A high-pressure drum at 10.5 MPa; is there a phosphate dosing unit in case of tube blockage? Help analyze the reasons for it
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High-pressure boiler, with a drum pressure of 10.5 MPa. The national standard requires phosphate levels to be between 2–6 mg/L. The pH value of the water in the boiler drum is generally normal, staying within the range of 9.0–9.5; the conductivity is between 20–45 μs/cm, while the silica level is between 300–500 μg/L. There are 2 plunger pumps, with a rated pressure of 12 MPa and a flow rate of 3.12 L/h. The outlet pressure of the phosphate pumps in Furnace No. 1 remains stable at 10.5 MPa. However, in Furnace No. 2, the outlet pressure of the phosphate pumps rose to 11.5 MPa and 12.5 MPa after just one month. Today, reverse steam cleaning at 300°C was carried out again, and a small amount of dark brownish water was observed. The management has decided that frequent reverse flushing poses risks, and they want to find out the cause of the blockage. What could be the reasons for the blockage? Relevant information: 1. Starting from June, anhydrous trisodium phosphate produced by Changzhou Chuanphos Chemical Industry was used; it has a 98% purity level, and is mixed with water at a concentration of 4% to produce a solution that is clear and transparent. (The theoretical solubility at 20°C is 12.1 g/100 g of water, or 12.1%). Additionally, according to what I’ve found, after a solid dissolves, its solubility is generally not affected by pressure, but it is greatly influenced by temperature. 2. The chemical dosing tank is made of 304 material; both boilers share the same dosing tank, but it is the phosphate pump of Boiler No. 2, which has the shortest chemical dosing pipeline, that gets clogged frequently. 3. DL/T 805 \"Guidelines for Steam and Water Chemistry in Thermal Power Plants\": Chemical dosing in the boiler water – it is necessary to ensure uniform dosing of chemicals in the water of the boiler drum. The chemical dosing pipe inside the drum should be arranged horizontally along the axial direction of the drum. It should be 10 to 20 cm lower than the continuous drain pipe. The outlet holes of the dosing pipe should be evenly distributed along the length of the drum, with dosing taking place from the middle of the drum. Our plant’s steam drum is 10 meters long; the blowdown pipe has a diameter of φ21, and the chemical dosing pipe also has a diameter of φ21. The blowdown pipe is 10 cm higher than the chemical dosing pipe, and they are located on opposite sides of each other, with a distance of 80 cm between them. The chemical dosing pipe is equipped with φ3-sized discharge holes. 4. Is it possible that the siphoning phenomenon in the chemical dosing pipeline is causing the pump to get clogged? Is it possible that the DN20 304 stainless steel pipeline is blocked by phosphate crystals? Could you help suggest other possible reasons and ideas?
What are the disadvantages of using steam at 300°C to perform backwashing on the stop valve designed to withstand a pressure of 150 MPa (located at the phosphate addition inlet of the drum)? Like valve flaps and things like that
Are your phosphate pipes made of stainless steel? Also, is the temperature for phosphates at room temperature? Too high dissolution crystallization (depends on the temperature solubility curve)
The phosphate pipeline is made of 304 material, with a DN20 diameter and a length of about 60 meters; the steam drum of the furnace that is blocked has the shortest dosing pipeline. Phosphates are added at room temperature; there are only 2 connected globe valves (with a pressure rating of 150 MPa) located 30 cm in front of the feed inlet for the drum dosing (left-side feed inlet). ——National standards specify that it is best to introduce phosphates at the middle part of the drum, which facilitates even distribution on the left and right sides. Could it be that crystallization has caused blockage in the 60m pipeline? I always think it’s the chemical dosing pipeline in the drum (about 9 meters long), whose outlet hole is too small – only φ3. What should be the typical size of such an opening? How far apart should dosing holes be placed?
I don’t think it’s a problem with backflow; our factory has also produced brown-colored water. I believe it’s an issue related to the dosage of chemicals used
Thank you all for your attention. At present, a preliminary conclusion has been reached: 1. For boilers in China with a pressure of over 9.81 MPa, the design approach involves adding chemicals in the middle of the steam drum; the pipes used for this purpose generally have a diameter of DN20–DN25, and their length along the axial direction of the steam drum is usually between 800–1000 mm. If both ends of these pipes are left open, sealing will not be possible. Additionally, honeycomb-shaped holes are provided on these pipes to facilitate the release of chemicals, with the diameter of these holes typically being φ4 or φ5. 2. The chemical is fed on the left side of the Sumitomo drum (unlike the design concept of feeding from the middle), and with such a long chemical feed pipeline, relying solely on small honeycomb-shaped openings for discharge, it is easy for blockages to occur and pressure to build up (unlike the design with direct openings of DN20 at both ends). Before starting maintenance work on the power plant, the honeycomb pores need to be cleared using a small needle or similar tool each time. 3. During the next maintenance session, check whether the above conditions exist! Thank you!