This overview provides recommendations for the safe design of piping systems for hydrogen distribution.
Hydrogen is a highly volatile liquid with a high tendency to leak. It is a very dangerous and deadly combination of tendencies, a volatile liquid that is difficult to control. These are trends to consider when choosing materials, gaskets and seals, as well as the design characteristics of such systems. These topics about the distribution of gaseous H2 are the focus of this discussion, not the production of H2, liquid H2, or liquid H2 (see right sidebar).
Here are a few key points to help you understand the mixture of hydrogen and H2-air. Hydrogen burns in two ways: deflagration and explosion.
deflagration. Deflagration is a common combustion mode in which flames travel through the mixture at subsonic speeds. This occurs, for example, when a free cloud of hydrogen-air mixture is ignited by a small ignition source. In this case, the flame will move at a speed of ten to several hundred feet per second. The rapid expansion of hot gas creates pressure waves whose strength is proportional to the size of the cloud. In some cases, the force of the shock wave can be enough to damage building structures and other objects in its path and cause injury.
explode. When it exploded, flames and shock waves traveled through the mixture at supersonic speeds. The pressure ratio in a detonation wave is much greater than in a detonation. Due to the increased force, the explosion is more dangerous for people, buildings and nearby objects. Normal deflagration causes an explosion when ignited in a confined space. In such a narrow area, ignition can be caused by the least amount of energy. But for the detonation of a hydrogen-air mixture in an unlimited space, a more powerful ignition source is required.
The pressure ratio across the detonation wave in a hydrogen-air mixture is about 20. At atmospheric pressure, a ratio of 20 is 300 psi. When this pressure wave collides with a stationary object, the pressure ratio increases to 40-60. This is due to the reflection of a pressure wave from a stationary obstacle.
Tendency to leak. Due to its low viscosity and low molecular weight, H2 gas has a high tendency to leak and even permeate or penetrate various materials.
Hydrogen is 8 times lighter than natural gas, 14 times lighter than air, 22 times lighter than propane and 57 times lighter than gasoline vapor. This means that when installed outdoors, the H2 gas will quickly rise and dissipate, reducing any signs of even leaks. But it can be a double-edged sword. An explosion may occur if welding is to be performed on an outdoor installation above or downwind of an H2 leak without a leak detection study prior to welding. In an enclosed space, H2 gas can rise and accumulate from the ceiling down, a condition that allows it to build up to large volumes before being more likely to come into contact with ignition sources near the ground.
Accidental fire. Self-ignition is a phenomenon in which a mixture of gases or vapors ignites spontaneously without an external source of ignition. It is also known as “spontaneous combustion” or “spontaneous combustion”. Self-ignition depends on temperature, not pressure.
The autoignition temperature is the minimum temperature at which a fuel will spontaneously ignite prior to ignition in the absence of an external source of ignition upon contact with air or an oxidizing agent. The autoignition temperature of a single powder is the temperature at which it spontaneously ignites in the absence of an oxidizing agent. The self-ignition temperature of gaseous H2 in air is 585°C.
The ignition energy is the energy required to initiate the propagation of a flame through a combustible mixture. Minimum ignition energy is the minimum energy required to ignite a particular combustible mixture at a particular temperature and pressure. Minimum spark ignition energy for gaseous H2 in 1 atm of air = 1.9 × 10–8 BTU (0.02 mJ).
Explosive limits are the maximum and minimum concentrations of vapors, mists or dusts in air or oxygen at which an explosion occurs. The size and geometry of the environment, as well as the concentration of the fuel, controls the limits. “Explosion limit” is sometimes used as a synonym for “explosion limit”.
The explosive limits for H2 mixtures in air are 18.3 vol.% (lower limit) and 59 vol.% (upper limit).
When designing piping systems (Figure 1), the first step is to determine the building materials needed for each type of fluid. And each fluid will be classified in accordance with ASME B31.3 paragraph. 300(b)(1) states, “The owner is also responsible for determining class D, M, high pressure, and high purity piping, and determining whether a particular quality system should be used.”
Fluid categorization defines the degree of testing and the type of testing required, as well as many other requirements based on the fluid category. The responsibility of the owner for this usually falls to the engineering department of the owner or an outsourced engineer.
While the B31.3 Process Piping Code does not tell the owner which material to use for a particular fluid, it does provide guidance on strength, thickness, and material connection requirements. There are also two statements in the introduction to the code that clearly state:
And expand on the first paragraph above, paragraph B31.3. 300(b)(1) also states: “The owner of a pipeline installation is solely responsible for complying with this Code and for establishing the design, construction, inspection, inspection, and testing requirements governing all fluid handling or process of which the pipeline is a part. Installation.” So, after laying down some ground rules for liability and requirements for defining fluid service categories, let’s see where hydrogen gas fits in.
Because hydrogen gas acts as a volatile liquid with leaks, hydrogen gas can be considered a normal liquid or a Class M liquid under category B31.3 for liquid service. As stated above, the classification of fluid handling is an owner requirement, provided it meets the guidelines for the selected categories described in B31.3, paragraph 3. 300.2 Definitions in the section “Hydraulic services”. The following are definitions for normal fluid service and Class M fluid service:
“Normal Fluid Service: Fluid service applicable to most piping subject to this code, i.e. not subject to regulations for classes D, M, high temperature, high pressure, or high fluid cleanliness.
(1) The toxicity of the fluid is so great that a single exposure to a very small amount of the fluid caused by a leak can cause serious permanent injury to those who inhale or come into contact with it, even if immediate recovery measures are taken. taken
(2) After considering pipeline design, experience, operating conditions, and location, the owner determines that the requirements for normal use of the fluid are not sufficient to provide the tightness necessary to protect personnel from exposure. ”
In the above definition of M, hydrogen gas does not meet the criteria of paragraph (1) because it is not considered a toxic liquid. However, by applying subsection (2), the Code permits the classification of hydraulic systems in class M after due consideration of “…piping design, experience, operating conditions and location…” The owner permits the determination of normal fluid handling. The requirements are insufficient to meet the need for a higher level of integrity in the design, construction, inspection, inspection and testing of hydrogen gas piping systems.
Please refer to Table 1 before discussing High Temperature Hydrogen Corrosion (HTHA). Codes, standards, and regulations are listed in this table, which includes six documents on the topic of hydrogen embrittlement (HE), a common corrosion anomaly that includes HTHA. OH can occur at low and high temperatures. Considered a form of corrosion, it can be initiated in several ways and also affect a wide range of materials.
HE has various forms, which can be divided into hydrogen cracking (HAC), hydrogen stress cracking (HSC), stress corrosion cracking (SCC), hydrogen corrosion cracking (HACC), hydrogen bubbling (HB), hydrogen cracking (HIC). )), stress oriented hydrogen cracking (SOHIC), progressive cracking (SWC), sulfide stress cracking (SSC), soft zone cracking (SZC), and high temperature hydrogen corrosion (HTHA).
In its simplest form, hydrogen embrittlement is a mechanism for the destruction of metal grain boundaries, resulting in reduced ductility due to the penetration of atomic hydrogen. The ways in which this occurs are varied and are partly defined by their respective names, such as HTHA, where simultaneous high temperature and high pressure hydrogen is needed for embrittlement, and SSC, where atomic hydrogen is produced as closed-gases and hydrogen. due to acid corrosion, they seep into metal cases, which can lead to brittleness. But the overall result is the same as for all cases of hydrogen embrittlement described above, where the strength of the metal is reduced by embrittlement below its allowable stress range, which in turn sets the stage for a potentially catastrophic event given the volatility of the liquid.
In addition to wall thickness and mechanical joint performance, there are two main factors to consider when selecting materials for H2 gas service: 1. Exposure to high temperature hydrogen (HTHA) and 2. Serious concerns about potential leakage. Both topics are currently under discussion.
Unlike molecular hydrogen, atomic hydrogen can expand, exposing the hydrogen to high temperatures and pressures, creating the basis for potential HTHA. Under these conditions, atomic hydrogen is able to diffuse into carbon steel piping materials or equipment, where it reacts with carbon in metallic solution to form methane gas at grain boundaries. Unable to escape, the gas expands, creating cracks and crevices in the walls of pipes or vessels – this is HTGA. You can clearly see the HTHA results in Figure 2 where cracks and cracks are evident in the 8″ wall. The portion of nominal size (NPS) pipe that fails under these conditions.
Carbon steel can be used for hydrogen service when the operating temperature is maintained below 500°F. As mentioned above, HTHA occurs when hydrogen gas is held at high partial pressure and high temperature. Carbon steel is not recommended when the hydrogen partial pressure is expected to be around 3000 psi and the temperature is above about 450°F (which is the accident condition in Figure 2).
As can be seen from the modified Nelson plot in Figure 3, partly taken from API 941, high temperature has the greatest effect on hydrogen forcing. Hydrogen gas partial pressure can exceed 1000 psi when used with carbon steels operating at temperatures up to 500°F.
Figure 3. This modified Nelson chart (adapted from API 941) can be used to select suitable materials for hydrogen service at various temperatures.
On fig. 3 shows the choice of steels that are guaranteed to avoid hydrogen attack, depending on the operating temperature and partial pressure of hydrogen. Austenitic stainless steels are insensitive to HTHA and are satisfactory materials at all temperatures and pressures.
Austenitic 316/316L stainless steel is the most practical material for hydrogen applications and has a proven track record. While post-weld heat treatment (PWHT) is recommended for carbon steels to calcinate residual hydrogen during welding and reduce heat affected zone (HAZ) hardness after welding, it is not required for austenitic stainless steels.
Thermothermal effects caused by heat treatment and welding have little effect on the mechanical properties of austenitic stainless steels. However, cold working can improve the mechanical properties of austenitic stainless steels, such as strength and hardness. When bending and forming pipes from austenitic stainless steel, their mechanical properties change, including the decrease in the plasticity of the material.
If austenitic stainless steel requires cold forming, solution annealing (heating to approximately 1045°C followed by quenching or rapid cooling) will restore the mechanical properties of the material to their original values. It will also eliminate the alloy segregation, sensitization and sigma phase achieved after cold working. When performing solution annealing, be aware that rapid cooling can put residual stress back into the material if not handled properly.
Refer to tables GR-2.1.1-1 Piping and Tubing Assembly Material Specification Index and GR-2.1.1-2 Piping Material Specification Index in ASME B31 for acceptable material selections for H2 service. pipes are a good place to start.
With a standard atomic weight of 1.008 atomic mass units (a.m.u.), hydrogen is the lightest and smallest element on the periodic table, and therefore has a high propensity to leak, with potentially devastating consequences, I might add. Therefore, the gas pipeline system must be designed in such a way as to limit mechanical type connections and improve those connections that are really needed.
When limiting potential leak points, the system should be fully welded, except for flanged connections on equipment, piping elements and fittings. Threaded connections should be avoided as far as possible, if not completely. If threaded connections cannot be avoided for any reason, it is recommended to fully engage them without thread sealant and then seal the weld. When using carbon steel pipe, the pipe joints must be butt welded and post weld heat treated (PWHT). After welding, pipes in the heat-affected zone (HAZ) are exposed to hydrogen attack even at ambient temperature. While hydrogen attack occurs primarily at high temperatures, the PWHT stage will completely reduce, if not eliminate, this possibility even under ambient conditions.
The weak point of the all-welded system is the flange connection. To ensure a high degree of tightness in flange connections, Kammprofile gaskets (fig. 4) or another form of gaskets should be used. Made in almost the same way by several manufacturers, this pad is very forgiving. It consists of toothed all-metal rings sandwiched between soft, deformable sealing materials. The teeth concentrate the load of the bolt in a smaller area to provide a tight fit with less stress. It is designed in such a way that it can compensate for uneven flange surfaces as well as fluctuating operating conditions.
Figure 4. Kammprofile gaskets have a metal core bonded on both sides with a soft filler.
Another important factor in the integrity of the system is the valve. Leaks around the stem seal and body flanges are a real problem. To prevent this, it is recommended to select a valve with a bellows seal.
Use 1 inch. School 80 carbon steel pipe, in our example below, given manufacturing tolerances, corrosion and mechanical tolerances in accordance with ASTM A106 Gr B, the maximum allowable working pressure (MAWP) can be calculated in two steps at temperatures up to 300°F (Note : The reason for “…for temperatures up to 300ºF…” is because the allowable stress (S) of ASTM A106 Gr B material starts to deteriorate when the temperature exceeds 300ºF.(S), so Equation (1) requires Adjust to temperatures above 300ºF.)
Referring to formula (1), the first step is to calculate the pipeline theoretical burst pressure.
T = pipe wall thickness minus mechanical, corrosion and manufacturing tolerances, in inches.
The second part of the process is to calculate the maximum allowable working pressure Pa of the pipeline by applying the safety factor S f to the result P according to equation (2):
Thus, when using 1″ school 80 material, the burst pressure is calculated as follows:
A safety Sf of 4 is then applied in accordance with the ASME Pressure Vessel Recommendations Section VIII-1 2019, Paragraph 8. UG-101 calculated as follows:
The resulting MAWP value is 810 psi. inch refers to pipe only. The flange connection or component with the lowest rating in the system will be the determining factor in determining the allowable pressure in the system.
Per ASME B16.5, the maximum allowable working pressure for 150 carbon steel flange fittings is 285 psi. inch at -20°F to 100°F. Class 300 has a maximum allowable working pressure of 740 psi. This will be the pressure limit factor of the system according to the material specification example below. Also, only in hydrostatic tests, these values ​​can exceed 1.5 times.
As an example of a basic carbon steel material specification, an H2 gas service line specification operating at an ambient temperature below a design pressure of 740 psi. inch, may contain the material requirements shown in Table 2. The following are types that may require attention to be included in the specification:
Apart from the piping itself, there are many elements that make up the piping system such as fittings, valves, line equipment, etc. While many of these elements will be put together in a pipeline to discuss them in detail, this will require more pages than can be accommodated. This article.