EPANET: How to do a 2000 gpm Fire Flow Demand with 20 psi Minimum Pressure 2

[a_25y]

Greetings,

I am working on a very simple model. I have attached my EPANET file. I would greatly appreciate any constructive help!

What I know is that:
[li]I want to simulate a fire flow demand of 2000 gpm with a Peak Daily Demand of 1.76 gpm on a cul-de-sac. I have set my nodes with a base demand of 1.76 gpm.[/li]
[li]I have 8 inch diameter service pipes up through the cul-de-sac and an existing 12 inch diameter main pipe connected to my tank.[/li]
[li]My elevations are set according to our topographic results. [/li]
[li]I need to have a minimum pressure of 20 psi throughout the system. [/li]
[li]I need to have a connection pressure of 33 psi on my "SouthConnect2" node. [/li]
[li]My model will fail if I set my demand pattern multiplier to 1136.36 with negative pressures occurring at hour 1(I set this value because 1.76 gpm * 1136.36 = 2000 gpm approximately). Therefore, I reduced it to 150 and the model successfully runs just to see if there were any flaws in the pipe network itself and it appears that there are none. [/li]

What I don't know is:
[li] If my tank dimensions are the reason why this fails at 2000 gpm. I had set up a HUGE tank (ex. diameter = 10,000) to see if the system fails due to the tank being emptied out but regardless of how large I make the tank, the system fails with a demand pattern multiplier of 1136.36 at hour 1. [/li]
[li] If my demand pattern is done correctly to simulate a FFD of 2000 gpm. [/li]
[li] If having an extra node after my Fire Hydrant node is necessary. [/li]
[li] If my layout needs to be altered. [/li]
[li] Why do I get negative pressures at hour 1 with a demand pattern multiplier of 1136.36? [/li]

Please help me on this model. It is frustrating because this is not a complex model at all. I will greatly appreciate your help.

Thank you!

 

[cvg]

well, with 2,000 gpm through an 8 inch pipe, you have a velocity of 13 fps and friction loss is very high. it is likely that your system just cant handle it and that you are exceeding the capacity of your pipes. try lowering the fire flow until you reach your 20 psi limit. that will be the maximum flow you can handle.

 

[a_25y]

Thank you cvg.

Very true. I tried 12 inch pipes throughout and my max fire flow demand pattern multiplier is around 410. How do I reach a 2000 gpm FFD without modifying my pipes to an extremely large diameter (ex. 24", 27", 30" etc) to reduce friction loss? I would understand increasing the pipe diameter for commercial zones but this is a residential zone.

 

[adammal44]

There aren't alot of options here to reduce friction.

You can try to get smoother pipes, but the friction coefficient won't move the needle too much.
You could try an alternative suppression fluid with a lower density, but I'm not sure it's allowed by the fire code.

I don't understand your statement "I would understand increasing the pipe diameter for commercial zones but this is a residential zone." Physics applies everywhere.

 

[cvg]

well you are not giving a lot of information (and I have not tried to open your data file), but I would assume two things could be in play here:

total demand in the system with the baseline flow and the fireflow is high enough to exceed the capacity of your backbone system.

The elevation of your hydrant that is being modeled is high and the static pressure is low to begin with.

It should be easy enough to trace the flow from the hydrant back to the tank and to see what the flows, friction losses and resulting pressures are in those pipes.

 

[fel3]

A couple other points:

A fire flow of 2000 gpm should be taken out at two fire hydrants (1000 gpm each), rather than just one. Here's the rule that Los Angeles County used (may still use) that I learned back in the early 1980s:
- 1000 gpm = one FH
- 1250 gpm to 2000 gpm = two hydrants, 1000 gpm at the FH being examined and the balance at the next nearest FH
- 2250 gpm to 3000 gpm = three hydrants, two at 1000 gpm each (including the FH being examined), with the balance at the next nearest FH
- 3000 gpm to 5000 gpm = three hydrants with the flow equally split between them. The middle hydrant is the one you are testing.

The usual rule is a minimum 20 psi residual at the flowing hydrant. However, between hydrant lateral losses, hydrant losses, and the elevation difference between the ground over the main and the hydrant outlet, you can lose the equivalent of around 5 psi, so I usually target a minimum residual pressure of 25 psi at the node on the main line that serves the FH in questions (I usually don't include the FH lateral and FH assembly in the model).

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[a_25y]

Thank you adammal44. Correct, even with smoother pipes, this will not fix the problem. Correct, I do not think that is allowed by the fire code either. My statement's intention was to say it would not be ideal to increase my pipe diameter to 30", for example, for a residential development area.

Thank you cvg. Hopefully, my file can provide any lacking information on my part. Say assumption 1 is true, is my only solution decreasing my FFD and increasing my pipe diameter to ensure I do not exceed the capacity of my system? Say assumption 2 is true, how do I modify my elevation and static pressure on my hydrant in EPANET? How exactly do you trace the flow? Hazen-Willam's Formula?

 

[a_25y]

Thank you fel3. Is that even possible/realistic to do in a simple residential neighborhood model as this? To put 2 FH? Do I just input two nodes and bind them together at the end as FH? Do I set the demand pattern multiplier as 1000 for the FH being examined? What multiplier do I set the other FH? Will this appropriately simulate a FFD of 2000 gpm and a PDD of 1.76 gpm?

I suspect my problem has to do with having the end of my cul-de-sac being much high than my "SouthConnect2" node resulting in not having enough water to reach the end of my cul-de-sac.

 

[cvg]

typically fire hydrants are spaced about 500 - 600 feet apart, so on a small cul-de-sac, I would not expect to have two hydrants. Given that a hydrant can handle about 1,000 gpm, I would not expect 2,000 gpm for residential fire flow with only one hydrant. 750 - 1,000 is typical for single family, detached construction. For higher density development, perhaps a maximum of 1,500 gpm.

I suspect part of the problem is that you do not have a looped water main in the cul-de-sac.

 

[a_25y]

I agree cvg. I do not expect to put two hydrants on a small cul-de-sac. I understand now what the parameters of one fire hydrants are now. If I understand correctly, you are saying that my EPANET model I can expect at most 1500 gpm from one FH. Then why do fire codes and state regulations require a FFD of 2000 gpm if EPANET is not able to model this (especially since this model is only modeling FFD and PDD for 14 homes. This doesn't seem like I am asking EPANET to accomplish an outrageous task)?

I have increased all my pipe diameters to 12" and that allows my a demand pattern multiplier of 500. Which is relatively close to 1000 gpm (500*1.76gpm = 880 gpm).

I have tried looping my cul-de-sac to the adjacent cul-de-sac to the north and my model still fails miserably.

I'm brainstorming other solutions and I thought of one maybe I can get some feedback: I have my file set as "Auto-Length ON". If I were to turn "Auto-Length OFF" and re-do and reconnect my pipes, the default length will be 1000 feet. Could this increase of length allow more capacity throughout my system?

What I am understanding from everybody's input is that:
[li]My system can't handle the demand of 2000gpm.[/li]
[li]My FH nodes can't simulate a FFD of 2000gpm.[/li]

So is this an impossible model for EPANET to simulate? Should I do hand-calculations instead? How do I go about this?

 

[coloeng]

a_25y,

I looked at your model and you only have a static pressure of 34.67 psi at the fire hydrant when your tank is full. Based on your model, there is almost no way you will be able to get 2,000 gpm fireflow without a very large pipe. 34.67 psi as a static pressure is not even reasonable in normal demand conditions for most entities let alone trying to get a 2,000 gpm fireflow. Is this a real situation? Is this a new or existing development? What is the data in your model based on (elevations, demands, pipe lengths, tank elevations)? Because your demands at the nodes are so insignificant in relation to your 2,000 gpm fireflow and you don't have any looping in your model, why don't you just use a spreadsheet?

 

[a_25y]

Yes, I understand. To answer your question coloeng.

I have no fixed data on how I designed my tank. I am basically in the dark when it comes to designing my tank. I wanted my tank to act as the source of water from the existing water line (which would be my 12" pipe connect to my tank). But I am not sure how to modify this? I was advised in the past to use a reservoir instead but that doesn't allow me to modify my "SouthConnect2" Node connection pressure. Is 34.67 psi too high or too low?

This is a new development. I want to connect this pipe system to an existing water line down the cul-de-sac.

The elevations are based on our surveyor's topography results.

The demands are based on Herriman City, UT requirements. (PDD = 1.76 gpm, FDD = 2000 gpm, "SouthConnect2" Node Connection Pressure = 36 psi)

The pipe lengths are done relative to my scale on my backdrop image. I scaled the backdrop image accordingly(1 inch=80 feet).

I was adjusting my tank elevation until I attained a connection pressure of 36 psi on my "SouthConnect2" Node.


I like your suggestion. What method/formulas do I use on a spreadsheet? I am just so puzzled as to why EPANET cannot model this if this is a SUPER simple.

 

[cvg]

unfortunately, your problem is not with epanet, it is with your design. you wont get better results with a spreadsheet.

I would not accept any design that only provides 34 psi static pressure to the system (with a full tank). 40 psi is often used as a minimum pressure for peak daily flow.
your tank needs to be much higher in order to serve this pressure zone.

I don't think your idea to connect this development via the existing 12 inch line will work without relying on a booster pump. suggest you build a tank on a higher hill, pump the water up to that and feed your new subdivision off a higher tank.

 

[a_25y]

I see, cvg.

I am increasing my static pressure in my tank and I am achieving pressures above 20 psi in my system!

Now, I am observing that as I increase my static pressure in my tank, my connection pressure at node "SouthConnect2" increases proportionally as well. Does this mean due to my design, I will not be able to attain a connection pressure of 36 psi?

Basically, with respect to your pump suggestion, we cannot supply water to this new development without a pump.

My current design layout is shown below:
(FH)--(House)--(House)--...--(House)--(SouthConnect2)--(Tank)

To implement your pump suggestion, is this what you had in mind? If not, where/how?:
(FH)--(House)--(House)--...--(House)--(SouthConnect2)-(PUMP)-(Tank)

 

[cvg]

pump water from the 12 inch line to the tank
distribute water from the tank to the new subdivision by gravity
the elevation of the new tank should be about 100 - 200 feet (or more) higher than the new subdivision

 

[coloeng]

For the EPANet specific questions from your original post:
1) if you use a demand multiplier of 1136.36 it will increase your demand at every node with the 1.76 gpm PDD to 2,000 gpm. If you have 16 lots you have now set your demand at 32,000 gpm. Instead of using a demand multiplier you just need to set a demand of 2,000 gpm at your node with the firehydrant, set the 1.76 PDD at your other nodes and don't use a demand multiplier.
2) The model is running correctly. You get negative pressures because as you are emptying your tank, the head available decreases. The pressure at your nodes for each time step is simply the "Available Head" - "Friction Losses". EPANet doesn't care if pressures go negative or not.

Is the 36 psi the static pressure at "SouthConnect2" or is it the residual pressure with the 2,000 gpm fire flow + Peak Daily Demand? What you really need is a system curve (pressure vs. flow) for SouthConnection2. You could then model this as a pump using the system curve as your pump curve.

 

[a_25y]

Thank you coloeng.

1- I fixed the demand on my nodes. FH = 2000 gpm, Houses = 1.76 gpm. Do I set my "SouthConnect2" node as 1.76 gpm as well? Right now it is set as zero.

My tank is at 5253 ft, initial level = 110 ft, max level = 150 ft, and diameter = 100 ft with a static pressure of 47.66 psi.

The requirement of 36 psi at "SouthConnect2" is the connection residual pressure with 2000 gpm fire flow AND Peak Daily Demand of 1.76 gpm.


2- That is good to know. I am not sure how to ensure my tank is not emptied.
I was thinking of using simple controls like (for example):
"LINK SouthConnect2 open if NODE Tank1 BELOW 75
LINK SouthConnect2 closed if NODE Tank1 ABOVE 140"

With all these changes made; I get "Negative pressures at 10:00 hrs - 24:00 hrs".
If I understand correctly, you propose a system curve for "SouthConnect2" Do I set the pump curve on that node without actually inserting a pump? Do I use some random pump model and use its pump curve?

 

[coloeng]

a_25y,

You need to be working with actual values for the system curve at "SouthConnect2". These values are generated either by fire hydrant test data for the existing distribution system or an existing calibrated hydraulic model of the existing distribution system. If you don't have this actual data, you are just guessing at what can be provided by the existing system.

The system curve would be a series of flow vs residual pressure values at "SouthConnect2", i.e. Flow = 0, Pressure = y1; Flow = 400 gpm, Pressure = y2; Flow = 800 gpm, Pressure = y3; etc. The more data points you have the more reliable your model will be. You would then replace "SouthConnect2" with a pump, using the system curve as your pump curve and draw from a reservoir that has the same water surface elevation as the pump and a short virtually frictionless pipe connected between the pump and reservoir.

If there is another engineer or technician in your office or firm that you can talk to that has experience with hydraulic modeling, you should really talk to them.

 

[a_25y]

So I have reached a solution:
1 - No, a pump won't fix this problem. It is impractical. The city officials would not allow this. When is the FFD needed? In a state of emergency. What if the power goes out and the pump is not able to work? We are not able to provide the FFD to put out a fire.

2 - The best solution is to have the connection pressure at the node before the tank modified at a value where the whole system is above 20 psi (which would be around 50 psi or so). This new value would then have to be sent to the city for approval. We want to keep the pipe line no bigger than 12".

Thank you to everybody who took their time into helping!