WaterCad - Inputting Hydrant Flows?

[czvanhyning]

I am brand new to WaterCad. I've done a portion of the tutorials, plus some research on the internet. Unfortunately, I've found little useful information on using the software and I have a lot of "why" questions. Additionally, the software is new to our office and no one knows how to use it.

I need to determine the static pressures and fire flows (@ 20 psi residual) of the fire hydrants on a new site. I have created the new network in Watercad using the correct pipe lengths and elevations. I connected the network to the existing system by a reservoir and pump as suggested in the tutorial. Is this the correct method to connect using the data from a fire flow test? If so (or not), how do I translate the flow data into pump data?

The data I have is follows:
FLOWING HYDRANT:85 psi static, 45 Pitot, 1130 GPM
RESIDUAL HYDRANT:90 psi static, 60 psi residual

Any assistance would be greatly appreciated!

 

[sam74]

Sounds like you are doing it correctly. If you are not using this link as a reference I would suggest you should. It explains a lot of things very well.

http://www.haestad.com/library/books/awdm/online/wwhelp/wwhimpl/java/html/wwhelp.htm

 

[czvanhyning]

Thank you for the link. I am not using this as a reference currently. On briefly reviewing the info it looks to be very informative - I will definitely delve into reading!

I did solve my question though. I did away with the pump and just used the reservoir. The elevation of the reservoir was set to the sum of the ground elevation and the static pressure head. The resulting static pressures at the fire hydrants seemed reasonable based on the site conditions. Next I need to determine the fire flows of the hydrants. I am going to try making a child scenario to do this.

Thanks again!

 

[francesca]

Yes, you did right by getting rid of the pump. The fire flow condition does not have to be a child scenario as all your changes will be on the fire flow tab. A child scenario would be where you changed one of the other flows or geometric properties. The fire flow analysis is a different type of run.

WaterCAD ships with a rudimentary user guide, which should be enough to get you through the fireflow stuff. Also, if you have tech support, in the pre-Bentley days the support was pretty good.

 

[bws2]

Hi, I know this is somewhat of an old thread, but I have a similar question and thought it would be better to post here than to start a new thread.

Why would it be correct to eliminate the pump and set the reservoir elevation equal to the elevation of the hydrant + the static head? Doesn't this ignore the friction losses between the source and the hydrant? It's my understanding that the hydrant flow test should be performed so that the residual hydrant is downstream (for lack of a better term) of the connection and the flowed hydrant be upstream. From what I understand this is to estimate the available flow and the friction losses.

Also isn't the purpose of the pump curve to approximate the equation:

Q(r) = Q(f) x ((h(r)^0.54)/(h(f)^0.54))

I'm not very familiar with the fire flow option but I have a similar situation and have set up scenarios for each hydrant and assigned 1000 gpm demand at each fire hydrant node. When I run these scenarios doesn't the program just get the available pressure at 1000gpm from the pump curve and subtract headlosses to determine the residual pressure at the hydrant?

I know this isn't really worded so much as a question but I am looking for someone to tell me where and why I am wrong.

Thanks

 

[francesca]

Generally speaking, water reticulation networks are based on static pressures at water towers throughout the distribution network. Only in isolated places near the edge of the distribution network are booster pumps employed to provide pressure to customers. However, when you're designing a subdivision and need to know the residual pressure at the nearest node to where you want to connect, the utility will do a pressure test at the nearest fire hydrant. They provide you with a pressure in psi, and you need to convert it into something that WaterCAD can handle.

The easiest way to do this, and the way recommended by the WaterCAD manual, IIRC, is to place a reservoir at the location of the fire hydrant tested and adjust the elevation of the reservoir to match the pressure in psi that was determined in the test. If your real world situation is to have a pump downstream of the fire hydrant, then by all means use the correct pump curve, but to try to approximate a pump curve to match a pressure at a point and flow is to my garbled weekend brain not possible (needing 3 points to plot a curve when you have just two - static psi and residual psi).

 

[bws2]

Again correct me if I am wrong but I think you can convert the psi to feet of head using the weight of water (62.4 lb/ft^2). The terms static and residual both refer to static pressure.

The equation I mentioned is a form of the Hazen Williams equation(I think, my weekend brain is garbled as well) which allows you to calculate the 3rd point of the pump curve.

 

[bws2]

Sorry - weight of water = 62.4 lb/ft^3