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Air Valve - Vapor Volume Calculation

Dear All,

I have modeled my case using real air valve data. At one place it's showing large vapor volume. Surge is under the permissible limit.

Can a air valve handle this volume or what should I do to reduce the vapor volume ?

Air valve spec details also attached.

Model has add one 50mm dia double acting air valve at the pump house and 100mm dia. Triple acting air valves at about 250-300m, 800-850m, 1150m distances.

Looking forward to hearing from you.

Thank You.

Ishanka Wimalaweera

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  • If you are looking at time zero in the screenshot provided, this shows that there is a large negative pressure at the high point in question, even in the initial conditions. Would this actually happen in the real system, or would you have an air valve at the high point that would stay open in the initial conditions, with part-full/gravity flow in the downstream piping? If the latter, then the initial HGL at the high point would be zero and I would suggest modeling the high point as a Discharge To Atmosphere element, as there would be an air-gap there that the transient waves would not pass. See the following articles for more:

    User Notification "Initial pressure less than vapor pressure. At the pipe end(s), the elevation(s) or head(s) are incorrect"

    Modeling Reference - Discharge To Atmosphere

    Otherwise - the volume shown in your first screenshot would be air volume, if it occurs at an air valve location. This can be expected in a case where the hydraulic grade falls below the air valve elevation, as air will be introduced. Whether or not the volume of air that you see is "acceptable" or not is based on your own judgment, but I would point out two things:

    1. The exact max volume you see may not necessarily be accurate, because of the assumptions with air and vapor tracking. Meaning, if you are modeling a pump shutdown, the air pocket may increase in size as long as the pump remains off, since HAMMER does not track how far the air pocket moves along the adjacent pipes.
    2. The impact of the air release tends to be the most important factor. If the air is released out of the air valve too quickly, the adjacent water columns will slam together causing an "upsurge" wave. You may not have run the simulation long enough or you may need to turn the pump back on to see this happen. So, you may need to model a pump shutdown followed by startup.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

    Answer Verified By: Sushma Choure 

  • Dear Jesse,

    Thank you for the reply. We have used 5000L surge tank at the start. I hope this would be the reason for the negative surge at the initiation. I will run the suggested scenarios by you.

    Best Regards,

    Ishanka

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