Structural requirements for all-titanium containers

Full titanium container refers to the main parts, such as shell, head and takeover are made of titanium, secondary parts can be made of non-titanium, for example, live sleeve flanges and their connecting bolts can also be made of carbon steel.

The minimum thickness of all titanium container shell is 2mm, mainly considering meeting the thickness requirements of welding process and ensuring geometric dimension tolerance, meeting the stiffness requirements required in the manufacturing, transportation and lifting process; and saving titanium material and reducing the cost.

Because the mechanical strength of titanium decreases significantly at a temperature greater than or equal to 200℃, and the elastic modulus of titanium is low, the whole titanium structure is not suitable for application in high temperature, high pressure or medium pressure and large devices.

The allowable temperature of all titanium pressure vessels should not exceed 250℃, and it is considered that it is relatively economical to choose small and medium-sized vessels with pressure at 0.5MPa and temperature below 150℃. Pure titanium may not be economical when the thickness is more than 13mm.

Although the all-titanium container is similar to the stainless steel in the structural design, due to some special properties of the titanium material itself, it is unique in the design, processing and manufacturing, so the following points should be paid attention to in the structural design:

1) When designing the welding structure, the welding part must be convenient for the operation of the hydrogen arc welding welding tool, and all the welding joint areas at high temperature (above 400℃) can be effectively protected.

Titanium, in the molten state, can combine with almost any element, so special protection must be taken during the welding and thermal processing processes. In order to achieve the effective protection purpose, the structure shape of the parts should be simple, and the takeover hole on the shell is as vertical as possible to the axis of the shell, so as to protect the fixture for convenient production and better protection effect.

2)Strictly avoid steel and titanium mutual melting welding structure. Because iron and other metals melted in the titanium weld will form a hard and brittle intermediate metal compounds, greatly reduce the weld plasticity, in addition to explosion welding and brazing, titanium and steel cannot be welded.

3)The blunt edge gap of the butt-welded joint shall be appropriate. The blunt edge gap of the butt welding joint of the all-titanium pressure vessel is smaller than that of the steel, which is due to the high melting point, poor thermal conductivity, small thermal capacity and large resistance coefficient, and large metal fluidity of the welding melting tank.

4)Titanium containers shall be designed to ensure structural continuity and smooth transition of welded joints to avoid stress concentration.

5)The bending and turning of titanium parts shall adopt a large bending radius (compared with steel), and a small pipe expansion rate shall be adopted.

6)Industrial pure titanium is prone to gap corrosion in some media, in the design and treatment of containers in contact with these media, should be as far as possible to avoid cracks and stagnant flow area, in the gap with gap corrosion resistant titanium alloy (such as titanium palladium alloy) or coating.

7)When designing and treating containers in contact with a conductive corrosion medium, if titanium is found in contact with other metals that can cause electrocouple corrosion, structural measures should be taken (such as a third material as a transition layer) or using anode protection.

8)When designing equipment prone to corrosion, the flow rate of corrosion medium should be lower than the critical flow rate, and try to avoid sudden changes in flow rate or flow direction; or set up a protective baffle in areas prone to corrosion and abrasion.

①When the medium has corrosion or abrasion and ρ v2> 740kg / (m s2) or the medium has no corrosion or abrasion, but ρ v2> 2355kg / (m s2) (ρ is the medium density, kg/m3, v is the line speed of the material flow, m/s).

②A protective plate shall be provided when the corrosion medium enters the equipment along the tangential or the inlet pipe alignment wall and the distance is less than 2 times the outer diameter of the pipe.