Modelling Air Flow through Pneumatic Valves: A Brief Review with an Experimental Case Study

Round 1

Reviewer 1 Report

The author reviewed the most common models for pneumatic valve flow, beginning at the very foundations of the currently accepted models.

The topic is very important to pneumatic valves engineers. The topic fits the scope of the journal Eng.

The results of this manuscript are well presented and organized. The presented results of the performed modeling work and experiment work have scientific meaning.

I recommend to accept this manuscript after the minor revisions.

1.       This manuscript uses a large number of equations and general model descriptions to identify accuracy of the models. My concern is, as a review paper, this manuscript may address the research status and problems is not solve. And pointing out the research topic in the future. I thought this manuscript could be a research paper with focus on the model accuracies.

2.       More detailed research work or reference may be referred in this review. For example, when describe the some research argued its accuracy in line 34.

3.       The treatment of incompressible or compressible flow in pneumatic valve design. The references should given. The simple assumption of fluid incompressibility can be safely applied for the usual pressure range in industrial applications when the flow direction is towards pressurization of the receiving pneumatic element.

4.       More references may be cited. Could the references in this submitted journal or similar papers of MDPI publishers be relevant and be referred?

5.       Geometry and size of the tests in figure 8 should be given.

6.       Interestingly, an even simpler relation provided a more accurate match with experimental values when the flow direction was directed towards depressurizing the air tank. Could this be studied and some relation or equations have been studied in published references?

7.       There are too much ‘remarks’ in the paper, line 107 and line 189. Why not use a sub tittle to replace the remarks?

Author Response

The author reviewed the most common models for pneumatic valve flow, beginning at the very foundations of the currently accepted models. 

The topic is very important to pneumatic valves engineers. The topic fits the scope of the journal Eng.

The results of this manuscript are well presented and organized. The presented results of the performed modeling work and experiment work have scientific meaning.

I recommend to accept this manuscript after the minor revisions.

I thank the Reviewer for the positive comments!

1. This manuscript uses a large number of equations and general model descriptions to identify accuracy of the models. My concern is, as a review paper, this manuscript may address the research status and problems is not solve. And pointing out the research topic in the future. I thought this manuscript could be a research paper with focus on the model accuracies.

I thank the Reviewer for the suggestion. I have agreed with the point made and changed the paper category accordingly. 

2. More detailed research work or reference may be referred in this review. For example, when describe the some research argued its accuracy in line 34.

As a matter of fact, only one of the references listed in the paper explicitly contest the accuracy of the ISO 6358 equations. Line 34 has been changed to state the point clearly.

3. The treatment of incompressible or compressible flow in pneumatic valve design. The references should given. The simple assumption of fluid incompressibility can be safely applied for the usual pressure range in industrial applications when the flow direction is towards pressurization of the receiving pneumatic element.

Since compressible and incompressible models produce similar results in some particular situations, incompressible models are almost never referenced as a basis for valve flow design. To state the point clearly, I have added some lines to the “Conclusions” section.  

4. More references may be cited. Could the references in this submitted journal or similar papers of MDPI publishers be relevant and be referred?

In reply to the Reviewer’s request, four new MDPI references have been added.

5. Geometry and size of the tests in figure 8 should be given.

In this particular aspect, and with my sincere apologies, I am not inclined to proceed as suggested. The reason is that including the geometry of valves would be difficult, besides adding the need to reproduce industrial drawings in the paper, which would demand copyright permissions. Because of that, industrial references to each component have been given in Table 3. I hope that this reasoning reads satisfactory to the Reviewer.

5. Interestingly, an even simpler relation provided a more accurate match with experimental values when the flow direction was directed towards depressurizing the air tank. Could this be studied and some relation or equations have been studied in published references?

I absolutely agree with the Reviewer and, as indicated in the text, I am not aware of any particular paper that provides a suitable explanation for such phenomenon. Yet, as also mentioned in the Conclusions, I intend to investigate it further in the future. In summary, I believe that this could be studied but I have not found any significant treatment of the subject so far – this is why no reference other than [7] has been given.    

7. There are too much ‘remarks’ in the paper, line 107 and line 189. Why not use a sub tittle to replace the remarks?

In reply to the Reviewer’s comment, the “Remarks” section after equation (17) has been removed. I believe that the text is more nicely presented now.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear author,

The article about basic thermodynamics nicely describes a "simple" problem of filling or discharging gas volume. I think, that the most interesting point, is that the charging and discharging are different processes with different dynamics and relations. This might be highlighted in the abstract and a deeper discussion should be involved.

The asymmetry in charging/discharging occurs in surprisingly often. One example is the fluid jet (see e.g. a youtube movie https://youtu.be/BiuOKTng8jE), which can play some role in this problem as well.

Remarks:

- In the whole work, You should use strictly the absolute pressure. I understand, that the apparatus measures Gauge pressure, but it has to be recalculated in order to be consistent across the article, mainly with the base thermodynamic equation and the adiabatic law.

-line 66 "... we might factor pressure losses into C..." strange sentence

- line 76, it is clear, Your readers are not Your students...

- equation (6) Why T = T_0 in discharging? This needs some discussion.

- Remark 3: The expansion into vacuum is problematic due to phase transitions. When the expansion is fast, thus there is no time to thermalization, thus the process is adiabatic. Imagine expanding air at temperature 300 K, then at pressure ratio p/p0 = 0,7 the temperature is 293 K (i.e. triple point of water and some water condenses even from dry air), then at p/p0 = 0.22, the CO2 freezes out, at p/p0 = 0.015, oxygen liquifies... When expanding into vacuum, no gas is ideal gas!

Figure 3: legend missing

equations 18, 19.: I think, that You define too many symbols, it is not needed

Figure 4: make the y-axis comparable with figure 5, KT is function of temperature

Equation 21: Why there is no γ? This equation looks strange to me and it should be argued similarly as the previous equations. What it is based on?

line 251. missing number of equation You are talking about.

Equation (26): how behaves the T? It is isothermal process, or T is function of p and γ?

I really like Your experiment: it is simple and straightforward and it shows the physics. However a small discussion might be included. How it is with the temperature in the tank: it is massive and the walls have to have significant thermal capacity which turns the charging process towards isothermal. The Δp in discharging has to be considered in respect tot he point immediately behind the orifice - the output is too far and there will be significant contribution from pressure loses in the tubes. This should be compensated in the results. And the discharging is no discharging into vacuum, thus there will be moment, when the supercritical flow changes into subcritical (and it will not depend on the pressure ratio outside due to the pressure loses in tubes)

I think, that after including the mentioned points into discussion, the article will be ready to publish. 

Author Response

Dear author,

The article about basic thermodynamics nicely describes a "simple" problem of filling or discharging gas volume. I think, that the most interesting point, is that the charging and discharging are different processes with different dynamics and relations. This might be highlighted in the abstract and a deeper discussion should be involved.

In reply to the Reviewer’s observation, the abstract has been modified, as well as the main text.

The asymmetry in charging/discharging occurs in surprisingly often. One example is the fluid jet (see e.g. a youtube movie https://youtu.be/BiuOKTng8jE), which can play some role in this problem as well.

I thank the Reviewer for the valuable comment and reference

Remarks:

- In the whole work, You should use strictly the absolute pressure. I understand, that the apparatus measures Gauge pressure, but it has to be recalculated in order to be consistent across the article, mainly with the base thermodynamic equation and the adiabatic law.

I understand the Reviewer’s concern. However, the only places where gauge pressures are explicitly used are in the vertical axis of figures 7,9 and 10. Nicely redrawing these figures would involve running every simulation again (they were created with Scilab). Due to my own time limitations, I wonder if the Reviewer could relax this particular requirement. As for the rest of the paper, a careful revision has been made to assure that only absolute pressures are used in all the equations.

-line 66 "... we might factor pressure losses into C..." strange sentence

The line has been changed into “the coefficient  can be experimentally adjusted to include pressure losses” 

- line 76, it is clear, Your readers are not Your students...

I recognize the Reviewer’s concern. However, given the comments of the other Reviewers, I kindly ask for leaving the text as it is.

- equation (6) Why T = T_0 in discharging? This needs some discussion.

Equations (6) can be easily obtained from (5), as mentioned in the text. On the other hand, (5) can be found in the classic Fluid Mechanics textbook, given in reference [1]. I must say, I am a little confused with the Reviewer´s request here… I do apologize for it.

- Remark 3: The expansion into vacuum is problematic due to phase transitions. When the expansion is fast, thus there is no time to thermalization, thus the process is adiabatic. Imagine expanding air at temperature 300 K, then at pressure ratio p/p0 = 0,7 the temperature is 293 K (i.e. triple point of water and some water condenses even from dry air), then at p/p0 = 0.22, the CO2 freezes out, at p/p0 = 0.015, oxygen liquifies... When expanding into vacuum, no gas is ideal gas!

I thank the Reviewer for the extra piece of information. I have added some lines to the text to make it richer.

Figure 3: legend missing

I thank the Reviewer for the observation. The caption has been duly adjusted

equations 18, 19.: I think, that You define too many symbols, it is not needed

I recognize the Reviewer’s observation as a valid one. However, given the comments of other Reviewers, I kindly ask to leave the text as it is.

Figure 4: make the y-axis comparable with figure 5, KT is function of temperature

The figure has been changed to accommodate the Reviewer’s suggestion. The mass flow has been plotted instead. However, for simplicity, the temperature has been assumed as constant.

Equation 21: Why there is no γ? This equation looks strange to me and it should be argued similarly as the previous equations. What it is based on?

Equation (21) is the equation suggested by the ISO 6358 Standards, as duly referenced in the text. In fact, it comes from equation (20), which is, in turn, based on Figure 5.    

line 251. missing number of equation You are talking about.

I thank the Reviewer for noting. The equation number has been inserted.

Equation (26): how behaves the T? It is isothermal process, or T is function of p and γ?

In the text, it is explicitly mentioned that the process occurs at a constant temperature.” Considering that the air tank volume is V and that it remains at a constant temperature, T, the following equation is obtained … “

I really like Your experiment: it is simple and straightforward and it shows the physics. However a small discussion might be included. How it is with the temperature in the tank: it is massive and the walls have to have significant thermal capacity which turns the charging process towards isothermal. The Δp in discharging has to be considered in respect tot he point immediately behind the orifice - the output is too far and there will be significant contribution from pressure loses in the tubes. This should be compensated in the results. And the discharging is no discharging into vacuum, thus there will be moment, when the supercritical flow changes into subcritical (and it will not depend on the pressure ratio outside due to the pressure loses in tubes)

I recognize that the paper is not scientifically accurate as to factor in all the suggested parameters. Yes, temperature is an issue and measuring it accurately would be a formidable task (temperature changes spatially within the tank and within the plastic tubes). But extreme accuracy has never been the proposal. The paper is rather concerned about day-to-day use of pneumatic models for the valves and, as such, a number of simplifying assumptions have been made (most of them are implicit throughout the text). Likewise, discharging into vacuum is completely out of the scope of the experiments. The goal of this paper is far less intrepid. I am confident that the Reviewer will understand these limitations.  

I think, that after including the mentioned points into discussion, the article will be ready to publish. 

Again, I thank the Reviewer for the valuable comments and assure I did my best to reply to each one of them. Hopefully my answers have been satisfactory.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The article is very well written, with a clear structure and message. The language is concise.

It was a pleasure to read, I believe it is ready to be published. I was only able to find a few very minor things:

Line 28:
"treat the variables involved in an average way" - maybe "averaged" would sound better?

Line 214:
"by Lord Rayleigh in 1916 [3] who, experimentally" - the comma should be placed before "who", instead.

Line 240:
"used, , is the ISO 6358" - duplicate comma.

Line 322:
"In the Table" - I believe there should be either "In the Table 1", or "in the table".

Line 345:
"in Table 2 for ?(?) , adjusting constants" - extra space before the comma.

Author Response

The article is very well written, with a clear structure and message. The language is concise.

It was a pleasure to read, I believe it is ready to be published. I was only able to find a few very minor things:

I thank the Reviewer for the positive comments!

Line 28:
"treat the variables involved in an average way" - maybe "averaged" would sound better?

I agree with the Reviewer. The text has been adjusted accordingly to the suggestion

Line 214:
"by Lord Rayleigh in 1916 [3] who, experimentally" - the comma should be placed before "who", instead.

I thank the Reviewer for the observation. The comma has now been placed correctly

Line 240:
"used, , is the ISO 6358" - duplicate comma.

I thank the Reviewer for the observation. The duplicate comma has been removed

Line 322:
"In the Table" - I believe there should be either "In the Table 1", or "in the table".

I thank the Reviewer for the observation. I have changed the text into "in the table"

Line 345:
"in Table 2 for ?(?) , adjusting constants" - extra space before the comma.

I thank the Reviewer for the observation. The extra space has been removed

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Dear author,

the paper is ready to be published.