An Investigation into the Volumetric Flow Rate Requirement of Hydrogen Transportation in Existing Natural Gas Pipelines and Its Safety Implications

S/N

REVIEWERS’ COMMENTS

AUTHORS’ RESPONSE

1.       

“I would like to see more explanations in the materials and methods section. More details on the simulation, EOS used and why, the Beggs and Brill model and its limitations”.

An elaborate section of the Materials and Methods section has been provided featuring a comprehensive description of the Model development in Section 3. Section 3.1 presents the EOS employed and the justifications for that. Section 2.5.2 now presents the generic form of the Beggs and Brill model along with its limitation. 

2.       

“Figure 3 (the generic pipeline model) not clear, please provide more explanations”

The figure has been expanded (zoomed out)to provide a clear view of it. The study now contains an elaborate Study Methodology Section, Section 3 with more explanations on the materials and methods. Table 1 provides the pipeline parameters.

3.       

“Equation 6 the limits of the integral are from p1 to p1??”

Justify the EOS used.

This was an oversight. The equation no longer features in the revised manuscript in the new Results Section.

Section 3.1 presents the EOS used in the revised experiment/manuscript and the justification for that.

4.       

“Give details on the mixing rules used in the preparation of Figure 4 and justifications”.

The mixing rule inherent in the EOS was used. The theory of which was presented in Section 2.5.1 and Section 3.1 along with citations and justifications.

5.       

“Equation 7 starts with API RP 14E”…..lower c in equation, upper C in defining

No, this was mentioned there to show that it was sourced from there. That has been corrected and the IEEE citation is now used consistently throughout the work.

The discrepancy with the use of “c” is an oversight that has been corrected. It is now Equation 12 in the revised model (See Pages 12 and 17)

6.       

“How did you come up with the maximum allowable velocity?”…”figure 5 is not clear”

The previous value of 20m/s was obtained from Literature. However, since it is not a fixed value, the API RP 14E formula has been used to determine the new value of 14m/s that is peculiar to the range of parameters being used in this study. See Section 2.4.6 and  Section 3.3.2

7.       

“Gas density trends…mixing rule”

Please refer to Bullet number 4 above.

8.       

“Gas velocity trend,… which gas or the mixture”

For the mixture. The initial caption was ambiguous; it is now better captioned. See Figure 7.

9.       

“Several spelling mistakes throughout the paper”

These have all been attended to.

S/N

REVIEWERS’ REMARK/COMMENTS

AUTHORS’ RESPONSE

1.       

“(i) This paper investigates the impact of blending hydrogen into a natural gas pipeline; (ii) the authors claim that ‘This study explored the technical and safety concern around compressibility, erosional velocity, pressure drop and density of the hydrogen blends in the short and medium-term and pure hydrogen in the long term; (iii) ‘it is also claimed that the paper “could provide new insights into how compressor stations requirements and metering stations can be positioned to handle the flow behaviour of gases”’. In short, these points are not addressed by the authors”

The authors acknowledge these limitation in the old manuscript as the study model were not effectively reported. In the new manuscript: (i) The objectives are clearly defined in the Introduction Section (Section 1.); (ii) the methodology and model are now tailored towards meeting those objectives and; (iii) the claims in the Conclusion are strictly on from the results obtained and targeted at the objectives specified in the Introductory Sections. The manuscripts Abstract and Titles have been changed to reflect the modified contents, experiments, results and conclusion.

2.       

“The Literature Review is Weak and there are not enough technical details about the recent approaches to gas network modelling. The lit. review should discuss the published research works in a critical manner showing research gap that the authors will address”

The Literature Review has now been strengthened. Section 2.5 is now dedicated to recent approaches in gas network modelling and it features research gaps, like the assumption of single components flow streams, that the revised manuscript addressed.

3.       

“The Methods Section is non-existent. The approach of modelling one pipe is overly simplistic and no description”.

An elaborate Section 3 has now been provided which comprehensively and concisely describes the study.

The approach of modelling one pipe is sufficient enough to capture the objective of the study in the revised manuscript.

All aspect of the simulation workflow has now been technically reported.

4.       

“The limited results are poorly presented and do not show anything novel or new”

The revised manuscript and experiment has been targeted towards clear objectives: (i) showed how blending natural gas with hydrogen affects the velocity profiles and compressibility behaviour of the flow stream as different proportions of hydrogen and natural gas flow and; (ii) show that this can cause safety concerns as industry limits of some flow parameters – in this case, the erosional velocity -  can be exceeded by blending or increasing the hydrogen flow rate to provide the same amount of energy as natural gas. The authors did not find any precedence to this. No previous gas network modelling has been targeted at this.

5.       

“The Conclusion is weak without answering the research question and the challenge designed in the abstract and introduction section”.

The Conclusions, Abstract and Introduction sections are now in unison. The conclusion now answers the questions in the abstract and introduction sections. Strong affirmations as to the relevance and results of the study are now in the conclusion.