Abstract: In the industrial production process, flow measurement is relatively complicated, and it is not easy to obtain parameters. With the continuous development of industrial technology, especially the chemical industry's need for stability control, the role of flow meter in industrial sites has become increasingly important. Because mass flow meter have obvious advantages in flow meters, as field application meters with high measurement accuracy and many measurement parameters, they are widely used in chemical, steel, power and other industries. Due to the complexity of on-site operating conditions, as well as asymmetrical technical parameters provided by users, various problems have occurred in the use of mass flow meters in the field. At present, the cost of mass flow meters is relatively high, or trade settlement is involved, which requires manufacturers to be more careful in the selection stage. This article summarizes the many problems encountered in this area of work, and provides some experiences and methods for users to obtain a better product experience.

Matters needing attention in the selection of mass flow meter

According to the actual situation of the project and technical parameters, from the perspective of meeting the needs of the user and saving costs, choose a mass flow meter with the appropriate shape, material, nominal diameter, nominal pressure, and installation method, and pay attention to the following aspects:

1 media aspect

The difference between the mass flowmeter's measuring medium will mainly affect the caliber, accuracy, material and other aspects of the flow meter.

1) Media status

It is mainly divided into gas and liquid (gas-liquid two-phase may also occur in special cases). It will involve the accuracy requirements of the instrument and the caliber and material of the flow tube.

2) Accuracy

Gas is generally 0.5%, while liquid is generally 0.2%, with high accuracy of 0.1%.

2 Calibre

The specific diameter of each manufacturer's flow meter will correspond to the corresponding flow range. While satisfying the user's flow measurement, the viscosity of the medium must also be considered. This index relates to the pressure loss mentioned later. Sometimes a larger diameter sensor is selected to meet the user's requirements for pressure loss.

3 Material aspects

The main consideration is the corrosivity of the medium. It is often necessary to consult the corrosion manual and select the corresponding anti-corrosive material according to the characteristics of the medium to extend the service life of the flow meter. Please note that the guidelines provided are for reference only. Small changes in fluid properties (such as temperature, concentration, impurities, etc.) are more likely to affect the compatibility of wetted parts. Material compatibility selection is the sole responsibility of the end user.

4 In terms of operating temperature and pressure

These two technical parameters are related to whether the flow meter can be used normally and the safety issues.

1) Operating temperature

In general, if it exceeds 180 ℃, the type is selected as a split type, and the sensor-related components are selected as a high-temperature type. In other cases, it can be selected as an integrated type. It is damaged due to high temperature, which causes the flow meter to malfunction.

2) Operating pressure

The mass flowmeter's flow tube and flange will have a certain pressure, for example, the flange has 1.6MPa, 2.5MPa, 4MPa, 6.3MPa, etc., the flow tube has 10MPa, 35MPa, etc. The nominal pressure of the flange must be greater than the operating pressure of the user, leaving a part of the redundancy.

5 Security aspects

According to the requirements of the user, combining explosion-proof and non-explosion-proof according to the user's site conditions.

6 Flow range

In general, there will be several calibers or series that meet the user's flow range. It is necessary to consider the cost, pressure loss, diameter reduction and other factors at the same time, and choose the appropriate caliber.

7 Precision requirements

It involves aspects of accuracy, including the state of the medium, the large and small flows that need to be met, and so on.

8 Pressure loss requirements

Coriolis mass flow meters are mainly used to measure the mass flow rate of the medium or the volume flow rate calculated by the output. The selection parameters provided by the process professional only involve the mass flow rate or volume flow of the medium, so the influence of the flow rate of the medium is ignored. Coriolis mass flow meter have a small measuring tube amplitude, which can be regarded as no moving parts and no obstructing parts.The measurement value is not affected by the flow field in the pipe, so it can measure a variety of non-Newtonian fluids as well as viscous and particulate-containing Slurry, and the flow-to-range ratio can be large. However, the large measurement value of a Coriolis mass flow meter with a certain nominal diameter is actually limited, and the medium flow rate is a factor.

Pay attention to the problem of measuring the flow velocity of the medium, in fact, pay attention to the problem of the pressure loss caused by the flow meter. In addition, many Coriolis mass flow meters now use curved measuring tubes, such as triangles, which tend to form "turbulence". Therefore, the Coriolis mass flow meter may produce a large pressure loss, and the pressure loss can even be unbearable (0.1 MPa ~ 0.2MPa). In addition, a large pressure loss can easily cause "cavitation" and damage the measuring tube of the flow meter.

In summary, when selecting a Coriolis mass flow meter, it is necessary to consider whether its pressure loss is allowed by the entire process flow. The requirement to limit pressure loss also limits the upper limit of the medium flow rate under a certain nominal diameter; or under the requirement of ensuring the upper limit of the mass flow or volume flow of the medium, a larger nominal diameter is selected to expand the circulation area. In addition, the nominal diameter of the flow meter is small, the measurement accuracy is high, and the price is relatively cheap; the larger the nominal diameter, although the pressure loss of the flow meter is reduced, the measurement accuracy is reduced and the purchase cost is increased. Therefore, it is necessary to strike a balance between accuracy and pressure loss to calculate a suitable nominal diameter.

9 Reduction

While selecting the model, we must also pay attention to the size of the original pipeline of the user. When selecting the corresponding caliber according to other technical parameters, consider factors such as pressure loss and on-site construction, and also take into account the problem of shrinkage. In general, the diameter of the pipe can be reduced by one level, and the reduced diameter is half of the pipe diameter. For example, if the user's pipe diameter is DN80, the smaller diameter of the flow meter can be DN40. Occasionally, there are special situations, that is, if the accuracy requirements cannot be met from the flow in order to ensure the diameter reduction requirements, it is necessary to consult with the user to confirm the site conditions and requirements.

1 0 Special needs

Sometimes, for example, when measuring water-containing crude oil, users often need to know data such as water content, which requires that the selected mass flow meter contain a concentration measurement function. Other requirements such as dual current output, with RS485, HART, Profibus DP / PA communication, etc.