Low Temperature Type Shell and Tube Heat Exchangers for Oil Refineries in Iraq

Low Temperature Type Shell and Tube Heat Exchangers for Oil Refineries in Iraq

Low-temperature shell-and-tube heat exchangers are core heat exchange units specifically designed for industrial sectors such as oil refining and chemical processing, with a design temperature typically at or below -20°C. These units employ a classic shell-and-tube configuration, featuring multiple heat exchange tube bundles arranged within a cylindrical shell. The tube bundles are secured at both ends to tube sheets, creating separate tube-side and shell-side channels to facilitate efficient heat transfer between the two media. In response to the harsh operating conditions of Iraqi refineries—characterised by high temperatures, high pressure and high corrosion—this product has undergone comprehensive optimisation in terms of material selection, structural design and core welding processes, ensuring long-term stable operation under extreme environmental conditions.

Key features of the core welding process

1. Butt welding of the shell and tube sheet:

To reduce localised stresses, the shell and tube sheet are joined using butt welding, ensuring a smooth transition at the joint and avoiding abrupt changes in cross-section. Prior to welding, a rigorous welding procedure qualification is carried out, including Charpy low-temperature impact testing on the weld and the heat-affected zone. Following welding, ultrasonic or surface testing is performed on at least 50% of the weld to ensure the absence of defects. The assembly is then heat treated.

Note that the weld must be fully penetrated, and the metal must be allowed to cool between each weld pass. As the flanges are relatively thick, the temperature drops rapidly during welding. If preheating is insufficient, gases will not have time to escape, leading to porosity, which in turn will affect the hydrostatic test.

2. ‘Expansion + welding’ composite joint between the tube and tube sheet:

This is the core of this product’s manufacturing process. Neither welding nor expansion alone can meet the requirements for high strength, fatigue resistance and a tight seal. We employ either an ‘expand-then-weld’ or ‘weld-then-expand’ process (depending on the specific operating conditions):

Expansion: Hydraulic expansion technology is used, with computer control ensuring uniform plastic deformation of the tube wall to achieve a tight fit with the tube sheet hole. This effectively eliminates the gap between the tube and the tube sheet hole, fundamentally preventing crevice corrosion and significantly enhancing the joint’s fatigue resistance. Rigorous pre-expansion tests are conducted prior to formal expansion, precisely controlling the expansion ratio within the optimal range of 0.9% to 2.2% to avoid under- or over-expansion.

Expansion Rate Formula:
E = [(d - b) - (c - a)] / (a - b) x 100%

Where:
- E: Expansion rate
- a: Outer diameter (OD) of the tube before expansion
- b: Inner diameter (ID) of the tube before expansion
- c: Inner diameter (ID) of the tubesheet hole
- d: Inner diameter (ID) of the tube after expansion

Welding: Following expansion, either a sealing weld or a strength weld is performed. For thin-walled tubes, precision welding methods such as pulsed TIG welding are employed. By controlling parameters such as peak current, base current and welding speed (e.g. peak current 115–120 A, welding speed 95–101 mm/min), heat input is precisely regulated to prevent burn-through of the tube sheet or heat exchanger tubes. For applications with more stringent requirements, internal bore welding technology may be employed to achieve full penetration welding, further eliminating end gaps and providing higher resistance to vibration fatigue.