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How to size a centrifugal pump in SewerCAD?

Hi there.

I come from the pumping industry in Australia and trialing the use of SewerCAD at the moment, we use steady-state modeling via excel sheets and also some of the other modeling tools available in the market. 

I have gone through the exercises available on Youtube and also from your learn server, where it doesn't actually represent a real-life problem, the pump curves are not just available for a project as you show, and in most cases, for a model it is the responsibility of the designer to choose the pump/s with the help of suppliers, for this, the designer is expected to provide a duty point ie. flow and required head. 

please see attached for a simplified real-life problem where we are required to work the pump duty point so we can select a pump. We can do this in various other ways as I have mentioned above and what I want to please know is how to tackle this from SewerCAD. How would you model a system to work out the pump's duty point without placing the pump? 


PDF

explanation of the scenario:

we have a wet well and the pump sets are submersible centrifugal sewer pumps. 

the incoming gravity pipework is 225mm Ductile Iron pipework, ID is 240mm. The peak incoming flow is 12L/s, and our requirement is the match this flow via the pump selection. ie. the pump flow rate has to be min 12L/s. The levels are provided in the model and based on this flow rate we have selected the forced main as 125mm PE100 PN16 (ID is 102mm)

In some of the other modeling tools, we can place the pump as a fixed flow pump and when we run the model it would develop the required duty point.

while searching about this topic, I had come across this: https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/16879/estimating-a-pump-curve-for-a-model but not too sure if this works or not. 

your help in this regard is appreciated and believe this will help many. 

Cheers,

Imran

Parents
  • The overall steps work like this.

     

    1. Pick a design flow for the pump. The value should be a little higher than the peak inflow. The inflow will vary over time and will rarely high the peak.

     

    1. Size the force main such that the velocity is greater than 2 ft/s and less than 8 ft/s when the pump is on.

     

    1. Generate the system head curve for the pump. Generate the system head curve. You need to use the GVF-Convex solver for this. This calculation will show you the head you need to provide at you design flow rate.

     

    1. Now pick several alternative pumps which produce that flow at that head. Run each through the model (use scenario manager). Determine the operating point of reach pump and see what the pump efficiency is at that operating point.

     

    1. Check the operation by running an extended period simulation to see how the pump will cycle, not just at peak time but on normal days. If it is cycling too frequently, you may need to upsize the wet well or downsize if the cycle is too slow.

     

    1. Those steps should get you a reasonable pump. However, to do a really want to pick the best pump use WaterGEMS’ scenario energy costing tool to determine the present worth of energy cost. The biggest component of life cycle cost is usually energy and sometimes the least expensive pump is the most costly to run.
  • Hi Tom, 

    appreciate your response to this, 

    I am quite familiar with everything except item #3, I am not sure how to model this in SewerCAD without giving the pump curve, if you or someone could explain this that would be beneficial. 
    Thanks in advance 

    Imran

  • Hi Tom, 

    if I have candidate pumps available I know how to create a system head curve, my question is regarding the first step, how do we know the required system head if the flow is known (I can do this in various other ways but I would like to know how this is modeled in SewerCAD).

    So once the required duty point is known we can obtain the most suitable pump in the market / based on client specifications. I have been trying to work out how to model a system when you don't have a pump selection and find that duty point if flow is known (as I have explained in my initial post).

    Cheers.

  • The wiki below should answer your questions.

    https://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/24627/understanding-system-head-curves-in-watergems-watercad-and-sewercad

    If you want to find the head manually, you can remove the pump from the model and put an inflow at the pump discharge point equal to the design flow. The model will tell you the head needed. Subtract the head inside the wet well and that remaining head is the head the pump needs to add.

  • Hi Tom, thanks for your help so far, and I have gone through the wiki you have provided, and understanding it relating to SewerCAD seems to be a bit hard for me especially as I have recently started with SewerCAD,

    I have quickly mocked up a model and have attached it here (this is not relevant to my initial info provided earlier), can you please have a look and let me know how would you find out the head needs to be if the required flow is to be around 10L/s? 

    the catchment has a 500 population with 180L/d as the average per capita and I have used 5 as the factor for peak wet flows. That comes closer to 5.2L/s flow into the pumping station, but due to the scouring requirement on the forcemain the pumped flow rate has to be a min of 10L/s,

    How would you change the model to get the required head without including a pump curve?

    Sorry about my queries as I am trying to get to the bottom of it with your help,

    appreciate your help. 

    Cheers.

     TestPSsystemhead.stsw

  • Hello Imran,

    I can see that you have only shared the "TestPSystemhead.stsw" file. We would also need the "TestPSystemhead.stsw.sqlite' file to open the model. The best option is to go to File > Save as Package and share it with us.


    Regards,

    Yashodhan Joshi

  • Hi Yashodhan,

    thanks for your response, please see below. 

    Hope I have packaged it correctly. 

    Cheers.

    Imran

    TestPSsystemhead.stswpkg.zip

Reply Children
  • Hi Imran,

    I checked your model but it doesn't have any pump curve defined. If your intention was to determine the pump curve see the following steps I followed. I have sent you the modified model via private message.

    1. Using the information from the earlier article you went through (Estimating Pump Curve) I setup the model in such a way that I could estimate the head required. See the scenario "Without Pump" in the shared model. With the difference in HGL at J-1 and J-2 being 13.68 m and flow required as 10 L/s (as per your minimum required) I could setup Test Pump Curve-1.

    2. I ran this model with pump in scenario "With Pump @13.68 m head / 10 L/s". In this scenario I checked the System Head Curve as per the article shared by Tom. The system head curve gave me value of head as approximately 18.75 m and flow of 5.2 L/s (which is what is actually calculated).

    3. With these values I setup Test Pump Curve-2 and ran the scenario "With Pump@ 16.75 m head / 5.2 L/s".

    You can refine the pump curve like this iteratively. What the system head curve shows are the required values. In your case, you require 10 L/s as flow due to scouring requirement, so you can change the pump curves accordingly.

    I have leveraged the Scenarios and Alternatives functionality in our products here.

    Also, since you are new to SewerCAD, I would also recommend you go through the below article which points to various learning resources;

    Learning Resource Guide for New Users of SewerCAD

    Let me know if this helps.


    Regards,

    Yashodhan Joshi

    Answer Verified By: Imran Saheer 

  • Thank you Yashodhan for your time and effort to clarify this issue, this is what I like about Bentley, a fantastic team to respond to whenever you needed the most.