A brief analysis of soft maintenance of differential pressure flow measurement system

1 Introduction

The differential pressure flow measurement system refers to a flow measurement system that uses the differential pressure principle to measure flow. This system is one of the more reliable flow measurement methods in the world today. It is generally composed of secondary instruments such as a throttling device (differential pressure generator), a differential pressure transmitter (differential pressure gauge), and a flow integrator. The characteristics of this flow measurement system are:

The principles and theoretical research are very mature;
the degree of standardization is high, with GB/2624 and ISO5167 standard documents;
the structure is simple, the cost and maintenance cost are low;
the ability to adapt to the environment is strong;
if the maintenance and management are proper, higher measurement accuracy and reliability can be obtained.

The above characteristics are unmatched by other forms of flow meters, so differential pressure flow meters account for a relatively high proportion in flow measurement, about 85%. Although some new flow meters have been put on the market in recent years, due to reasons such as structure, cost, and use conditions, differential pressure flow meters still cannot shake the position of flow measurement. Therefore, the correct use and maintenance of this measurement system are still issues that automation instrument professionals must pay great attention to.

2. Maintenance and use status of differential pressure flow measurement system

The differential pressure flow measurement systems currently in use are mostly limited to hardware management and maintenance due to the limitations of the professional and technical level of maintenance personnel. The main work content is nothing more than checking whether the throttling device, that is, the pulse pressure pipe valve of the differential pressure generator, is leaking or blocked, whether the data processors such as the flow integrator are working normally and regularly calibrating them, etc. If no problems are found in the above maintenance checks, even if the data displayed by the system has obvious deviations, it is generally considered that the system is normal.

I think that the maintenance of differential pressure flow measurement system is far from enough to be limited to the management and maintenance of hardware. What is more important is to perform soft maintenance on the system. The so-called soft maintenance is to track, analyze, study and calculate the operating parameters of the differential pressure flow measurement system, and repeatedly revise the technical parameters of the system according to the calculation results to make it as close to the actual operating conditions as possible. In fact, soft maintenance is a process of making the technical parameters of the system infinitely close to the actual operating conditions.

3 Necessity of soft maintenance of differential pressure flow measurement system

For any differential pressure flow measurement system, its design calculation is based on the medium type, temperature, pressure, flow rate, pipeline geometry and other working condition parameters provided by the customer, that is, a flow measurement system is calculated and designed and manufactured based on a set of parameters provided by the customer. The system can only guarantee the accuracy and reliability of the measurement results if it operates under the working conditions adopted in the design calculation. If the actual use conditions deviate from the design conditions, the system measurement data cannot guarantee accuracy and reliability. However, most of the differential pressure flow measurement systems in actual use are different from the design conditions, and some are even very different. Therefore, soft maintenance must be performed on them, and this soft maintenance should run through the entire process of system use.

4 Impact of deviation between operating conditions and design conditions on system measurement accuracy and reliability

4.1 The actual operating flow value deviates from the design value

The actual operating flow value deviates from the design flow value, which is a common problem. In most cases, the design flow value is set relatively high, but the actual operating flow value is lower. The main reasons that affect the accuracy and reliability of system measurement are:

(1) Since the accuracy and reliability of the throttling device are calculated based on the commonly used flow rate, which is generally about 70% to 80% of the scale flow rate, if it deviates far from the commonly used flow rate, it will cause a large change in the outflow coefficient C, and its accuracy and reliability will be greatly reduced, resulting in poor accuracy and reliability of the system measurement.

(2) The reliability of the differential pressure transmitter (differential pressure gauge) is calculated based on the reference error, and its error distribution is shown in the table.
Table 1 Differential pressure gauge error analysis table (differential pressure gauge is based on 0.5 level)

It can be seen from the table that if the system operates below 50%, the relative error of the differential pressure gauge is ±2%~±6%, and the impact on the flow rate is ±1%~±3%. At the same time, since the Reynolds number is a function of the flow value (see the formula below), the deviation of the flow value will inevitably cause the change of the Reynolds number, which will inevitably cause the change of the outflow coefficient C. If these effects are concentrated together, the cumulative error will be relatively large. This will also cause the accuracy and reliability of the system measurement to be poor. [page]

4.2 Medium temperature and pressure deviate from the design value

When designing and calculating the throttling device, the flow rate of the medium fluid (including gas, liquid and steam) is the mass flow rate (or volume flow rate) under the design conditions, that is, the mass flow rate (or volume flow rate) under certain specific conditions. This specific condition includes many parameters, mainly temperature and pressure parameters. Under this specific condition (temperature, pressure, etc.), the density of the medium fluid is certain. If the main parameter values ​​of the medium fluid such as temperature and pressure deviate from the design value during actual operation, the density of the medium fluid will inevitably change. The flow calculation formula is

In the formula, qm is the mass flow rate; β is the ratio of the inner diameter of the throttling device to the inner diameter of the pipe; C is the outflow coefficient; is the flow coefficient; ε is the expandability coefficient of the fluid upstream of the throttling device; d is the inner diameter of the throttling device under working conditions; △P is the static pressure difference between the upstream and downstream of the throttling device; ρ is the density of the fluid upstream of the throttling device.

The outflow coefficient C of the throttling device (composed of an orifice plate) is given by the Stolz equation. When the angle connection is used to take the pressure:

Where ReD is the Reynolds number of the pipe; μ is the dynamic viscosity of the fluid; and D is the inner diameter of the pipe.

From the above calculation formula, it can be seen that the change in density of the medium fluid has a very intuitive effect on the flow value. On the other hand, for the convenience of process operation, the instrument indication value gives the volume flow rate under standard conditions, which is converted according to the density of the medium fluid. The change in the density of the medium fluid also changes its conversion relationship. Both effects will cause the increase of the measurement error of the throttling device. The role of adding a temperature and pressure correction link in some flow measurement systems is to reduce this effect. Of course, there are many factors that affect flow measurement, and it is not enough to just perform temperature and pressure compensation. Temperature and pressure compensation is only a means to ensure that the measurement is as accurate as possible.

The national standard for throttling devices stipulates that: it is assumed that an uncalibrated throttling device is geometrically and dynamically similar to a fully tested throttling device, that is, it meets the requirements of this standard and is within the uncertainty specified in this standard. For a throttling device designed and manufactured according to this principle, if the actual working conditions differ greatly from the values ​​used in the design calculation, the conditions of dynamic similarity cannot be met, and the uncertainty is not within the regulations, that is, the accuracy and reliability cannot be guaranteed.

There are many factors that affect the accuracy and reliability of differential pressure flow measurement systems in actual use. The above only analyzes the two aspects with the greatest impact. In order to use the differential pressure flow measurement system well, maintenance managers should track and investigate the system operation and solve problems in a timely manner. For large-scale material flow measurement systems, each measurement point of the flow measurement system should be analyzed and studied from the perspective of the material balance of the entire system, the control range of the flow value of each measurement point should be calculated, the gap between the measurement value and the control range should be repeatedly studied, and the cause of data deviation should be found and solved. This can further improve the accuracy of flow measurement and provide more reliable measurement data for production and operation.

5 Requirements for soft maintenance

The soft maintenance management of the differential pressure flow measurement system is a highly technical job, and personnel with a certain level of professional skills should be assigned to carry out this work. The work content mainly includes:

(1) Collect and keep various technical documents and information on system design, calculation, manufacturing and installation;

(2) After the system runs stably, investigate and analyze the system's operating conditions. If it is found that the operating conditions deviate from the design values, the system can be adjusted based on the calculation results;

(3) For systems with high frequency of operating condition changes, a temperature and pressure correction link can be added. If conditions permit, a computer can be used to make timely online corrections. In practical applications, it is possible to consider developing some system software that makes soft maintenance easy to implement, which can also greatly improve the overall technical level of the flow measurement system.

(4) During the system soft maintenance process, we should pay attention to learning lessons, summarizing experience, and constantly improving our soft maintenance level. At the same time, we should combine the system hardware maintenance to do a comprehensive and detailed maintenance of the system to ensure the normal operation of the system.

6 Conclusion

As enterprises move towards the market, their production and operation management will be further deepened, and flow measurement will become more important. Automation instrument professionals must strengthen the maintenance of flow measurement systems from both soft and hard aspects to meet the requirements of enterprises and enable flow measurement to play an important role. (end)