Application of titanium tube in desalination equipment
Fresh water accounts for about 3% of the earth’s water resources. It is a major trend for the world to obtain new fresh water resources through desalination. At present, desalination has become the main way to obtain water resources in areas like the Middle East that lack of water resources.
Desalination in the world
By 1993, there were 9014 desalination equipment in 5738 regions in the world, with a total capacity of 1.624x107m3/d. The total capacity in the Middle East alone was 8.91x106m3/d, accounting for 55%, and that in the United States was 2.37x106m3/d, accounting for 5%.
As early as the 1950s, desalination has been used to produce fresh water. The main methods of desalination are:
- ① Evaporation method: multi-stage flash, single-stage flash, vertical multi effect, horizontal multi effect, immersion tube, vapor compression;
- ② Membrane method: electrodialysis, reverse osmosis;
- ③ Compound method.
Among them, evaporation method accounts for 60%, reverse osmosis method accounts for 33%, and electrodialysis method accounts for 5.5%.
Application of titanium in desalination equipment
Heat conducting pipe in seawater desalination equipment
The original heat pipe of desalination equipment mainly uses copper alloy pipe, which has been replaced by titanium pipe with high reliability and free of maintenance due to its many shortcomings
Wall thickness of titanium tube
The wall thickness of the heat-transfer tube is determined by the use conditions, tube sheet materials, construction capacity of expanding operation, welding technology at the tube end, etc. because the diameter of the heat-transfer tube is small and the strength requirements are not high, the tube with thinner wall thickness is used in actual use, generally, the wall thickness of copper alloy tube is 0.9mm-1.2mm; instead of titanium tube, the thin-walled welded tube with 0.3mm wall thickness can be used in the place with less corrosiveness.
Thermal conductivity of titanium tube
Due to the different materials of heat conducting tubes, the heat conductivity is also different. For example, titanium is 17w / (m · K), aluminum brass is loow / (m · K), 90 / 10 white copper is 47w (m · K), 70 / 30 white copper is 29w / (M · K). Therefore, the heat conducting effect of heat conducting tubes can be controlled by the change of wall thickness. Among the above materials, titanium has the least thermal conductivity. For example, if thin-walled titanium welded pipe is used, its thermal conductivity is worse than that of aluminum brass, but it is equivalent to 90 / 10 white copper and better than 70 / 30 white copper.
Economy of titanium tube
The unit mass price of titanium pipe is 2-6 times more than that of copper alloy, but simultaneous interpreting the cost performance, the titanium tube price can be counterbalanced with the copper alloy tube. Because of the low titanium density and the same wall thickness, the titanium tube with the same length is only 50% of the copper alloy tube. When the thickness of the titanium tube is 50% of the copper alloy tube, the titanium tube with the same heat transfer area is only 1 / 4. of the copper alloy tube, at the current price level. The overall price of thin-walled titanium welded pipe is the same as that of aluminum copper pipe, which is cheaper than that of white copper pipe.
Development and application of thin-walled titanium welded pipe in Japan
The successful development of titanium strip rolling technology has become the basis of the mass production of titanium welded pipe. In the 1960s, titanium wire was used in the production of mercury based caustic soda in Japan; in the early 1990s, in order to prevent pollution, the production process of caustic soda was improved. With the adoption of diaphragm method, more than 700 tons of titanium strips were applied. Taking this opportunity, Japan developed the technology of continuous production of hot-rolled and cold-rolled titanium strips, and established the technology for desalination of seawater and thin-walled welded titanium tubes of power plant condenser The production technology of thin-walled welded pipe is developed according to the batch production system with coil.
The power plant condensers produced by Hitachi, Mitsubishi and Toshiba use 0.5mm thick titanium welded pipes. The desalination units produced by Mitsubishi, Kawasaki, Hitachi, Mitsui and Kobe Steel use 0.5mm-0.7mm thick titanium welded pipes.
Titanium welded pipe has been widely used in seawater desalination, ironmaking, shipping, petroleum refining, chemical industry and other fields. In 1983, in 16 years, Japan produced 4038t thin-walled titanium welded pipe for desalination equipment all over the world, which has not been damaged by seawater corrosion.
Ventilation condenser and jet compressor
The real desalination equipment in Japan is 2650t / D seawater desalination equipment built by Matsushima carbon Mine Co., Ltd. in 1967. Due to the corrosion of Br – in seawater, the heat transfer tubes and tube plates of the ventilating condensers and jet compressors of the device can not be made of copper alloy. After the replacement of titanium, there is no failure caused by corrosion.
Heat release condenser
The multi-stage flash condenser uses sea water as cooling water to cool the water vapor produced by flash chambers at all levels. Because the sea water is often mixed with sediment and sea creatures, they are attached in the heat transfer tube and tube end, eroding the copper alloy tube. Therefore, titanium tubes are used on the heat transfer condensers of almost all MSF desalination equipment. Especially in order to kill the bacteria in the sea water, it is necessary to use titanium tube with good corrosion resistance when oxygen has to be injected.
Heat recovery condenser
The heat transfer area of the condenser in the heat recovery department is large. Due to economic reasons, copper alloy tubes are usually used now, and titanium tubes are only used in special occasions. For example, the medium containing ammonia or hydrogen sulfide has severe corrosion on copper alloy. In 1977, the 3600t / D MSF type desalination device exported to Germany was made of titanium instead of copper alloy because it was an accessory equipment of ammonia; due to the corrosion of sulfurized oxygen, the 3120t / D MSP type desalination device in Peru corroded the aluminum brass tube one year later, and finally replaced the whole heat transfer tube with titanium tube.
It is reported that 60000 titanium tubes are used in seawater desalination plant with a daily output of 100 tons. From 1967 to 1994, in the past 30 years, 52 sets of condensers and 7 sets of seawater desalination equipment for original energy level thermal power generation have been produced, with a total of 11000t titanium welded tubes.
Problems in use
When titanium is in contact with other metals, it can promote the corrosion of other metals. The prevention method is that titanium or sacrificial anode is used for both heat transfer tube and tube plate. Above 80 ℃, in order to prevent hydrogen absorption, fe-90% Ni alloy is used as sacrificial anode; below 80 ℃, coating or rubber lined steel plate is used.
The titanium tube is installed on the titanium tube plate by expanding tube method, and the interstitial corrosion can occur in the seawater with pH value of 8 at 100 ℃, but the copper alloy is used in the actual water chamber, even if the seawater temperature reaches 120 ℃, the interstitial corrosion will not occur. In reality, in order to improve the reliability of the equipment, pipe end welding is often used to prevent the gap corrosion when the temperature is more than 100 ℃.
In sea water above 80 ℃, titanium may absorb hydrogen; when cathodic protection is applied, hydrogen will be absorbed when over protection is applied. If fe-9% NQ alloy is used as sacrificial anode plate, titanium hydrogen absorption will not occur.
Due to the thin wall of titanium tube, the damage caused by tube vibration should be paid attention to when replacing copper alloy tube. This problem can be solved by using a method smaller than that of copper alloy tube.
Source: China Titanium Pipe Manufacturer: www.titaniuminfogroup.com