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Gas Blowby Case

Gas Blowby is the discharge of gas from a process component through a liquid outlet. It can be caused;
- Failure of a liquid level control system.
- Or in advertent opening of the control valve by pass.

One of the effects of gas blow by can be overpressure in a downstream component. Of course, the low level is the detectable condition that indicates gas blowby may be occurred.

This picture below show gas blowby case

For instance in a separator system, to determine whether this case applicable or not, check design pressure of the equipment in downstream control valve. If the design pressure is equal, the gas blowby will not cause overpressure in the downstream equipment. Conversely, If the design pressure of the downstream equipment is lower than the upstream equipment, when control valve fail, the gas will flow to the downstream equipment and cause overpressure.

Please note, gas blowby case is applicable if only the design pressure of downstream equipment lower than the upsteam pressure.

The required load capacity of PSV can be determined based on the mass balance in the system after control valve fail is occurred.

Here is just simple case.

The most often questions which is raised by new process engineer in calculation load capacity gas blowby is the formula for determining the maximum flow through the control valve

The maximum flow through control valve is depends on the control valve Cv and the pressure drop across the control valve. The following picture show an example formula from one of control valve Vendor; please note, each Vendor has their own formula that might different in each other. I give this formula to show you what the variable that has affect to flowrate through the control valve

My friend, for detail calculation in project stage, I suggest you to use INSTRUCAL software for determining the maximum flow through the control valve.

After the maximum flow through control valve is calculated, the required capacity of PSV at downstream equipment can be calculated (for simple, Load = Maximum flow stream 3 – Normal flow stream 4)

Thank you for reading my friend, that’s all I can share to you today. Correct me if I am wrong…

" Selamat liburan panjang :D , semoga ada pencerahan hidup yg bermanfaat yang kita dapatkan... ( don't know how to express this in english :D) "

21 comments:

  1. Your article is very useful. I am also currently working on overpressure protection for gas blow-by case from Separator to Condensate Surge Drum.

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  2. Can we avoid gas blowby case by simply putting SDV at liquid outlet line? This SDV will close should low level shutdown occurs. Thanks

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    Replies
    1. NO, you cannot take credit of SDV at the liquid outlet line which will close at the low level to avoid Gas blowby case. you need to analyze gas blow by case though you have SDV at the liquid line.

      Delete
    2. Ashok, I agreed with you.

      Matsu, for addition , if the liquid level transmitter fail or something wrong with level transmitter, SDV might be will not be closed even the liquid level become zero. Therefore gas blow by case still applicable.

      Delete
    3. If a SDV is added to the liquid outlet line it needs to be completely independent from the normal process control layer for level control. Hence the SDV initiator will be an independent level transmitter thus forming a Safety Instrumented Function (SIF). This would then mean that credit can be taken for SDV (this is common practice in industry and HAZOPs).

      Delete
  3. If there is a pump before the control valve at the outlet stream, do gas blowby case still applicable?

    ReplyDelete
    Replies
    1. Ahmoi,
      I understand that you imagine the system is pressure vessel which liquid level is controlled by control valve installed at discharge pump line.
      When the level control valve fail open, my understanding the pump will operate at higher load, Pump has a protection regarding the overload or over speed,.Therefore pump will trip and gas blowby will not be occurred.

      Delete
  4. this is very good article however i would like to add that what will be the pressure drop through control valve in gas blowby case where CV is selected based on liquid pressure drop.

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    Replies
    1. The pressure drop of control valve is in correlation with relieving condition ( or some engineer consider set pressure condition)
      The more important point is how to determine the maximum flow through the control valve with certain Cv value ( which is designed for liquid but after blowby occurred, the servise fluid is gas phase)

      Delete
  5. hi. How can I know if conditions are critical or subcritical?

    i'm lost

    ReplyDelete
    Replies
    1. Elza, please read this post;
      http://process-eng.blogspot.com/2011/03/critical-subcritical.html.

      should you still have any comment, please..

      Delete
  6. hi gents,
    about taking credit of the SDV, it could be but the credit factor is very low (on the SIL assessment), the credit should be taken by the increasing of design criteria of the lines and equipment's after the LCV (level control valve), and the installation of appropriate Pressure relief system (PSV's for example), some engineer combine between the calculation of the CV of the LCV and the relief system capacity in order to reduce the cost of the relief system but on safety point of view is not good because under some scenario of washout case or the change of the LCV it became a serious problem; the best is to calculate the relief system based to full gas flow through the pipe without considering the LCV.

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  7. I do not really understand why blow by case can only be considered if design pressure at downstream equipment is lower than upstream one. Can you clarify please ?

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    Replies
    1. I ask this question because I think that even if the design pressure of downstream and upstream are the same, in case of blowby of gas, there will be an accumulation of gas in the dowstream equipment and thus an overpressurisation. I really not understand..

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    2. The cause of this case is failure of control valve. The flowrate thru the control valve is depend on the pressure drop. If the design pressure of downstream and upstream system is the same, the pressure downstream of the control valve will never reach the design pressure.

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  8. Thank you very much. It is clear now !

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  9. This comment has been removed by the author.

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