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WaterCAD Multiple Pumps

Hello,

I have a network of 110 grinder pumps that connect to a force main. The grinder pumps are a combination of a 32.5 and 70 gallon storage tanks and the grinder pumps only turn on when they have reached tank capacity. I also have a master pump stations that serves as a re-pump to the force main. I am trying to determine how many grinder pumps can run at the same at peak hour (maybe from 7am to 8am for example). How do i model this in watercad?

I am using 2.5 persons or 250 GPD usage per grinder pump. I understand i will have to assume how much of the 250 gallons is used at peak hour (7am to 8am example).

Thank you for your help!!

Senhad

Parents
  • how many grinder pumps can run at the same at peak hour

    The short answer is "All of them could be running".

    They may not actually be delivering much individually, but if the wetwells are above the "turn on" level then the controls will tell the pumps to energize and won't stop until the wetwell is pumped down.  As more and more wetwells reach the "turn on" level, their corresponding pumps will energize and the output of every other operating pump in the system will be reduced by some amount.  They will then run at some reduced capacity, with the ones closer to the discharge end of the force main pumping a little more than the ones at the far end of the system.

    If you set up your pumps, wetwells and controls and run it as an EPS, WaterCad will tell you how your system will react.

    What I would be interested in is what happens to wet well levels as the pumps cycle on and off.  As I stated above, if a lot of pumps are running then none of them will be outputting much flow, and consequently your wetwell levels may continue to rise.  What I would want to find out is if any wetwell levels rise to above the overflow elevation.  If they do then you have a problem.

    It sounds like this should be a fairly straightforward system to model, although it may take some time to set up depending upon the topography of your system, how many different pump curves you have, and how many different control settings you have.  In my experience getting the controls set up properly will be the most time consuming (and most frustrating) part of the job .

    Good luck.

  • Adam, thank you for the reply. 

    I just want to add that I have modeled all the grinder pumps but i am new to this so i want to assure myself that i did it the right way. My grinder pump modeling is as follow; resevoir-pump-juntion-forcemain. Yo your question regarding what would happen to the grinder pump wetwell, the grinder pumps have a control switch that turns off the pumps if they are forced to pump against high head but also only turn on when the tank is full. Ultimately the pumps that are the furthest away from the discharge end will be prone to high head and will ultimately shut down and flood from my assumption. I do not know what the shutoff parameters (high head) are for the grinder pumps.

    My model is all set up. I technically have 2 zones i am working with. Zone 1 has a 110 grinder pumps connected to a forcemain ranging from 1.5"-4" diameter and one master pump station that is a re-pump station to zone 1. Zone 2 goes from repump station for zone 1 to a second pump station that discharges to an outlet (county line), forcemain in zone 2 range from 4"-6"in diameter (from pump station in zone 1 to pump station in zone 2) and then 8" outlet to county line from pump station in zone 2 . Zone 2 has an additional 29 grinder pumps. My goal is to determine the efficiency of this system for full development (110 + 29 grinder pumps). 

    I have a total of 4 curves, 2 curves for the 2 pump stations and 2 curves for two different grinder pump stations (32.5 gallon and 70 gallon tank). Although my goal is to analyze the entire system i am more concern with zone 1 due to higher number of grinder pumps connected to a smaller forcemain diameter and also the longest forcemain of the two. 

    I have run a simulation on the model and analyzed the results and what you mentioned above agrees with the results i am getting. As i mentioned i am new to this but i would essentially like to know how many grinder pumps will turn on and run during a peak hour, say morning at 7 am to 8 am. Considering that grinder pumps only turn on when they reach capacity (32.5 gallons or 70 gallons). And they will not all turn on at the same time due to to the fact that they only turn on when the tanks are full. I will also have make some assumptions and say each grinder pump will receive for example 50 gallons from 7am to 8 am. So i guess my question is to find out if i can set up parameters for the grinder pumps to turn on only when tanks are full and if can assign assumption to say between 7am to 8am each grinder pump will receive 50 gallons within this hour?

    Right now when i run my analysis, it is essentially running all the grinder pumps at once and like you said some are pumping and some are not. But this does not align with the assumption that not all of the grinder pumps will turn on at the same time, at least i don't think it is .

    My apologies for not providing more detail and possibly making it more confusing.

    Thank you again.

  • Mr Walski,

    Thank you, this was helpful.

  • I am assuming the pressure switch is there so the pump doesn't overheat. The end result would be the same whether the switch was there or not. If the pump overheats, it no longer works and would flood is my guess, which is same case if the pump turns off due to high pressure. I will run an EPS without the switches off and see the outcome.

    Thank you.

  • Hello Senhad,

    We have an existing article for incorporating the maximum pumps allowed to run for different grinder pump numbers based on the E/One Probability Method. Table 3, Low Pressure Sewer Systems Using Environment One Grinder Pumps has details of maximum simultaneous grinder cores for give number of grinder pumps.

    Here is the article which can guide you into incorporating this: Modeling Grinder Pumps

    However, the methodology described in the above article uses Extreme Flow Factors to represent this in SewerCAD. Do you have access to modeling this system in SewerCAD / SewerGEMS? It uses the GVF-Convex Solver to model this particular case.

    Let me know if this helps. 


    Regards,

    Yashodhan Joshi

  • Yashodhan,

    Thank you for your information. I don't have access to Sewer CAD, only WaterCAD. I am modeling an E-One grinder pumps and do have a question about setting up the pump curve in WaterCAD. Since the E-One pump pump curve is a near vertical head vs. discharge. What is the best way to set up in WaterCAD using a multiple points to let WaterCAD know the shutoff condition (The head the pump produces at 0 flow) for the E-One Extreme Series? This is reffering to Gary Adams information given earlier.

    I would like to set my shutoff at 185 feet TDH but that is not at zero flow but instead somewhere in 7.8 GPM according to the E-One curve. Zero discharge is off the pump curve and would be a lot higher than the pump is rated if the line is produced to zero GPM.

    Right now i am using a multiple point to establish my curve and using 0.0,7.8,11.0,15.0 for my flow (GPM) and using 185, 185, 92, 0 respectively. Am i on the right track with this ?

    Thank you

  • Hello Senhad,

    One approach you can follow here since you are using WaterCAD can be as below;

    1. Instead of individual pumps and wet-wells (tanks) in WaterCAD model these flows as inflows (negative demands) at junctions to force a particular flow downstream. For example, if a point has 2 grinder pumps set up each pumping at (say) 11 gpm, add a negative demand of -22 gpm at that point to force this inflow downstream. Do this for all the points / junctions where the grinder pumps are setup.

    2. Once setup calculate the total negative demand you have. If the total negative demand comes to about - 1045 gpm i.e. 1045 gpm of flow is being forced into the system, correlate this with the number of grinder pump cores you have. So, if each grinder pump is having 11 gpm of output, that means there are 1045 / 11 = 95 pumps.

    3. Refer the Table 3 I shared above and find out the maximum number of grinder pumps operating simultaneously. For 95 grinder pumps only 8 can operate simultaneously as per the table. So, the multiplying factor would be 8 / 95 = 0.084 for the total demand.

    4. Using demand adjustments use the factor of 0.084 to multiply all demands since only 8 grinder pumps will operate simultaneously. 

    5. Compute the model and observe the flows. If a junction has -22 gpm of demand (forced inflow), then with a factor of 0.084 it would be -1.848 gpm. Since there are two grinder pumps each one has an output of 0.9 gpm.

    6. Generate the system head curve for your model and observe the head vs flow relationship for 95 grinder pumps operating. This will give you an idea of how the overall system is responding with only 8 pumps operating simultaneously at a time.

    The above approach will only provide a rough basis for understanding the system response in an overall way. If we try to setup the system as in SewerCAD it would be very complicated with individual demands for each pump combination and then having demand multipliers for each of them.


    Regards,

    Yashodhan Joshi

Reply
  • Hello Senhad,

    One approach you can follow here since you are using WaterCAD can be as below;

    1. Instead of individual pumps and wet-wells (tanks) in WaterCAD model these flows as inflows (negative demands) at junctions to force a particular flow downstream. For example, if a point has 2 grinder pumps set up each pumping at (say) 11 gpm, add a negative demand of -22 gpm at that point to force this inflow downstream. Do this for all the points / junctions where the grinder pumps are setup.

    2. Once setup calculate the total negative demand you have. If the total negative demand comes to about - 1045 gpm i.e. 1045 gpm of flow is being forced into the system, correlate this with the number of grinder pump cores you have. So, if each grinder pump is having 11 gpm of output, that means there are 1045 / 11 = 95 pumps.

    3. Refer the Table 3 I shared above and find out the maximum number of grinder pumps operating simultaneously. For 95 grinder pumps only 8 can operate simultaneously as per the table. So, the multiplying factor would be 8 / 95 = 0.084 for the total demand.

    4. Using demand adjustments use the factor of 0.084 to multiply all demands since only 8 grinder pumps will operate simultaneously. 

    5. Compute the model and observe the flows. If a junction has -22 gpm of demand (forced inflow), then with a factor of 0.084 it would be -1.848 gpm. Since there are two grinder pumps each one has an output of 0.9 gpm.

    6. Generate the system head curve for your model and observe the head vs flow relationship for 95 grinder pumps operating. This will give you an idea of how the overall system is responding with only 8 pumps operating simultaneously at a time.

    The above approach will only provide a rough basis for understanding the system response in an overall way. If we try to setup the system as in SewerCAD it would be very complicated with individual demands for each pump combination and then having demand multipliers for each of them.


    Regards,

    Yashodhan Joshi

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