With the rapid development of China’s economy, the instruments in the metallurgical industry have continued to improve to a comprehensive level. As a result, the application of thermal gas mass flowmeter in the production of the metallurgical industry has become more and more popular. Therefore, this article will mainly discuss the working principle of the thermal gas mass flow meter and its application and other issues frequently encountered in the design work of the Metallurgical Industry.
Thermal gas mass flow meter in gas flow measurement of the metallurgical industry has a wide application; it is usually used for accurate measurement of gas flow with the constant temperature difference method, it has the advantages of slight pressure loss, high degree of digitalization, easy installation, precise measurement, etc. This paper will elaborate on its principle and application in the metallurgical industry.
Development of Thermal Gas Mass Flowmeter
Early thermal mass flowmeter heating coil and temperature measurement elements directly in contact with the fluid is a contact flowmeter, but due to the prevalence of corrosion, wear, and explosion-proof problems in the field environment, its industrial applications are minimal.
By the 1950s, people proposed a boundary layer flowmeter without contact with the fluid, which overcomes the shortcomings of contact flowmeter, the measurement results by the media parameters (such as thermal conductivity, viscosity, etc.) the impact of environmental vibration.
By the 1970s, the development of a thermal gas mass flow meter based on the measurement of fluid temperature distribution had been widely used due to its unique advantages in the industry. Still, it has a fatal shortcoming, and it can only detect small flow. For small diameter measurement, it is helpless for large diameter fluid measurement. Based on these flowmeters, people have designed a kind of insertion gas mass flowmeter, and its appearance makes it possible to measure the flow of large diameter gas. After decades of efforts of scientific and technical personnel, thermal gas mass flowmeter has become increasingly mature, is accepted and used by various industries.
Thermal Gas Mass Flowmeter operating principle:
Thermal gas mass flow meters are divided into two main principles: the thermal gas mass flow meter and the Golden Rule gas mass flow meter. The thermal gas mass flow meter is mainly used to measure minor diameter gases. In contrast, the Golden Rule gas mass flow meter is more often used for the measurement of gas flow in large and medium diameter pipelines.
Principle of operation of Thermal Gas Mass Flowmeter:
Two sets of coil windings for heating and temperature measurement are wound symmetrically around the outer wall of the measuring tube to transfer heat to the fluid through the tube wall. When the flow rate is 0, the temperature of the measuring tube is symmetrically distributed according to the centerline, and the measuring bridge is in equilibrium. As the fluid flows, the fluid brings some of the heat from the upstream section downstream, so the temperature of the upstream section drops and the downstream area rises, with the highest temperature point moving downstream from the centerline. The bridge measures the average difference in temperature between the two sets of coils, and the mass flow rate can be derived from the following equation.
From the above working principle, it can be seen that when the heating power is a constant value, the temperature difference and the flow rate have a certain mathematical relationship, from the change in temperature difference can reflect the change in flow rate. Conversely, when the temperature difference is a constant value, the change in heating power can also skip the change in flow rate. Therefore, the first measurement method is usually called the regular power method, and the second is the consistent temperature difference method. Due to this principle, the detection element can be made smaller so often manufactured into the insertion instrument. In the large and medium diameter pipeline, flow measurement is more widely used.
Advantages of Thermal Gas Mass Flowmeter:
Thermal gas mass flow meter due to the unique principle, the structure of the reason makes it has some conventional flow meter does not have the characteristics.
- Measuring tube diameter range is a significant feature, from DN1 mm to DN5000 mm above the diameter of the thermal gas mass flowmeters are applied.
- An extensive range ratio is also a feature of the thermal gas mass flow meter, a conventional throttling device; due to the introduction of the open link, its range ratio is difficult to break 10:1. In contrast, the thermal gas mass flow meter can usually reach 100:1, some high-end thermal gas mass flow meters can even go 1000:1.
- In the measurement process, the pressure loss is slight is another advantage of the thermal gas mass flow meter. Its pressure loss is almost negligible.
With modern advances in materials technology, thermal gas mass flow meters and the part of the measurement medium contact can use more advanced materials, such as stainless steel, Hastelloy, Monel, etc. With these alloy materials, thermal gas mass flowmeters can be used in a broader range of working conditions. Some corrosive and high-temperature gases can also be measured by it. The thermal gas mass flow meter is also excellent in the face of a high-pressure measurement environment, can withstand up to 70MPa pressure. The thermal gas mass flow meter is constructed as a solid unit without any moving parts, avoiding wear, fractures, and fatigue cracks in the mechanical aspects of the meter due to the high speed of the fluid passing through. In addition, the thermal gas mass flow meter is usually equipped with a protection device, which does not cause any damage to the meter even if the actual flow rate measurement is out of the process range.
Typical applications of Thermal Mass Gas Flowmeters in the Metallurgical Industry
In the metallurgical industry, the flow of gases in large and medium diameter pipelines often needs to be measured, such as blast furnace gas, converter gas, etc.
The flow measurement of these gases generally has the following characteristics:
- A large pipe diameter required slight pressure loss in the measuring device.
- Short straight pipe sections
- Because of the reserved expansion of production scale, the processing pipeline diameter is large, and the gas flow rate is generally low.
- Old pipelines have high gas flow rates due to the expansion of the production scale.
- Frequent changes in working pressure & temperature as well as flow rate.
- Short maintenance time, which required fast completion of commissioning
- Process pipelines work all year round, requiring installation with gas and pressure.
- Individual conditions exist in need of positive and negative flow measurement needs.
- For process requirements or measurement and settlement requirements, flow needs to set the temperature, pressure correction.
In the above working conditions, traditional flow measurements devices such as orifice plates, venturi tubes, and other challenging to meet requirements of production, conventional flow measurement devices in the straight section length, measuring range, etc. can not meet the criteria, in this case, the thermal mass flow meter can still meet the conditions of use, everyday use.
The metallurgical industry there are also many small-diameter pipelines where the flow of gases often needs to be measured, such as the measurement of the flow of oxygen and argon in the steelmaking process.
The flow measurement of these gases generally has the following characteristics:
- Small pipe diameter which requiring slight pressure loss of the measuring device.
- High measurement accuracy requirements are directly affecting product quality.
- Process changes, the flowmeter parameters need to be adjusted at any time.
- The site environment is harsh.
- The gas is relatively pure, but in extreme cases, there are dirty substances.
- Considering reserved expansion of production scale, the processing pipeline diameter is large, and the gas flow rate is generally low.
- The maintenance time is short and requires quick completion of calibration and purging.
- For process requirements or measurement and settlement requirements, flow needs to set temperature and pressure correction.
In the above conditions, the thermal mass flowmeter has many applications. Flow measurement in minor diameter conditions to ensure the accuracy of gas measurement has been the shortcomings of conventional flow meters, but this condition precisely to meets the amount of thermal gas mass flowmeter use conditions. The use of thermal gas mass flowmeter to improve product quality, protect the equipment operating rate, shorten the maintenance time has practical significance.
The selection of Thermal Gas Mass Flowmeter
Thermal gas mass flow meters have many advantages. Still, there are also requirements and restrictions in the selection, only to meet the relevant conditions, thermal mass flow meters can work reliably and correctly.
- The client needs to supply the fluid’s parameters, including the fluid type, measurement range, physical properties (density, viscosity, corrosiveness, etc.), the range of temperature and pressure changes, and the commonly used temperature and pressure. For example, when the specific heat capacity of the gas at constant pressure () changes will lead to changes in the range of the instrument.
- Clients need to tell the flowmeter factory the process requirements and working conditions, including accuracy levels, allowable pressure loss, and whether the fluid is one-way or two-way. For example, when the liquid is pulsating flow or require the instrument response speed is very fast (thermal mass flow meter response time is relatively long, the time constant is generally 2 ~ 5s, the faster response to 0.5s, some models up to several seconds, more than ten seconds or even dozens of seconds. So, we need to select the thermal mass flowmeter carefully.
- Understand the installation conditions of the flowmeter, such as the flowmeter before and after the straight pipe section conditions; there is no downtime to repair the flowmeter conditions, whether the flowmeter needs to be plugged online.
- Consider energy saving. The conventional flowmeter will cause irrecoverable pressure loss, directly increase production costs, so give priority to thermal mass flowmeter such as low operating costs instrument.
- Investment limits. Cannot unilaterally consider the many advantages of the thermal mass flowmeter without regard to investment limits.
The thermal gas mass flow meter is an excellent gas flow measurement instrument; it can be used in high temperature, high pressure, high corrosive, and other extreme environments. Moreover, it has good adaptability for various pipe diameters , so that the thermal gas mass flow meter is widely used in the metallurgical industry. However, the clients should communicate with the flow meter manufacturer for the thermal gas mass flow meter characteristics and then combine them with the site’s actual requirements. This way can ensure the correct design selection and give full play to the high reliability of the thermal gas mass flow, easy installation, and other advantages.