Pre-tightening control using electric heating or hydraulic stretching
At present, the primary method for bolt pre-tightening involves using either manual or mechanical means for standard bolts that don’t require strict control. In such cases, torque wrenches or power-assisted tools are typically used to ensure the correct level of tightening, with precise requirements on torque values or bolt elongation. For long-term applications, especially in high-stress environments, electric heating or hydraulic stretching methods are often employed depending on the specific conditions. For example, at Shenyang Pump Co., Ltd., the early approach was mainly electric heating, but now the company has shifted toward hydraulic stretching.
In the production of high-pressure boiler feed pumps, electric heating is commonly used for pre-tightening. Nuclear pumps, including main nuclear pumps, as well as subcritical and supercritical high-pressure boiler feed pumps, are usually pre-tightened using hydraulic stretching techniques. These methods provide more accurate and consistent results compared to traditional approaches.
The electric heating method requires drilling into the bolt, making it suitable for larger diameter bolts. On the other hand, the hydraulic stretching method demands a longer thread length and sufficient space around the bolt, which makes it ideal for pumps with more room. However, many pumps do not meet these criteria. For instance, older single-shell boiler feed pumps and petrochemical hydro-pumps have large bolt diameters and limited space, so they can only be tightened using standard wrenches.
Modern high-pressure boiler feed pumps, particularly those with a double-shell design, have larger cylinder diameters and more available space, making them better suited for hydraulic stretching. While domestic pump manufacturers initially used electric heating, foreign companies have largely transitioned to hydraulic stretching in recent years.
The hydraulic stretching method typically involves a high-pressure manual (plunger) pump, along with a pressure gauge, transition joints, a three-way (or five-way) valve, high-pressure oil hoses, quick connectors, and a dedicated hydraulic tensioner for the bolt. This system allows for precise and controlled pre-tightening, ensuring optimal performance and safety in critical applications.
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