Structural requirements for all-titanium containers

An all-titanium vessel means that the main parts, such as the shell, head and nozzles, are made of titanium, while the secondary parts may not be made of titanium. For example, a lob flange and its connecting bolts may also be made of carbon steel.  

The minimum thickness of the shell of the all-titanium vessel is 2mm, which is mainly considered to meet the requirements of thickness of welding process and geometric tolerance during manufacturing, and to meet the stiffness requirements in the process of manufacturing, transportation and lifting.  And save titanium material, reduce cost. 

Design selection principle

Because the mechanical strength of titanium material decreases significantly when the temperature is greater than or equal to 200℃, and the elastic modulus of titanium is low, it is not suitable for the application of all-titanium structure in high temperature, high pressure or medium pressure and large equipment.   

The allowable temperature of all titanium pressure vessels should not exceed 250℃. It is considered that it is economical to choose all titanium structure for small and medium sized vessels with pressure of 0.5MPa and temperature below 150℃.  When the thickness is greater than 13mm, it may not be economical to use pure titanium.  

Structural requirement 

Although the structure design of all-titanium vessel is somewhat similar to that of stainless steel, due to some special properties of titanium material itself, it has its uniqueness in design and manufacturing. Therefore, the following points should be paid attention to in structural design:   

1) When designing the welding structure, it is necessary to make the welding part convenient for the operation of the HYDROGEN arc welding tool, and ensure that all the welding joint area at high temperature (above 400℃) can be effectively protected.  

Titanium can combine with almost any element in melting state, so special protection must be taken in welding and hot working process.  In order to achieve the purpose of effective protection, the structure shape of parts should be simple, and the nozzle opening on the shell should be perpendicular to the axis of the shell as far as possible, so that the production of protection fixture is convenient and the protection effect is better.   

2) Strictly avoid the welding structure of steel and titanium mutual fusion.  Because iron and other metals melted in the titanium weld will form a hard and brittle intermediate metal compound, greatly reduce the weld plasticity, in addition to explosive welding and brazing, titanium and steel can not be welded.   

3) Blunt edge clearance of butt welding joint should be appropriate.  The blunt edge clearance of butt welding joint of all-titanium pressure vessel is smaller than that of steel, which is due to the high melting point, poor thermal conductivity, low heat capacity and high resistivity of titanium and the high fluidity of welding pool metal. 

4) The design of titanium vessels should ensure the continuity of structure and smooth transition of welded joints, and avoid stress concentration as far as possible.   

5) The bending and flanging of titanium parts should adopt a larger bending radius (compared with steel), and when expanding tubes, a smaller expansion rate should be used.  

6) Industrial pure titanium is easy to produce gap corrosion in some media. In the design and treatment of containers in contact with these media, cracks and stagnant areas should be avoided as far as possible, and crack corrosion resistant titanium alloys (such as titanium palladium alloy) or coating should be used in the gap.  

7) In the design and treatment of containers in contact with conductive corrosive media, if it is found that contact between titanium and other metals can lead to galvanic corrosion, structural measures should be taken (such as using a third material as a transition layer) or anodic protection.  8) In the design of equipment prone to corrosion, the flow rate of corrosive medium should be lower than the critical flow rate, and try to avoid sudden changes in flow rate or direction;  Or in the easy to produce corrosion and erosion of the part of the installation of protective baffle.  

① When the medium has corrosion or abrasion and ρ V2 >740kg/(m·s2) or no corrosion or abrasion of the medium, but ρ V2 >2355kg/(m·s2) (ρ is the density of the medium, kg/m3, V is the linear velocity of the material flow, M /s), the material should be set at the entrance of the anti-impact plate.   

② When the corrosive medium enters the equipment along the tangential direction, or the inlet pipe is facing the wall, and the distance between them is less than 2 times the outer diameter of the pipe, the protection plate should be set.