Application of titanium alloy material in marine valve
Titanium and its alloys have many outstanding advantages, such as low density, high strength, good corrosion resistance and pressure resistance. At present, they have been widely used in aviation, aerospace, petroleum, chemical industry, metallurgy, electric power, medicine, health and instrument industries. In order to meet the needs of deep sea vessels, ships and other fields of marine engineering, in the development process of titanium alloy valve, the product design is standardized with new technology, new materials and new standards, which meets the requirements of national high-tech product policies. It is a new and promising material application technology, thus improving the valve quality and product performance.
Corrosion resistance of titanium
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Titanium is a metal with high chemical activity, which has excellent corrosion resistance to most corrosive media. Titanium has a great affinity with oxygen. When it contacts with atmosphere or any medium containing oxygen, a strong and compact passive oxide film is formed on its surface. This film is very stable, if there is mechanical damage, it will immediately re form (as long as there is a certain amount of oxygen).
There are several special corrosion forms of titanium, such as high temperature corrosion, stress corrosion, spot corrosion, crevice corrosion, galvanic corrosion and corrosion fatigue, etc., but the application of titanium in ship valves is mainly galvanic corrosion.
Galvanic corrosion is when two different metals form galvanic couple in an electrolyte, the corrosion of the metal as anode can be accelerated. The amount of corrosion depends on the electrode potential difference between materials, the area ratio of anode and cathode and the polarization characteristics of both. Titanium is different from general materials, it is blunt in many media, and shows a potential similar to that of blunt 18-8 stainless steel. The potential of titanium in seawater is – 0.10v (for saturated calomel electrode). When titanium contacts with Monel alloy, Hastelloy C, 18-8 stainless steel and aluminum bronze tube sheet with Ni, the potential difference is very small and there is no galvanic corrosion. But also in seawater, when titanium contacts with aluminum, zinc and carbon steel, aluminum, zinc and other metals are corroded. However, their corrosion rate is smaller than that caused by contact with stainless steel. The degree of corrosion of these metals varies with the ratio of titanium to the surface area of the metal in contact with titanium. When the surface area ratio of the metal in contact with titanium is 1:10 (i.e. the area ratio of anode to cathode is 1:10), the metal in contact will corrode quickly. On the contrary, when the ratio of the surface area of the metal and titanium in contact with titanium is 10:1, the corrosion rate is greatly reduced. In low concentration reducing acid (such as sulfuric acid or hydrochloric acid), there is similar situation of seawater. In addition, titanium has a strong polarization tendency, which means that the couple containing titanium as cathode only produces small current. Due to the existence of oxide film on the surface of titanium, the corrosion rate will not increase when contacting with other metals. Therefore, the comparison of galvanic corrosion should be paid attention to in other materials of titanium valves for ships.
Valve structure features
(1) The valve body flow passage of titanium alloy valve adopts the form of Figure.1 (taking titanium alloy check valve as an example) to reduce the direct flushing of medium on the sealing surface and the eddy current area caused by the sharp contraction and expansion of the passage.
(2) Titanium material is more expensive. In order to use titanium material economically and reasonably, titanium parts shall not be used as much as possible in the parts not in contact with corrosive medium.
(3) The mechanical properties of titanium change with temperature, and its value is larger than that of carbon steel or alloy steel. The strength index decreases with the increase of temperature. For example, the tensile strength and yield strength at 250-300 ℃ are about 50% of those at room temperature. Therefore, even if the design temperature is not high, the strength value should be selected according to the design temperature. The yield strength ratio of titanium is high and the endurance strength is good. Therefore, when the design temperature is below 316 ℃, the tensile strength value at this temperature is often the key to determine the design strength index. The strength index of industrial pure titanium can not be improved by heat treatment. The thermal expansion coefficient of titanium is small. When titanium and other materials are used together, the stress caused by the difference of expansion value should be paid attention to.
(4) The plastic deformation range of industrial pure titanium and α – titanium alloy is small and easy to crack. However, increasing the deformation speed or decreasing the deformation temperature may lead to the fracture in machining. Therefore, it is better not to design the cold work parts with large deformation. Where flanging is required, the bending radius shall be as large as possible. The clearance tolerance between the tube connected by strength expansion and the tube hole of tube sheet shall be small to avoid titanium tube expansion crack.
(5) When choosing titanium bolts, we must consider the problem of stress relaxation due to the creep phenomenon of titanium at room temperature. It is better not to use titanium bolts as the connection of forced seal. When titanium bolts are required due to corrosion resistance, the structural design shall consider that it is easy to tighten the bolts regularly to ensure sealing. Titanium, like stainless steel, also has friction, adhesion and occlusion. Titanium thread is easy to bite, which can be solved by using different materials or large clearance thread matching or using appropriate lubricant. The internal thread that needs to be tapped shall be used less, and the turning thread structure with undercut shall be selected as far as possible. Similarly, the adjustment and action of titanium valve controlled by rotating the stem of handwheel shall be ensured by the material performance and the processability of each part.
(6) In order to avoid crevice corrosion of titanium in some media, it is necessary to eliminate crevices and recesses where water can be stored. In particular, the cracks between titanium and tetrafluorocarbons are more susceptible to corrosion than those between titanium and titanium, because titanium is not resistant to corrosion for solutions containing a small amount of soluble fluoride, which causes blunt damage. Therefore, special care should be taken when using polytetrafluoroethylene plastic pads, fluorine-containing rubber washers and adhesives.
(7) Due to the high chemical activity and special physical and mechanical properties of titanium, general cutting methods can be used for its cutting. However, compared with other commonly used metals, there are special requirements. Master certain processing technology and pay attention to it during the processing. In order to prevent excessive temperature rise, a lower cutting speed should be adopted. Generally speaking, the cutting speed of steel parts with the same hardness should be 25% – 50% or lower. With a large amount of feed, the effect of feed on temperature rise is very small. Do not stop cutting in the process of cutting, otherwise it will cause work hardening or produce sintering, extrusion and damage to the tool. The cutting edge of the tool should be sharp, otherwise the tool is very easy to wear, because the wear of the cutting tool is not proportional to the cutting amount, and the wear is very small at the beginning. Once the tool becomes blunt, the wear will increase immediately. Use enough cutting fluid (5% sodium chloride solution, emulsified oil with oil water volume ratio of 1:20, etc.) to lubricate and cool, reduce the heat on the blade, and wash away the chips.
Structure design of valve
The stop valve adopts the plane sealing structure, and the sealing auxiliary stop of the valve body and disc is also directly processed (Fig.2).
Fig.2 Sealing surface structure
As the control mechanism of fluid, the sealing function of valve disc and valve seat is very important. According to the different working conditions of the valve, the sealing surface of the valve is generally required to have good scratch resistance, corrosion resistance, suitable hardness, good thermal structure stability, crack resistance and reasonable technology. Therefore, titanium alloy ti60260-300hb is selected as the valve disc and valve seat stop materials, and the allowable specific pressure of sealing surface is 45MPa.
Upper seal setting
In order to improve the sealing quality, the upper sealing structure is set, that is, when the stop valve is fully opened, the chamfering fit between the disc nut and the valve cover is set, and the chamfering accuracy is required to be high.
Sealing of valve body and bonnet
Rubber gasket is used for sealing between flange and bonnet in valve body. Its sealing function is mainly achieved by the pre pressure during installation and the deformation of sealing ring caused by the pressure of working medium during operation.
The size of stuffing box is designed according to GB standard, and the packing gland and packing seat are serialized. When installing the packing, avoid scratching the valve stem, and press the packing gland evenly. The valve stem is easy to cause pitting corrosion and abrasion due to the action of the packing, which is one of the reasons for the leakage of the packing. The solution is to reduce the content of chloride ion and sulfur ion in the packing, or to adopt corrosion inhibiting packing and improve the smoothness of the valve stem. The friction resistance of the packing must be overcome when the valve rod is in motion. According to the analysis, polytetrafluoroethylene split wire braided packing is selected as the packing. The material has high temperature resistance, self-lubricating and chemical corrosion resistance, good flexibility, impermeability, low friction coefficient, and improves the anti leakage ability of the valve.
The structure size, form, material, middle flange structure, connecting bolt size and medium flow channel of valve body shall be designed according to GB standard. Due to the high price of titanium, the thin-walled formula is used to calculate the thickness, and the strength of the valve body and the number of connecting bolts of the middle flange are checked.
The valve cover and the valve body constitute the pressure bearing shell, and the technical parameters such as the medium pressure and temperature that the valve cover bears are basically the same as the valve body. Therefore, both of them have common features in design and calculation methods. The structure size and form of the bonnet shall be in accordance with GB standard, and the design calculation of the flange size in the bonnet shall be the same as that in the valve body.
When the medium flows in from the bottom of the valve disc, the maximum axial load of the valve rod is in the closed position, and the valve rod is under pressure. When the medium flows in from above the disc, the maximum load is in the open position, and then the valve rod is pulled.
The rapid development of valve industry is promoted by the development of petroleum, chemical industry, aerospace, ship and ocean. The structure, material and production technology of titanium alloy valve are further developed and studied, which improves the reliability of valve and the life of pipeline control equipment, and meets the requirements of various fields for the development of valve technology.
Source: China Valve Provider: www.titaniuminfogroup.com