Safety measures on oxygen pipeline

Safety measures on oxygen pipeline

Two accidents of oxygen pipeline burning explosion occurred in China’s steel industry in 2021 are introduced , and the mechanism of burning explosion in oxygen pipeline is briefly analyzed. Standard requirements and corresponding measures in aspects of oxygen pipeline flow rate control , material selection , pipe and valve selection and pipeline construction , which should be followed to ensure the safety of oxygen pipeline , are described in details , according to the national standard “Safety technical regulation for oxygen and relative gases produced with cryogenic method”. Finally , the experience of safe use of oxygen pipelines is presented.

Two oxygen pipeline explosion accidents

Oxygen pipeline combustion and explosion accidents are frequent, with huge losses and many lessons. In 2008, two more typical oxygen pipeline combustion and explosion accidents occurred, which is worthy of vigilance.
At 09:00 on February 22, 2008, the oxygen regulating valve in the oxygen surge chamber of 300T converter and the upstream oxygen filter and pipeline of a new steelmaking plant 2# of a large iron and Steel CoMPany burned and exploded, with flame, belonging to chemical explosion. A hole was burned out in the regulating valve, the filter was damaged, and the equipment was seriously damaged. Fortunately, no one was hurt. Check the oxygen filter of another converter and find a large amount of rust, welding slag and electrode head. These are construction leftovers, which are not purged before putting into operation, and are all blocked in front of the oxygen filter. These foreign matters rub and collide with the steel pipe in the high-purity, high-pressure and high-speed oxygen flow, and become the excitation source of combustion and explosion. The oxygen filter is not cleaned regularly, and a large number of foreign matters are blocked and rubbed, which is also the inducement of combustion and explosion. It can be seen that the untreated oxygen filter has become a potential safety hazard. When the accident happened, the oxygen pipeline, valves and pipe fittings were ignited first, the temperature increased sharply, the wall thickness was thinned, the strength decreased rapidly, and the explosion occurred because they could not bear the oxygen pressure.
At about 22:00 on March 27, 2008, after the oxygen pipe network of an iron and steel coMPany was transformed with the main plant, an oxygen valve (DN300 ball valve) and pipeline explosion accident occurred when sending oxygen to users and opening the valve, and two valve operators died on the spot. The cause of the accident was that the oxygen ball valve opened too fast under large differential pressure (the pressure in front of the valve was 119MPa and the pressure behind the valve was 0) (it should be opened slowly below 013MPa differential pressure as required), and there were many foreign matters such as rust and welding slag in the oxygen pipeline. Because the new and old pipelines were staggered and the construction period was tight, it was not purged. At the same time, it did not formulate a proper oxygen delivery scheme. When opening the valve, the high-pressure pure oxygen flow entrained rust The welding slag iMPacts or rubs the oxygen valve and pipeline at high speed, igniting the oxygen valve and pipeline and causing explosion.

Combustion and explosion mechanism of oxygen pipeline

Based on the analysis of the mechanism of “three elements” of combustion and explosion, the material of oxygen pipeline is generally carbon steel or stainless steel. Because it contains carbon, it is a combustible material, and the heat release during ferrite combustion is large and the temperature rises rapidly. The high-purity and high-pressure oxygen transported in the oxygen pipeline is a very strong oxidant. The higher the purity, the higher the pressure, the stronger the oxidation and the more dangerous it is. There are many kinds of excitation energy causing combustion and explosion of oxygen pipeline:

• (1) When the valve is suddenly opened between the high and low pressure sections, the oxygen in the low pressure section is compressed rapidly. Due to the fast speed, there is no time to dissipate heat, forming the so-called “adiabatic compression”, and the local temperature rises sharply, becoming the ignition energy;
• (2) When opening and closing the valve, the iMPact and extrusion of the valve disc and valve seat and the friction between valve components;
• (3) High speed moving material particles (such as rust, dust, welding slag and impurities) rub against the pipe wall, iMPact each other and iMPact and collision at valves, elbows, bifurcation heads, reducing pipes and weld beads;
• (4) External high temperature such as heating surface, flame and radiant heat;
• (5) Electrostatic induction;
• (6) Grease ignition;
• (7) Catalytic action of rust and iron powder, etc.

It is the actual need of industrial production to transport high-purity and high-pressure oxygen in carbon steel and stainless steel oxygen pipelines. In order to eliminate or reduce the combustion and explosion accidents of oxygen pipeline, necessary measures shall be taken in various links such as design, manufacturing, installation, use and management to prevent the formation of excitation energy, which is the key of oxygen pipeline safety technology.

Oxygen pipeline flow rate control

The control of the maximum allowable flow rate of oxygen in oxygen pipeline is a very important safety problem. If the flow rate is too high, the friction between high-pressure pure oxygen and steel pipe wall, the friction and collision between high-pressure pure oxygen and impurity particles and steel pipe wall will cause the temperature of oxygen pipe to rise, which will lead to combustion and explosion accidents, which has been verified by facts. Foreign test reports show that there are iron powder or incompletely oxidized ferrous oxide powder in carbon steel pipes. Their ignition temperature in pure oxygen is only 300 ℃ – 400 ℃, which decreases with the increase of oxygen pressure and particle size reduction. The combustion of these particles leads to the fire of carbon steel pipes. Although stainless steel does not rust, it contains a large amount of iron and a small amount of combustible carbon, and its thermal conductivity is poor. It is only 1/3 of that of carbon steel. It is not easy to dissipate heat. It can still ignite when there are excitation energy such as friction and iMPact.
According to the safety technical code for the production of oxygen and related gases by deep freezing method (GB16912-2008), the maximum allowable flow rate of oxygen in the oxygen pipeline shall not exceed the range specified in the standard according to the pipeline material and working pressure (see table. 1).
Table.1 maximum allowable flow rate of oxygen in pipeline V

Texture of material	Working pressure P/MPa
	p ≤0.1	0.1 < p ≤1.0	1.0 < p ≤3.0	3.0 < p ≤10.0	10.0 < p < 15.0	p ≥15.0
Carbon steel	Determined according to the pressure drop of piping system	20m/s	15m/s	Not allow	Not allow	Not allow
Austenitic stainless steel		30m/s	25m/s	PV ≤ 45MPa · M /S (iMPact occasion) PV ≤ 80MPa · M /S (non iMPact occasion)	415m /S (iMPact occasion) 810m /S (non iMPact occasion)	415m/s 810m /S (non iMPact occasion)

Note:

• (1) The maximum allowable flow rate refers to the actual flow rate at the lowest working pressure and the highest working temperature of the pipe system.
• (2) IMPact occasion and non iMPact occasion: the position where the fluid flow direction suddenly changes or vortices are generated, resulting in the iMPact of particles in the fluid on the pipe wall. Such position is called iMPact occasion; Otherwise, it is called non iMPact occasion.
• (3) For copper and copper alloys (except aluminum containing copper alloys), nickel and nickel copper alloys, the flow rate is not limited when the pressure drop is allowed under the condition of less than or equal to 21.0MPa.

(1) The maximum allowable flow rate of oxygen in the pipeline is related to the working pressure and pipeline material. Flow rate refers to the actual flow rate of oxygen in the pipe under a certain working state, which is related to the pressure, temperature and flow under the working state. The maximum allowable flow rate refers to the actual flow rate at the lowest working pressure and the highest working temperature of the pipe system. The diameter of oxygen pipeline shall meet the requirements of safe flow rate (maximum allowable flow rate) under peak load, and leave room to ensure safety. The liquid oxygen pipeline generally adopts stainless steel pipeline or copper and copper alloy pipeline. Due to the low temperature state, the liquid oxygen flow rate is not strictly limited.
(2) The design and data selection in Table 1 not only draws on the domestic design and operation experience in recent ten years, but also draws on the scientific research achievements and data of the United States, Germany, France, Britain, Russia and Japan, especially a large number of scientific test data from the European Industrial Gas Association (Eiga). The European Industrial Gas Association has conducted a large number of ignition tests for different materials of oxygen pipelines under different oxygen pressures, and combined with its engineering practice experience, has compiled the standard of oxygen pipeline system. In the standard of oxygen pipeline system, oxygen pipelines and components are scientifically divided into “iMPact occasion” and “non iMPact occasion”, and then the maximum allowable flow rate of oxygen is determined according to oxygen pressure and pipeline material according to a large number of test results. The standard has been recognized by peers in Europe, the United States and Japan.
Make the oxygen flow direction change suddenly or produce vortex, so as to cause the iMPact of particles and foreign matters entrained in oxygen on the pipe wall. Such a position is called iMPact occasion. The iMPact occasion is prone to generate excitation energy and cause combustion and explosion. It is a dangerous occasion. The maximum allowable flow rate of oxygen shall be strictly controlled and small. Such as pressed butt welded tee (when oxygen flows from branch pipe to main pipe), threaded reducer, field welded tee, short radius elbow (bending radius is less than 1.5 times of pipe diameter), reducer with reduction ratio greater than 3 (when oxygen flows from large end to small end), mitered elbow, outlet pipe of vent valve and safety valve, stop valve, needle valve, check valve, pressure reducing valve, regulating valve Within the pipeline range of 8 times the diameter of the bypass valve and its outlet end, when the ball valve or plug valve is opened and closed, the valve plate, filter and orifice plate of the butterfly valve are iMPacted.
Non above positions are non iMPact occasions, which are safer, and the maximum allowable flow rate of oxygen is controlled more loosely. For example, straight pipe section, factory made butt welded tee (when oxygen flows from the main pipe to the branch pipe), long radius elbow (bending radius is greater than or equal to 115 times the pipe diameter), reducing pipe with reduction ratio less than or equal to 3, ball valve and cock are all non iMPact occasions when they are fully open.
(3) The higher the working pressure of oxygen, the greater the risk and the smaller the maximum allowable flow rate. Table 1 divides the working pressure range into 6:

• ① Oxygen with working pressure P ≤ 0.1MPa has low ignition risk and belongs to low-pressure oxygen pipeline. Its flow rate is determined according to the pressure drop of the pipeline system.
• ② In the range of 0.1MPa < p ≤ 1.0MPa, the maximum allowable oxygen flow rates of carbon steel and austenitic stainless steel oxygen pipelines are 20m/s and 30m/s respectively, which are added according to the needs of shipbuilding, mechanism and other industries.
• ③ In the range of 1.0MPa < p ≤ 3.0MPa, the maximum allowable oxygen flow rates of carbon steel and austenitic stainless steel oxygen pipelines are 15m/s and 25m/s respectively, which is a common oxygen pressure range in iron and steel, metallurgy and other industries. After more than 10 years of practice, it has been proved that as long as the oxygen flow rate is strictly controlled, the oxygen pipeline can operate safely and economically, and the data is scientific and reliable.
• ④ 3.0MPa < p ≤ 10.0MPa, which is the commonly used oxygen pressure range in the chemical industry. Due to the high oxygen pressure and high risk, carbon steel pipes prone to rust are not allowed to be used. For austenitic stainless steel pipe, the iMPact occasion and non iMPact occasion can also be scientifically distinguished, and the maximum allowable flow rate can be determined according to the limit of the product value of oxygen pressure and maximum allowable flow rate. For high-risk iMPact occasions, PV ≤ 45MPa · M/S is specified; For non iMPact occasions with less risk, PV ≤ 80MPa · M/S is specified. If the oxygen pressure is the same, the maximum allowable flow rate calculated in the iMPact case is smaller than that in the non iMPact case; For the same occasion, the higher the pressure, the smaller the calculated maximum allowable flow rate.
• ⑤ The range of 10.0MPa < p < 15.0MPa is mainly used in the chemical industry, which is a new oxygen pressure range in GB16912-2008 standard. Of course, carbon steel pipe can not be used as oxygen pipeline for this oxygen pressure. When austenitic stainless steel pipe is used, the maximum allowable flow rate of oxygen in iMPact and non iMPact occasions is 415m/s and 810m/s respectively, and the flow rate control is very small. In fact, this value is also the flow rate calculated by substituting 10.0MPa pressure into the previous pressure interval formula.
• ⑥ The range of P ≥ 15.0MPa is mainly used for filling steel cylinders. Carbon steel pipes are not allowed for this kind of high-pressure oxygen. When austenitic stainless steel pipes are used, regardless of iMPact or non iMPact occasions, the maximum allowable flow rate of oxygen is set at 415m/s, which is strictly controlled.

(4) The oxygen pipe material directly limits the maximum allowable flow rate of oxygen. Copper and copper alloys (except aluminum containing copper alloys, which are combustible under specific conditions), nickel and nickel copper alloys do not contain carbon elements. They only melt at high temperature and will not burn. Friction and iMPact do not cause fire. They are non-proliferation combustion materials. The fire does not spread and is safe. Therefore, they can be used in high-pressure and dangerous parts with high flow rate; When the pressure P ≤ 21.0MPa, the oxygen flow rate is unlimited when the pressure drop is allowed. Carbon steel pipe has low combustion temperature, fast combustion speed, poor anti combustion ability, and small maximum allowable flow rate of oxygen. It is used in general parts. The combustion resistance and combustion rate of austenitic stainless steel are between copper and carbon steel and do not rust. Therefore, the maximum allowable flow rate of oxygen is higher than that of carbon steel pipe and lower than that of copper pipe. It is used in important, dangerous and high oxygen pressure parts. The combustion temperature, combustion speed and anti combustion ability of some metals in oxygen are shown in Table 2.
Table.2 combustion temperature, combustion speed and anti combustion ability of some metals in oxygen

Metal	Pressure/MPa				Anti combustion ability	Combustion speed
	0	3.5	7	12.5
	Combustion temperature/℃
Copper	1035	886	836	800	Strongest 1	Only fusible but incombustible 1
Stainless steel	1367	–	–	–	2	3
Mild steel	1278	1.06	1018	928	3	Max 4
Iron	981	826	741	630	–	–
Aluminium	661	–	–	–	Minimum 4	2

Note: The numbers 1, 2, 3 and 4 in the column of anti combustion ability and combustion speed are safety grade evaluation.

Material selection of oxygen pipeline

The material selection of oxygen pipeline is a vital safety factor. With the increase of working pressure and flow rate of oxygen pipeline, the pipeline material has also developed from carbon steel and stainless steel to copper and copper alloy or nickel and nickel alloy. See Table 3 for specific selection regulations (Table 3 is quoted from GB16912-2008 standard).
(1) Table 3 divides the oxygen working pressure into four sections:

• ① Oxygen pressure pipeline with P ≤ 0.6MPa.
• ② The oxygen pressure pipeline in the range of 3.0MPa is widely used in iron and steel, metallurgy, shipbuilding, mechanism and other industries.
• ③ The oxygen pressure pipeline in the range of 3.0MPa < p ≤ 10MPa is mainly used in the chemical industry, with high oxygen pressure and strict requirements for pipeline material.
• ④ Oxygen pressure with P > 10MPa.

Force pipeline is mainly used in steel cylinder filling and some chemical industries. The oxygen pressure is very high and the pipeline material is more strictly limited. Practice has proved that it is convenient and applicable to be divided into four pressure ranges.
(2) Behind the valve, the range of 5 times nominal diameter (not less than 115m) behind the main pipe valve, one system branch pipe valve and workshop inlet valve, 5 times nominal diameter (not less than 115m) before and after the regulating valve group, inside the oxygen pressure workshop, behind the release valve, wet oxygen transmission, oxygen filling platform, bus bar and other iMPact occasions, important places, dangerous locations, multiple accident sources, high-pressure facilities, The material selection of oxygen pipeline shall be strict.
Table. 3 selection of oxygen pipeline materials

Working pressure P/MPa	p ≤0.6		0.6 < p ≤3.0		3.0 < p ≤10		p > 10		Liquid oxygen pipeline
Pipe category	General place	After the valve in the frequent operation area on the distribution main pipe and after the blow off valve.	General place	Range of 5 times nominal diameter (not less than 115m) behind the valve; Within the range of 5 times of nominal diameter (not less than 115m) at the front and rear of the control valve group; Inside the oxygen pressure workshop; After the bleed valve; Wet oxygen transport.	General place	Range of 5 times nominal diameter (not less than 115m) behind the valve; Within the range of 5 times of nominal diameter (not less than 115m) at the front and rear of the control valve group; Inside the oxygen pressure workshop; After the bleed valve; Wet oxygen transport.	General place	Oxygen filling table and busbar.
Steel coil welded pipe	✓	×	×	×	×	×	×	×	×
Welded steel pipe (GB/T 3091, SY/T 5037)	✓	×	×	×	×	×	×	×	×
Seamless steel pipe (GB/T 8163)	✓	×	✓	×	×	×	×	×	×
Stainless steel welded steel pipe (GB/T 12771)	✓	✓	✓	✓	×	×	×	×	×
Stainless steel plate welded pipe	✓	✓	✓	✓	×	×	×	×	×
Stainless steel seamless steel pipe (GB/T 14976)	✓	✓	✓	✓	✓	×	✓	×	✓
Copper and copper alloy extruded pipe (YS/T 662)	✓	✓	✓	✓	✓	✓	✓	✓	✓
Copper and copper alloy drawn tubes (GB/T 1527)	✓	✓	✓	✓	✓	✓	✓	✓	✓
Nickel and nickel alloys	✓	✓	✓	✓	✓	✓	✓	✓	✓

Note:

• (1) “<” is allowed“ ×” Not allowed.
• (2) Carbon steel plate coiled welded pipe should be used when the working pressure is less than 0.1MPa and the pipe diameter exceeds the pipe diameter of existing welded steel pipe and seamless steel pipe products.
• (3) Valves in the table refer to main pipe valves, branch pipe valves for one system and workshop inlet valves.
• (4) The oxygen pipeline in the oxygen storage tank area should be made of stainless steel.
• (5) Copper alloy pipes with working pressure greater than 3.0MPa do not include aluminum containing copper alloy.

(3) Material selection of oxygen pipeline: the characteristics of various materials of oxygen pipeline in Table 3 have been described in the previous section, which can be referred to.

• ① Carbon steel plate coiled welded pipe should be used when the working pressure P is less than 0.1MPa and the pipe diameter exceeds the pipe diameter of existing welded steel pipe and seamless steel pipe products.
• ② Welded steel pipes are suitable for general places with P ≤ 0.6MPa.
• ③ Seamless steel pipe is suitable for general places with P ≤ 3.0MPa.
• ④ Stainless steel welded steel pipe and stainless steel plate welded pipe are applicable to all places with P ≤ 3.0MPa. For stainless steel plate welded pipe, the inner wall weld shall be polished.
• ⑤ Stainless steel seamless steel pipes are suitable for all places with P ≤ 3.0MPa and general places with P > 3.0MPa. Oxygen pipes with P > 10MPa are basically used for cylinder filling. The main pipe from the outlet of high-pressure oxygen compressor or liquid oxygen pump to the filling table is a general place. Stainless steel pipes can be used and have mature use experience. However, large-diameter copper alloy or nickel alloy pipes are scarce, expensive and rarely used. For Copper alloy or nickel alloy pipes shall be used for the pipes of the filling table itself and the oxygen busbar. Stainless steel seamless pipes are low temperature resistant and high strength, and are suitable for liquid oxygen pipes.
• ⑥ Copper and copper alloy extruded pipes, copper and copper alloy drawn pipes, nickel and nickel alloy pipes are applicable to all places and liquid oxygen pipelines with P ≤ 21MPa due to their excellent flame and explosion resistance and low temperature resistance.

(4) Within the range of 5 times the nominal diameter (not less than 115m) behind the oxygen valve, the air flow is disordered and the iMPact is severe; 5 times the nominal diameter (not less than 115m) before and after the control valve group Within the scope, because the diameter of the oxygen regulating valve is often 1-2 grades smaller than the pipe diameter, there are variable diameter pipes in the front and back, the flow velocity at the valve port can reach subsonic speed, the air flow iMPact is very severe, and the friction heat generation is serious, which is a multiple source of combustion and explosion accidents; the internal position of the oxygen pressure workshop is important; the flow velocity behind the release valve is fast, which is an iMPact occasion; the wet oxygen transmission pipeline is easy to rust, which is a dangerous position. In these unusual places, when the oxygen pressure P ≤ 3.0MPa Stainless steel pipes (welding, coil welding and seamless) shall be used. When oxygen pressure P > 3.0MPa, copper and copper alloy or nickel and nickel alloy pipes shall be used.
The oxygen storage tank area with P ≤ 3.0MPa is an important part, and the oxygen pipeline in the area should adopt stainless steel pipe. The copper alloy pipe used for the oxygen pipeline with P > 3.0MPa does not include aluminum containing copper alloy because it is combustible under specific conditions. The oxygen pipeline with P < 0.1MPa and large diameter can adopt aluminum alloy pipe, which is suitable for the pipeline between the air separation equipment and the inlet of large oxygen compressor because it is not rusty and resistant to low temperature.
After the boundary valve of the oxygen plant (station, workshop) is sent, the oxygen main pipe is sent to a system branch valve, after entering the workshop valve, the oxygen pipe before and after the regulating valve group and the regulating valve shall be provided with a fire retardant copper pipe section.
When the valve chamber or protective wall is not set separately for the oxygen control valve group, copper alloy (except aluminum containing copper alloy) or nickel alloy pipes shall be used within the pipeline range of 8 times the nominal diameter of the control valve in front of and behind the oxygen control valve, which can be safely fused in case of accident and will not spread combustion. Low temperature resistant stainless steel seamless steel pipe, copper and copper alloy pipe, nickel and nickel alloy pipe can be used as liquid oxygen pipeline.

Selection of oxygen pipe fittings

Selection of elbow, reducer and tee on oxygen pipeline

Elbows, reducers and tees on oxygen pipes are prone to oxygen flow iMPact and severe friction. Some are iMPact occasions and are multiple sources of combustion and explosion accidents. Rolled (pressed) products with smooth inner wall, uniform wall thickness and regular groove should be selected to avoid accidents.

• (1) It is strictly prohibited to use folded elbows for the elbows of oxygen pipes, because the oxygen flow at the folds is disordered and the iMPact friction is severe, which is a frequent accident position. When cold bending or hot bending carbon steel elbows are used, the bending radius shall not be less than 5 times the nominal diameter to make the oxygen flow turn slowly; folds are not allowed during pipe bending operation to reduce friction and iMPact. When pressed butt welding elbows are used, long half length elbows should be selected Diameter elbow, that is, the bending radius is greater than or equal to 115 times the nominal diameter of the pipeline, which becomes a non iMPact occasion with less risk. For steel rolled welded pipe with working pressure P ≤ 0.1MPa, miter elbow (commonly known as shrimp waist elbow) is allowed due to low pressure and low risk The 90 ° elbow shall adopt the pipe fittings made of two-stage pipe or more in the middle to make the steering transition as smooth as possible, and the inner wall of the elbow shall be smooth without sharp edges, burrs and weld beading, so as to reduce friction and iMPact.
• (2) The reducing pipe of oxygen pipeline should be pressed butt welded pipe parts. When welding is used, the length of the reducing part should not be less than 3 times of the difference between the outer diameters of the pipes at both ends, so as to make the oxygen rheology smooth and slow. The inner wall of the reducing pipe should be smooth without sharp edges, burrs and weld beading, so as to reduce friction and iMPact and prevent combustion and explosion accidents.
• (3) The tee of oxygen pipeline should be pressed butt welding. When it cannot be obtained, it should be prefabricated in the factory or on site. It should be processed to no acute angle, protruding part and weld bead to ensure safety. It is not allowed to open or plug in the site, because it is difficult to make the pipe free of acute angle, protruding part, weld bead and weld slag, which is easy to cause accidents.

Selection of flange and gasket on oxygen pipeline

The flange on the oxygen pipeline shall ensure the strength and processing accuracy, and shall be selected in strict accordance with the relevant current national and industrial standards. The gasket of the pipeline flange shall not use combustibles, and the sealing performance shall be good. The higher the oxygen pressure, the higher the risk, and the stricter the requirements for the gasket (see table. 4). Table. 4 is quoted from GB16912-2008 standard.

Table.4 gaskets for oxygen pipe flanges

Working pressure P/MPa p/ MPa	Gasket
p ≤0.6	PTFE gasket, flexible graphite composite gasket.
0.6 < p ≤3.0	Spiral wound gasket, PTFE gasket, flexible graphite composite gasket.
3.0 < p ≤10	Spiral wound gasket, annealed softened copper gasket, nickel and nickel alloy gasket.
p > 10	Annealed softened copper gasket, nickel and nickel alloy gasket.

The oxygen working pressure is divided into four sections in table. 4, which is consistent with the oxygen pressure section selected for the oxygen pipe material in table. 3, which is scientific, reasonable, convenient, applicable and matched with each other.

• ① For low-pressure oxygen pipeline with P ≤ 0.6MPa, PTFE gasket and flexible graphite composite gasket can be used as flange gasket.
• ② 0.6MPa < p ≤ 3.0MPa, which is a common oxygen pressure range in iron and steel, metallurgy, shipbuilding and mechanism industries. The oxygen pressure is slightly higher. In addition to the above two gaskets, spiral wound gaskets are best used for the pipeline.
• ③ 3.0MPa < p ≤ 10MPa is the common oxygen pressure range in the chemical industry. The oxygen pressure is high. The spiral wound gasket can be used for the pipeline. When the oxygen pressure is close to the upper limit, it is best to use annealed softened copper gasket or nickel and nickel alloy gasket.
• ④ P > 10MPa, most of the oxygen pressure is used for cylinder filling and a few chemical departments, with high pressure and high risk. Its pipeline can only use annealed softened copper gasket or nickel and nickel alloy gasket.

Connection of oxygen pipeline

The oxygen pipeline shall be connected firmly by welding to reduce leakage. However, flange or thread can be used at the connection with equipment and valves to facilitate connection. Teflon film shall be used as filler at the threaded connection. It is strictly prohibited to use lead red hemp, cotton thread or other greasy materials as filler, because these materials are combustible and will ignite when oxygen leaks and become a fire source.

Oxygen filter

Filters that can be cleaned regularly shall be set at the inlet of the oxygen compressor and in front of the oxygen regulating valve. Filters shall be set at the inlet of the oxygen compressor to prevent rust, debris, particles, etc. from entering the cylinder or casing, causing equipment damage and fire. Filters shall also be set in front of the oxygen regulating valve. Because the diameter of the regulating valve is generally 1-2 grades smaller than that of the pipeline, oxygen throttling, high flow rate and close to subsonic speed, it must be prevented When sundries, rust and particles flow through the regulating valve at high speed, they rub and iMPact, resulting in fire. The oxygen filter shall be cleaned regularly, and its cycle shall be determined by the user according to the actual operation. If not cleaned, rust, sundries and particles will accumulate in the filter, increase resistance, form eddy current, generate heat during friction and iMPact, damage the filter screen and cause combustion and explosion accidents. Such accidents have occurred for many times Get up.
The shell of the oxygen filter shall be made of stainless steel or copper and copper alloy with high strength and no rust, and the filter internals shall be made of copper and copper alloy that will not catch fire during friction and iMPact. In addition to meeting the filtering function, the filter screen shall also have sufficient strength to prevent the filter screen from breaking and losing the filtering function; moreover, the debris of the filter screen will also affect the normal operation of the oxygen compressor or regulating valve. The filter screen should be fire-free and stable Nickel copper alloy (i.e. Monel alloy) with high strength, followed by copper alloy without fire (except aluminum copper alloy) The mesh size should be 60-80 mesh, which is the preferred data proved by practice. If the hole is too large, the filtering effect is poor, and if the hole is too small, the metal wire is too fine, the strength is low, and it is easy to break. The effective filtering area of the filter element should be more than twice the cross-sectional area of the pipeline, so as to ensure that the filter element can pass the design flow in case of partial blockage.

Selection of oxygen pipeline valves

The oxygen pipeline valve is an important key equipment on the oxygen pipe network and the source of accidents, so it should be selected with caution. The oxygen pipeline valve should be a special oxygen valve, and its specificity and characteristics are reflected in: the valve body in contact with oxygen should be copper alloy or nickel alloy with good combustion resistance and no spark during friction and iMPact, which is generally silicon brass (stainless steel, cast steel or nodular cast iron can be used for low oxygen pressure); the sealing letter shall be made of flame-retardant or flame-retardant materials, such as graphitized asbestos, polytetrafluoroethylene, expanded graphite, etc.; the valve shall be strictly degreased and tightly packed to prevent pollution, and marked with “special for oxygen” The valve shall undergo strict and standardized strength test and air tightness test to prevent leakage; large diameter oxygen valves are generally equipped with pressure equalizing small bypass valve to facilitate operation and ensure the safety of valve opening.

• (1) It is forbidden to use gate valve for oxygen pipeline with working pressure P > 0.1MPa. Because the gate chute is easy to accumulate rust and is not tightly closed, squeezing the chute rust when operating the valve is easy to form excitation energy. The air flow entrains rust, particle friction and iMPact the pipe wall, which is easy to cause combustion and explosion accidents. There are many lessons from this kind of accident. Moreover, the double flap gate valve is easy to drop thallium, which is very unsafe.
• (2) The manual oxygen valve with nominal pressure ≥ 1.0MPa and nominal diameter ≥ 150mm should choose the valve with bypass. Because this valve is already an oxygen valve with high pressure and large diameter. If there is no small pressure equalizing bypass valve, it is easy to open the main valve too violently. The large pressure difference causes the sudden rise of oxygen pressure at the downstream side of the valve outlet, the friction and temperature rise of high-speed oxygen flow, forming adiabatic compression and igniting the oxygen pipeline. If there is small pressure equalizing bypass valve, it is easy to open the main valve too violently For bypass valve, first slowly open the small valve to equalize the pressure, and then slowly increase the pressure at the downstream side of the outlet to reduce the pressure difference with the inlet side, so as to avoid excessive flow rate, high temperature rise and adiabatic compression, which will eventually lead to combustion and explosion accidents. When the pressure difference between the upstream and downstream sides of the main valve is ≤ 013MPa, then open the main valve to ensure safety. The higher the pressure of the oxygen valve, the larger the diameter and the more dangerous it is, the valve with small pressure equalizing bypass should be selected. This is Safety requirements, Europe, the United States and Japan have already formulated norms, which is also China’s successful experience.
• (3) The selection of oxygen valve materials (see Table.5) should consider not only strength and economy, but also safety. Table 5 is quoted from GB16912-2008 standard.

Table.5 requirements for valve material selection

Working pressure P/MPa p/ MPa	Material Science
p ≤0.6	The valve body and bonnet are made of malleable cast iron, nodular cast iron or cast steel; The valve stem is made of stainless steel; The valve disc is made of stainless steel.
0.6 < p ≤10	Stainless steel, copper alloy or combination of stainless steel and copper alloy (copper alloy is preferred), nickel and nickel alloy.
p > 10	Copper alloy, nickel and nickel alloy are used.

Note:

• (1) Material of pressure or flow regulating valve with working pressure above 0.1MPa
• The material shall be stainless steel or copper alloy or a combination of the above two.
• (2) The sealing packing of the valve shall be PTFE or flexible graphite
• Material.

Table. 5 divides the working pressure into three sections:

• ① P ≤ 0.6MPa, low oxygen pressure and low risk. Malleable cast iron, nodular cast iron or cast steel can be used for valve body and bonnet. Moving parts such as valve stem and disc have high requirements and are made of stainless steel.
• ② 0.6MPa < p ≤ 10MPa, high oxygen pressure, wide range and wide application range, involving all industries. High requirements. The valve material is stainless steel, copper alloy or a combination of stainless steel and copper alloy (copper alloy is preferred) , nickel and nickel alloy. Stainless steel does not rust, copper alloy, nickel and nickel alloy do not catch fire during oxygen friction and iMPact, with good flame retardancy and high safety. The higher the oxygen pressure, the more copper alloy or nickel and nickel alloy valves should be selected.
• ③ If P > 10MPa, it belongs to high-pressure oxygen and has great risk. The valve material can only be copper alloy, nickel and nickel alloy. For the pressure or flow regulating valve with working pressure P > 0.1MPa, because its throttling and expansion conditions belong to iMPact occasions and have great risk, the valve material should be stainless steel at least. When the oxygen pressure is high, it should be combined with stainless steel and copper alloy. If the oxygen pressure is high, it should be copper alloy valve.
• The sealing packing of the valve shall be made of flame-retardant or flame-retardant polytetrafluoroethylene or flexible graphite.

(4) The frequently operated oxygen valve with nominal pressure ≥ 1.0MPa and nominal diameter ≥ 150mm is already an oxygen valve with high pressure and large diameter. Pneumatic remote control valve should be used for remote operation to avoid injury in case of accident and reduce the labor intensity of operators.

Construction requirements for oxygen pipeline

The construction of oxygen pipeline shall not only strictly comply with relevant national standards for pressure pipeline construction, but also comply with its special safety regulations, such as strict rust removal, degreasing, welding, flaw detection, pressure and air tightness test, purging, etc. after construction, it shall also be strictly accepted. If these links are not strictly controlled, it will lead to oxygen pipeline combustion and explosion accidents, with huge losses and painful lessons.

Cleanliness requirements

Before installation, oxygen pipes, valves and pipe fittings shall be inspected in accordance with the requirements of code for construction and acceptance of industrial metal pipeline engineering (GB50235) (oxygen shall be treated as combustible fluid), and their cleanliness shall also meet the following requirements:
(1) Carbon steel oxygen pipes and pipe fittings shall be strictly derusted, and derusting can be carried out by sand blasting, acid pickling and other methods. Burr, weld beading, sticky sand, rust and other combustibles shall be completely removed from the surface in contact with oxygen to keep the inner wall smooth and clean. During pipe derusting, the original color of metal shall appear.
(2) All parts in contact with oxygen such as oxygen pipes and valves shall be strictly derusted and degreased before installation and after maintenance.
(3) The degreasing treatment of oxygen pipes, valves and other parts in contact with oxygen shall be carried out in accordance with the requirements of code for construction and acceptance of degreasing Engineering (HG20202) (including the surface of all components in contact with fluid). If there are different requirements in the engineering design documents, the provisions of the engineering design documents shall be followed. Degreasing can be carried out with inorganic non combustible cleaning agent, dichloroethane, trichloroethylene and other solvents, and shall be checked by ultraviolet inspection method, camphor inspection method or solvent analysis method until it is qualified.
The degreased carbon steel oxygen pipe shall be passivated immediately or filled with dry nitrogen to close the pipe orifice. For the pipeline subject to hydrostatic test, the inner wall of the pipe shall be passivated after degreasing to prevent corrosion. The pipeline components after degreasing shall be sealed after being purged with nitrogen or air to prevent re contamination, and the residual degreasing medium and oxygen shall be prevented from forming a dangerous mixture.
During and after installation, effective measures shall be taken to prevent the oxygen pipeline from being polluted by grease, and prevent combustibles, rust chips, welding slag, sand and other sundries from entering or leaving in the pipe, and strict inspection shall be carried out.

Requirements for pipeline installation, welding, construction and acceptance

The installation, welding, construction and acceptance of oxygen pipeline shall not only meet the requirements of GB50235 standard and code for construction and acceptance of field equipment and industrial pipeline welding engineering (GB50236), but also meet the following requirements:

• (1) When welding carbon steel and stainless steel oxygen pipes, argon arc welding shall be used for priming to facilitate penetration and prevent the formation of welding slag on the inner wall of the pipe.
• (2) Mechanical methods shall be adopted for pipe cutting and groove processing, and gas welding cutting and beveling are not allowed to ensure processing accuracy and facilitate welding.
• (3) The length of pipe prefabrication should not be too long. It should be easy to check the installation, welding, cleanliness and quality of the internal and external surfaces of the pipe.
• (4) Radiographic inspection shall be adopted for the weld inspection of pipeline. When hydrostatic test is adopted, the inspection quantity and standard shall be in accordance with relevant requirements (see Table 6). Table. 6 is quoted from GB16912-2008 standard.

Table.6 inspection requirements for oxygen pipeline welds

Design pressure P/MPa	Radiographic scale	Weld quality assessment
p > 4.0	100%	Class II
1.0 < p ≤4.0	40% (fixed welded junction); 15% (rotate the welded junction).	Class II
p ≤1.0	10%	Class II
Cryogenic liquid pipeline	100%	Class II

Note: the weld quality assessment standard is radiography of metal fusion welded joints (GB/t3323).

Table. 6 divides the design pressure P into three pressure ranges. The higher the pressure, the greater the risk, the higher the welding quality requirements, and the higher the weld quality Х The larger the proportion of radiographic testing, the higher the requirements for weld quality grade. When gas is used for pressure test, the risk is high and the weld is not safe
The proportion of radiographic inspection shall be increased. The specific requirements are as follows: when the design pressure is not greater than 0.6MPa, the inspection proportion shall not be less than 15%, and the weld quality grade shall not be lower than grade III; When the design pressure is greater than 0.6MPa and less than or equal to 4.0MPa, the detection proportion is 100%, and the weld quality grade is not lower than grade II.
(5) For welds that are not required to undergo NDT, the quality inspector shall conduct visual inspection on the visible parts of all welds, and the quality shall comply with the relevant provisions of GB50236 standard.

Pressure and air tightness test requirements

After the installation of the oxygen pipeline, the pressure and air tightness test shall be carried out to check the strength and tightness of the pipeline. The pressure and air tightness test requirements shall comply with the following provisions:

• (1) The pressure test medium of oxygen pipeline shall be clean water without oil or dry air and nitrogen. It is strictly prohibited to use oxygen as the test medium. When nitrogen is used as the test medium, attention shall be paid to safety to prevent suffocation accidents. When the design pressure of oxygen pipeline is greater than 4.0MPa, it is forbidden to use gas for pressure test. Once the air pressure test is exploded, the gas expands rapidly and has great power. The entrained pipe fragments fly everywhere, causing damage to nearby equipment, casualties and great harm. After the hydrostatic test of oxygen pipeline, it shall be dried in time to prevent corrosion. During hydrostatic test of austenitic stainless steel pipeline, the chloride ion content in water shall not exceed 25g/m3. Because chloride ion will cause stress corrosion of austenitic stainless steel. Otherwise, corresponding measures shall be taken. After the water is drained, dry it with oil-free dry nitrogen.
• (2) The pipeline test pressure shall be calculated based on the design pressure.
• (3) During the pressure test of the pipeline, the hydrostatic test pressure shall be equal to 115 times the design pressure, and the buried pipeline shall not be less than 014MPa; The pneumatic test pressure is equal to 1115 times the design pressure and not less than 0.1MPa. The test methods and requirements shall comply with the provisions of GB50235 standard. During the strength test, the pressure shall be stabilized for 10 minutes after reaching the test pressure, and then reduced to the design pressure. It is qualified if the pressure does not drop and there is no leakage.
• (4) After the oxygen pipeline pressure test is qualified, the air tightness test shall be carried out. The test medium shall be oil-free, dry and clean air or nitrogen, and the test pressure shall be equal to the design pressure of the pipeline. The air tightness test is stricter, more scientific and safer than the previous “ventilation leak detection” of oxygen pipeline system. If the leakage rate fails to meet the requirements, the leakage point must be found out to eliminate hidden dangers and ensure safety.

In addition to the air tightness test methods and requirements according to GB50235 standard, the leakage rate shall also be calculated. The gas pressure in the pipeline shall be maintained for 24 hours after reaching the design pressure, and the average hourly leakage rate a of indoor and trench pipelines shall not exceed 0125% (since oxygen leakage is not easy to spread indoors or trench, it is dangerous, so it is strictly required); For outdoor pipelines, it shall not exceed 015% (because oxygen leaks outdoors, with good ventilation, easy diffusion and low risk, the requirements are lower than before). The leakage rate a is calculated according to equations (1) and (2):
When the nominal diameter of pipeline DN ≤ 013m:

When pipe nominal diameter DN > 013m:

In the formula:

• P1 – absolute pressure at the beginning of the test, MPa;
• P2 – absolute pressure at the end of the test, MPa;
• T1 – temperature at the beginning of the test, ℃;
• T2 – temperature at the end of the test, ℃;
• DN – nominal diameter of pipeline, M.

Requirements before putting oxygen pipeline into use

Before the oxygen pipeline is put into use after installation, maintenance or long-term shutdown, the residual water, iron filings and sundries in the pipeline shall be purged with oil-free dry air or nitrogen until there is no rust, dust or other sundries. The purging speed shall not be less than 20m/s and not lower than the design flow rate of oxygen pipeline. The flow rate shall be large and the flow rate shall be high to ensure the purging effect. Check the nozzle with white cloth or target plate coated with white paint. It is qualified if there is no scrap iron, welding slag, rust, dust, particles, foreign matters and moisture, so as to ensure the safety after oxygen supply operation. Therefore, it is common to cause combustion and explosion accidents due to neglect or inadequate work, with huge losses and heavy lessons.
It is forbidden to purge the pipeline with oxygen to avoid combustion and explosion accidents. “Blasting and purging method” for oxygen pipeline: when it is difficult to obtain large purging gas source or interfere with production, blasting and purging can be carried out for oxygen pipeline. That is, set a blasting port at the purging port at one end of the pipeline, fix the blasting plate (single-layer, multi-layer rubber asbestos plate or cardboard, etc. according to the pressure level), fill the pipe with oil-free dry nitrogen or air, when the pressure rises to the blasting pressure of the blasting plate, the blasting plate will break, the high-pressure air flow will be blown out from the blasting port, the pressure in the pipeline will decrease instantaneously, and the pressure (gauge pressure) at the blasting port is close to 0. Due to the large pressure difference inside and outside the pipe (equal to the blasting pressure) and the rapid expansion of nitrogen or air, the purging gas produces a great flow rate, even close to the sound speed, and has great kinetic energy. It is used to purge the inner wall of the pipe to remove rust, welding slag, sundries and moisture.
Blasting purging method has the advantages of high purging quality, simplicity, safety and reliability, no iMPact on production, low cost and short construction period. At present, it has been widely used. In the implementation, full attention should be paid to the determination of blasting pressure, the selection and reinforcement of blasting opening and the safety warning around blasting opening. It should be noted that while blasting and purging, adding an appropriate amount of degreasing agent into the pipe can achieve the purpose of degreasing, with good effect and successful experience.

Pipeline paint color identification

Pipeline paint color identification is not only to distinguish and identify the medium in the pipeline, but also to meet the safety needs and avoid accidents caused by mischarging and misuse. It is a safety sign. According to the basic identification colors, identification symbols and safety signs of industrial pipelines (GB7231) and GB16912-2008 standard, the paint color of oxygen pipeline is light blue, and other common industrial pipeline paint colors are: light gray of air pipeline, bright red of steam pipeline, light yellow of nitrogen pipeline, brown of sewage nitrogen pipeline, red with white ring of hydrogen pipeline, silver gray of argon pipeline, brilliant green of water supply pipeline bright green plus black ring for sewer pipeline, yellow plus black ring for oil inlet pipeline, yellow plus black ring for oil outlet pipeline, red plus black ring for heating and thawing gas pipeline, red and hanging signs for fire water pipeline, etc.
In the past, the basic identification color of industrial pipelines was set very extensively, which failed to be strictly distinguished, which once led to accidents. If the oxygen pipeline is sky blue and the compressed air pipeline is dark blue, the paint color cannot be distinguished over time. During electric welding construction in summer, someone mistakenly opened the oxygen valve and blew compressed air to cool down, resulting in a major safety accident of burning one person and burning one person.

Safe use of oxygen pipeline

(1) The oxygen pipeline shall be inspected and maintained frequently. Carbon steel pipes should be derusted and painted once every 3-5 years to prevent corrosion. The carbon steel main pipe should be purged once every 5 years to prevent the increase and accumulation of rust and fire. The pipe wall thickness should be measured once every 5 years, mainly to measure the pipe behind the elbow and regulating valve, so as to ensure the safe thickness of these easily scoured and worn parts. Check the safety grounding device once a year.
(2) Safety devices such as safety valves and pressure gauges on oxygen pipelines shall be inspected regularly once a year, which shall be sensitive and accurate to prevent overpressure and oxygen leakage.
(3) The oxygen pipeline shall not be connected and used indiscriminately. It is strictly prohibited to blow air with oxygen, use oxygen to make a furnace, etc. Do not strike fire and arc on the running oxygen pipeline to avoid fire.
(4) Before the hot work of oxygen pipeline, a hot work plan shall be formulated, including the person in charge, operation flow chart, operation plan, safety measures, personnel division, supervisor and tester, etc. The scheme shall be approved by the competent safety department and the hot work permit procedures shall be handled. The fire can be carried out only when the oxygen content is less than 23%. Fire explosion and casualties caused by insufficient attention to hot work, imperfect scheme, unfulfilled responsibility, lax measures (especially isolation from oxygen source blind plate), untimely and inadequate laboratory supervision are common, which should be paid special attention.
(5) When major operations are carried out on the oxygen pipeline, a detailed operation scheme (including process, method, step, time, position, division of labor, scope, responsibility, monitoring and confirmation, etc.) must be formulated in advance and can be implemented only after being approved and implemented by the relevant competent department.
(6) The manual oxygen valve shall be opened slowly, and the operator shall stand at the side of the valve to prevent injury. When opening the oxygen valve with small bypass valve, first open the small bypass valve to slowly charge the low-pressure section behind the valve. When the pressure difference between the high and low-pressure sections at the upstream and downstream sides of the main valve is ≤ 013MPa, then open the main valve. If the oxygen valve is suddenly and quickly opened between the high and low pressure sections, the low pressure section will be rapidly oxygenated and compressed. Due to the fast speed, there is no time to dissipate heat, forming adiabatic compression, and the local temperature rises sharply, becoming the ignition energy.
Set the oxygen pressure before and after the sudden opening of the oxygen valve (i.e. the oxygen pressure before and after adiabatic compression) as P1 (0.1MPa) and P2 (113MPa) respectively, and the oxygen temperature before and after the sudden opening of the oxygen valve (i.e. the oxygen temperature before and after adiabatic compression) as T1 (30 ℃, i.e 303k) and T2, and the oxygen adiabatic index k is 1139. According to the adiabatic compression formula, the temperature T2 of the low-pressure section after adiabatic compression can be calculated as:

This temperature is sufficient to ignite iron powder, rust or organic packing materials in the pipeline. It is dangerous to open the oxygen valve quickly. The greater the pressure difference between the front and back of the valve, the more dangerous it is.
(7) The oxygen valve for regulating pressure or flow shall be high. Generally, copper alloy or stainless steel valve shall be used and operated by pneumatic remote control to avoid accidents and injury. Therefore, non regulating valves shall not be used for regulation.
(8) Since the fire of oxygen pipeline or valve spreads to the upstream side, the upstream air source shall be cut off immediately in case of accident to prevent further expansion of the accident.
(9) Ordinary grease is strictly prohibited for the lubrication of oxygen valve and valve stem threads, and qualified silicon oil, silicon grease or molybdenum disulfide without fire shall be used.
(10) When the oxygen pipeline is equipped with liquid oxygen vaporization supplementary facilities, do not allow low-temperature liquid oxygen to enter the normal temperature oxygen pipeline, so as to avoid freezing crack of ordinary carbon steel pipeline or overpressure malignant combustion and explosion accidents such as violent vaporization of liquid oxygen, violent expansion of volume (800 times that under normal pressure) and surge of pressure.
(11) The pipeline oxygen shall be transported by dry oxygen, and the water leakage of the cooler of the oxygen compressor shall be handled in time to prevent the inner wall of the oxygen pipeline from rusting and causing combustion and explosion accidents.
(12) The oxygen filter in the oxygen pipeline system shall be cleaned regularly to remove rust and foreign matters and eliminate combustion and explosion accidents. After the construction and maintenance of the oxygen pipeline system, the oxygen filter must be checked and cleaned before oxygen delivery, and the oxygen can be delivered only after confirming that there are no sundries inside, so as to prevent the accumulation of foreign matters such as rust and welding slag from becoming potential accidents and ensure the safety of oxygen delivery.
(13) The unit using the oxygen pipeline shall be responsible for the safety management of its own oxygen pipeline. Implement relevant safety laws, regulations and technical specifications and standards of oxygen pipeline. Establish and improve the oxygen pipeline safety management rules and regulations of the unit. Appoint full-time or part-time professional technicians to be responsible for the safety management of oxygen pipeline. Establish the technical archives of oxygen pipeline, formulate the regular inspection plan of oxygen pipeline, arrange the regular inspection and maintenance of auxiliary instruments and safety protection devices, check and rectify the hidden dangers of oxygen pipeline system, and implement professional management of oxygen pipeline. Oxygen pipeline operators and inspectors shall receive safety technical training, and take up their posts with certificates after passing the examination.
Author: madafang

Source: China Oxygen Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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