Function and Process Requirements of the Bottom Gas Seal Machine for Gas Cylinder

Function and Process Requirements of the Bottom Gas Seal Machine for Gas Cylinders
Bottom Gas Seal Machine for Gas Cylinders: Function, Process Requirements, and Standard Specifications
A. Core Function of the Bottom Gas Seal Machine for Gas Cylinders
1. Special Inspection of Bottom Sealing Defects
For seamless steel cylinders and aluminum alloy gas cylinders, the bottom is formed by bottom rotary pressing. The bottom of the entire gas cylinder is the most prone area to micro cracks, delamination, rotary folding, pinholes, and uneven wall thickness; the entire machine can only detect major leaks, while minor bottom micro-seepages are very difficult to detect.
The equipment applies local pressure to the bottom from the inside and conducts a gas tightness screening on the rotary sealing area alone, accurately detecting:
Rotary folds, interlayers, micro pinholes, subtle贯穿 cracks;
Thin-bottom wall thickness, micro leakage channels caused by heat processing;
Penetrating damages caused by bottom milling and grinding.
2. Pre-emptive Screening of Non-conforming Products to Reduce Post-process Losses
Process Position： Bottom Rotary Pressing → Bottom Milling Cleaning → Bottom Gas Seal → Heat Treatment → Whole Machine Water Pressure / Gas Seal
Screen out bottom leakage cylinders in advance to avoid their flow into heat treatment, machining, flaw detection, spraying sections, reducing waste of semi-finished products, processing, and coating costs.
3. Meeting the Mandatory Inspection Requirements for Special Equipment Certification
According to GB/T 17258, GB/T 5099, and GB/T 12137, special rotary press gas cylinders must have a special test for bottom sealing; without this process, it is impossible to obtain the gas cylinder manufacturing license; OEMs, CNG / hydrogen storage cylinder customers are required to conduct 100% individual bottom gas seal testing.
4. Distinguishing "Local Bottom Leakage" from "Bottle Body / Bottle Mouth Leakage"
The whole machine gas seal leakage cannot determine the leakage point; the bottom gas seal machine only applies pressure to the bottom. Once leakage occurs, it can directly determine the defect in the rotary sealing area, facilitating the traceability of rotary press process parameters (heating temperature, rotary speed, feed rate, mold).
5. Two Detection Modes for Production Lines
Water immersion observation method： Invert the gas cylinder, immerse the bottom in the water tank, and visually observe continuous bubbles;
Pressure attenuation / helium inspection method： Automated production lines use pressure sensors or helium mass spectrometry to quantitatively detect micro leakage rates, suitable for high-speed assembly lines for automatic determination and data storage.
6. Process Verification and Quality Traceability
Automatically record the steel grade of each gas cylinder, test pressure, holding time, pressure drop / leakage rate, and generate traceable inspection reports to meet the requirements for special equipment supervision and inspection archiving.
B. General Process Requirements
(1) Pre-treatment Requirements for Workpieces Before Testing
The bottom surface of the gas cylinder must be free of aluminum chips / iron chips, oil stains, water stains, oxide scale; after shot blasting and bottom milling, it must be blown clean and dried; otherwise, bubble interference will affect judgment;
The bottom must have no obvious dents, scratches, or deep slots from milling;
The inside of the cylinder must have no water accumulation or metal debris to prevent blockage of the pressure sealing head;
The environmental temperature is 5-40℃, and the temperature difference between water and cylinder body is ≤5℃, to avoid pressure fluctuations caused by temperature differences and misjudgment.
(2) Medium and Pressure Parameter Requirements
Test Medium
Conventional industrial gas cylinders： Dry and clean compressed air / nitrogen;
Oxygen, oxidizing gas cylinders： Oil-free and water-free high-purity nitrogen, prohibited with grease;
Hydrogen storage, high-purity gas cylinders： Prefer helium for leak detection, with higher sensitivity.
Test Pressure (in two types of standards)
CNG vehicle gas cylinders (GB/T 17258)： Gas seal pressure = nominal working pressure (e.g., 20MPa cylinder pressurized to 20MPa);
Industrial steel / seamless aluminum gas cylinders (GB/T 5099)： Bottom gas seal is commonly 0.8-1.0 times the nominal pressure; some manufacturers use 1.2 times to increase the screening strictness;
Pressure increase rate： ≤0.5MPa/s,严禁 rapid impact to prevent micro cracks from expanding.
Pressure application area specification
The pressure application area of the bottom sealing head must be ≥ 1/16 of the total area of the bottom, with a pressure application diameter ≥ 20mm, to ensure uniform pressure is applied to the rotary sealing area and no local pressure distortion. Soap solution coating method: Maintain pressure for ≥ 1 minute, and observe throughout the process to ensure no continuous bubbles at the bottom;
Water immersion method: The highest point of the gas cylinder should be immersed in water ≥ 50mm, maintain pressure and observe for ≥ 1 minute, and for mass production lines, the standard is ≥ 2 minutes;
(4) Qualified determination criteria
Water immersion / Soap solution coating: During the entire pressure maintenance process, no continuous bubbles should be emitted, and sporadic single bubbles should not continuously occur as qualified; if continuous bubbles occur, it should be scrapped directly;
Pressure decay method: The pressure drop within the specified pressure maintenance time should be ≤ the allowable value (normal ≤ 0.5% of the test pressure);
Leakage cylinder must not be reworked and repaired; it should be directly isolated and scrapped.
(5) Hardware process requirements for the bottom gas tightness machine of gas cylinders
Sealing tooling: Polyurethane high-pressure resistant sealing head, suitable for different bottle bottom curvatures, without damaging the workpiece;
Pressure monitoring: Dual pressure gauges, accuracy ≥ 0.4 grade, range of 1.5 to 3 times the test pressure, with pressure sensor for real-time collection;
Safety configuration: High-pressure overflow valve, explosion-proof protective cover, overpressure automatic relief, emergency stop interlock;
(6) Process steps of operation procedures
Material loading and positioning: The gas cylinder is inverted and clamped, the sealing head of the sealing head is in close contact with the inner side of the bottle bottom;
Pre-charging at low pressure: First, leak detection tooling is sealed at 3 to 5 MPa for leak detection; if the tooling leaks, the machine stops and re-clamps;
Uniformly increase pressure to the test pressure;
Steady pressure maintenance and pressure measurement (water immersion visual inspection / pressure collection / helium inspection scanning);
Qualified: Pressure relief, material discharge into the next process; Unqualified: Sound and light alarm, automatic分流 isolation;
Pressure relief recovery： High-pressure gas recovery to reduce gas usage costs.
(7) Safety process requirements
Isolation and protection of the test area, personnel should not directly face the bottle bottom;
Medium is dry and free of oil to prevent high-pressure gas explosion;
Daily calibration of pressure gauges and leak detectors, retention of calibration records;
Timely replacement of sealing parts for leakage and gas pipelines to avoid pressure deviation.
C. Inspection requirements for the bottom gas tightness machine of gas cylinders
Pressure application position： Only local pressure application on the inner side of the bottle bottom
Inspection focus： Micro leakage of the rotary seal, micro pinholes
Process position： After rotary milling of the bottom and before heat treatment
Defect location determination： Defects are directly determined to be on the bottle bottom
Necessity of evidence collection： Mandatory for seamless rotary gas cylinders
D. Key standards basis
GB/T 12137-2025 "Gas cylinder gas tightness test method";
GB/T 17258-2022 "Automobile compressed natural gas steel cylinder" bottom sealingity test clauses;
GB/T 5099 "Steel seamless gas cylinder";
ISO 9809 "Manufacturing inspection specification for seamless gas cylinders".