How to model a connection to an existing system

[Darrin Smith]

Hi.

I am new to EPAnet having used WaterCAD mostly in the past.  I am doing a very simple project but I am really struggling with how to model the connection to the existing system.  In the past with WaterCAD I would use a reservoir at the connection elevation connected to a pump with a 3-pump curve that emulated the pressure drop in the system.

I have something similar with EPAnet setup, but it seems to be ignoring the pump completely.  There is not added head through the pump and I end up getting negative pressures as the head is simply equal to the elevation of the reservoir.  Also, when I look at the pump output, it is saying it has a flow of 5766.34 with no units.  Is this a flowrate in gpm?  My total demand on the system is 462.75 gpm, so I have no idea what this number means.

I know in WaterCAD you had to set the elevation of your pump, but I do not see an option for that in epanet.  Should the 3-point curve add in the elevation?  This does not seem right and produces a very strange curve to be honest.

Any help would be greatly appreciated.

Thanks

[Santiago Arnalich]

Hi Darren,

If I got right what you want to do, you can just use a reservoir in the connection point (no pumps!).

Then you create a head pattern representing the pressures at different times of the day in your connection point. This is done in Browser/Data/Patterns. You add multipliers to vary the head with time as you would do with demand. Name it, for example, 1.

Then back in the reservoir properties you would add "1" in the field Head pattern.

Hope it helps

Santiago Arnalich
Coordinador
+34 671700686
skype: sarnalich
...
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Consultancy & training for Development

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[Darrin Smith]

Santiago -

Thank you for the quick reply.  I am not sure that the straight forward connection to the reservoir will work.  In short, what I am trying to do is model tying into a complex system with a single point.  So with this single point, I am trying to emulate what occurs in the system as more water is pulled through it.  For instance I have a static pressure of 130 psi, but when I flow 3000 gpm at this point this drops to 90 psi.  Calculating a 3rd point, I was able to emulate this pressure drop using a 3-point curve in waterCAD.  I have something similar in epanet, but my pump doesn't seem to be doing anything.  I have raised the reservoir elevation up to represent the HGL at the connection point, but I don't believe this will properly represent the system as I draw large flows out.

In essence there is not so much a pressure change with time, it is more the pressure changes with flow being drawn out of the existing system.  A simple reservoir will always supply me with the same amount of pressure (at the connection point) no matter how much water is being pulled.

Not sure if that makes sense or if I am missing something.  Honestly, I have not been able to find any really good documentation for the pump on epanet.  From my understanding, a pump element should simply add so much pressure to the system based upon the intersection of the flowrate on a pump curve.  This does not seem to be happening with my simple model.

Thanks again for your reply and any insight that you could give me.

[Santiago Arnalich]

Hi Darrin,

You are right, I may have omitted explaining something in my first email.

When I have done that in the past, I model the first system with the consumption of the new system concentrated in the connection point. That is how you get the yield responsive pressures. You record the pressures over your cycle period and then you use them as heads in reservoir of the second system.

As for pumps what are your troubles? Can you send a file?

I may be able to see it tomorrow.

Santiago Arnalich
Coordinador
+34 671700686
skype: sarnalich
...
www.arnalich.com
Consultancy & training for Development

On Sun, Apr 8, 2012 at 9:35 PM, Darrin Smith <dar...@gmail.com> wrote:

Santiago -

Thank you for the quick reply.  I am not sure that the straight forward connection to the reservoir will work.  In short, what I am trying to do is model tying into a complex system with a single point.  So with this single point, I am trying to emulate what occurs in the system as more water is pulled through it.  For instance I have a static pressure of 130 psi, but when I flow 3000 gpm at this point this drops to 90 psi.  Calculating a 3rd point, I was able to emulate this pressure drop using a 3-point curve in waterCAD.  I have something similar in epanet, but my pump doesn't seem to be doing anything.  I have raised the reservoir elevation up to represent the HGL at the connection point, but I don't believe this will properly represent the system as I draw large flows out.

In essence there is not so much a pressure change with time, it is more the pressure changes with flow being drawn out of the existing system.  A simple reservoir will always supply me with the same amount of pressure (at the connection point) no matter how much water is being pulled.

Not sure if that makes sense or if I am missing something.  Honestly, I have not been able to find any really good documentation for the pump on epanet.  From my understanding, a pump element should simply add so much pressure to the system based upon the intersection of the flowrate on a pump curve.  This does not seem to be happening with my simple model.

Thanks again for your reply and any insight that you could give me.

On Sun, Apr 8, 2012 at 1:04 PM, Santiago Arnalich <coordi...@arnalich.com> wrote:

Hi Darren,

If I got right what you want to do, you can just use a reservoir in the connection point (no pumps!).

Then you create a head pattern representing the pressures at different times of the day in your connection point. This is done in Browser/Data/Patterns. You add multipliers to vary the head with time as you would do with demand. Name it, for example, 1.

Then back in the reservoir properties you would add "1" in the field Head pattern.

Hope it helps

[Roy Flynn]

AFTERNOON.

EPANET will not let you place an elevation on the pump. When using a pump you need to use a reservoir to fix a known elevation head. Reservoirs have endless capacity to take accept or send water without changing water surface elevations. Pumps have to be connected to a node and set your elevation to a node.

If I understand your description and you have input pump curve number in pump parameterst should take care of your problem.

roy

Roy Flynn, PE
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>>> Darrin Smith <dar...@gmail.com> 4/8/2012 2:48 PM >>>
Hi.

Thanks

--

[Darrin Smith]

Roy,

Okay - this may be where I have an issue.  Should I set my reservoir elevation to the static pressure head then?  This doesn't seem right to me, because then my pump would be adding to this HGL.

Santiago - I am attaching my very rudimentary model as well.  It is not complete, but I was just trying to see if my connection point was working.  I have closed the south connection point, just so I could see what is going on with the north connection.

Thanks again for your help gentlemen.

[Patrick Moore]

I think how you choose to model this depends on what you are trying to replicate

1) If you are trying to replicate the upstream headloss that occurs as the flow to your modeled region then rather than a pump which add's head I would use a general purpose valve that can simulate different headloss as the flow increases.  I would model a reservoir set a the highest head seen during low demand conditions and then add a General purpose valve using a headloss vs. flow curve to represent the upstream headloss seen as flow increases to represent the headloss upstream of the point of connection.

2) If the system head is know to fluctuate you can also set a reservoir but have the head set to match a pattern representing how the system head is known to vary over the course of the simulation.  I would still then also represent the GPV to increase the headloss to represent upstream headloss during higher flow conditions.

I would never use a pump in this situation as pumps are for adding head not in representing headloss.  It is usually best to use a valve to represent headloss vs flow using a general purpose valve (GPV).

My 2 cents!

Pat Moore