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Pressure in pipes around 110-115psi. How to reduce? And How to add a Fire Demand Water Meter to model?

I have modelled a system on WaterCAD. The issue with the model is that the pipes are all flowing at around 110-115psi. I tried accounting for all the bends, valves etc through the minor loss tab for each pipe. One thing that may be affecting the model might be the pump. I defined the pump with the help of the information from the existing hydrants, like static pressure, flow etc. I selected the 3-point option to define my pump curve. But if I define my pump curve with the 1-point option, the flow in the pipes come down to 75-85psi, which is acceptable. Isn't the 1-point option for the pump definitions inaccurate?

I also had a question regarding adding a fire demand water meter to the model. How do I add this meter. It might also help reduce the pressure in the system. Any suggestions?

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  • Do you have demands (and/or storage) added to the model, and is the network balanced? (check for red timesteps with corresponding "network unbalanced" user notifications). If there are no downstream demands or storage, the pump will be operating at its shutoff head, which can be very sensitive with a 1-point curve vs. a 3-point curve.

    Regarding "adding a fire demand water meter" - if you're referring to accounting for the headloss across a meter device, that can be accounted for in the headloss coefficient section of the adjacent pipe properties, or by using a GPV node element similar to modeling a backflow preventer. If you mean that you want to add the demand/outflow from opening a hydrant for fire fighting, then as Mark said you would enter it at the node that represents that outflow point (junction, hydrant, etc). In the node properties you will see a "demand collection", which opens the demand entry dialog where you can add one or more demands consisting of a base flow plus a pattern (for use during an Extended Period Simulation).


    Regards,

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

  • Yes all the demands minor loss coefficients for bends and valves have been added in the respective pipe properties. While computing, it says that the system is balanced, but a message in the user notification says that "Negative pressures in system at one or more time steps." Source: Pressure Engine. This message is for the pump.

    For the water meter, I could not find the headloss coefficient section in the pipes properties. Do you mean minor losses? I was thinking that there might be a good amount of pressure loss through the fire demand water meter, which may reduce the pressure in the whole system. I tried using the GPV node on the pipe as well, but I need to know how I should obtain the headloss curve for the GPV. Just to make sure, the values shouldn't be the same values as I entered for the pump curve, right?

  • Hi Sayan, just to be sure, are you saying that you have added all normal demands throughout the downstream system nodes (in which case the pump should be operating away from the shutoff head). Or, are trying to model "static" conditions, with no demands in the model? This means the pumps would be operating at the shutoff head, essentially against a closed off system if there are no demands or downstream storage. Per my previous reply, this would explain the sensitivity to the pump definition, as the further away you get from the duty point/mid point of the pump curve, the more approximated the 1-point curve will be. Meaning, the 1-point curve extrapolates a curve based on the one point you provide, and on the far left side of the curve, it could end up far away from the exact shutoff head point that you would define with a 3-point curve. I would check the pump flow value to see if it is zero or if it is equal to the expected downstream demands.

    When you add the downstream flow, the pump would then be operating closer to that mid-point (more flow = less head = less pressure as you observed) and so the results would be closer between the two pump definitions.

    Regarding the water meter losses - yes, it would be the minor loss coefficient field in the pipe properties. You would need to obtain information about the characteristics of the meter in order to select an appropriate minor loss coefficient (K) or GPV headloss curve to model it. You might be able to get some information from reference books or the meter manufacturer. regarding your comment on the GPV headloss curve and the pump - correct, they would not be the same. The pump curve defines how much head the pump can add for a range of flows, and the GPV headloss curve defines the drop in head across the valve for various flow values.

    Regarding the negative pressure at the pump, this might be due to the elevation of an upstream reservoir that I assume you have. See bottom of this article: Troubleshooting negative pressures at pumps, junctions, & other node elements

    If all of the above does not help, it may be best to send in the model for review, with any directions needed to observe the issue:

    Sharing Hydraulic Model Files on the Haestad Forum


    Regards,

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

  • I have added all the demands required by the lots. In this case, it is 2gpm per lot. And I have added junctions at various points, with demands. For example, if a junction ends after 12 lots, the demand in the junction would be 12 lots x 2 gpm = 24 gpm. Please correct me if this is not how it is done in the model.

    For Fire Flow, I am taking the furthest hydrant and putting a value of 1000 gpm in the demand section of the properties. The pressure in the overall system drops to 65-75psi when I compute the model, which makes sense.

    Please correct me, even if I add the water meter, would the pressure drop extensively, considering it is just a minor loss that the water meter would be causing?

    The negative pressure at the pump has been corrected. You were right.

    Thanks!

  • Ok, thanks for the clarification. Yes, adding a fire demand would expect to cause a drop in pressure as the pump shifts on its curve and friction losses increase. As the name implies "minor" losses should add a minor amount of headloss. These are sometimes omitted.


    Regards,

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

Reply
  • Ok, thanks for the clarification. Yes, adding a fire demand would expect to cause a drop in pressure as the pump shifts on its curve and friction losses increase. As the name implies "minor" losses should add a minor amount of headloss. These are sometimes omitted.


    Regards,

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

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