Abstract: Through the analysis of the use of polyethylene pipe filters, this article briefly introduces their improvement situation. The equipment runs well and improves current efficiency.
Keywords: Salt water PE filter improvement
Our factory has an annual production capacity of 130000 tons of diaphragm caustic soda, using 214 sets of 30-IQ diaphragm electrolytic cells with a basic operating current of 50-52 kA
Previously, the refined salt water sent to the electrolytic cell by our factory was only filtered once through a valve free filter device, and the pipeline for transporting refined salt water was made of ordinary carbon steel pipes, making it difficult to guarantee the quality of the salt water. In November 1998, our factory put three DJS-150 saltwater secondary filtration devices into production, which achieved good results in improving saltwater quality, improving the operation of the electric tank, and increasing current efficiency.
But after running for 4 months, the filtering device encountered problems that hindered the stable operation of the electrolytic cell. In response to this, our factory has collaborated with the production plant of the saltwater secondary filtration device to tackle the problem and carry out technical improvements, ensuring the stable operation of the device.
1. There are problems
After March 1999, serious corrosion began to occur on the housings and collection tanks of three filters, and the polyethylene pipes (hereinafter referred to as PE pipes) of the filters aged and became brittle. Although our factory has undergone multiple maintenance, the saltwater filters still cannot maintain normal use. Among them, the No. 3 filter was stopped in July 1999, and in January and February 2000, due to a large number of filter pipes falling off during operation, the refined saltwater entering the electrolytic cell became turbid several times, and the quality of the saltwater in the electrolytic cell cannot be obtained, which will inevitably have a significant impact on the current efficiency.
2 Reasons Analysis
2.1 Unreasonable structure and material of filter equipment
Although the filter cylinder is lined with rubber, the short circuiting of the saltwater inlet and outlet of the cylinder is not lined with rubber, resulting in severe corrosion of the equipment body; At the same time, due to the use of 304 material for the internal collection tank and PE pipe joints, they are severely corroded by chloride ions and stray currents in saltwater, causing weld cracking and a large number of joints and PE pipes in the collection tank to fall off, making it impossible to maintain normal use.
2.2 Unreasonable connection method of filter tube
The connection between the filter tube and the collection tank adopts a sleeve joint with a clamp. The ordinary stainless steel outer clamp of the filter tube is not resistant to saltwater corrosion and is prone to detachment, which further increases the frequency of filter tube detachment.
2.3 Unreasonable selection of filter tubes
PE pipes have certain defects in saltwater filtration. Due to the easy entry of some particles into the inner wall of the pipe during filtration, they cannot be effectively blown out during backwashing, resulting in blockage and greatly reducing the effective area of the filter pipe. The service life is significantly shortened, and the backwashing frequency has been reduced from once every 24 hours to once every 6-8 hours. The reliability of the equipment has decreased, and the labor intensity of the operators has also increased.
2.4 Unreasonable use temperature
Due to process defects, the 60 ℃ refined brine is preheated to 75 ℃ through a hydrogen brine heat exchanger before entering the secondary filtration device. PE pipes exhibit accelerated aging and brittleness at a saltwater temperature of 75 ℃. In November 1999, it was discovered that most of the PE pipes in two filters were aging, severely cracked and detached. By March 2000, almost all PE pipes had aged and could not be used normally.
2.5 Poor reliability of pneumatic switchgear
The filter uses a pneumatic switch device to open the bottom manhole cover for exhaust during backwashing. However, the reliability of its bottom pneumatic quick opening device is poor, and the manhole cover often cannot be opened during use. Moreover, after backwashing and resetting the manhole cover, the seal is not tight enough, which can easily lead to leakage.
3. Take measures
To improve the quality of saltwater, reduce tank voltage, and enhance current efficiency, we have retrofitted three saltwater filters.
3.1 Improve the structure and material of the filter
After replacing all the short circuits of the filter, the equipment is fully lined with PE, and the internal stainless steel collection tank is eliminated. Instead, a rubber lined flower plate structure is used, which is made of 40mm steel plate drilled with rubber on the surface. Due to changes in the filtering structure, the original liquid outlet has been changed from the side to the top, and the pipeline has been rearranged. After improvement, the equipment is not only resistant to saltwater corrosion, but also solves the problem of saltwater bypass caused by easy detachment of the original joint, and the quality of saltwater cannot be guaranteed.
3.2 Improving the Connection Method of Filter Tubes
Use a fixed joint to connect with the flower plate hole, and the connection between the joint and the filter tube is achieved through threaded hot tensioning. Additionally, the material of the fixed joint has been changed from stainless steel to polypropylene. Through the improvement of the connection method, the connection of the filter tube is very reliable, and no one has fallen off since use.
3.3 Improving the Filter Tube
In response to the problem of some particles easily entering the inner wall of the filter tube and causing pipe blockage, we replaced all the old and used PE tubes. Based on the actual situation, we improved the original specification of the PE tube from a diameter of Φ 31x20 with an average pore size of 31-38 µ m to a diameter of Φ 38 x 20 PE tube with an average pore size of 39-45 µ m. After increasing the pore size of the filter tube, the problem of pipe blockage was solved and the backwash cycle was extended. The backwash frequency of the filter has now been extended to once every 24 hours, greatly reducing the labor intensity of the operator. Increasing the wall thickness of the PE pipe not only improves the strength of the filter pipe, but also compensates for the shortcomings of enlarging the filter pipe holes.
3.4 Transform the process flow and reduce the operating temperature
The operating temperature of PE pipes should be controlled below 60 ℃. In November 1999, we carried out a process renovation. After the renovation, the brine section was transported to the advanced secondary filtration device for electrolytic refined brine, and then preheated through a hydrogen brine heat exchanger and a steam brine heat exchanger before entering the electrolytic cell. In this way, the temperature of the refined saltwater entering the secondary saltwater filter has decreased from the original 75 ℃ to around 60 ℃, greatly extending the service life of the PE pipe and ensuring the long-term stable operation of the filter. See Figure 10
3.5 Improving switch devices to enhance reliability
Cancel the pneumatic quick opening device with poor reliability, replace the movable manhole at the bottom of the filter with a fixed manhole, and add a Dg200 discharge port at the manhole, controlled by a manual butterfly valve. It is very convenient for backwashing and solves the problem of easy leakage at the bottom of the filter.
4. Usage and improvement effects
By using and improving the saltwater secondary filter, the quality of saltwater has been improved and the current efficiency has been enhanced. After the major overhaul in our factory in November 1998, we began using this filter, and the current efficiency significantly increased. In 1999, the current efficiency for the whole year increased by 1.75% compared to 1998, and the monthly current efficiency was greater than 92.7%. After the major overhaul in November 1999, three improved secondary filters produced significant benefits. In 2000, the current efficiency of the electrolytic cell increased by 0.23% compared to 1999. Three filters are currently running well, and there are no issues with the PE filter tubes.