Combined with the design and production practice of slurry pipelines in PVC devices, the slurry pipelines are very easy to be blocked, resulting in frequent disassembly and flushing on site. The principles and precautions of slurry pipeline piping are explained from multiple aspects such as pipeline venting, main pipeline drainage, regulating valves, bypass pipe settings, start-up and shutdown flushing, and purging. Combined with the actual results of the project, a feasible piping basis is provided to avoid the problem of easy blockage of slurry pipelines in production.

Introduction

The suspension polymerization method of polyvinyl chloride (PVC) uses vinyl chloride as raw material and desalted water as suspension medium. In the presence of chemicals such as dispersants and initiators, the vinyl chloride monomer from outside the boundary is subjected to suspension polymerization reaction, slurry stripping, and centrifugal drying to obtain the product polyvinyl chloride powder. The PVC device is divided into a feeding unit, a polymerization unit, a slurry stripping unit, a VCM recovery unit, a drying unit, a product transportation and storage, and a packaging unit.

After polymerization in the polymerization kettle, the PVC slurry is transported to the top of the stripping tower by the slurry feed pump. Before entering the tower, it exchanges heat with the hot slurry coming out of the bottom of the stripping tower in the slurry heat exchanger. Steam is added from the bottom of the stripping tower, and VCM is removed from the slurry. The PVC slurry is dispersed on the tower plate, in good contact with the steam, and kept in a boiling state. The hot slurry after VCM removal is continuously discharged from the bottom of the stripping tower, cooled in the slurry heat exchanger, and then enters the drying unit. It was found from the design, construction, start-up and production processes of multiple PVC projects that the slurry pipelines in and out of the slurry stripping tower are very easy to block, and some even need to be disassembled and cleaned once a week. In view of the frequent operation problems, the following discusses the precautions, design principles, and piping requirements for the design and piping of slurry pipelines.

Design requirements for slurry pipelines

The entire slurry pipeline should follow the principle of step-by-step high or step-by-step low to ensure that there is no bag shape, especially the lower bag shape (see Figure 1). When the pipeline is gravity flow, the pipeline should have a slope (the slope is in accordance with the requirements of PID) and slope towards the container. The pipeline between equipment should be the shortest and the elbows should be minimized. If a turn is required, a large radius elbow should be used, and the curvature radius of the elbow should be б▌4DN. However, some projects will use large radius elbows instead of elbows considering cost issues.

Pipe design for slurry pipelines

In order to avoid dead corners in pipe fittings, pressure gauges and safety valves should not be installed on slurry pipelines, and venting and draining should not be installed as much as possible. If the pump port design requires the installation of a pressure gauge, it should be connected vertically at the top of the horizontal pipeline, and the connecting pipe should be the shortest, and horizontal connection is not allowed.

In order to reduce the dead corners of the slurry pipeline, the valve on the slurry main pipe should be a ball valve, and the valve should be installed horizontally, and the vent valve or drain valve should be installed as close to the main pipe as possible. Many projects use the method of welding the neck flange directly to the pipe platform to eliminate the short pipe connection, which can also better prevent the accumulation of slurry in the short pipe and cause blockage. The method of tangentially leading the drain pipe from the bottom of the slurry pipeline main pipe is widely used when the operating space is limited. The drain valve should also be as close to the main pipe as possible to make the drain pipe as short as possible.

The drain pipe is drained on site, and the weld reinforcement ring is relatively high. The drain method requires that the pressure of the pipeline should not be too large. Because of the easy adhesion of the slurry, the inner surface of the slurry pipeline welding needs to be processed smoothly to prevent the slurry from adhering to the inner wall.

The regulating valve on the pipeline should be set at the high point of the pipeline, so that the slurry can settle from both sides of the valve when the valve is closed, and it is not easy to block the valve.

It is best not to set a bypass pipe for the regulating valve on the slurry pipeline. When it is necessary to set it, the bypass pipe should be 45бу with the main pipe, and the bypass pipe between the bypass valve and the main pipeline should be as short as possible to reduce slurry accumulation.

Slurry pipelines are very easy to accumulate and clog pipelines and pipe fittings when parking. Flushing and purging pipelines should be set at the low point of the pipeline before the regulating valve and near the inlet and outlet of the equipment so that the slurry can be completely discharged from the pipeline. The flushing and purging pipelines require access to a special three-way ball valve from the top of the slurry pipe. However, in the actual production of several PVC projects in China, the top access is basically met, and the connecting pipe is the shortest, and it is impossible to use a special three-way ball valve for all projects.

The slurry heated by the heat exchanger in front of the tower is expanded and then enters the slurry stripping tower, mainly to prevent clogging at the pipe mouth of the equipment. The length and requirements of the expansion should be strictly in accordance with the PID requirements when arranging the pipeline. To ensure the smooth operation of the stripping tower, the hot water pipe entering the tower should be directly led out from the main pipe into the stripping tower, especially not allowed to be connected from the lower part of the main pipe. The discharge pipe at the bottom of the stripping tower should be as short as possible, which requires the slurry removal pump to be arranged as close to the stripping tower as possible.

Larger branch connections of slurry pipelines should use Y-type tees, and the flow direction of the branch pipes should follow the flow direction of the main pipeline, which can prevent slurry blockage.

Conclusion

Regarding the piping of slurry pipelines, in addition to the above-mentioned design of no bags, shortest venting connection pipes, tangential drainage of the main pipeline, and increased flushing and purging during start-up and shutdown, the clogging problem of small-diameter slurry pipelines can also be reduced by expanding the diameter. Some projects use large-radius elbows, and disassembly flanges are added at both ends of the elbows to facilitate disassembly and cleaning. In addition, pipeline heating is also a method, but the conditions and costs of heating must be considered comprehensively. From the design, construction, and production of the stripping process of several polyvinyl chloride units in China, being familiar with the characteristics of slurry pipelines and following the principles and methods of slurry pipeline piping can solve the problem of easy blockage and frequent disassembly of slurry pipelines.

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