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Negative pressure equal to vapor pressure developing at pump suction head readings only found in extreme pressure report

Dear All,

I am working on hammer bentley version 10.3.3.72 and whenever i generate the extreme pressures report, i get negative pressure equal to vapor pressure at the pump suction side. Is this normal, and if so, is there a means to eliminate such negative pressure on the suction side. or this is something that is a glitch in the software and should be disregarded.

Please advise

Parents
  • Hello Ayman,

    Your colleague also submitted a Service Request for this. Here is a copy of the reply that I just sent:

    The initial pressure at the suction nodes is already very low – less than one meter. This is because the upstream boundary reservoir is set to an elevation only half a meter above the elevation of the pump and the suction nodes. It will be almost impossible to maintain positive pressure on the pump suction side during a transient because it only takes a drop of half a meter to cause the pressures to become negative. Meaning, the initial hydraulic grade at the suction node and pump suction side is 831.5 m while the physical elevation is 831.0 m, so only a 0.5 m drop will result in a negative pressure.

    Typically you would design a pump in the initial conditions (steady state or EPS in WaterCAD/GEMS) to have adequate NPSH so that this situation does not happen. You will need to increase the initial suction pressures (for example raise the elevation of the reservoir) in order to have a chance of maintaining positive suction pressure (and avoid cavitation) during a transient event.

    Note that node elevations in HAMMER need to be set to the actual pipe elevation and not ground elevation, for an accurate calculation of pressures. Reservoir elevation represents the boundary hydraulic grade (for example water level in a tank or well which is assumed to stay at a constant value for the transient simulation). See:

    https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/9454/what-does-node-elevation-represent-in-watercad-watergems-and-hammer


    Regards,

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

  • Well, i wish it is that simple. The negative pressure that i get i do not read from the nodes list, but from the extreme pressures report. Some files that i work on have no negative pressure in the node list but have negative pressure, precisely vapor pressure in the extreme pressure report. 

  • Ayman,

    Can you provide screenshots or detailed steps to reproduce, so I can provide a more clear answer to your question? (what specifically do you mean by "nodes list" and which field are you looking at and comparing to the extreme pressures report?)

    The root cause of the negative pressures during the transient simulation is due to the explanation in my previous reply regarding the very low initial pressure.

    If you are saying that you see a difference in the minimum pressure reported in different places, it could be due to the minimum pressure occurring between reporting timesteps. See:

    Results from the Extreme Pressure and Heads table are different from results found in the transient profiles

    Transient maximum result is different from the highest value seen in a time history graph

    If this does not help, please provide additional details to help us understand your specific question/concern so that we can provide a more specific answer to address it.


    Regards,

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

Reply Children
  • I have sent the info to your colleague Mr. Deepjyoti Chakraborty. If you can provide me with your email i can send you the info

  • Deepjyoti forwarded me that email which resulted in me opening the Service Request. The email points out the large negative pressures but it is not clear what your specific question is beyond understanding why there are such large negative pressures. The answer to that has been provided further above - the initial pressures on the suction side are too low, so even a very small transient would easily cause them to become negative. You can visualize this better by creating a short profile that just covers from the reservoir through one pump, then animate it in the Transient Results Viewer. 

    You will need to check the conditions on the upstream side of the pumps to check for elevations that were not entered correctly (see for example my point about using pipe elevation, not ground elevation) and use your engineering judgement to decide how to prevent negative pressures from occurring. 

    A pump shutdown results in the water column suddenly stopping, which means the suction side would tend to experience an "upsurge" as the water column suddenly stops, but that wave quickly reflects off the boundary conditions, resulting in a negative pressure. This typically happens rapidly due to the short length of pipe on the suction side. In short, seeing negative pressure on the suction side is expected in a sudden pump shutdown transient event if the initial pressure is not adequate. When the pressure falls to the vapor pressure limit, a vapor pocket will form and the pressure cannot drop any lower.

    If you need further assistance, please provide the specific steps to reproduce and clarify your question either by posting the information to this thread, or by responding to my email from your Service Request (the email would be from Support@Bentley.com) 


    Regards,

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

  • So what you are saying is that the negative pressure developing at the suction side is normal and un avoidable considering the conditions set in the file. 

    But this does not make sense. If i have a 3 m high water tank with adequate volume, then i would still experience the same problem. What you are suggesting is having an elevated tank. 

  • Yes, the results are "real" based on the data input so if you do not expect these results, then there may be a problem with the data input. It is highly unusual to have such low initial suction pressures and that is the main contributor to the negative pressure problem, but there could certainly be other problems with the model setup compared to the real world conditions that are also impacting the negative pressures.

    Generally speaking when your pump shuts down, the positive surge wave reflects back to the pump suction side causing a negative pressure, but the severity/magnitude of this effect depends on many factors, including the pump characteristics. At the time of pump shutdown, HAMMER assumes that electrical power is cut, and the pump spins down based on its inertia. See more explanation here and here. So, the primary factor influencing pump shutdown results is often the pump inertia set in the Transient tab of the pump definition. As an example to illustrate, if you set the inertia ten times higher, you will see the pump impeller takes longer to spin down and thus the change in momentum of the water column is reduced and the minimum pressure on the pump suction side is greatly improved. This highlights the need to confirm the correct pump inertia. Consider the points mentioned in this article.

    Note that with a hydropneumatic tank on the downstream side, check valve slam tends to be a main concern. The pump check valve assumes instant closure upon first detection of reverse flow. If you need to see a slower-closing check valve (which can sometimes produce more conservative results due to reverse velocity), set the pumps as control valves with a large closure delay like 9999 seconds, and use check valve node elements downstream of the pumps. See: 

    Modeling a Check valve or Control valve in a pump for transient analysis

    Modeling Reference - Check Valves in HAMMER

    Elevations are also a large factor here. For the pump elevation of 831 m - is that the ground elevation or the pipe elevation? If for example that is actually the ground elevation but the pipe elevation is much deeper, than the pressure results are essentially not accurate since they would be the pressure as measured from the ground, and the negative pressure conditions might not actually occur.

    Some other observations in the model:

    1. The check valves set in the pipes downstream of the pumps are not necessary because you have configured a check valve built into the pumps.
    2. The transient calculation options are set to produce output for all elements, which can cause the report period to be increased, making it more difficult to view detailed results. See: Improving performance of transient simulations (recommended reporting calculation options)
    3. The vapor pressure transient calculation option was set to zero, which causes HAMMER to use the default of -10 m. 


    Regards,

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

  • I have been trying out the items pointed out in your reply, and what I got is the following:

    • I will always be getting negative pressure on the pump suction side, unless I have an elevation on the tank of around 30m or more
    • This negative pressure will only be appearing for a fraction of a second on the suction side
    • I have reduced the suggested time step by the software from 0.01 to 0.003, this seems to do improve the results a bit

    So my conclusion is that negative pressure cannot be eliminated no matter what we do if the current conditions in the model, which represents the live situation on site, are kept. Is that right? and if so, can this condition be lived with, as we modeled the extreme condition where the tank is almost empty, and pumps are working. the water column in the tank is around 3.5 to 4 meters.

    I need to issue a statement to my client to tell him what to do.