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Thread: Which equipment should I go for?

  1. #1
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    Question Which equipment should I go for?

    Hi all forum members. I am a mechanical engineering student. This is my last year and I am studying on my senior design project. This is about "automatic inflatable yacht fenders". I put a link below in case you may not have seen/heard anything like that:

    2070731.jpg

    As you see it is inflatable and its main function is to absorb berthing energy of yachts/ships etc. In my design project, these fenders&whole mechanism (while deflated) are placed in the hull of ship and while berthing, all fenders start to step out. Then, inflation starts. However, I'm stuck in equipment choice. What should I choose for inflation operation? An air compressor or an air blower or any other tool that can be useful for this operation?

    Besides, I assume that the inflation progress of whole fenders is done in a minute (Fender dimensions~Φ320 mm-1050 mm). The approximate volume of a fender is 3 cubic feet. For 10 fenders = 3 ft3 x 10 = 30 ft3. An 30 CFM air compressor will work for me? Is that a true calculation method? Should I look for other specs?

    Lastly, I should deflate/vacuum the air in fenders when I'm done. Does that require any other tool/equipment like vacuum pump or something? Sorry for my poor English.

    Excuse my ignorance about this topic. Thanks in advance.
    Last edited by a.yuksel; 11-06-2013 at 05:57 AM.

  2. #2
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    Because this is a school project I am going to restrict my response but I will tell you that the cfm required depends upon the pressure required in the fenders to sufficiently absorb the berthing impact without flattening a fender. Once you have determined the required inflated pressure and the resulting cubic feet of air required for all of the fenders at that pressure then you can determine the cfm required at that pressure. Then this should be converted to cfm at standard conditions (atmospheric pressure and temperature) = scfm = cfm x inflated pressure / 14.7 for the compressor or blower sizing.

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    Question

    Firstly, thanks a lot for your interest.

    Actually, I have a little trouble when calculating the energy to be absorbed by a fender?

    fmathEquation.jpeg
    I think, by considering this principle I can find absorbed energy by one fender (correct me if I'm wrong, please). However, how am I supposed to calculate volume change of the fender dV?

    Regards.

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    I was concerned about that part of the problem but had to assume you might have determined that. Your equation is correct and if you were compressing a piston in a cylinder then obviously the solution would be straight forward. Otherwise, I need to know the configuration of your planned fender if I am to offer much or any guidance.

    If your fender is a sausage configuration similar to most conventional fenders then it depends upon the deformation of the fender and the elasticity of the fender's material. If the material is flexible but relatively inelastic as with a fabric reinforced elastomer or polymer then you will need to calculate the change in the fender's internal volume as its cylindrical X-section is flattened to progessively flatter oval x-sections.
    Last edited by JAlberts; 11-06-2013 at 05:06 PM.

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    Yes, the fender is as you think, conventional (sausage configuration) fender and we haven't determined an exact material for the bladder which will be filled with air. (Actually, we should deeply investigate that issue, too) Maybe that time I might want your helps again. By the way, I couldn't still understand the volume reduction process exactly (The last sentence). (Maybe it's because of my English)

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    Let me try to explain my last sentence better, when your fender is unloaded its cross section is a circle O but as a force is being applied it will begin to flatten, first to an oval 0 shape and then eventually until it nearly completely flattened.

    Think of a piece of tubing made of rubber impregnated cloth with both ends sealed lying on a table and you are pressing down on it with your hand. As the tubing gets flatter and flatter the tubing cross section area and therefore the volume inside the tubing will reduce and the air pressure in the tubing will rise.

    I hope this helps.

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    Quote Originally Posted by JAlberts View Post
    Let me try to explain my last sentence better, when your fender is unloaded its cross section is a circle O but as a force is being applied it will begin to flatten, first to an oval 0 shape and then eventually until it nearly completely flattened.

    Think of a piece of tubing made of rubber impregnated cloth with both ends sealed lying on a table and you are pressing down on it with your hand. As the tubing gets flatter and flatter the tubing cross section area and therefore the volume inside the tubing will reduce and the air pressure in the tubing will rise.

    I hope this helps.
    Haaa, now I understand better, thanks. But still confused about its math :( . V2 is going to 0?

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    Hi again,

    If I use an air blower for inflating and deflating all fenders, should I use any pneumatic system component like air dryer etc. ? What type of valve (or equipment) should I use to stop air flow after fenders are inflated enough?

    Regards.

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    V2 will approach 0 and the internal air pressure in the fender will approach infinity as the volume approaches 0. So, in reality the fender will continue to flatten to the point that either the impact energy of the ship against the pier is absorbed or the internal fender pressure rises to the the point that the fender material fails and the fender(s) burst.

    An air dryer is always a good idea to keep moisture out of the air handling system and from collecting in your fenders and is particularly important for the wet marine environment where your fenders will be applied.

    As for the control of the air supply to the fenders, use an pressure switch to turn off the air blower or compressor when the air pressure in the fenders has reached your desired value.

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    You really explain very good, but there are still some points that I am having trouble to understand. As the boat touch a fender the cross section will start to transform from O shape to 0 shape. At the same time, fender will stretch longitudinally. Therefore, there will be volume reduction. Is that possible to calculate the volume reduction of a fender? I am trying to say if it was a rigid vessel (piston cylinder/like image below), it would be easier to calculate the work done. But this situation seemed more complicated to me. Besides, in the work formula above, pressure will not be constant. So, I need to know the pressure as a function of volume (V), don't I?

    Boyles_Law_animated.gif

    Moreover, mechanical behaviour characteristics (e.g. deflection) of elastomers are not like metals. Am I wrong? This should be another sophisticated issue.

    Really appreciate your help.

    Regards.
    Last edited by a.yuksel; 11-20-2013 at 07:08 AM. Reason: Attach Images Direct

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    If the fender is made from a fabric reinforced elastomer then the fender will deflect but not stretch, so there will be no elongation of the fender as flattens.

    The volume of each successive flattened oval = the area of each successive flattened oval x the length of the original fender.

    The change in pressure is calculated as follows: Pgauge deflected = [(Pgauge original + 14.7) x V original / V deflected] -14.7
    where: P original = the absolute inital pressure in the fender = P gauge (which is the pressure you would read on a pressure gauge + the atmospheric pressure at sea level)
    Pgauge deflected = the fender pressure you would read on a pressure gauge at each of the deflected fender volumes
    V original = the volume of your fender without any loading
    V deflected = the volume of the fender at each of your deflection oval volumes

    The above calculations are generally standard knowledge to someone in about the third year in their university engineering education so it is becoming clear that you have bravely chosen a very challenging project for someone with your current educational background.

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    Oh, that was very humiliating, sir.

    If you are talking about "Pgage = Pabsolute − Patm", "p1V1=p2V2" and "Volume = CrossSection Area x Height", thanks but I already knew those, as you would appreciate. I think, I couldn't express my questions well. Also, that wasn't a project I chose, it was given to me by instructors.

    Anyway, nevermind. Thanks for your help attempts.
    Last edited by a.yuksel; 11-20-2013 at 08:56 AM.

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    a.yuksel, If you feel my comment was in some way humiliating I seriously appologize because it was definitely not intended in that manner.

    In fact, your project you would be very challenging for any experienced engineer, including myself, with over 20 years of background in pneumatics and fluid mechanics related high pressure and high temperature safety equipment design and development. For example, adequately addressing the energy absorption issue of the fender(s) is going to be challenging due to varying vessel weights, berthing speeds and angles, regardless of your technical background.

    My statement truly meant that it appeared to me that from some of your questions you had taken on an ambitious project related to your prior course contents. Further, if this is a project that was assigned to you by your instructor it is his responsibility, while giving you a challenging problem, to also insure that you have the prerequisite courses that allow you to successfully complete the task given a bit of assistance.

    Since this is a school project and the general rule of the forum is to be very restrictive in supporting student projects, I would not have provided you with the support I have if I thought your questions were trivial or the answers should be simple for you.

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    You're tutor is expecting a lot from you. The amount of variables in a project like this would give me a heart attack now, let alone when I was a student.

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    Ok, I was being very touchy, I guess, since a month is left to submit the project. Also, you're right, in fact, I don't have any of minimum system requirements to run this project. Especially, designing control systems will be challenging.

    I hope, I would overcome this process safe and sound.

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    a.yuksel, with respect to the above discussion, I have realized that in raising the fender's energy absorption issue I may have inadvertantly directed you to an element of the project that was neither envisioned or expected by your professor as a part of your assignment.

    Before you consume more time on this element of the project I would recommend you discuss your identifcation of this issue to see his response. Your professor may intend the project to focus on the fender deployment and storage systems and mechanisms rather than the complex issue of the fender's energy absorption. One bit of evidence of your professor's project focus intent is whether he included a ship weight and berthing speed in his description of the project because both of these are absolutely necessary as a basis for designing your fender(s) for a specific energy absorption capability.

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