Pipe Flange Bolt Tightening in LNG Projects: Key Considerations

Abstract: Using the LNG unloading pipeline at a receiving terminal as an example, this article examines the calculation of flange bolt preloading torque and the requirement for secondary tightening during the pipeline pre-cooling process. Applying a proper preload torque effectively prevents flange leakage, ensures construction quality, and provides a solid foundation for the safe operation of the plant. LNG (liquefied natural gas) is characterized by extremely low temperatures. In the event of a leak, it rapidly vaporizes into natural gas, which is highly flammable and explosive. During the design of an LNG receiving terminal, flange connections are kept to a minimum. For example, valves larger than 2 inches on LNG pipelines are typically welded instead of flanged. However, for inspection and maintenance purposes, flange connections are often installed at equipment outlets. Flange leakage typically occurs due to failure of the sealing surface. Factors such as improper installation, uneven gasket load distribution, bolt deformation, gasket misalignment, damage or deformation of the flange sealing surface, and inadequate pipe support or restraint can all compromise the integrity of the flange–gasket–bolt system. Insufficient sealing pressure resulting from inadequate tightening force or uneven bolt load distribution is one of the most common causes of flange leakage. To prevent LNG leaks, flange bolts must be correctly tightened using a torque wrench. This article uses the LNG unloading pipeline at a receiving terminal as a case study to illustrate the calculation of flange bolt preload torque and the associated tightening procedure.

The unloading pipeline at an LNG receiving terminal is a large-diameter pipeline with a nominal diameter of 38", a design pressure of 1.86 MPa, an operating pressure of 0.3 MPa, a design temperature of -165°C, and an operating temperature of -160°C. The discharge pipeline extends from the wharf, passes through the trestle, pipe rack, and tank platform, climbs along the exterior wall of the tank, and finally connects to the tank nozzle. All valves on the discharge pipeline are welded instead of flanged. At the wharf, the connection between the unloading arm and the discharge pipe is made using a flange. However, because the pipe diameter is only 16", it must be connected to the 38" main discharge line via a reducer. In areas such as the trestle, pipe rack, and tank platform, flange connections can be avoided. In the tank area, however, the interface between the tank nozzle and the discharge pipe must be flanged to allow for pressure testing, inspection, and maintenance. If welded, subsequent maintenance would be extremely difficult. Due to the large diameter of the discharge pipe, temperature differences between its upper and lower surfaces, and the substantial load on the tank nozzle, the risk of flange leakage is relatively high. Given the LNG design temperature of -165°C, the flange material chosen for this project is A182-F304/304L. Based on the pressure–temperature rating, a CL150 flange can withstand up to 1.9 MPa at -165 °C, whereas a CL300 flange can withstand up to 4.96 MPa. For this project, CL150 A182-F304/304L flanges are used for the discharge pipe, although some LNG receiving terminals employ CL300 flanges to provide a higher safety margin and minimize the risk of leakage. In principle, flange sealing reliability depends on proper bolt tightening. When the nuts are tightened, the studs stretch, clamping the flange and compressing the gasket. Excessive tightening force, however, can cause bolt failure or gasket damage, whereas insufficient tightening force leads to inadequate gasket compression and friction, resulting in slippage and leakage. Therefore, accurately calculating the required bolt-tightening torque is crucial for ensuring reliable flange sealing performance.

Reliable sealing, a simple structure, and ease of assembly and disassembly are fundamental requirements for flange connections. Two conditions must be met: mechanical integrity (adequate strength) and tightness (effective sealing). The key parameters of the LNG discharge pipe flanges and connecting bolts are listed in Table 1.

Table 1 Flange and Bolt Parameters

During pre-tightening, the gasket surface must be fully embedded in the flange sealing surface. However, over-tightening must be avoided, as it can cause the gasket to lose its elasticity and enter a plastic state. In this state, the gasket loses its resilience, and any deformation of the flange surface can create gaps, resulting in leakage. For this project, the discharge pipe flange is fitted with a spiral-wound gasket, which is typically circular in shape. Based on their structural characteristics, spiral-wound gaskets can be classified as basic, inner-ring, outer-ring, or inner-and-outer-ring types. The spiral-wound gasket used in the LNG discharge pipe for this project is of the inner-and-outer-ring type. Its specific parameters are listed in Table 2.

Table 2 Spiral Wound Gasket Parameters

In flange joints, all components—gaskets, studs, and nuts—must be installed correctly. Proper installation ensures a reliable seal, whereas improper installation can lead to leakage. Whether the flange is new or used, carefully clean the sealing surface to remove any debris, rust, or oil before installing the gasket. For large-diameter flanges, do not remove the gasket’s protective packaging until it is ready to be positioned for installation. Check the alignment of the flanges. If there is excessive spacing, angular deviation, or misalignment, adjust the pipeline before installing the gasket. Do not rely on the gasket to correct these issues. After placing the gasket between the two flanges, use appropriate tools to correct any misalignment. The studs should pass smoothly through the flange bolt holes, and after tightening, the studs and nuts should sit flush. The gasket must be centered—or nearly centered—within the flange. If it is misaligned, take appropriate corrective measures before tightening the bolts, paying special attention when installing gaskets on horizontal pipelines. Tighten the flange studs in a symmetrical sequence. The preload torque must reach the specified value, and the torque wrench used should be pre-calibrated with an accuracy tolerance of ±5%. After reaching the calculated bolt tightening torque, recheck all bolts in a clockwise sequence to ensure uniform tightening.

LNG pipelines must be precooled before commissioning. At LNG receiving terminals, the typical precooling media are liquid nitrogen and LNG, with most facilities opting for liquid nitrogen because it presents a lower risk of liquid regeneration. Liquid nitrogen is delivered to the site by tank truck. Due to the large diameter and considerable length of the discharge pipeline, a substantial volume of liquid nitrogen is required. To ensure smooth precooling, only the tank trucks scheduled for immediate unloading are allowed inside the station; all others must wait outside. The main discharge pipe has a diameter of 38 inches, while the drain pipe measures 4 inches in diameter. After passing through the liquid nitrogen vaporizer, the liquid nitrogen enters the drain pipe and then flows into the discharge pipeline. A pressure gauge and thermometer are installed between the cryogenic nitrogen source and the drain pipe to monitor and control the precooling rate. The discharge pipeline in the pipe gallery should not exceed 100 m in length, and thermometers should be installed at regular intervals. Typically, three thermometers are installed between the tank riser and the tank outlet. These are surface thermometers, and each measurement point records temperatures from both the upper and lower surfaces of the pipe. By monitoring these thermometers, the liquid nitrogen flow rate can be adjusted to maintain a controlled precooling rate. During precooling, the temperature should be reduced at a maximum rate of 10°C per hour, and the temperature difference between the top and bottom of the pipe, as measured by the surface thermometers, should not exceed 50°C. This helps prevent pipeline deformation and twisting. If the cooling rate is too rapid, excessive temperature differences across the flanges can lead to flange cracking. Uneven temperatures between the top and bottom of the pipe can cause flange distortion, gasket failure, and bolt loosening, potentially resulting in nitrogen leakage. A nitrogen leak is typically indicated by a hissing sound and visible white vapor at the leak site. Such leaks present serious safety hazards, including risks of frostbite and asphyxiation, and necessitate the immediate suspension of precooling operations. Once precooling begins, the pipeline must be inspected for any displacement and to ensure the integrity of flange seals. The flange connecting the discharge pipeline to the liquid nitrogen vaporizer will be the first to reach -50°C, followed by the valves on the lower and upper tank platforms, the valves on the tank roof, and finally the tank outlet flange. Inspections should be conducted as each component reaches -50°C, with flange sealing verified using a calibrated torque wrench or a copper hammer. If the bolts are noticeably loose, they must be retightened to the specified preload torque. For minor adjustments, bolts can be cold-tightened when the temperature further decreases to -70°C. A portable combustible gas detector should also be used to inspect the flanges for any signs of leakage. When the thermometer near the vaporizer connection reads -70°C, the discharge pipe flange must be cold-tightened, followed by inspection of the valves on the lower and upper tank platforms. Although the connection between the valve body and the pipeline is welded, inspection handholes are provided and sealed with flanges. As the temperature drops, the valve body contracts, which may cause the handhole bolts to loosen. These bolts must be rechecked and tightened as needed. On-site inspections confirmed that both the handhole and the operating lever flange fasteners had loosened, and they were retightened during the precooling process. The tank outlet flange is the last to reach -70°C. At this stage, inspections usually reveal considerable bolt slack, which must be corrected by tightening the bolts with a torque wrench to the calculated preload torque. After the pipeline reaches the final temperature, cold-tightening is repeated for the discharge pipe flange, the valves on various tank platforms, the valve handholes, and the operating lever connections to ensure reliable sealing across the entire system.

After the discharge pipeline has been precooked, it is filled. In newly constructed LNG receiving terminals, the pipeline is filled with LNG supplied from ship unloading operations at the dock. In expanded LNG receiving terminals, the pipeline can be filled with LNG supplied through a low-pressure transmission pipeline connected to the discharge line. The physical properties of LNG in the discharge pipeline are summarized in Table 3.

Table 3 LNG Physical Parameters

After the pipeline is filled with LNG, its overall weight increases, which consequently raises the load on the flanges. This change in flange stress impacts the ability of the flange sealing surfaces to maintain adequate sealing pressure. During the filling process, the accumulation of liquid at the bottom of the pipeline increases the load on the flanges, causing uneven flange stress and a higher risk of flange leakage. Additionally, because LNG does not arrive simultaneously along the pipeline, the loads on nearby supports and hangers vary. These load changes can exert force on the flanges, further increasing the risk of leakage. Therefore, during the filling process, it is essential to enhance flange monitoring and conduct thorough on-site inspections. White fog formation at the site should be monitored as a visual indication of potential leaks. On-site combustible gas detectors must be used to detect any gas leaks, and portable combustible gas alarms should be focused on the flange locations. If a leak is detected, LNG filling must be halted, the affected pipeline section isolated, and corrective measures—such as tightening flange fasteners or replacing gaskets—implemented before resuming operations. During the precooling process, the bolt pre-tightening force decreases as the temperature drops, reducing the initial preload. This requires secondary bolt tightening: initially a cold tightening at -70°C, followed by a second tightening at the final operating temperature. For better process control, bolt tightening can be carried out in three stages: an initial cold tightening at -50°C, a second at -90°C, and a final tightening at the ultimate operating temperature.

Given the extremely low temperatures and high operational risks of LNG pipelines, ensuring their safety is of paramount importance. Proper tightening of flange bolts is crucial to prevent leakage and ensure the safe operation of the pipeline. Using a 38" discharge pipe at an LNG receiving terminal as a case study, this article calculates the required bolt tightening torque, which was determined to be 1,216 N·m for this project. During the pre-cooling process, secondary tightening of the bolts was carried out, and no flange leakage was observed. The project is currently operating normally, with no flange leaks reported.