Analysis of hazard area associated with hydrogen gas transmission pipelines

Hydrogen is considered to be the most important future energy carrier in many applications reducing significantly greenhouse gas emissions, but the safety issues associated with hydrogen applications need to be investigated and fully understood to be applicable as the carrier. Generally, the locations of hydrogen production and consumption are different. Hydrogen must be transported from the point of production to the point of use. Pipeline delivery is cheaper than all other methods for large quantities of hydrogen. The rupture of a hydrogen pipeline can lead to outcomes that can pose a significant threat to people and property in the immediate vicinity of the failure point. In this work, a simplified equation of hazard analysis is proposed for the pipeline transporting hydrogen, which relates the diameter, the operating pressure and the length of the pipeline to the size of the affected area in the event of a failure of the pipeline. The dominant hazards are thermal radiation from sustained fire and shock pressure from gas cloud explosion. For a transmission pipeline of hydrogen gas, the hazard area from the fire is slightly larger than by the other event. The hazard area is directly proportional to the operating pressure raised to the power one-half, and to the pipeline diameter. This simplified equation to estimate the hazard area will be a useful tool for safety management of hydrogen gas transmission pipelines.     

Recommendations on X80 steel for the design of hydrogen gas transmission pipelines

By limiting the pipes thickness necessary to sustain high pressure, high-strength steels could prove economically relevant for transmitting large gas quantities in pipelines on long distance. Up to now, the existing hydrogen pipelines have used lower-strength steels to avoid any hydrogen embrittlement. The CATHY-GDF project, funded by the French National Agency for Research, explored the ability of an industrial X80 grade for the transmission of pressurized hydrogen gas in large diameter pipelines. This project has developed experimental facilities to test the material under hydrogen gas pressure. Indeed, tensile, toughness, crack propagation and disc rupture tests have been performed. From these results, the effect of hydrogen pressure on the size of some critical defects has been analyzed allowing proposing some recommendations on the design of X80 pipe for hydrogen transport. Cost of Hydrogen transport could be several times higher than natural gas one for a given energy amount. Moreover, building hydrogen pipeline using high grade steels could induce a 10 to 40% cost benefit instead of using low grade steels, despite their lower hydrogen susceptibility.     

Modelling approaches to acoustic cavitation in transmission pipelines

A general treatment of acoustic cavitation was presented, including both fluid dynamics instabilities that can occur at cavitation inception as well as non-equilibrium thermal and mechanical effects during bubble dynamics. Different approaches to cavitation modelling were considered and compared.A novel barotropic cavitation model has been developed, based on the partial differential equations governing the mass-conservation and momentum balance. The fluid has been taken as a homogenous mixture of a pure liquid, its vapor and a quantity of gas, both dissolved and undissolved. The analytical expression for the vapor source term driving cavitation has been carried out by means of the energy conservation equation and a general formula for the sound speed in homogeneous bubbly flows has been derived.A recently developed conservative, implicit, high-resolution, second-order accurate numerical scheme was applied to solve the equations governing the pipe flow. The resultant computational algorithm was assessed through comparison with experimental data referring to a system made up of a pipe connecting two constant-pressure reservoirs of water. The model predictions were examined and discussed in order to underline the most interesting fluid-dynamic phenomena, such as the dynamics of shock waves arising at cavitation collapse.The influence of the frequency-dependent friction on the simulation of the pressure wave dynamics in the presence of cavitation was also analyzed and discussed.     

Long-distance renewable hydrogen transmission via cables and pipelines

Intermittency is one of the main obstacles that inhibit the wide adoption of the renewable energy in the power sector. Small-scale fluctuations can be tackled by short-term energy storage system, whereas long-term or seasonal intermittencies rely on large-scale energy management solutions. Besides the supply and demand mismatch in temporal domain, renewable energy sources are usually far away from consumption points. To connect the energy sources to the demand cost-effectively, cable transmission is usually the default option, and considering the long distance, other emerging energy carriers such as hydrogen could be a feasible option. However, there is handful studies on the quantitative evaluation of the long-distance energy transmission cost. This paper investigated the economic feasibility of renewable energy transmission via routes of power cable and gas pipeline. In the direct power transmission case, renewable energy is transmitted via HVDC cable and then converted to hydrogen for convenient storage. The alternative case converts renewable energy into hydrogen at the source and transports the hydrogen in the gas pipeline to consumers. Existing data available from public domain are used for cost estimation. Results show that the improvements of capacity factor and transmission scale are the most cost-effective approach to make the renewable hydrogen economically viable. At 4000 km of transmission distance, renewable hydrogen LCOE of 7 US$/kg and 9 US$/kg are achievable for the corresponding optimum cases, respectively.     

Condensation heat transfer on tubes in actual flue gas

In order to improve boiler efficiency, latent heat recovery from flue gas is a very important concept. Condensation heat transfer on horizontal stainless‐steel tubes was investigated experimentally by using an actual flue gas from a natural gas boiler. The experiment was conducted at different air ratios of the flue gas and a wide range of tube wall temperatures. The condensation pattern was similar to a dropwise condensation near the dew point. By decreasing the wall temperature, the wall region covered with a thin liquid film increased. The heat and mass transfer behavior was well predicted with the analogy correlation at the high‐wall‐temperature region. At the low‐wall‐temperature region, the total heat transfer was higher than that predicted by the analogy correlation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 139–151, 2001     

Simulation of compressible flow in high pressure buried gas pipelines

The aim of this work is to analyze the gas flow in high pressure buried pipelines subjected to wall friction and heat transfer. The governing equations for one-dimensional compressible pipe flow are derived and solved numerically. The effects of friction, heat transfer from the wall and inlet temperature on various parameters such as pressure, temperature, Mach number and mass flow rate of the gas are investigated. The numerical scheme and numerical solution was confirmed by some previous numerical studies and available experimental data. The results show that the rate of heat transfer has not a considerable effect on inflow Mach number, but it can reduce the choking length in larger f_DL/D values. The temperature loss will also increase in this case, if smaller pressure drop is desired along the pipe. The results also indicate that for f_DL/D = 150, decreasing the rate of heat transfer from the pipe wall, indicated here by Biot number from 100 to 0.001, will cause an increase of about 7% in the rate of mass flow carried by the pipeline, while for f_DL/D = 50, the change in the rate of mass flow has not a considerable effect. Furthermore, the mass flow rate of choked flow could be increased if the gas flow is cooled before entrance to the pipe.     

Condensation from gas–vapour mixtures in small non-circular tubes

Careful measurements have been made during condensation of steam from steam–air mixtures flowing in a small, flattened, horizontal tube. The ranges of the relevant variables covered (inlet temperature, pressure, air mole fraction and mixture mass flow rate) were chosen to simulate those occurring in an exhaust heat-exchanger tube of a proposed fuel-cell engine. The experimental tube was cooled by water in laminar counter flow to simulate the external heat-transfer coefficient (air flowing over fins) in the application. The total heat-transfer rate was found from the mass flow rate and temperature rise of the coolant. The tube wall temperature was measured by thermocouples attached in grooves along its length. Special arrangements were made to ensure good mixing of the coolant (in laminar flow) prior to measuring the inlet and outlet temperatures. The condensate was separated using a cyclone at exit from the tube. A simple model was developed to predict local and total heat-transfer and condensation rates and local bulk vapour composition, temperature and pressure along the tube in terms of the inlet parameters and the wall temperature distribution. The measured heat-transfer and condensation rates for the tube were found to be in good agreement with the calculated values without having recourse to empirical adjustment.     

A new gas release model for a homogeneous liquid–gas mixture flow in pipelines

The gas release phenomenon, resulting from a rapid decompression in a homogeneous gas–liquid flow is expressed by multiplying the mixture density by a degassing coefficient G_r. The effect of this coefficient is calculated by using the classical conservation equations of fluid mechanics and diffusion laws. These equations are solved by an improved new two time step finite difference scheme. The method of characteristics is used at the boundaries. The theoretical results obtained are in good agreement with experimental data and confirm the gas release effect on the flow parameters.     

Condensation of vapor in the presence of non-condensable gas in condensers

This paper presents a set of differential and algebraic equations that model heat and mass transfer in condensers in which a mixture of water vapor and non-condensable gas is cooled. The model has been used to predict the condensation rate, the bulk temperatures of the coolant and the gas–vapor mixture, and the surface temperatures of the condenser wall. The predicted results for counter flow tube condensers are compared with three sets of published experimental data for system in which air is the non-condensable gas. It is found that the predicted condensation rates and coolant bulk temperatures agree very well with all the three sets of experimental data, the predicted wall temperatures agree reasonably well with the experimental results, and the agreement between the predictions and the experimental results on the bulk temperature of the air–vapor mixture is excellent for one set of the experimental data, reasonable for the second set of experimental data, but poor for the third set of experimental data. It is suggested that the poor agreement between the predicted and measured bulk temperatures of the mixture for the third set of experimental data arises from the experimental errors. The results from this study show that when modeling vapor condensation in the presence of a non-condensable gas, a simple model for the mixture channel alone may not be sufficient since neither the temperature nor the heat flux at the wall can be assumed to be constant. The results also show that the wall temperature in the coolant channel can be quite high, and careful modeling of the heat transfer in the coolant channel is needed in order to achieve good agreement between the model predictions and the experimental results.     

Minimum thickness for circumferential sleeve repair fillet welds in corroded gas pipelines

The minimum weldable pipe wall thickness for sleeve repair welds is numerically assessed in this work, as a function of pressure during the welding operations of a corroded gas pipeline, according to the approach by Battelle. The minimum weldable thickness is found to increase when the flow rate of the transported gas in the section being repaired increases. Integrity of the repairs is assessed, and alternative measures to momentarily increase the flow in the area of the repair are evaluated.     

Fracture propagation control in gas pipelines: A survey of relevant studies

This paper reviews the problem of crack propagation in gas pressurised pipelines. After summarising the fracture control philosophy for pipelines, the main terminology and basic concepts are defined.

A review of the brittle fracture problem summarises Battelle and British Gas studies relating fracture appearance and crack behaviour, and the background to the adoption of the DWTT fracture appearance criterion in specifications is detailed. The application of the strain energy theory to full-scale test results is discussed. A balance between strain energy stored in the pipe wall and fracture energy explains why brittle fractures arrest if the stress is below a critical level.

The problem of ductile fracture propagation is reviewed and the results of full-size tests carried out by British Gas to provide toughness levels for fracture arrest are presented. The nature of ductile fracture propagation is discussed with reference to analogies with fracture initiation. Steady state, constant velocity, fractures imply specific shapes of the G (energy available)-velocity characteristic curves. In particular, a falling G with increasing fracture velocity is necessary. Some published theories to account for this effect are summarised and it is concluded that considerations of inertia in the fractured pipe wall provide the required relationship. Laboratory toughness tests for assessing brittle and ductile fracture resistance in relation to pipeline fracture behaviour are discussed. The relevance of fracture mechanics tests, particularly R curve analysis, is highlighted.

The paper concludes with a list of areas of work which warrant further study.     

Remaining load carrying capacity of gas pipelines damaged by surface corrosion

The influence of surface corrosive defects on the remaining working ability of transit gas pipelines of big diameters is discussed. An analytical and a FEM-numerical approach to the problem are presented. Finally a comparison of some of the authors' experimental results (to verify the theoretical solutions) with ANSI/ASME Code recommendations is given.     

Full scale experimental analysis of stress states in sleeve repairs of gas pipelines

This study discusses the experimental determination of stress states in sleeve repairs of underground gas pipelines. Work was done to define the effects of the reduction of pressure during welding, the load and place of positioning clamps, the length of the repair sleeve, and the use of O'ring-based devices to prevent gas leakage. Tests were carried out in reinforcements, welded with internal pressure equal to 60, 80 and 100% of the service pressure. High stresses were generated in tests carried out with short sleeves and O'rings, and occurred once the sleeve was fully welded and the pipeline pressure re-established. Maximum stresses, up to 270 MPa, were generated after about 1 min following closing of venting valves, on tests with artificial gas leaks. From the results of these experimental studies, it is concluded that several operative aspects could be optimised, to minimise the stresses in the reinforcements and to reduce the risk of failures.     

Condensation of a vapor-noncondensable gas mixture within a horizontal air-cooled tube

The equations governing the coupled heat and mass transport mechanism are derived for the condensation of a pure vapor—non condensable gas mixture within a horizontal finned tube cooled by air in cross flow. Assuming that temperatures of both the gaseous and liquid phase vary linearly in a short length tube element, where the problem is posed, a couple of equations are obtained for the exit temperatures of the streams as a function of the inlet and the interphase ones. These equations can be handled iteratively as a sub-routine of a simulation program already implemented by Urbicain and Paloschi (1). The heat transfer mechanism is governed by an overall coefficient U defined between the main gaseous stream and the cooling air, calculated from the individual resistances which operate on two different sections of the condensing film. The procedure has been succesively tested on a water vapor-carbon dioxide finned air cooled condenser and represents a generalization of the step by step program mentioned above.     

A review of technical and regulatory limits for hydrogen blending in natural gas pipelines

There is rising interest globally in the use of hydrogen for the provision of electricity or heat to industry, transport, and other applications in low-carbon energy systems. While there is attention to build out dedicated hydrogen infrastructure in the long-term, blending hydrogen into the existing natural gas pipeline network is also thought to be a promising strategy for incorporating hydrogen in the near-term. However, hydrogen injection into the existing gas grid poses additional challenges and considerations related to the ability of current gas infrastructure to operate with blended hydrogen levels. This review paper focuses on analyzing the current understanding of how much hydrogen can be integrated into the gas grid from an operational perspective and identifies areas where more research is needed. The review discusses the technical limits in hydrogen blending for both transmission and distribution networks; facilities in both systems are analyzed with respect to critical operational parameters, such as decrease in energy density, increased flow speed and pressure losses. Safety related challenges such as, embrittlement, leakage and combustion are also discussed. The review also summarizes current regulatory limits to hydrogen blending in different countries, including ongoing or proposed pilot hydrogen blending projects.     

Welding of multilayer pipes in the manufacture and construction of high pressure gas pipelines

The technology of welding large diameter multilayer pipes in shop production is described. Circumferential and lap welding of such pipes to eliminate pore and other defect formations are considered in detail, with data on welding processes, conditions and materials. Experience in the construction of multilayer high pressure gas pipeline sections is described.     

Capital structure in LNG infrastructures and gas pipelines projects: Empirical evidences and methodological issues

This paper provides new empirical insights on the capital structure of project-financed LNG infrastructures and gas pipeline projects, by using data relating to projects whose financial close occurred between June 2004 and March 2011. Most results are consistent with the basic view of risk-averse funds suppliers. Especially, the projects located in risky countries and larger projects tend to exhibit lower debt ratios and less-concentrated equity ownerships. In addition, regasification projects appear to have a more diluted equity ownership. Methodological issues raised by the financing of these projects are also examined from a capital-budgeting perspective. In particular, the equity residual method, usually used by industrial practitioners to value these projects, should be adjusted.     

Evaluation of the safe separation distances of hydrogen-blended natural gas pipelines in a jet fire scenario

The desire for sustainable development in various countries has increased the use of hydrogen energy. Considering cost and time savings, the introduction of hydrogen into existing natural gas pipelines is an excellent option, and the failure consequences of hydrogen blending in natural gas pipelines should be considered. In this study, a solid flame model is used to calculate the thermal radiation intensity of a hydrogen-blended natural gas jet fire. A method is proposed to modify the calculation of the view factor in the near field, and parameters such as the specific heat capacity and calorific value of pure gas are replaced by the parameters of the mixed gas. The data of the Thornton and modified models are compared with the experimental results, and the modified model result is found to be more accurate. Using the modified model, the variations in different hydrogen blending ratios, internal pressures, and pipe diameters with the safe separation distance of the thermal radiation intensity in a pipeline accident are investigated, and the relationships between them are analyzed.     

长距离输气管线压气站负荷优化调度

介绍了建立优化调度所需的燃气轮机、离心压缩机和输气管线模型的方法。提出了对于长距离输气管线增压站负荷调度的分级优化方法：首先根据输气量和“最高压力原则”确定全管线压力的最佳分配，使运行的增压站数目最少；然后根据各站的压比优化分配各增压站内压缩机组间的负荷，使总燃料消耗量最低。计算结果证明分级优化方法是有效的。