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Peer-Review Record

Entrapped Air Removal by Hydraulic Means in Gravity Water Systems in Small Diameter Pipelines

Water 2023, 15(16), 2870;
by Emilio Quintana-Molina 1, Jorge Víctor Prado-Hernández 2,*, Joaquim Monserrat-Viscarri 3 and José Rodolfo Quintana-Molina 4
Reviewer 1:
Reviewer 3:
Water 2023, 15(16), 2870;
Submission received: 1 July 2023 / Revised: 4 August 2023 / Accepted: 5 August 2023 / Published: 9 August 2023

Round 1

Reviewer 1 Report

I have read the paper with some interest but also some frustration.   The interest arises because the topic of the paper is interesting.   The frustration arises become of several things I do not understand.

1   1. I do not understand the basic set up in that the key diagrams in Figure 1 and Figure 2 imply a physical setting that seems highly unlikely or impossible.   First there doesn’t seem to be enough head at the bottom of the descending section to induce flow through the gate valve. Second it seems to imply this is a steady or equilibrium situation and it seems it would transient at best.  That is, if there is any non-zero Beta value it would deplete the entrapped air in the descending limb and the pocket would retreat until air could no longer be removed.  What am I missing here?

22.  But the whole set-up seems contrived.   It is one thing to have small diameter lines, which in my experience are still highly rare, but the whole set up seems small scale and rather unlikely.   Most gravity flow systems are supplying considerable pressure, and rather than air being release from solution it would be absorbed over time, particularly for a filling scenario.   Indeed, I don’t think the paper is really about removal of entrapped air as much as about filling small diameter lines.  The scale issues would seem to complicate any issues of generality or applicability.   Certainly more discussion of the key assumptions and limitations of the approach is essential.

33.    On a related issue, even for operations that avoid air valves, they would normally put a manual bleed or vent valve at the top of the high point.  Such a valve would be opened only long enough to bleed out the entrapped air, thus avoiding the head loss.   Without such approach, depending on the exact configuration, a complete air lock could easily occur.   (I have often operated aquariums for tropic fish.   These have (small diameter!) external filters.  If air get caught at the high point of the aquarium wall the flow to the filter simply stops until the air is removed.) 

44.    I do not understand why Beta is not a function of time.  I cannot imagine the kind of steady process that is envisioned in the key plots.

55.    These seem to be to be rather large issues.  There are some smaller ones in thing like language and mathematics that are trivial by comparison.   But, if the authors undertake a revision, they should be careful to use sin or sine for the sine function and not the Spanish sen.  They need to be quite careful in language and should introduce only enough discussion that is relevant to their current paper.   The Introduction, for example, introduces several air-water issues that are not really considered in the paper.

If my understanding is faulty, I apologize and invite the authors to clarify what they are doing and what it means.   I have not been able to get these points clear as I read the paper.

See above

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The current paper presents an analysis of entrapped air removal in gravity systems for small internal diameters, where the authors mentioned that the there is a lack of literature for the values used in this studied. In addition, the authors should be mentioned other comments related to the new contributions of this paper to the current literature.

Review the experimental module section since I cannot identify in Figure 1 where is located the PVC pipe and the analyzed angles.

Include a photograph of the experimental facility used in this study. Include a scheme with dimensions where the readers can see the analyzed variables and initial conditions. Where the flexible pipe mentioned in line 114 is located? Please, you should show in detail all elements and devices.

Specify the used device to measure the air volume changed over time.

Review the air-water interface presented in Figure 1 since the water level should be below of the current level to have the configuration of the air pocket size.

I would like to see experimental tests considering the ultrasonic flow meter each one-second for the different experimental facility. Normally, to see the water flow evolution a lower resolution should be used. I think that the device cannot capture the evolution of water flow since the event occurs very fast.

The air pocket can be simulated using a thermodynamic law (pressure x air pocket volume ^ polytropic coefficient = constant); however, the authors do not mention the limitations of the Bernuolli equation for transient events with entrapped air pocket. I consider that this approximation can be only used for small pipelines. Please, include a complete list of limitation and hypothesis of the used models. Please, consider that you have using an approximation that is neglecting the inertia of the system (dv/dt). In my experience with this approximation, you can only detect the final behavior of the air pocket, but you cannot determine the behavior along of the occurrence of the hydraulic event.

Provide information regarding the initial condition of the air pocket pressure (it is a at atmospheric condition, other).

Review equation presented in line 196.

In the sentence “The considered PVC pipeline roughness was 0.0015 mm” is missing the word ABSOLUTE.

Specify how opening valve values ranged between 10 and 15 % were computed.

I recommend the use of a CFD model to show to the readers a complete description of the phenomena. With this kind of contribution, the authors can improve this paper.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report


Review of “Entrapped Air Removal by Hydraulic Means in Gravity Water Systems in Small Diameter Pipelines”

The paper addresses the entrapped air issue in steady and unsteady conditions. Several experimental tests have been carried out.

General considerations

1.       The manuscript is not well structured.

2.       The English grammar and style must be reviewed.

The abstract needs a full revision, please consider rewriting the abstract.

The introduction can be enriched by adding publications with entrapped air scope e.g.,[1-5]


Line 81 – the latter can be achieved by integrating tailored air removal models, please explain

Line 102 – 0.37Kw please check units

Figure 1 and 2, please explain the main differences between Figure 1 and 2, and as far as possible, use the same nomenclature throughout the paper, piezometric 1 should be the same throughout the paper, and so on, otherwise is very confusing and not consistence

Line 194 – one way ANOVA, please explain and add equations if needed.

Line 196 – Tukey’s test, please explain and add equations.

Table 1, mention the number of the equation in the energy loss generated by entrapped air calculated.

Why figures 4-5-6 are needed??? Doesn’t Figure 7 has all the information?

Line 294, the downward angle \theta, please mention the figure where is presented.

Figure 8 – please explain why this is happening.

Table 2 – what do d, g, and V stands for? Please explain.

Also, present the Fr equation

Table 3 and Table 4, present all the parameters used

Figure 10 should be in the first part of the paper and not in the conclusions. Please consider removing this figure from the conclusions to the introduction, of a introduce a section to add this information.

Conclusions section – this section should be rewritten adding all the information obtained with all this experimental data.

In general, the paper needs major revision in order to comply with the high standards of Water – MDPI.


1.             Paternina-Verona, D.A., et al., Rapid Filling Analysis with an Entrapped Air Pocket in Water Pipelines Using a 3D CFD Model. Water, 2023. 15(5).

2.             Vasconcelos, J.G. and G.M. Leite, Pressure surges generated by entrapped air pockets in stormwater systems, in World Environmental and Water Resources Congress 2011. 2011, American Society of Civil Engineers: Palm Springs, California, United States.

3.             Martins, N.M.C., et al., Maximum transient pressures in a rapidly filling pipeline with entrapped air using a CFD model. Journal of Hydraulic Research, 2017. 55(4): p. 506-519.

4.             Koelle, E., Critical volume of entrapped air in hydraulic circuits, in 8th International Round Table on Hydraulic Transients in Power Stations, I.-I.A.f.H. Research, Editor. 1987: Madeira. p. 87-102.

5.             Martin, C.S., Entrapped air in pipelines, in Second International Conference on Pressure Surges. 1976: London, England. p. 15-28.




English very difficult to understand/incomprehensible

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The new version of the paper has some improvements but three issues in my view still need attention.

The first one mentioned before and I am not satisfied with the authors response.  The paper mentioned several times constant flow rate and their methodology and results somewhat implicitly assume this.  I cannot imagine how their experiments design and analysis will justify this answer.   As air is removed from the descending limb, there will be a strong tendency for friction to deceased and flow rate to increase.    Whether it increases sufficiently to remove all the air will depend on many factors.  This reality needs to be better discussed and acknowledged.

The paper implies a relative simple monotonic relationship between clearing velocity and the slope of the descending limb.   I think this will break down as the slope is increased, due to a different mechanism of two-phase flow making slug flow more likely.   Again, the limitations of the current study need be emphasized in this respect.

Finally, the very nature of the stratification assumed is air and water flow rate sensitive, and again the limitations of the study should be clearly articulated.

Some recent work has been done by Elias Tasca in Brazil and some of these recent papers might be added to the reference list. 

Somewhat better; still choppy in a few places. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

(re)Review of “Entrapped Air Removal by Hydraulic Means in Gravity Water Systems in Small Diameter Pipelines”

The paper addresses the entrapped air issue in steady and unsteady conditions. It was design and assembled an experimental setup to carry out this research.


General consideration of the (re)revised manuscript.

1.       As mentioned before the English grammar and style must be revised. The text is confusing due to the style which I give only a few examples:

a.       Line 106 – experimental module, please consider using experimental model/setup.

b.       Line 107 – the experimental setup was designed to generate (maybe study?) air accumulation zones …

c.        Line 109 – (…) constant head reservoir, a supply reservoir/tank (…)

d.       Figure 1 – where does the water go? Recirculation?? The scheme does not represent the recirculation

e.       Figure 1 – does it make sense have a pipe not connected, with changes in the diameter, in a schematic? Please clarify this figure.

f.        Line 109 – piezometers, please specify where they are located and why.

g.       Line 110 – regulation valves – please specify

h.       Line 113 was, please consider using is

i.         Line 122 body, please consider using volume

In general, the manuscript needs to be fully revised in term of style and English grammar.

2.       Figures 1 and 2 are the same, please add the information of figure 2 to figure 1, identify the water in gray.

3.       Line 171, w is subscript as in line 174, please be coherent.

4.       Line 176, water flow?? What does this mean?? Discharge? Flow rate?

5.       Line 176, ultrasonic water meter?? Ultrasonic meter, please consider removing the word water.

6.       One-way ANOVA and Tukey’s test, please explain and add more information.

7.       Again, figures 4-5-6 can be removed since figure 7 gives all the information.

8.       Figure 10 – where are the results obtained in this research? There is no comparison with the results obtained in this research. If so, please identify them in the legend, otherwise, change the position of a figure comparing results of literature.

as mentioned, the english grammar and mainly the style must be improved.

Author Response

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Author Response File: Author Response.pdf

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