Discussion on the Application of Coal Slurry Flow Meter in Texaco Gasifier

Introduction

With the continuous development of coal chemical projects in China, the application of Texaco gasifier coal slurry flowmeter is becoming more and more extensive. The two methanol projects used as the background of this article both adopt the Texaco gasifier process, with a coal slurry concentration of about 62% (wt/wt), an average particle size of about 31μm, a pipeline of DN150, a pressure level of 600lbs, and a production capacity of 200,000t. However, since the commissioning, the operation effect of the original imported flowmeter has been unsatisfactory, and large fluctuations often occur. The vertical pipe flowmeter fluctuates more frequently, and sometimes the horizontal pipe flowmeter also fluctuates together. This can easily lead to accidental tripping, causing great losses to the enterprise and greatly increasing the labor intensity of relevant departments.

In response to the problem of tripping accidents caused by fluctuations in coal slurry flowmeters, the causes of the fluctuations of coal slurry flowmeters were analyzed, and the special structure sensors and new generation AC excitation converters produced by Shanghai Willtech were used to effectively prevent tripping accidents caused by fluctuations, and the expected application effect was achieved.

1 Analysis of the causes of fluctuations

According to the principles of electromagnetic flowmeters, relevant literature, and long-term application experience, the fundamental cause of flow field fluctuations depends on the flow field and flow state of the fluid in the electromagnetic flowmeter measuring tube [1]. The corresponding relationship between the flow rate and flow velocity of the DN150 measuring tube is shown in Table 1. During normal production, the flow rate is generally around 25m3/h, and the corresponding flow velocity is only 0.4m/s. The fluid flow rate is low, while the coal slurry concentration is as high as 60% or more, which easily leads to an unstable flow field in the pipeline, and the instability of the vertical pipe and the horizontal pipe is different [2].

In the vertical pipe, the flow velocity in the center is relatively fast, while the flow velocity around is relatively slow, which makes a layer of slow-flowing coal slurry adhere to the surface of the pipe wall. If this adhesion layer is so slow that it is almost motionless and relatively thick, it will cover the electrode, and the flow output of the converter will decrease, or even drop to zero. Since the coal slurry pump works in a pulsating manner, the coal slurry pipeline also vibrates, and the vertical pipe is very high, the coal slurry attached to the inner pipe wall is in an unstable state, and avalanche-like phenomena will occur from time to time in the pipeline, and this avalanche phenomenon is not necessarily only in one place. The schematic diagram of the coal slurry flow state is shown in Figure 1. If this avalanche phenomenon occurs in the measuring tube of the flowmeter, it will cause fluctuations in the converter output. The output fluctuation cycle caused by the "avalanche" is relatively fast, and the damping time is increased, which can play a certain stabilizing role.

Fig.1 Schematic diagram of coal slurry flow state

In the production process, the most difficult to deal with is another kind of fluctuation. The fluctuation cycle is very slow, but the amplitude is large, large enough to cause the car to jump. To explain this phenomenon clearly, it is necessary to introduce the concept of weight function [3-6]. Roughly speaking, the weight function gives the contribution rate of the induced electromotive force generated by the fluid cutting the magnetic lines of force at different positions to the electrode signal. The schematic diagram of the weight function is shown in Figure 2.

Figure 2 Schematic diagram of weight function

Figure 2 shows the weight function distribution of the electrode plane, with the center point weight being 1. As can be seen from Figure 2, the closer to the electrode, the greater the weight; conversely, when the guide tube turns 90°, the weight is the smallest.

Two distributions of coal slurry flow velocity on the electrode plane are shown in Figure 3, and the distribution of its weight function is the same as Figure 2. [page]

Figure 3 Schematic diagram of uneven flow velocity distribution

The ideal flow field distribution should be axially symmetrical, with a slightly larger center and slightly slower surroundings. At this time, the flow measured by the electromagnetic flowmeter is accurate and stable (except for the influence of slurry noise). If the flow field is not ideal, the flow velocity distribution in the cross section of the measuring tube is uneven, which may cause fluctuations in the flow measurement output.

As shown in Figure 3 (a), the flow velocity is fast where the weight function is high, and the flow velocity is slow where the weight function is low. At this time, the converter flow output increases; as shown in Figure 3 (b), the flow velocity is fast where the weight function is low, and the flow velocity is slow where the weight is high, and even the electrode is covered. At this time, the converter output decreases, or even approaches zero.

The cause of the fluctuation of the cross tube is different from the above analysis. Its fluctuation is mainly caused by siltation. If the siltation surface does not exceed the electrode, the converter output will increase due to the reduction of the cross section of the measuring tube and the increase of the flow velocity; if the siltation surface exceeds the electrode, the converter output will decrease. Since this siltation is unstable, the siltation surface rises and disappears from time to time, which causes a long period of fluctuation in the flow output. Generally speaking, the cross tube performs better in the early stage of feeding, and the cross tube will fluctuate after a period of time.

2 Solution

Before this, most people believed that the reason for the flowmeter fluctuation was slurry noise or even electrode noise. Since the period of slurry noise and electrode noise is relatively short, the method of dealing with fluctuation is nothing more than increasing the damping time. Facts have proved that this method cannot fundamentally solve the problem of jumping caused by the fluctuation of flowmeter output.

After the above analysis, we believe that the fundamental reason for the fluctuation of flowmeter output lies in the flow field and flow state, which is characterized by a relatively long fluctuation period of several seconds or even tens of seconds. To improve the flow field and flow state, it is necessary to start with the design of sensor structure. Therefore, a sensor with a venturi shape is used, which not only increases the flow rate, but also plays a certain degree of rectification role. Application experience shows that when the flow rate is about 1m/s, the flow field is better, and the sensor designed in this way plays a key role in eliminating large-scale long-period abnormal fluctuations.

In order to solve the abnormal fluctuation of the flowmeter, it is not enough to just change the structure of the sensor. Since the original imported electromagnetic flowmeter is square wave excitation, the so-called low-noise electrode and the more expensive ETFE lining cannot effectively solve the problem of the converter overcoming the inherent deficiency of slurry noise. In addition, the slurry noise increases with the increase of flow rate. After the diameter is reduced, if a square wave excitation converter is used, although large-scale long-period fluctuations can be avoided, the problem of fast-period fluctuations cannot be solved. More seriously, this fluctuation may also cause the car to jump.

In order to make the converter output stable, the square wave excitation converter can only increase the damping time. However, the safety interlock scheme of the Texaco gasifier has strict restrictions on the damping time of the flowmeter. According to calculations by professionals, if the coal slurry pipeline is blocked, the oxygen is cut off for only 8 to 10 seconds. If the damping time of the flowmeter is too large, there will be a risk of over-oxygen explosion.

From the above analysis, it can be seen that the Texaco process requires the coal slurry flowmeter to have not only stable output (to prevent accidental jump) but also fast response speed (once the slurry is cut off, the safety interlock is quickly activated to cut off oxygen to prevent furnace bricks from burning or over-oxygen explosion). Since the square wave excitation electromagnetic flowmeter itself has defects in measuring slurry, an AC excitation converter should be selected. The new generation of AC excitation converters produced by Shanghai Welltech uses a high-order bandpass filter, which can achieve stable and fast flow measurement output. In our actual project, a damping time of 0.2s has been used for more than two years of safe and reliable operation.

Since the cause of the fluctuation of the horizontal pipe is different from that of the vertical pipe, the treatment method is also different. A domestic electromagnetic flowmeter manufacturer once imitated Welltech's vertical pipe flowmeter and designed a reduced diameter and applied it to the horizontal pipe, but the actual effect was not ideal. The horizontal pipe solution provided by Welltech is shown in Figure 4.

Figure 4 Schematic diagram of cross-tube fluctuation solution

As shown in Figure 4, the measuring tube of the flowmeter is half-moon shaped, and the upper part is blocked, which is mainly to prevent the lower interception from causing pipeline blockage. In order to improve the flow field, guide plates are also designed upstream and downstream of the flowmeter. The converter still uses the second-generation AC excitation technology.

3 Application effect

The solution proposed in this article has been successfully applied to two domestic coal chemical enterprises. First, the solution is applied to the vertical pipe with relatively frequent fluctuations, and the horizontal pipe still uses the original imported flowmeter. A certain enterprise has caused dozens of tripping accidents a year due to the frequent fluctuations of the original imported flowmeter. Since the stability of the coal slurry flowmeter provided by Willty is far superior to other similar products, the enterprise has never caused a false tripping due to flowmeter fluctuations since it was put into operation. Figure 5 shows a comparison chart of the flow output stability obtained from the screenshot of the monitoring system workstation. It can be seen from Figure 5 that when the inlet flowmeter located in the horizontal pipe fluctuates greatly, the output of the vertical pipe Willty flowmeter remains stable. At the same time, the process personnel use other parameters such as furnace temperature for comprehensive judgment, which also proves the stability of the vertical pipe flowmeter output.

Figure 5 Flow output stability comparison chart

In order to accurately compare the output speed of different electromagnetic flowmeters, two electromagnetic flowmeters were installed in the same pipeline for testing. The horizontal pipe was the imported flowmeter and the vertical pipe was the Willty flowmeter. During the test, the coal slurry pump was suddenly turned off, then turned on again after a period of time, and the speed of the coal slurry pump was gradually increased. Figure 6 shows the current output results of the two electromagnetic flowmeters recorded simultaneously by the DI-730 data acquisition device screenshot. Each grid of the horizontal axis in Figure 6 is 3s. It can be seen from Figure 6 that the Willty flowmeter takes about 6s from the slurry cut to the flow output being zero. From the descent process, it can be estimated that the filter time constant is less than 2s, which fully meets the safety interlock requirements; while the reaction time of the other imported flowmeter exceeds 30s. [page]

Figure 6 Flow output speed comparison chart

The Willty coal slurry flowmeter has been operating reliably for more than three years, and its service life is not inferior to that of imported products.

The horizontal pipe solution was studied and tested after the vertical pipe was successfully applied. The original imported flowmeter installed on the horizontal pipe generally began to fluctuate greatly after about a week of use. The Willty flowmeter reached a stable state about 8 hours after being put into use. At the beginning, the flow output was very stable. After about 8 hours, the fluctuation became slightly larger, but there was no large abnormal fluctuation. After more than half a year of use, the horizontal pipe solution has passed the acceptance and the effect is better than the original imported flowmeter. However, the installation of the flowmeter guide plate is relatively complicated. Willty has improved the design and the new solution will be put into trial soon.

4 Clarification of misunderstandings

In actual applications, there are usually some misunderstandings in the selection and application of electromagnetic flowmeters in Texaco process water-coal slurry by relevant technical personnel [7-8]. They are summarized as follows.

① The view that "coal slurry has large wear, so wear-resistant ETFE lining is used" is inaccurate. ETFE mainly solves the problem of adhesion to metal. Although the raw materials of ETFE are cheap, its current processing technology is complicated. The cost of using it to make linings is higher than that of PFA, and there is no evidence that ETFE has better wear resistance than PTFE. The sensors produced by Shanghai Weitai use hard rubber linings. After three years of effective use on site, no obvious wear has been found.

② The view that "low-noise electrodes are used, so the fluctuation is small" is inaccurate. The shape of the electrode is indeed related to the noise level. Because the electrodes of the original imported flowmeters were scaled in a coal chemical enterprise, the flowmeters were often removed and the electrodes were polished with crystal sandpaper. Shanghai Weitai used self-cleaning electrodes (i.e., pointed electrodes) to effectively solve the scaling problem. Practical applications show that although the stability of the flowmeter using self-cleaning electrodes is slightly worse than that of the spherical electrodes, there has been no abnormal fluctuation. Therefore, we believe that low-noise electrodes are not a key technology in solving the abnormal fluctuations in coal slurry flow output.

③ The view that "the original imported flowmeter has low installation requirements, and '5D in front and 2D in the back' is enough" is inaccurate. When calibrating in the laboratory, the straight pipe section is required to be relatively long (up to 10D); in application, generally "5D in front and 3D in the back" is sufficient, which is not only applicable to imported flowmeters. If the diameter is reduced, the straight pipe section requirement can be further reduced. In addition, at this stage, there is basically no closed-loop control for coal slurry flowmeters, and the requirements for accuracy are not very high. The key is to ensure that the safety interlock is in an effective state to avoid abnormal fluctuations causing false tripping.

④ The view that "the flow velocity of the original imported flowmeter has little effect on the flow rate, and a flow velocity of 0.3m/s is applicable" is inaccurate. This statement is very misleading. Practical application experience shows that when the flow velocity is low, especially when the flow velocity is lower than 0.5m/s, the coal slurry flowmeter is prone to fluctuations. Therefore, this view is inaccurate.

⑤ The view of "simple diameter reduction" is inaccurate. We once reduced the diameter of the pipeline according to the diameter reduction scheme of Shanghai Willtai and installed a flowmeter with a smaller diameter, but the actual use effect was not as good as the scheme proposed in this article. On the one hand, due to the involvement of pipeline modification, high-pressure flanges and pressure vessel-level welding, the comprehensive cost is not low; on the other hand, when the diameter is reduced on the pipeline, the length of the small diameter will be much greater than the diameter reduction on the flowmeter, resulting in increased pressure loss. In addition, if the converter is not replaced, many results are unpredictable.

⑥ The view that "the original imported flowmeter has a lot of performance, so the risk is small" is inaccurate. More performance and good performance are two concepts, and there is no causal relationship between the two. Due to historical reasons, the market share of the original imported flowmeter is relatively high. Although there are many good performances, there are also bad performances. Once the fluctuation causes the car to jump out of the car, the loss is very large. According to incomplete statistics, because the fluctuation of the coal slurry flowmeter causes the car to jump out of the car, the loss of a 200,000t methanol production line is about 300,000 yuan; the loss of a 600,000t methanol production line is about 800,000 yuan. This is also one of the reasons why the price of a good quality coal slurry flowmeter remains high. We used two brands of imported flowmeters, and there were cases where they broke down in eight months, and there were also cases where three sets broke down in a year. The solution provided by Shanghai Weiertai has carried out a number of targeted designs for the special working conditions of the Texaco gasifier site, and has been used safely and reliably for more than three years.

In summary, we believe that the appropriate choice is the best choice for practical application. In the measurement application of coal slurry flow in Texaco gasifier, the fluid parameters involved in the flow field and flow state are relatively complex, and the source of coal and the formula of coal slurry are also different. Therefore, the application effects of different coal slurry flowmeters are also quite different. The same flowmeter is prone to fluctuations in some occasions, but has better application effects in other occasions. This is caused by the fact that the stable working range of the selected coal slurry flowmeter is not wide enough. In order to

minimize or avoid false tripping and ensure that the safety interlock is in an effective state, a coal slurry flowmeter with a wider stable working range should be selected, and the selection of a coal slurry flowmeter that is both stable and fast is to meet the basic requirements of the process.

5 Conclusion

Since the measurement of water-coal slurry flow is a difficult point in the field of Texaco process coal chemical industry, peers in this field have made a lot of work and efforts to actively seek effective solutions.

This paper takes two Texaco gasification furnace processes as the application background. The fluctuation range of the coal slurry flowmeter reading is large, which leads to the inability to effectively put the two important interlocks of oxygen-coal ratio and coal slurry flow into use, thus becoming the biggest safety hazard in coal chemical production. In order to solve this problem, the author has carefully analyzed the misunderstandings in the current application field of coal slurry flowmeters, put forward his own unique insights and opinions, and further improved the water-coal slurry flow measurement method in the field of Texaco process coal chemical industry [9-11].

This paper received the guidance and help of Dr. Zhang Zhenshan, and I would like to express my deep gratitude.

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