WaterCAD Dead End Hydrant Model

[bschlang]

I'm not extremely familiar with WaterCAD v8 and working on a smaller commercial site that needs a service stub and fire hydrant. Existing infrastructure consists of a 16" main in the frontage road to the project with an 8" lateral stubbed at the project entrance.

I've set up a model with a reservoir at an elevation equal to 65 psi. The model goes, reservoir - 16" line - junction(1) - 8" line - junction(2) - 6" line - junction(3). Junction(3) has a small demand added to the system. I've run my typical scenarios and all the pressures stay within the reason of the 65 psi, everything looks good. Now to run my fire flow of 1500 gpm. I cannot get 1500 gpm at junction(2) or junction(3). This seemed unreasonable as the total distance from the 16" is less than 100 ft. It appears that my flow is being limited at junctions(2 & 3). For example I can only pull 780 gpm off junction(3).

I thought I should have adequate capacity to pull over 2500 gpm through a 6" pipe off an 8" off a 16", with sufficient capacity. I increased all the pipe sizes to 24" except the 6" and can still only pull 800 gpm off. It seems as though there is something limiting my amount of flow.

Any advice will be greatly appreciated. I don't know if WaterCAD has some limiting capabilities regarding dead ends and reservoirs. I've looped the system and was able to get greater flows, but it is not true representation of the model.

Thank you in advance.

Brett

 

[bimr]

There does not seem to be any problem with the model. You are not going to get 2500 gpm through a 6-Inch pipe. That would be a velocity of upwards of 25 ft/sec. Try a larger pipe size.

 

[bschlang]

This may be a silly question, but then how would standard fire flows (ranging anywhere from 1500 gpm to 3500 gpm) be required from a hydrant that is typically off a 6" lateral?

I ran another model with a hydrant attached to a 16" main with plenty of pressure and attached to reservoir and I agree with what you say it just doesn't seem like I can get that type of flow.

However, I ran yet another model and looped the system and I was able to get approximately 40% more flow through the 6", but not quite enough, but it also not an accurate model. The velocities were also well below 10 ft/s.

So if the 6" pipe size limits the amount of flow why would it change if the system was looped. I don't understand the principles at play here with WaterCAD and I greatly appreciate anymore imput.

Thanks.

 

[bimr]

Each hydrant will typically only supply 600 to 800 gpm.

The water pressure at the hydrant should be not less than approximately 20 psig when the hydrant is being used.

Note that fire truchs are pumping as well. The suction side of the fire pumps are connected to hydrants.

Looping water mains helps with the pressure loss because it reduces the upstream pressure losses.

 

[bschlang]

Thank you, so that means in order to meet a fire flow requirement of 1500 gpm I would likely always need 2 hydrants?

So then in WaterCAD what is the best way to model hydrants, should you add actual 6" lines with hydrants and look to total flows based on the specified hydrant spacing. Or pull fire flows off mains so that your results and data show your fire flow is met.

Also out of curiosity when a fire truck is connected to a hydrant will more flow be available at that hydrant based on the suction pump? Would that ever be modeled in WaterCAD?

Thanks so much for your help.

 

[cvg]

6 inch lines to each hydrant should be very short, from the street to sidewalk and no further. However, you can get a fire hydrant with an 8 inch barrel and do away with the 6 inch line. Bear in mind, you only have 65 psi which is not that much to work with. Also, you will need a flow test to confirm your fireflow is met. Better to do it now instead of later.

 

[bimr]

It will depend on the available pressure and water main size.

http://firehydrant.org/index.html

has informatin on hydrants. Here is an example.

A fire brigade may have smaller engines equipped with medium diameter hose and 750 GPM (2850 L/min) pumps, however with a suitable water supply system, the brigade may upgrade to engines equipped with large diameter hose and pumps with a capacity of 1250 GPM (4732 L/min) or greater.

 

[bschlang]

Thank you guys for your input.

I'm a little awestruck, because I thought the majority of hydrants in local developments were rated for flows above 1000 gpm but most designs I see are with 6" laterals.

Anyways thanks you for all the info, I will use it wisely.

 

[fel3]

For modeling purposes, here is how I generally distribute the fire flows. This is based on how the Los Angeles County Fire Department required it years ago. Back then, fire flows started at 500 gpm and incremented by 250 gpm above that to maximum of 5000 gpm. Local codes differ, but the same type of procedure applies.

500 gpm <= Q <= 1000 gpm: use 1 hydrant.
1000 gpm < Q <= 2000 gpm: use 2 hydrants with 1000 gpm at the most critical and the balance at the other hydrant.
2000 gpm < Q <= 3000 gpm: use 3 hydrants with 1000 gpm at the two most critical and the balance at the third hydrant.
3000 gpm < Q <= 5000 gpm: use 3 hydrants up to 1667 gpm at each, starting with the most critical.

Of course, it is hard to get the larger flows through a 6" hydrant lateral and hydrant assembly, but for modeling you want to be conservative. BTW, 1667 gpm through a 6" pipe is almost 19 fps; 1000 gpm is 11.4 fps. Many agencies want to keep emergency flows below 12 to 15 fps.

Another thing: we typically model only the distribution system and not the hydrant lateral unless it's a long one, but even if we model the hydrant lateral, we don't model the hydrant itself. At 1000 gpm, a typical hydrant will lose about 2 to 2.5 psi, and at 1500 gpm it might be in the 6 to 8 psi range. (I've done lab testing of fire hydrants for a client.) Consequently, if my fire flow requirement is 20 psi minimum at the flowing hydrant, I target anywhere from 24 to 30 psi minimum in the system.

I hope this helps.

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