What are the advantages of vacuum drying method in the drying process of stainless steel pipes

In the drying process of stainless steel pipes in the LNG system, the core objective is to keep the moisture content within the pipes at an extremely low level (typically required to be ≤ 100mg/m³, and in some harsh conditions ≤ 50mg/m³), to prevent water from freezing and blocking the pipes, damaging the valve seals, or reacting with the impurities in LNG to form corrosive media (such as H₂O + CO₂ → carbonic acid, which intensifies the pitting corrosion of stainless steel). The vacuum drying method is one of the mainstream technologies in the current industry. Its advantages stem from its precise adaptation to "low temperature, low residue, and high safety" LNG conditions. This can be specifically explored from dimensions such as drying efficiency, drying depth, safety, and compatibility: 

1. Extremely deep drying depth: Meeting the "ultra-low moisture" core requirement of the LNG system

The tolerance of the LNG system to moisture is much lower than that of conventional industrial pipelines (such as the moisture requirement for ordinary gas pipelines is ≤ 500mg/m³), and the vacuum drying method, through a negative pressure environment to accelerate the phase change and removal of moisture, can achieve a drying depth that other drying methods (such as purging drying and adsorption drying) cannot reach: 

Principle of operation: In a vacuum environment (typically, the pressure inside the pipeline is reduced to ≤ 10 Pa, or even ≤ 1 Pa), the saturated vapor pressure of water significantly decreases (for example, at normal pressure, the boiling point of water is 100℃, while at 1 Pa negative pressure, the boiling point can drop to -19℃). Even if there is residual liquid water or adsorbed water inside the pipeline, it can rapidly vaporize at room temperature (or slightly heated, usually ≤ 80℃) and form water vapor, which is then directly removed by the vacuum pump. 

Actual effect: It can stably reduce the moisture content in the pipeline to 30-80mg/m³, fully meeting and even exceeding the strict standards of the LNG system. In contrast, traditional nitrogen purging and drying is limited by the "ability to carry moisture", usually only reaching 200-500mg/m³. It requires multiple purgings to approach the qualified level, and it is difficult to completely remove the residual moisture in the folds on the pipeline inner wall and the depressions at the welds. 

II. High drying efficiency: Shortens construction period and reduces system commissioning costs

LNG projects usually involve long-distance pipelines (such as the transportation pipelines from the terminal to the storage tanks, with single segments reaching several hundred meters) and complex pipe fittings (such as elbows, three-way valves, etc.). The drying efficiency directly affects the overall construction period of the project. The efficiency of vacuum drying is reflected in: 

No need for repeated cycles: Compared to the "nitrogen purging - detection - re-purging" cycle mode (which requires several hours for a single purge and may need 3-5 cycles), the vacuum drying method uses the process of "vacuum extraction → pressure maintenance detection → re-vacuum extraction", and the single-stage pipeline drying time can usually be controlled within 24-48 hours (depending on the pipeline volume and the power of the vacuum equipment), with an efficiency increase of over 50%. 

Targeted removal of "dead-end moisture": The weld beads on the inner walls of pipelines, the gaps of oxide scales, the recesses of valve cores, etc., are "dead-end" areas that are weak points in the process of purging and drying (the nitrogen gas flow is difficult to reach these areas). However, the negative pressure generated by vacuum will form a "global suction force", prompting the moisture in these dead-end areas to vaporize and flow towards the negative pressure area, thus avoiding the failure of drying due to the residual moisture in certain areas. 

III. No residual medium risk: Ensuring LNG purity and pipeline safety

LNG is a high-purity cryogenic medium (with methane content typically ≥ 95%). It is strictly prohibited for the pipeline to retain any dry medium (such as nitrogen used for purging or molecular sieve powder used for adsorption drying). Otherwise, it may lead to a decrease in LNG purity, a reduction in combustion efficiency, and even pose safety hazards. The vacuum drying method fundamentally eliminates the "residual medium" issue: 

No introduction of external media: The entire drying process is achieved solely through "removal of air + removal of moisture", without injecting any gases (such as nitrogen) or solids (such as adsorbents) into the pipeline. After drying, the pipeline only retains extremely small amounts of inert gas (mainly the remaining unextracted nitrogen, with a content typically ≤ 0.1%), and no additional "replacement and cleaning" steps are required. This directly meets the purity requirements for LNG transportation. 

Prevent secondary pollution of the adsorbent: In the adsorption drying method, molecular sieves and other adsorbents need to be placed in the pipeline. If the adsorbent particles are damaged or fall off, they may enter the downstream equipment (such as storage tanks, pumps, and gasifiers) along with the LNG, blocking filters or wearing down mechanical components. However, in the vacuum drying method, there is no such risk, and it provides better protection for the inner walls of the pipeline and downstream equipment. 

4. Good compatibility with pipeline materials: No risk of corrosion or damage

Although stainless steel pipes (such as 316L) have excellent corrosion resistance, they may still experience performance degradation if exposed to corrosive media or high temperatures during the drying process. The friendliness of the vacuum drying method towards pipeline materials is reflected in: 

Low-temperature / Normal-temperature operation: During the drying process, only the outer wall of the pipeline needs to be slightly heated (usually ≤ 80℃, achieved through electric heating or hot air circulation). This temperature is much lower than the "sensitization temperature" of stainless steel (the sensitization temperature of 316L is approximately 450-850℃). This ensures that the stainless steel pipe maintains its low-temperature toughness and corrosion resistance, avoiding the precipitation of Cr₂₃C₆ at the grain boundaries (i.e., "sensitization corrosion") caused by high temperatures. 

Without electrochemical corrosion conditions: During nitrogen purging and drying, if there is a trace amount of oxygen or moisture in the nitrogen, a "wet oxygen environment" may form on the inner walls of the pipeline, causing slight electrochemical corrosion. In a vacuum environment, the oxygen content is extremely low (≤0.01%), and the moisture is continuously removed, without the "electrolyte (water)" and "oxidant (oxygen)" required for corrosion reactions, completely eliminating the risk of corrosion during the drying process. 

V. Direct verification of drying effect: Facilitates quality control

The verification of LNG system drying quality is a crucial step. The "visual" verification advantage of the vacuum drying method is significant, and it can avoid the problem of "false drying": 

The degree of dryness can be directly determined through the "pressure retention test": During the drying process, when the vacuum level inside the pipeline reaches the target value (such as ≤ 10Pa), the vacuum pump is turned off for "pressure retention". If the vacuum level does not show any significant increase within the specified time (such as 2 hours) (usually requiring the pressure to rise by ≤ 1Pa/h), it indicates that the moisture in the pipeline has been largely removed (there is no pressure increase caused by the evaporation of moisture); while for the blowing and drying process, it needs to be indirectly judged by "detecting the gas dew point with a dew point meter", which is greatly affected by environmental temperature and air flow speed, and may lead to a misjudgment of "the outlet meets the standard but there is still residual moisture inside the pipeline". 

Data traceability: During the vacuum drying process, parameters such as the pumping capacity of the vacuum pump, pressure changes in the pipeline, and the heating temperature of the outer wall can be recorded in real time, forming a complete drying report. This facilitates quality traceability for the owner and the supervisory party, and complies with the "full-process compliance" management requirements of the LNG project. 

VI. Adaptation to complex pipeline layouts: No "blind zone" restrictions

The stainless steel pipes in the LNG system often include long straight sections, multiple curved bends, and multiple branch fittings (such as manifolds). The traditional drying method is prone to being limited by the layout, while the "negative pressure global coverage" feature of vacuum drying can adapt to complex scenarios: 

No need to rely on airflow direction: Nitrogen purge drying should be designed with "inlet - outlet" based on the slope and direction of the pipeline; otherwise, "airflow dead zones" may form in low-lying sections or blind ends. While vacuum drying achieves uniform negative pressure on the entire pipeline by setting vacuum pump interfaces at both ends or key nodes, consistent drying effects can be achieved regardless of straight pipes, curved pipes, or branch pipes. 

Suitable for large-diameter / long-distance pipelines: For large-diameter pipelines with a diameter of DN500 or above, or long-distance pipelines with a length exceeding 1000 meters, the vacuum drying method can achieve this by using the "segmental vacuum extraction + intermediate supplementary extraction" approach. This avoids the decline in drying efficiency caused by the large volume of the pipeline. The blow-off drying method requires a significant increase in nitrogen flow (high energy consumption) and is difficult to ensure the drying effect at the remote end. 

Summary: Comparison of Advantages of Vacuum Drying Method vs. Other Mainstream Drying Methods

Vacuum Drying Method 

Drying depth (mg/m³): 30 - 80 (suitable for demanding conditions) 

Drying efficiency: High (24 - 48 hours per segment) 

Residual medium risk: None (only trace amounts of inert gas) 

Pipeline compatibility: Good (normal temperature / low temperature, non-corrosive) 

Complex layout adaptability: Excellent (global negative pressure, no dead zones) 

Nitrogen purge and drying 

Drying depth (mg/m³): 200 - 500 (requires multiple purges) 

Drying efficiency: Low (72 - 120 hours per segment) 

Residual medium risk: There may be residual nitrogen (needs to be purged) 

Pipeline compatibility: General (May cause wet oxygen corrosion) 

Complex layout adaptability: Poor (Dependent on airflow, prone to blind spots) 

Adsorption drying method 

Drying depth (mg/m³): 50 - 100 (with easy residual adsorbent) 

Drying efficiency: Medium (48 - 72 hours / period) 

Residual medium risk: There is a possibility of residual adsorbent powder. 

Pipeline compatibility: General (The adsorbent may wear down the inner wall) 

Complex layout adaptability: Medium (branch pipes require additional adsorption units) 

In conclusion, the vacuum drying method, with its comprehensive advantages of "deep drying + high efficiency with low residue + safety compatibility", perfectly meets the core requirements of LNG system stainless steel pipes for "ultra-low moisture, no pollution, and high reliability". It has become one of the preferred technologies for drying treatment in current LNG projects (such as receiving stations, storage tanks, and transportation pipelines).