WaterCAD Connecting to Exist System Issue 2

[aliciam]

Hi. I have built a model for a small subdivision. The only information I have from the city is:

Static Pressure = 22 psi
Residual Pressure = 55 psi

It doesn't seem like this is enough information. I hate to go ask my boss because this is my first week at the job. At the previous company I was always given a pressure and a flow.

Do I need more info or can I model with what has been given?

Thanks for any help.

 

[aliciam]

I am using WaterCAD to create the model.

 

[jgailla]

I don't think you have enough information to model anything without a flow. The pressure numbers (especially residual) are meaningless without a flow. Also, the static and residual are backwards. Static should be higher than residual.

 

[aliciam]

I thought so but wanted to make absolutely sure before I bring these issues up with the new boss.

 

[aliciam]

btw.. Thanks so much for your quick reply.

 

[francesca]

What you have been given (numbers reversed) is typical. You use the static pressure to model a tank and set the elevation (converting psi to feet of head and therefore the tank elevation). You will likely have to make your own estimate of flow through the system up to the point where your subdivision draws off. If the residual pressure currently is only 22 psi, you'd better hope it's downhill to your subdivision as you'll be struggling to meet their system requirements (typically 20-30 psi residual pressure with fire flow) without upsizing some lines.

 

[aliciam]

It's really flat down here on the coast.

I don't typically model these types of systems with a tank. I am using WaterCAD and usually do a reservoir with a pump. So, you think I should use the static (lower) pressure to set my design point on the well?

Where I used to work I would have been given just a pressure and a flow at the point I was drawing off.

 

[francesca]

Your static pressure should be the higher one as it's the pressure with zero flow (and therefore zero friction loss). If you convert psi to a pump, how do you get the pump curve? You have just two points (0 gpm @ static psi, fire flow @ residual psi)? The way the WaterCAD manual describes is to convert to a tank.

 

[aliciam]

The manual I have says to use a pump and reservoir. You need data from a fire flow test. I brought it up to my boss this morning and he gave me the full report he got the numbers from. I am off to read the report.

I am attaching the info I pulled off of the WaterCAD help menu.

 

[Rookie2]

The numbers do seem to be reversed but I disagree that that is typical.There is not zero friction loss in the system at the static hydrant (the flow is not zero). The static head is the residual pressure (static head minus friction losses in the system at whatever flow may be occurring in the system at the time of the flow test; there are other demands on the system).

The flow test gives you an estimate of the headlosses in the system (from the source) at a flow between two known points. The pump and reservoir sounds like the correct way to model this.

 

[aliciam]

Thanks. I went out and field verified the numbers and was able to determine that they were reversed. Thanks so much for all of your (everyone) help.