Structural requirements for all-titanium vessels

All titanium container

   All-titanium container refers to the main parts, such as the shell, head and connection tube, are made of titanium. The minor parts can be made of non-titanium. For example, the loose flange and its connecting bolts can also be made of carbon steel.

The minimum thickness of the all-titanium container shell is 2mm. The main consideration is to meet the requirements of the welding process for the thickness and to ensure the geometrical tolerances during the manufacturing, to meet the rigidity requirements required in the manufacturing, transportation and hoisting processes; and to save titanium, reduce costs.

  Design selection principle

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

   The allowable temperature of all-titanium pressure vessels should not exceed 250°C, and it is considered that it is more economical to use all-titanium structures for small and medium-sized vessels whose pressure is 0.5MPa and temperature is below 150°C. In consideration of investment costs, when the thickness is greater than 13mm, the use of pure titanium may be uneconomical.

  Structural requirements

   Although the structural design of the all-titanium container is somewhat similar to that of stainless steel, due to the special properties of the titanium itself, it has its own uniqueness in the design and processing. Therefore, the following points must 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 tool, and all the welding joint areas at high temperature (above 400℃) can be effectively protected.

  Titanium can combine with almost any element in the molten state, so special protection must be taken during welding and hot working. In order to achieve effective protection purposes, the structural shape of the parts should be simple, and the opening of the pipe on the shell should be as perpendicular to the axis of the shell as possible, so that the protection fixture is easy to manufacture and the protection effect is better.

   2) Strictly avoid the welded structure of steel and titanium. Since iron and other metals melted in the titanium weld will form a hard and brittle intermediate metal compound, which greatly reduces the weld plasticity, except for explosive welding and brazing, titanium and steel cannot be welded.

  3) The blunt edge clearance of butt welded joints should be appropriate. The blunt edge gap of the butt welded joint of all titanium pressure vessels is smaller than that of steel. This is due to the high melting point of titanium, poor thermal conductivity, small heat capacity, large resistivity and large fluidity of the weld pool metal.

  4) The design of the titanium vessel should ensure the continuity of the structure and the smooth transition of the welded joints, and try to avoid stress concentration.

  5) The bending and flanging of titanium parts should use a larger (compared to steel) bending radius, and a smaller expansion rate should be used when expanding the tube.

6) Industrial pure titanium is prone to crevice corrosion in some media. When designing and handling containers in contact with these media, try to avoid crevices and stagnation areas, and use crevice corrosion-resistant titanium alloys (such as Titanium-palladium alloy) or coating.

7) When designing and handling containers in contact with conductive corrosive media, if it is found that contact between titanium and other metals can cause galvanic corrosion, measures should be taken on the structure (such as using a third material as a transition layer) or adopt Anode protection.

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

①When the medium is corrosive or abrasive and ρv2>740kg/(m·s2) or the medium is non-corrosive or non-abrasive, but ρv2>2355kg/(m·s2) (ρ is the density of the medium, kg/m3, v is the material When the linear velocity of flow, m/s), the anti-scouring plate should be installed at the material inlet.

  ②When the corrosive medium enters the equipment along the tangential direction, or the inlet pipe is facing the wall of the device, and the distance between them is less than twice the outer diameter of the pipe, a protective plate should be provided. 

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