peroxide, flue gas, methane, butane, chlorine, gas, biogas, carbon dioxide, oxygen, compressed air, argon Gas, toluene, benzene, xylene, hydrogen sulfide, sulfur dioxide, ammonia, and the like.
1 Flow meter usage status
The customer company originally installed 17 flow meter devices, of which 4 flow meters for measuring steam were all orifice differential pressure flowmeters, and all were replaced with vortex flowmeters.
2 Problems of steam flow meters
The steam metering volume of the customer company has always been inaccurate in the measurement process. Due to the excess heat of the workshop, it is supplied to other places, and the energy consumption is calculated, energy consumption analysis and cost settlement are always generated with other gas units. In the dispute, the other party said that the amount of steam is extremely small, and the measurement data of the customer company here shows so much data, and there is always a rush.
The steam flowmeter of the customer company adopts the orifice differential pressure flowmeter when designing the plant. The orifice differential pressure flowmeter itself has certain weaknesses: the installation is relatively complicated, and it is necessary to install a three-valve group and a differential pressure transmitter. Condensation pipe, flow totalizer, temperature and pressure compensation need to install temperature, pressure transmitter; pressure pipe is prone to blockage, easy to run and leak, winter also needs heat preservation, etc., these reasons cause the instrument to install a large amount of work, The cost is high, the maintenance workload is also large; the orifice plate is a throttling structure, resulting in large pressure loss, and the energy consumption for long-term operation is also large; the geometrical dimensions of the orifice plate change during long-term operation, which easily causes the acute angle to become dull. It even becomes a bell-shaped shape, which affects the measurement accuracy of the system; the acute angle of the orifice material becomes dull and the outflow coefficient changes, so that the measurement error increases and the repair cannot be improved.
3 Flow meter selection
According to the flow meter working principle, the steam flow measurement method can be divided into two types: direct mass flowmeter and derivation type, also called indirect mass flowmeter. The former directly measures the mass function as a function of mass flow; the latter calculates the mass flow using the volume and other variable measuring instruments or two different measuring principle flowmeters. Nowadays, the most widely used is the deductive flowmeter, in which the orifice differential pressure flowmeter and the vortex flowmeter are the mainstream. Vortex flowmeters are mainly used for the flow measurement of industrial pipeline media, such as liquid, gas, steam and other media. It is characterized by small pressure loss, large measuring range and high precision. It is almost unaffected by parameters such as fluid density, pressure, temperature and viscosity when measuring the volume flow rate. There are no moving mechanical parts, so the reliability is high and the maintenance is small. Instrument parameters can be stable for a long time.
Common features: The measured objects are the volume flow of steam, and the advanced point equipment is equipped with temperature and pressure compensation, and the mass flow value is calculated by transformation. It is also possible to measure the flow rate of the liquid. The accuracy is generally 1.0% - 1.5%.
Advantages of the orifice flowmeter: The measuring range is optional, such as 10-30m3/h, or 30-90m3/h, all of which can be selected. Strong anti-vibration. It can measure high temperature and high pressure steam. The temperature is above 500° and the pressure is greater than 40 MPa.
Disadvantages: Installation is more complicated than other flowmeters: three valve sets, differential pressure transmitters, condensing tubes, flow totalizers are required, temperature and pressure compensation are required for installation temperature and pressure transmitter; range ratio is narrow: The domestic differential pressure transmitter is generally 1:3, the good imported transmitter is 1:6; the measurement has pressure loss.
Advantages of the vortex flowmeter: The range ratio is wide, and the range ratio of the intelligent vortex flowmeter is generally 1:9, 1:10. The measurement has almost no pressure loss. Easy to install: the mounting method is flange-to-flange or flanged.
Disadvantages: The range of the range is relatively fixed with the caliber, but the variable diameter can be used to measure some media with a small flow. The seismic performance is weak. Measuring high temperature and high pressure steam is not dominant, generally measuring the upper temperature limit of 350 °, pressure 4 MPa.
This shows that the selection of the flow meter is very important.
4 Comparison of investment returns
An imported vortex street is about 1~150,000 yuan, domestically produced within 10,000 yuan, and an orifice differential pressure flowmeter device includes differential pressure transmitter, orifice plate and flange, pressure guiding tube, valve, heat preservation. The cost of the box and the like must exceed the limit of 10,000 yuan.
The installation of the vortex flowmeter is very simple. It only needs to ensure that there are certain straight pipe sections before and after the flowmeter. The orifice flowmeter has long-term running measurement accuracy for straight sections, concentricity, and pressure guiding pipes. The design of the orifice plate is due to differential pressure. It is nonlinear with flow. When the flow rate is lower than 30%, the error increases and the gas is more serious. On the other hand, the long-term wear of the medium, the sharp angle becomes blunt, and the flow coefficient changes, which is also an important reason for the accuracy. . The special structure of the vortex street, when the accuracy is actually determined, the accuracy is almost constant.
In addition to the periodic calibration of the vortex flowmeter, there is generally no failure, no other maintenance costs are required, and the orifice flowmeter is different. It runs leaking, periodically drains, fills the isolation fluid, and replaces the guide. Pressure pipe, valve, heat preservation, cleaning orifice plate, etc., have a certain amount of maintenance work. There is also an input from the heat tracing system, which does not include the update of the differential pressure transmitter and the cost of the orifice plate renewal.
There are strict installation requirements, and the installation of these components is time-consuming and laborious, so the installation cost is several times that of the vortex street.
In other respects, the advantages of vortex flowmeters are more obvious, such as interchangeability, range, and reliability. With the continuous development of technology, the performance of the vortex flowmeter is even better.
In summary, the vortex flowmeter has a small pressure loss of the pipeline medium, a large range ratio, and high measurement accuracy. It is less affected by parameters such as fluid density, pressure, temperature, viscosity, etc. in the measurement of the volume flow rate, and there is no movable machinery. Parts, so the reliability is high, the maintenance is small, the instrument parameters can be stable for a long time, etc. Based on the above discussion, for the advantages of the vortex flowmeter, the workshop will replace the three differential pressure flowmeters that cannot be put into normal production into vortex flow. There is no problem during use, and the amount of maintenance is greatly reduced. The successful application in the company's steam flow meter measurement does have many benefits for flow measurement.