Contribution of Air Management to the Energy Efficiency of Water Pipelines
, Mohsen Besharat 2,*, Helena M. Ramos 3, Edevar Luvizotto, Jr. 1 and Bryan Karney 4
Round 1
Reviewer 1 Report
The influence of the entrapped air on the conveyance capacity of water pipelines is assessed in the paper. The subject is worth investigating and the paper may be considered for publication after a revision addressing the following issues.
The main issue is related to the insufficient assessment of the entrapped air on the energy efficiency as expected when reading the title and the abstract. Only the last paragraph of the section 3.4 discusses it without any methodology proposed and applied to assess it.
The negative effects of entrapped air would be of interest for the readers if quantitatively extracted from literature.
The use of lumped references as in [13-16], [17-31], [32-38] should be avoided to provide sufficient representation of the main contribution of each referenced paper.
Check the English carefully for grammar, spelling and syntax.
Author Response
Reviewer 1 comment: The influence of the entrapped air on the conveyance capacity of water pipelines is assessed in the paper. The subject is worth investigating and the paper may be considered for publication after a revision addressing the following issues.
Answer from authors: The authors are glad that the reviewer found merit in the subject matter of the paper.
Reviewer 1 comment: The main issue is related to the insufficient assessment of the entrapped air on the energy efficiency as expected when reading the title and the abstract. Only the last paragraph of the section 3.4 discusses it without any methodology proposed and applied to assess it.
Answer from authors: In the initial manuscript, the authors focused more on a qualitative assessment regarding efficiency issues due to entrapped air. To also include a quantitative and more in-depth analysis, the following new section was added to the paper:
“\subsubsection{Energy considerations}
Energy wise, as noted, the progressive accumulation of air in a water rising pipeline tends to reduce its conveyance capacity, making the pumping system to function outside its higher efficiency region. Moreover, for a pipeline without an air management strategy, to compensate for the air--related head--losses, either the pumping power needs to be increased or the pumping system needs to function for longer periods, increasing the electricity consumption. Unfortunately, head--losses caused by entrapped air are sometimes mistaken by increased pipeline friction or other causes and are unduly unaddressed.
For the case in Figure \ref{open_path_1}, at point 1, the efficiency of the pump is 81 per cent and the pumping power is 72 kW. At point 2, with a reduction in the pump’s rotational speed (to 90 per cent), the efficiency changes to 76 per cent, the pumping power becomes 49 kW, and the water discharge decreases from 396 to 290 m$^3$/h. Considering a hypothetical pumping functioning time of 9 hours per day, the necessary time to achieve the same daily volume would increase to about 12 hours. Considering the pumping power and new daily functioning time at point 2, the energy consumption would go from 645 to 608 kWh. Note that, from point 1 to point 2, there is still no entrapped air influencing the system.
At point 3, with the presence of an entrapped air pocket and 90 per cent of the rated rotational speed, the efficiency goes to 53 per cent, the pumping power goes to 39 kW, the water discharge goes to 145 m$^3$/h, and the system would be required to function for more than 24 hours, thus compromising the overall water delivery. The energy consumption would considerably increase at point 3 (going to 966 kWh). In the previous calculations, it was considered the usual pumping power equation ($P=9.8 Q H/\eta$, with $\eta$ as the pump efficiency and $P$ its power in kW) \cite{porto2006}. The reduction in efficiency due to a reduction in rotational speed was neglected. From this example, it becomes evident that adequately managing the air content in a pipeline is essential for its overall energy efficiency but also for the assurance of meeting the water conveyance requirements.”
Reviewer 1 comment: The negative effects of entrapped air would be of interest for the readers if quantitatively extracted from literature.
Answer from authors: The following was added to the paper to address this issue:
“According to Sorensen (2017) \cite{sorensen2017}, it is estimated that entrapped air in pipelines cause at least a 10 per cent increase in pumping costs. Sorensen (2017) \cite{sorensen2017} mentions, for example, that for a wastewater pumping station having five air valves, the strategic substitution of two of these valves with new air valves resulted into an increase of the pump rates from about 100 gpm to more than 400 gpm.”
Reviewer 1 comment: The use of lumped references as in [13-16], [17-31], [32-38] should be avoided to provide sufficient representation of the main contribution of each referenced paper.
Answer from authors: Thank you for the concern regarding the lumped references. The authors addressed this issue by rearranging references closer to the ideas they support or by removing non-essential references or not as recent references.
Reviewer 1 comment: Check the English carefully for grammar, spelling and syntax.
Answer from authors: A general review of the writing of the text was done.
Reviewer 2 Report
My impression after reading the paper is positive. The paper addresses an important and timely topic. The Introduction section needs to be completed. I strongly recommend a brief discussion and citation of the articles I have identified.
Comments for author File: 
Comments.pdf
Author Response
Reviewer 2 comment: My impression after reading the paper is positive. The paper addresses an important and timely topic. The Introduction section needs to be completed. I strongly recommend a brief discussion and citation of the articles I have identified.
Answer from authors: The references suggested by the reviewer are appreciated. It should be noted, however, that the reference Lauchlan et al. (2005), already included in the initial manuscript, encompasses the ideas in lines 30 to 36 (of the initial manuscript). After due consideration, the authors included some of the references suggested by the reviewer.
Reviewer 3 Report
A paragraph concerning the literature review should be considered to foster the main literature on the topic.
The discharge reduction model works really well, but needs some further explanation about implications for other research.
A paragraph before the conclusion could be considered to emphasize implications and suggestions for stakeholders.
Author Response
Reviewer 3 comment: A paragraph concerning the literature review should be considered to foster the main literature on the topic.
Answer from authors: The authors agree that fostering previous literature is important. Overviews of the relevant literature are presented in the introduction section and in the section related to the critical velocity for the hydraulic removal of air from descending pipe segments.
Reviewer 3 comment: The discharge reduction model works really well but needs some further explanation about implications for other research.
To give context to the model developed in the paper, the following paragraph was added at the end of section 3.2:
“The modelling approach presented herein is a development of the model found in Ramezani et al. (2016) \cite{ramezani2016}. Despite the simplifying assumptions, the current modelling approach aims at offering a general engineering intuition about the effect of entrapped air pockets in pressurized pipelines. The model includes some complications found when dealing with air and water in pressurized systems, including the compressibility of air and occurrence of channel flow inside the pressurized pipe. The modelling approach, however, does not include some potentially relevant aspects like the occurrence and effect of hydraulic jumps, air release from or dissolution in water, tracking air pocket movement along the line, and consideration of backwater curves.”
Reviewer 3 comment: A paragraph before the conclusion could be considered to emphasize implications and suggestions for stakeholders.
Answer from authors: Thank you for this great suggestion. The following paragraph was added in the concluding remarks section:
“Designers, operators and owners of water pipelines should consider the following complications that may arise from unaddressed entrapped air pockets in piping systems:
\begin{itemize}
\item The proclivity of a pipeline to the deleterious effects of entrapped air may significantly change over time. Initially well--behaved systems may get chronic air--related problems if not well maintained and adjusted to new operational circumstances.
\item Air valves should be carefully selected, sized, located and maintained to allow the necessary air exchanges in a pipeline system in its varied operating conditions during its lifespan.
\item It is often difficult to identify the size and location of air pockets in a pipeline. Nevertheless, from their effects, operators might be able to infer their presence.
\item Entrapped air pockets can result in appreciable pumping inefficiency and conveyance capacity reduction.
\item The water conveyance reduction caused by unaddressed air pockets can put the serviceability of the system at risk.
\end {itemize}
The key implication is that the diligence and awareness of air management considerations is an ongoing need if water conveyance systems are to perform their many functions during their decades of service.”
Reviewer 4 Report
In this manuscript, the the impact of entrapped air in the pressurized and gravitational water transmission pipelines is studied. The topic and research methodology is quite interesting and sufficient to be published in this journal. Also, the novelty, research questions and contribution is found to be adequate. This reviewer has no further recommendation and I suggest it can be published in this from.
Author Response
Reviewer 4 comment: In this manuscript, the impact of entrapped air in the pressurized and gravitational water transmission pipelines is studied. The topic and research methodology is quite interesting and sufficient to be published in this journal. Also, the novelty, research questions and contribution are found to be adequate. This reviewer has no further recommendation and I suggest it can be published in this from.
Answer from authors: The authors are grateful for the appreciation offered by the reviewer.
Round 2
Reviewer 1 Report
The authors addressed the queries and the paper may be considered for publication after addressing the following minor issues:
- the non SI measurement units must be clearly defined or replaced with SI units (e.g. gpm) all over in the manuscript
- % should be used instead of "per cent" all over in the manuscript
Author Response
The authors complied with both requests asked by Reviewer #1. The corresponding changes were done in the manuscript. Thank you.
