Watercad Help

[bmossman]

I have very limited knowledge on Water distribution (drainage is my forte). I have an existing 10" main to the south and a 6" main to the north that I'm required to tie into by the city. I have 2 hydrant tests, 2 along the 10" main on the south with these results:

FH1: Static=85; Pitot=34; Flow=1062
FH2: Static=55; Pitot=30; Flow=998

Here's how the model is setup:

I have a reservoir located at each FH location that is an equivalent head to the FH static pressure + the ground elevation. When I run the model I'm getting outrageous flows ranging 5,000-1000 gpm at 20psi residual. I know this isn't correct because using the Hazen Williams formula, FH1 should be flowing 1,779 gpm @ 20 psi while the model is saying 9,000 gpm @ 133 psi residual.

What am I doing wrong?

 

[CPENG78]

Its been a while since I have used WaterCAD but I believe it works in the same manner at Boss International WaterNetworks which is what I typically use.

In the past I have run the model in one of two ways:

Method 1 - Use Reservoir and Pump combination - Place the reservoir elevation at the invert elevation of your tie in point. Connect the reservoir to your tie-in location with a pump. The pump would be defined by a three point curve developed from your fire flow data. Place the appropriate
roughness coefficients on both existing pipes and proposed pipes of the model. Then place the appropriate demand, say Flow 1062 gpm for FH1 (with no demand on FH2) and then 998 gpm to FH2 (with no demand on FH1) to validate your model against your fire flow test. If these check then your pumps are adequately defined to throttle the water from the reservoir to your system (in addition to assued roughness coefficient values). You can not move on to place your expected demands to verify system adequacy.

Method 2 - Use a reservoir only - Place the reservoir at the expected elevation connected by a 'dummy pipe' to your system

The reservoir elevation based on FH1 would be:

85 psi x 2.31 = 196.35 ft

Therefore 196.35 ft + 2.5 ft (approximate elevation from FG to nozzle) + FG elevation = Reservoir Elevation Water Level.

Once the reservoir is defined based on this information, the place the appropriate roughness coefficients to both existing and proposed pipes. Then place the demand of 1062 gpm to verify the expected residual pressure is obtained. If the residual pressure is not obtained, then adjust the roughness coefficient as necessary to obtain the residual pressure from the test. Once it checks, then your system is ready for the service demands to be tested. You can develop your reservoir definition the same way based on FH2 but I would only use it as a check, you can run the test with one reservoir alone.

Regardless of which method you use, verify that your static head + nozzle height + FG elevation falls within the city's or county's tank operating water levels. If at all possible get the tank's water elevation on the day the test was run to further verify your static head numbers. This will ensure that you validate your test data as much as possible.

From the data you provided, it appears to me that there is quite a substantial drop in static head from FH1 to FH2. Now, I don't know the topographical features of your site or where these hydrants are located, but if this was a flat site I would be concerned that there may be either a bust in one of the readings or that there is more than one water system on the road, street, which each hydrant connected to a different system or tank.

Another item to look out for is to verify that the fire hydrant location where the fire flow was run matches the address for the hydrant you are showing on your model. I just completed an fire line analysis last week, when the agency that ran two fire flows at two different locations, had given me the wrong address for one of the hydrants. Furthermore, and even funnier they had no record of they themselves running the second test. When I requested to verify location they gave a surprised look and asked why. I explained I was just dotting my i's and crossing my t's. When they realized I had a fire flow result for a test they had ran themselves and they had no record of, the said "can we make a copy?" and then said "thank you".

Also don't forge to define minor losses on your system. By the way, I tend to prefer to use method 1 but I do use both depending on the site size and complexity.

 

[bmossman]

Thank you for the thorough response. I will try what you suggested above and will post a subsequent reply of the outcome....stay tuned.

 

[bmossman]

I've gone thru and setup the reservoir as you indicated and eliminated the other reservoirs so that I have one source. I ran the fireflow analysis and the results indicate that I have 8,718 gpm at the first junction near the reservoir (which I know should be around 1,800 gpm @ 20 psi). All other flows seem reasonable. I'm at a loss at what to do next to calibrate the model. I've uploaded the model if anyone is interested in taking a look at it. Any help is greatly appreciated.

http://files.engineering.com/getfil...0a8bfe21&file=Water_Hydraulic_Model_Opt-2.wtg

 

[CPENG78]

From the fire flow test, what is the residual pressure when flowing the 1062 gpm?

 

[bmossman]

The residual from the fire flow test flowing 1062 gpm is 60 psi.

 

[CPENG78]

I tried downloading your file and openning it with WaterCAD,and I am been asked for a file ending with wtg.mdb. I believe this is the database file correlated with your graphical model. Do you have that available?

 

[CPENG78]

Based on the data for FH1, your three point pump curve definition should be as follows

196.35 ft w/ 0 gpm
138.6 ft w/ 1062 gpm
46.2 ft w/ 1779 gpm

For model calibration, when you say you ran the model, did you:

1. Get 196.35 ft of head under the static condition (no demand?

2. Get 138.6 ft (60 psi) of head when you place the 1062 gpm demand at the same hydrant?

3. Get 46.2 ft (20 psi) of head when you placed the 1779 gpm demand at the same hydrant?

Upload the related watercad files and I will be happy to look at it.

 

[bmossman]

I located the .mdb file and also included a .dhw file. There were also multiple .out files but I'm assuming those are the results from the model.

I keep getting errors when attempting to upload the files above to this site, so I've posted them here:

http://www.mediafire.com/file/zmtidolbnyd/Water

Hydraulic Model Opt-2.wtg.mdb

http://www.mediafire.com/file/0wurgzqnwzb/Water

Hydraulic Model Opt-2.wtg.dwh

I really appreciate your help.

 

[bmossman]

I ran the model using just a reservoir (I wasn't sure how to set it up using a pump). For the head calculation, I took the ground el. (132) + 85 static pressure (196 ft) = 328 ft. I placed a temporary node right next to the reservoir and ran the model to verify the static pressure. It looked ok so I then applied my demands and ran the fire flow calc and that's when I noticed that my first junction had 9,000 gpm available, while the other internal hydrants had a round 1,500-2,000 gpm.

 

[bmossman]

CPENG78:

I setup a new temporary model using a reservoir, pump and a junction in an effort to simulate just the FH fire flow test with these values for the pump:

196.35 ft w/ 0 gpm
138.6 ft w/ 1062 gpm
46.2 ft w/ 1779 gpm

At first I was getting negative pump flows, then I changed the elevation of the reservoir to 0.1 ft higher than the pump and all pressures checked out compared to the pump data.

I then setup my other model that contains the proposed development identical to my temporary model and I keep getting negative pump flows. What exactly do the negative pump flows indicate and how do you typically resolve this issue?

 

[CPENG78]

I was able to download your files, but since it had been a while since I had used WaterCAD, I did not realize that my company cancelled our program license and therefore it does not let me run the model. Can you send me a quick sketch of your network that shows where you are placing the reservoir, pump and demands during the calibration of the model and during the fire flow run? I will attempt to run the model in the program I normally use (which I do have a license for) to see what could be causing the issues.

I have a feeling that it may be related to the elevations assigned at your nodes, including pump and reservoir.

Anyone outer with a WaterCAD license, please see if you have luck openning and running the files?

 

[bmossman]

I could never figure out what was wrong with the original model so I just built a new one with the pump definition listed above and it works! Thanks for all of your help!

 

[CPENG78]

Go figurre...sometimes its just better to start over. I'm glad that it worked for you and that I was able to help a little.