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Steps to calculate Air Consumption of a Pneumatic System

  1. Jun 27, 2017 #1
    Hi there, I'm new to this forum and this is my first post so nice to meet you all!
    Since English is my second language, I apologize for some odd sentences.

    I'm a 4th year university student and currently has an internship in a Press Shop of a car manufacturer in Thailand. The project has been given to me by the training staff to find a solution for quality problems in a specific part stamping operation due to the reason stated below.

    For a quality problem, a specific part has die marks (a scratch caused by lower die setting is not in a right position when upper die is stamping). A cause of the problem is two pneumatic cylinders installed in the lower die(in order to set the die in the right shape prepare for stamping) are not supplied by proper amount of compressed air within the set timing(in short, like pneumatic need to complete its cycle within 5 seconds for the lower die to set in a right shape, but right now the pneumatic spend like 10 seconds per cycle).

    My solution for the problem is to redesign pipeline, valves or add an air tank to the line. This idea is coming up because when I calculated the air flow rate using the current pipe size a result shows that compressed air flow is not sufficient for the current operation.

    The question is "How do I know that my calculation steps utilized are proper?" so I'm here to ask experts!

    Given Specs: Bore diameter 250mm , Stroke 200mm, Rod diameter 60mm , operate at 5 bar at STP.
    The cylinder is a Double-Acting type with required Extend Stroke 1.177 sec and Retract Stroke 2.200 sec.

    These are the steps how I design for air consumption, required flow rate and pipe/valve sizing.

    1. Calculate pneumatic air consumption in SCFM (SCFMtotal = SCFMextend + SCFMretract)

    jJPSJbn.jpg
    lcgEoj2.jpg
    Source: https://cross-automation.com/blog/pneumatic-cylinder-air-flow-requirements

    2. Calculate Flow Coefficient(Cv) to select a proper valve size.

    wWfey3f.jpg
    Source: http://www.globalspec.com/pfdetail/valves/flow

    3. Pipe Sizing and system specs (pipe size, length, fitting) by using SCFMtotal obtained from (1.) to select a proper Pipe Size and effective length.

    zPfV5Es.jpg
    Source: Parker Pneumatic

    Here's some additional questions:
    Do the calculation steps above sufficient enough to design for the system?
    Is an air tank needed in the system, what is its profit?

    Is there any improper calculation steps above, I'm waiting for your advice! :smile:
    Thanks in advance! and nice to meet you all again.
     
    Last edited: Jun 27, 2017
  2. jcsd
  3. Jun 28, 2017 #2

    Baluncore

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    Welcome to PF.
    Your english is good. You seem to have enough information to solve the problem.

    What are the present diameters and lengths of the compressed air delivery pipeline?
    What is the pressure of the compressed air at the inlet to the pipeline?

    A local air tank or reservoir might be an advantage, but may not be permitted by safety, it would also need a drain to remove water. A greater diameter pipe would increase flow rate and could also provide a significant local reservoir.
     
  4. Jun 28, 2017 #3
    Nice to meet you Baluncore, and thanks for the replied.

    For your questions, the compressed air header pipe is 3/4" and then reduced to 1/2" before connecting to a 1/2" 5ports/2positions solenoid valve installed to control the air flow into the pneumatic cylinder. The pressure at an inlet is also 5 bar.
     
  5. Jun 28, 2017 #4

    jack action

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    Gold Member

    You have the air flow needed during the pneumatic cylinder cycle and you also have the air flow needed during the process cycle (which may include a rest period for the pneumatic cylinder).

    If the power source can supply the cylinder air flow, then an air tank is not needed.

    If the power source can supply the process air flow but not the cylinder air flow, then an air tank will be helpful. It will fill up during the cylinder down time and be emptied when the cylinder will be in action.

    If the power source cannot supply the process air flow, then an air tank will not be helpful. You must increase your power source capacity.
     
  6. Jun 28, 2017 #5

    Baluncore

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    Is the speed problem recent or has it always been there?
    Have you checked if a filter in the supply line is blocked?
    Are the cylinder seals and mechanical linkages free to move quickly?
    Is the ½" solenoid valve assembly working correctly? Maybe the very small bleed holes in the valve are blocked, which can significantly increase the air valve switching time.

    It would appear that the reduction from ¾" to ½" line and solenoid valve is a bottle-neck prior to the cylinder. What size are the cylinder ports? Could you increase the size of the ½" air line and/or the solenoid valve?
     
  7. Jun 28, 2017 #6
    The problem occurs frequently these day. Other factors like the die getting older everyday(which mean a friction in moving components is increasing) is also a cause of the problem. Anyway, imo redesign the pipeline system will prevent future problems potential and also solve the current problem which is worth doing more or less. This study is just for collecting essential data and write a proposal to propose a higher level of management (my boss). Changing all the pipe size, adding some tank or purchasing a bigger solenoid valve can be done if the change is worth doing(and within affordable cost).
     
  8. Jun 29, 2017 #7
    Even so (and I concur, redesigning the pneumatic circuit is worthwhile) the observation that problems occur more frequently now than before is troubling.
    Look into this. A clogged filter element can cause your symptom, and will continue to degrade operations even after pipe and valve size is increased. 40 micron is the most common element size for general protection, but replacement elements are often available down to 5 micron. This suggests another possibility - does your company stock several different elements? A finer (higher flow resistance) element installed during machine maintenance would also cause this symptom.

    FluidPower World air filter article.

    As a troubleshooting aid, you might consider installing pressure gauges immediately upstream and downstream from the filter, positioned where they are both in the technician's field of view. When the filter isn't clogged, both inlet and outlet gauge pointers will indicate nearly identical pressures when the cylinders are at rest, and swing to nearly identical, lower pressures when the cylinders have been actuated. However, as the filter becomes progressively more clogged, the downstream pressure gauge will read considerably lower pressures during cylinder operation.

    Don't know if you have one, but I've also seen malfunctioning air oilers cause havoc when they cease to operate. There are plenty of 'dry' systems built without air oilers, and that's fine. The main drawback of a dry system is increased wear, and is often addressed by adding an in-line oiler. However, once an oiler has been installed it is necessary to keep it filled, and operational. If oil delivery ceases, it won't be long before residual oil within cylinders and solenoid valves begins to "gum up" as the volatiles evaporate.
     
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