The Flow Rate Control of the Oxygen Pipeline (Part One)

The control of the maximum allowable flow rate of oxygen in the oxygen pipeline is a very important issue for safety. When the flow rate is too fast, the temperature of the oxygen pipeline will rise due to friction between high-pressure pure oxygen and steel pipe walls, friction and collision between impurity particles and steel pipe walls, causing an explosion. This has been verified by facts. Foreign test reports indicate that iron powder or incompletely oxidized ferrous oxide powder in carbon steel pipelines has an ignition temperature of 300℃ to 400℃ in pure oxygen, and it decreases with the increase of oxygen pressure and the decrease of particle sizes. The combustion of these particles causes the carbon steel pipe to catch fire. Although stainless steel does not produce corrosion, it contains a large amount of iron and a small amount of combustible carbon, and has poor thermal conductivity. It is not easy to dissipate heat. It can still ignite when there is excitation energy such as friction and impact.
 
According to the Safety Technical Regulations for Producing Oxygen and Related Gases by Deep Freezing Methods (GB 16912-2008), the maximum allowable flow rate of oxygen in oxygen pipelines should not exceed the range specified by the standard according to the material and working pressure of the pipeline. See Table 1.
 
Table 1 Maximum allowable flow rates of oxygen in the pipeline

 
Please note that the maximum allowable flow rate refers to the actual flow rate at the lowest working pressure and highest working temperature of the piping system.
 
Impact and non-impact situation: the position where the fluid flow direction suddenly changes or the vortex is generated, which causes particles in the fluid to impact on the pipe wall. Such a location is called the impact situation; otherwise, it is called the non-impact situation. For copper and copper alloys (except copper alloys containing aluminum), nickel and nickel-copper alloys, under the condition of the pressure being less than or equal to 21.0MPa, the flow rate is not limited when the pressure drop is allowed.
 
1. The maximum allowable flow rate of oxygen in the pipe is related to the working pressure and material of the pipeline. The flow rate refers to the actual flow rate of oxygen in the tube under a certain working state, and is related to the pressure, temperature, and flow rate under the working state. The maximum allowable flow rate refers to the actual flow rate at the lowest working pressure and highest working temperature of the piping system. To determine the diameter of the oxygen pipeline, it is necessary to meet the requirements for the safe flow rate (the maximum allowable flow rate) under peak load conditions, and leave room to ensure safety. Liquid oxygen pipelines generally adopt stainless steel pipelines or copper and copper alloy pipelines. The liquid oxygen's flow rate is not strictly limited due to the low-temperature state.
 
2. The design and data selection in Table 1 not only learn from the design and operation experience in the past ten years in China, but also draws on the scientific research results and materials of the United States, Germany, France, the United Kingdom, Russia and Japan, especially a large number of scientific test data from the EIGA. The European Industrial Gas Association conducted a large number of fire tests on different materials of oxygen pipelines under different oxygen pressure, combined its engineering practice experience, and compiled the Oxygen Piping Systems standard. In the Oxygen Piping Systems, oxygen pipelines and components are scientifically divided into impact situations and non-impact situations. Then, after a lot of test results, the maximum allowable oxygen flow rate is determined according to the oxygen pressure and the pipe material. This standard has been recognized by European, American and Japanese counterparts.
 
The oxygen's flow direction is suddenly changed or vortex is generated, which causes particles and foreign objects entrained in the oxygen to impact the pipe wall. Such a location is called an impact situation. The impact situation is prone to generate excitation energy, causing combustion and explosion. It is a dangerous place. The maximum allowable flow rate of oxygen should be controlled strictly and small. Butt-welded tees (when oxygen flows from the branch pipe to the main pipe), threaded reducers, field welded tees, short radius elbows (bending radius less than 1.5 times the pipe diameter), reducers with a reduction ratio greater than 3 (oxygen flowing from the big end to the small end), the outlet pipe of vent valves and safety valves, globe valves, needle valves, check valves, pressure reducing valves, regulating valves, bypass valves and their outlet ends at a range of 8 times within the diameter of the pipeline, when the ball valve or plug valve is opened or closed, the valve plate, filter and orifice plate of the butterfly valve are all impact places.
 
The locations other than the above are non-impact locations, which are safer and the maximum allowable flow rate of oxygen is not very strict. Straight pipe sections, butt-welded tees manufactured by the factory (when oxygen flows from the main pipe to the branch pipe), long radius elbows (bending radius being greater than or equal to 1.5 times the pipe diameter), reducer pipes with a reduction ratio less than or equal to 3, ball valves and plug valves at the fully opened state are all non-impact situations.