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Modelling startup of VSD booster pumps in series - first pump already operating

I am trying to model a 'changeover' in operation for a transfer system with a number of booster pumps in series (Pump Stations A, B, and C) to determine any transient impacts.

Initially, both PSA and PSC are operating at a given flow rate. The client's intended procedure for starting up PSB is as follows

- ramp down PSA and PSC to 'idle' speed (~50% speed)

- turn off PSC

- turn on PSB and ramp up to idle speed

- turn on PSC and ramp up to idle speed

- ramp up all pumps to full speed (at a higher flow rate than the initial conditions with only PSA and PSC operating).

I have all of the relevant speed-change patterns. My problem is that if I try to use the variable speed/torque pump type, although the pumps all turn on at the correct time, I don't get any "use" out of PSB, as both PSA and PSC go back to their original flow/head conditions - I can't figure out how to increase the flow and head to the new desired conditions.

As a workaround, I tried to run this scenario in two parts, first assessing the shutdown to idle speed (no problems) and then looking at a startup, but then again ran into problems with PSA when I tried to start it at an initial speed factor of less than 1.0 and ramp it up to a new flow condition alongside the others. So the THIRD thing I tried was to run PSA at full speed, ramp it down to idle in an initial (dummy) time period, and then start up the other two pump stations and start recording my transient conditions. Again, I could not get the full three-pump flow.

Finally I have tried to do a complete pump startup, with all three pumps off to begin with, ramping up PSA first to create the desired initial condition before starting up the other pump. But to do this I have had to artificially adjust the system with a downstream PSV to keep the pipe full and prevent the occurrence of vapour pressures across the highpoints, so that although I can make all the pumps start up in accordance with the patterns, the overall HGL is artificially inflated. 

I hope this all makes sense - essentially I'm wondering whether what I'm trying to do (changing target flow rate of a speed-controlled VSD midway through a simulation) is actually possible?

Thanks, Liz

  • Hello Liz,

    We may need to see a copy of the model to getting a better understanding of the setup of the pumps and the model in general. However, there are a few things that you can look at to see if they help. First, for the pump that is initially off, make sure that the nominal head and flow are accurately entered. This head and flow is used by the program to assign the accurate four-quadrant pump curve used by the transient solver.

    Second, if the pumps that are initially one have a relative speed factor that is less than 1.0, you will need to make sure the pattern on the pump is correctly reflecting this. The following support solution has some information on this: communities.bentley.com/.../16427.initial-surge-occurs-when-the-initial-relative-speed-factor-is-less-than-1-0

    If this doesn't help with the issue, please provide a copy of the model files. There are two options for sharing your model files on BE Communities. If you would like the files to be visible to other members, compress the files into a zip file and upload them as an attachment using the ‘Advanced Reply editor’ before posting. If your data is confidential, you can follow the instructions in the link below to send it to us via Bentley Sharefile. Files uploaded to Sharefile can only be viewed by Bentley.

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

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

    Regards,
    Scott
  • Thanks Scott.

    As far as I can tell, the nominal head and flow are entered correctly as for a normal pump startup analysis. I wasn't aware of the relative speed factor / pattern multiplier difference as per the suggested, but that unfortunately still hasn't helped - I cannot make PSA run to the required conditions. I have uploaded the model to Sharefile, saved as "startup testing".

    Thanks, Liz
  • Hello Liz,

    The run time for this is much higher than you see in most HAMMER models, at 18000 seconds. Is there a way that this might be reduced? I only ask because it will otherwise take a while to test various options and see about getting viable results.

    Regards,
    Scott
  • Hi Scott,

    Yes, I know, it's a bit painful, but this is the startup regime that the client wants to test/confirm as it's a reflection of their current operating procedures. I guess for the purposes of making the model run correctly, we could reduce everything so that it happens faster? And then once the pumps are running as they need to, we can extend the timing again...
  • Hello Liz,

    A smaller run time would help with troubleshooting, but it would also impact the results. For instance, currently PS B takes 720 seconds to reach half speed. This is longer than model HAMMER runs, and the transient impact of such a slow buildup of speed will likely be pretty small.

    A better option may be to model these as separate events, since the time scales suggest that is what they are, at least in turns of transient time scales. For instance, model a case where PS C turns off. In a separate model, model PS B turning on, and so on. That would eliminate the need to force the pumps to operate in a single long model run and still will likely give you viable results for the individual events.

    Regards,
    Scott