HYDRAULIC EXPANSION EXPLAINED
Hydraulic expansion is a method of expanding heat exchanger tubes using direct hydraulic pressure to form the tube to tubesheet joint. It is the preferred method of expansion after strength weld per ASME code.
PROCESS & APPLICATIONS
Water pressure from an air driven pumping system is intensified to the desired high pressure and is transmitted through a small bore, flexible, high pressure tube to a lightweight hand-held mandrel holder. A mandrel directs the pressure to the inside diameter of the tube and is held within an accurately prescribed pressure zone and is applied over the full length of the joint in one step. The pressure zone is mathematically adjusted to reduce the damaging crevices at the edges of the tube sheet. Stress, crevice corrosion, and metallurgical changes are thus minimized.
Direct hydraulic expansion with pure water produces clean joints. Tool lubricant is not forced into the tube surface and there is no surface flaking or galling inside the tube.
A smooth transition from expanded to unexpanded areas, along with no change in tube material properties, greatly reduces strain hardening, tube fatigue, and stress induced corrosion cracking. Tube wall thickness and tube length change is minimal. Before contact is made with the tube sheet, a small reduction in tube wall thickness and length occurs because a constant volume of tube material is expanded to a larger diameter. However, no change occurs during the high pressure swaging, eliminating the shear strain and loss of seal between tube and sheet that is a characteristic of roller expanding technologies, regardless of the control mechanisms of the roller expander drive motor.
The Hydraulic expansion cycle is automatic and initiated by the operator. The tube is filled with water at a low pressure, intensified to the higher setting and held for the time required to accomplish a stable plastic condition of the tube. This dwell time is essential to stabilize exotic tube materials. After the pressure is released, water is drained back into the system's reservoir. All of this occurs in a time period of a few seconds. Actual pressure at the tube is read either on an analog or a digital pressure indicator.