Decompression Wave Speed Research Articles

Measurements of Decompression Wave Speed in Pure Carbon Dioxide and Comparison With Predictions by Equation of State

Carbon dioxide capture and storage (CCS) is one of the technologies that have been proposed to reduce emissions of carbon dioxide (CO2) to the atmosphere. CCS will require the transportation of the CO2 from the “capture” locations to the “storage” locations via large-scale pipeline projects. One of the key requirements for the design and operation of pipelines in all jurisdictions is fracture control. Supercritical CO2 is a particularly challenging fluid from this point of view, because its thermodynamic characteristics are such that a very high driving force for fracture can be sustained for a long time. Even though CO2 is not flammable, it is an asphyxiating gas that is denser than air, and can collect in low-lying areas. Additionally, it is well known that any pipeline rupture, regardless of the nature of the fluid it is transporting, has a damaging reputational, commercial, logistic, and end user impact. Therefore, it is as important to control fracture in a CO2 pipeline as in one transporting a flammable fluid. With materials specified appropriately for the prevention of brittle failure, the key element is the control of propagating ductile (or tearing) fracture. The determination of the required toughness for the arrest of ductile fracture requires knowledge of the decompression behavior of the contained fluid, which in turn requires accurate knowledge of its thermodynamic characteristics along the decompression isentrope. While thermodynamic models based on appropriate EOS (equations of state) are available that will, in principle, allow determination of the decompression wave speed, they, in general, have not been fully validated for very rapid transients following a rupture. This paper presents experimental results of the decompression wave speed obtained from shock tube tests conducted on pure CO2 from different initial conditions, and comparison with predictions by models based on GERG-2008, Peng-Robinson, and BWRS equations of state (EOS). These tests were conducted as a baseline before introducing various impurities.

Decompression wave speed in CO2 mixtures: CFD modelling with the GERG-2008 equation of state

The development of CO2 pipelines for Carbon Capture and Storage (CCS) raises new questions regarding the control of ductile fracture propagation and fracture arrest toughness criteria. The decompression behaviour in the fluid must be determined accurately in order to estimate the proper pipe toughness. However, anthropogenic CO2 may contain impurities that can modify the fluid decompression characteristics quite significantly. To determine the decompression wave speed in CO2 mixtures, the thermodynamic properties of these mixtures must be determined by using an accurate equation of state. In this paper we present a new decompression model developed using the Computational Fluid Dynamics (CFD) package ANSYS Fluent. The GERG-2008 Equation of State (EOS) was implemented into this model through User Defined Functions (UDF) to predict the thermodynamic properties of CO2 mixtures. The model predictions were in good agreement with the experimental data of two ‘shock tube’ tests. A range of representative CO2 mixtures was examined in terms of the changes in fluid properties from the initial conditions, with time and distance, immediately after a sudden pipeline opening at one end. Phase changes that may occur within the fluid due to condensation of ‘impurities’ in the fluid were also investigated. Simulations were also conducted to examine how the initial temperature and impurities would affect the decompression wave speed.

Experimental validation of GASDECOM for High Heating Value Processed Gas mixtures (58 MJ/m3) by specialized shock tube

One of the fundamental requirements of the design of pipelines is the control of propagating ductile fracture, in which the Battelle two-curve method still forms the basis of the analytical framework used throughout the industry. The GASDECOM (GAS DECOMpression) tool is typically used for calculating decompression wave speed, which is one of these two curves. It uses the BWRS (Benedict–Webb–Rubin–Starling) equation of state to idealize the decompression process as isentropic and one-dimensional. While this equation of state was developed and validated against a quite restricted range of gas compositions, GASDECOM continues to perform relatively well for compositions slightly outside the original range of BWRS. The present research was focused on examining the performance of GASDECOM for mixture compositions up to a High (gross) Heating Value (HHV) of 58 MJ/m3. Four tests were conducted using a specialized high pressure shock tube (42 m long, I.D. = 38.1 mm) to experimentally determine the decompression wave speeds and compare them to the predictions by GASDECOM. Two tests were conducted on a gas mixture of HHV = 52 MJ/m3 and the other two on even richer gas mixture of HHV = 58 MJ/m3, all were from nominal initial pressures of 15 MPa and initial temperatures of 40 °C. The results from these tests show that decompression wave speeds are consistent with predictions of GASDECOM for gases of HHV typical of the previously validated range of BWRS. Predictions of the saturation pressure represented by the plateau pressure in the decompression wave speed curve were also in good agreement with measurements despite the fact that they occurred close to the critical point of the respective mixture compositions.

Extension of the semi-empirical correlation for the effects of pipe diameter and internal surface roughness on the decompression wave speed to include High Heating Value Processed Gas mixtures

The decompression wave speed, which is used throughout the pipeline industry in connection with the Battelle two-curve method for the control of propagating ductile fracture, is typically calculated using GASDECOM (GASDECOMpression). GASDECOM, developed in the 1970's, idealizes the decompression process as isentropic and one-dimensional, taking no account of pipe wall frictional effects or pipe diameter. Previous shock tube tests showed that decompression wave speeds in smaller diameter and rough pipes are consistently slower than those predicted by GASDECOM for the same conditions of mixture composition and initial pressure and temperature. Previous analysis based on perturbation theory and the fundamental momentum equation revealed a correction term to be subtracted from the ‘idealized’ value of the decompression speed calculated by GASDECOM. One parameter in this correction term involves a dynamic spatial pressure gradient of the outflow at the rupture location. While this is difficult to obtain without a shock tube or actual rupture test, data from 14 shock tube tests, as well as from 14 full scale burst tests involving a variety of gas mixture compositions, were analyzed to correlate the variation of this pressure gradient with two characteristics of the gas mixture, namely; the molecular weight and the higher heating value (HHV). For lean to moderately-rich gas mixes, the developed semi-empirical correlation was found to fit very well the experimentally determined decompression wave speed curve. For extremely rich gas mixes, such as High Heating Value Processed Gas (HHVPG) mixtures of HHV up to 58 MJ/m3, it was found that it overestimates the correction term. Therefore, additional shock tube tests were conducted on (HHVPG) mixes, and the previously developed semi-empirical correlation was extended (revised) to account for such extremity in the richness of the gas mixtures. The newly developed semi-empirical correlation covers a wider range of natural gas mixtures from as lean as pure methane up to HHVPG mixtures of HHV = 58 MJ/m3.

Numerical analysis of phase behavior during rapid decompression of rich natural gases

The effect of the condensation process on the gas and liquid phase behavior during rapid decompression of rich natural gases is studied in the paper numerically. A one-dimensional mathematical model of transient thermal two-phase flow of compressible multi-component natural gas mixture and liquid phase in a shock tube is developed. The set of mass, momentum and enthalpy conservation equations are solved for the gas and liquid phases. The approach to model a liquid condensation process during rapid decompression of rich natural gas mixture is proposed. The mass transfer between the gas and the liquid is taken into account by introducing the appropriate terms into the governing equations. Thermo-physical properties of multi-component natural gas mixture are calculated by solving the Equation of State (EOS) in the form of the Soave–Redlich–Kwong (SRK-EOS) model. The proposed liquid condensation model is integrated into the GDP model. A simple case of GDP model, where the liquid was not considered, was extensively validated on base and dry natural gases. The proposed two-phase model is validated against the experiments where the decompression wave speed was measured in rich natural gases at low temperature. It shows a good agreement with the experimental data.

A one-dimensional mathematical model of multi-component fluid flow in pipes and its application to rapid decompression in dry natural gas mixtures

The paper presents a one-dimensional transient mathematical model of compressible thermal multi-component gas mixture flow in a shock tube. The set of mass, momentum and enthalpy conservation equations is solved for the gas phase. Thermo-physical properties of multi-component natural gas mixture are calculated by solving the Equation of State (EOS) in the form of the Soave-Redlich-Kwong (SRK-EOS) model. The proposed mathematical model is validated against the experiments where the decompression wave speed in dry natural gases was measured at low temperatures and shows a good agreement with the experimental data at high and low initial pressure. The effect of the initial temperature on rapid decompression process is investigated numerically using the proposed model. Numerical results show that the proposed model simulates the decompression in natural gases much better and accurate than other models, and shows a great potential because it can be extended on the case of gas–liquid two-phase flow in a shock tube.

A study of the effects of friction, heat transfer, and stream impurities on the decompression behavior in CO2 pipelines

AbstractA transient multi‐phase outflow model is employed to study the effects of heat transfer and friction on the decompression behavior in CO2 pipelines. The model's predictions are compared to measurements obtained from a number of shock tube experiments for gaseous phase CO2 as well its various mixtures, typical of those found in the different capture technologies. Particular attention is paid to studying the impact of the stream impurities on the CO2 mixtures saturation pressures and the decompression wave speeds given their direct influence on the pipeline's propensity to running ductile fractures. © 2012 Society of Chemical Industry and John Wiley & Sons, Ltd

Mathematical modelling on rapid decompression in base natural gas mixtures under rupturing

The paper presents a one-dimensional mathematical model of transient compressible thermal multi-component gas mixture flows in pipes. The set of the mass, momentum and enthalpy conservation equations is solved for gas phase in the model. Thermo-physical properties of multi-component gas mixture are calculated by solving the Equation of State (EOS) model. The Soave–Redlich–Kwong (SRK–EOS) model is chosen. Gas mixture viscosity is calculated on the basis of the Lee–Gonzales–Eakin (LGE) correlation. Numerical analysis on rapid decompression process in a shock tube having base natural gases is performed by using the proposed mathematical model. The model is successfully validated on the experimental measurements of the decompression wave speed in base natural gas mixtures. The proposed mathematical model shows a very good agreement with the experiments in a wide range of pressure values and predicts the decompression in base natural gases much better than analytical and mathematical models, which are available from the open source literature.

Transferability of decompression wave speed measured by a small-diameter shock tube to full size pipelines and implications for determining required fracture propagation resistance

The control of propagating ductile (or tearing) fracture is a fundamental requirement in the fracture control design of pipelines. The Battelle two-curve method developed in the early 1970s still forms the basis of the analytical framework used throughout the industry. GASDECOM is typically used for calculating decompression speed, and idealizes the decompression process as isentropic and one-dimensional, taking no account of frictional effects. While this approximation appears not to have been a major issue for large-diameter pipes and for moderate pressures (up to 12 MPa), there have been several recent full-scale burst tests at higher pressures and smaller diameters for which the measured decompression velocity has deviated progressively from the predicted values, in general towards lower velocities. The present research was focused on determining whether pipe diameter was a major factor that could limit the applicability of frictionless models such as GASDECOM. Since potential diameter effects are primarily related to wall friction, which in turn is related to the ratio of surface roughness-to-diameter, an experimental approach was developed based on keeping the diameter constant, at a sufficiently small value to allow for an economical experimental arrangement, and varying the internal roughness. A series of tests covering a range of nominal initial pressures from 10 to 21 MPa, and involving a very lean gas and three progressively richer compositions, were conducted using two specialized high-pressure shock tubes (42 m long, I.D. = 38.1 mm). The first is honed to an extremely smooth surface finish, in order to minimize frictional effects and better simulate the behaviour of larger-diameter pipelines, while the second has a higher internal surface roughness. The results show that decompression wave speeds in the rough tube are consistently slower than those in the smooth tube under the same conditions of mixture composition and initial pressure & temperature. Preliminary analysis based on perturbation theory and the fundamental momentum equation indicates that the primary reason for the slower decompression wave speed in the rough tube is the higher spatial gradient of pressure pertaining to the decompression wave dynamics, particularly at lower pressure ratios and higher gas velocities. The magnitude of the effect of the slower decompression speed on arrest toughness was then evaluated by a comparison involving several hypothetical pipeline designs, and was found to be potentially significant for pipe sizes DN 450 and smaller.

Measurement of Decompression Wave Speed in Rich Gas Mixtures at High Pressures (370 bars) Using a Specialized Rupture Tube

Measurements of decompression wave speed in conventional and rich natural gas mixtures following rupture of a high-pressure pipe have been conducted. A high-pressure stainless steel rupture tube (internal diameter=38.1 mm and 42 m long) has been constructed and instrumented with 16 high frequency-response pressure transducers mounted very close to the rupture end and along the length of the tube to capture the pressure-time traces of the decompression wave. Tests were conducted for initial pressures of 33–37 MPa-a and a temperature range of 21–68°C. The experimentally determined decompression wave speeds were compared with both GASDECOM and PIPEDECOM predictions with and without nonequilibrium condensation delays at phase crossing. The interception points of the isentropes representing the decompression process with the corresponding phase envelope of each mixture were correlated with the respective plateaus observed in the decompression wave speed profiles. Additionally, speeds of sound in the undisturbed gas mixtures at the initial pressures and temperatures were compared with predictions by five equations of state, namely, BWRS, AGA-8, Peng–Robinson, Soave–Redlich–Kwong, and Groupe Européen de Recherches Gaziéres. The measured gas decompression curves were used to predict the fracture arrest toughness needed to assure fracture control in natural gas pipelines. The rupture tube test results have shown that the Charpy fracture arrest values predicted using GASEDCOM are within +7% (conservative) and −11% (nonconservative) of the rupture tube predicted values. Similarly, PIPEDECOM with no temperature delay provides fracture arrest values that are within +13% and −20% of the rupture tube predicted values, while PIPEDECOM with a 1°C temperature delay provides fracture arrest values that are within 0% and −20% of the rupture tube predicted values. Ideally, it would be better if the predicted values by the equations of state were above the rupture tube predicted values to make the predictions conservative but that was not always the case.

Measurements of flow parameters and decompression wave speed following rupture of rich gas pipelines, and comparison with GASDECOM

Measurements of flow parameters and decompression wave speeds in a conventional gas mixture and two other rich gas mixtures following rupture of a high-pressure pipe have been conducted. A test rig, consisting of a stainless steel expansion tube (internal diameter ∼50 mm), has been used to give a length to diameter ratio of 3440 representing a long section of a pipeline. Pressure– and temperature–time traces obtained with high-frequency response transducers have been collected at various locations close to rupture and along the length of the tube. Tests were conducted at three pressure levels: 10, 14 and 20 MPa and at temperatures as low as −25 °C. Results indicate good agreement between measured and predicted decompression wave speed by GASDECOM. Additionally, gas temperature–time traces were also measured at different locations along the expansion tube. Since the temperature probes are not as fast in their response, a technique has been developed to reconcile the actual gas temperature from the measured data. The paper presents example results for lean, medium and rich gas mixtures at different initial conditions.

Determination of Decompression Wave Speed in Rich Gas Mixtures

AbstractExperimental data on the decompression wave speed of a conventional gas mixture and three other rich gas mixtures with varying C1 from 70 to 82% have been collected. An NPS 2 stainless steel expansion tube test rig has been constructed at TCPL Gas Dynamic Test Facility (GDTF) in Didsbury, AB, Canada, specifically for this task. Decompression wave was generated by a rupture disc at the end of the tube, which was instrumented with eight high frequency response pressure transducers located along the tube. The facility has the following three distinct capabilities: high initial pressures of up to 22 MPa, low initial temperature down to ‐20°C, and flexibility in testing any rich gas mixtures. The decompression wave speed data are presented in terms of the decompression expansion pressure ratio for all four gas mixtures and three initial line pressure of ∼ 10, 14 and 20 MPa. Comparison between experimentally obtained decompression wave speeds and those predicted by GASDECOM is also made for each test.

Rapid crack propagation in plastic water pipes: measurement of dynamic fracture resistance

A classical analysis of Irwin and Corten relates the minimum pressure to sustain crack propagation along a pipeline to the dimensions, elastic modulus and fracture resistance of the pipe wall. Although the model is inapplicable to gas pressurisation, it is here modified to become sufficiently accurate for water pressurised thickwalled pipe to provide the basis of a dynamic fracture resistance test. Results for three commonly used water pipe grade polymers agree well with the predictions of the ‘Thermal Decohesion’ model, and with other experimental data where available. Rapid crack propagation behaviour, in both test and service pipeline configurations, is dominated by the ratio of crack speed to fluid decompression wave speed. This ratio can be adjusted in a controlled test environment.