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WaterGEM the System Head is lower than the pump curve settings

I am updating an existing water system model for a city with about 20,000 population.  I am having two problems. I have searched the Bentley Community forum, but I have not be able to fix the two problems listed below. I'd appreciate the advice from expertise.

1. There are four pumps pumps at higher flow rate outside the pump curve due to low system head (see the attached 'User Notification' and 'Pump Result').

2. Fire flow test results were computed to calibrate the model. The difference between the actual and model result of the static pressure are greater than 5 psi (between 4-6 psi.)(The residual reading difference between the actual fire flow test and the model results are 2-4 psi.)

Following is a list of updates that have been made:

1. Imported new piping, wells, pumps, storage tanks using Model Builder from AutoCAD. All pipe's C-values were updated based on the construction years.

2. Update pump curves for new pumps

3. Realized that the elevations near the river banks do not match with an USGS map. Imported elevation using TRex, then subtracted 8' using Global Edit in Junction Flex Table. Made sure the elevation of pumps, wells, and tanks remained the same.

4. Added the hydrants where the fire flow tests were conducted. Set the elevations of the hydrants to be 9.25' above the watermain

Please let me know if I should provide more data.  Thank you in advance for reviewing my questions.

  • Hello Yoko,

    This warning message usually indicates that the pump is operating beyond the user-entered pump characteristic curve (pump definition). This will happen when the pump definition is not defined all the way out to zero head. WaterCAD and WaterGEMS will use extrapolation on the pump curve, so that it extends all the way to zero head. The pump can operate past the user entered range if the system conditions warrant it. The pump curve that you see in the system head curve viewer is an extrapolated pump curve.

    The warning message may indicate that the real pump may not be able to operate correctly in this condition. Basically, the system conditions are such that the head that the pump needs to add is less than the lowest head value in your pump curve.

    The following link has more information on this user notification: communities.bentley.com/.../24830.pumps-are-operating-at-a-much-higher-rate-than-the-design-point-pump-exceeds-the-maximum-operating-point-specified-for-the-pump-curve

    You should take a look at the demands in the system, as well as elevations at tanks or reservoirs downstream of the pumps. Demands and hydraulic grade at boundaries will have an impact of the pump results.

    If that doesn't help, let us know. We may need to see a copy of the model files. There are two options for sharing your model on Communities, whichever you choose please be sure to zip your files first. The first option is to attach the zip file containing your model to your reply on the forum using the Advanced Reply Editor (you'll find the link below and to the right of the reply box). If your data is confidential please use the instructions in the link below to send it via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley employees.

    communities.bentley.com/.../7079.be-communities-secure-file-upload

    If you upload the file to Sharefile, please post here with the name of the file so that we know it is available.​

    Regards,
    Scott
  • Scott,

    Thank you so much for your timely reply and advice. I reviewed the demands downstream of the well pumps.
    For Well Pump 2 and 8, they are near by Tower 2 and I wonder if that is why the system head is lower than the pump curve.
    For Well Pump 6, 7, and 10, there are many junctions at 0.79 gpm and some at 1.14 gpm, which are relatively high demands. However, I did not see a huge demand. For Well Pump 20, there is no abnormal high demand; however, the pump is connected with Well Pump 21 and I wonder, the way I connected is creating some problem.

    Since I did not find errors in demand, I will upload my model. Thank you so much for reviewing it. In a mean time, I wonder if I should ask the City to provide a real example of the system demand and pumping rate at each well pumps at certain time of the day. I'd appreciate your advice if this kind of data would help determine the error. Thanks so much again!

    Yoko
  • We offer some tools to help you visualize what is happening in your system. Usually, the problem lies with a tank level on the discharge side of the pump.

    Try drawing a profile (View > Profile) and follow the hydraulic grade lien from a tank on the suction side to one o the discharge side to see if there is an error in talk levels .

    Right click on pump in the model and pick , Pump Curve to look at the operating point.

    Right click on the pump and pick System Head Curve.

    These tools should help you find the error in your model.
  • Tom,

    Thank you so much for letting me know about tools in the profile. I somehow did not use this function before and was using more non-visual method. I have not found answers to my problems yet, but looking at profiles increased my understanding of what is going on in the model at deeper level. The profile view graphs will be useful when I discuss this matter with my project manager. Thank you so much!!
  • Hello Yoko,

    In looking at the model, I would also check the controls. Right now there are several pump that are turned off because of the controls applied to them. If these pumps were on, less flow would be required from the other pumps and they may then fall on the pump curves. If the controls are not supposed to be applied to the steady state run, please set the calculation option "Use simple controls during stead state?" to False. If the controls are supposed to be active, make sure that the tank elevations and the controls themselves are set properly. Profiles may help with this, especially if the pumps are meant to flow toward the pumps mentioned in the controls.

    Regards,
    Scott