WaterCad 2

[ximba1]

I am doing a waterCAD design and the county sent me the tie in water pressure = 53 psi
First questio - is this correct to converte psi to an elevation at the reservoir (Elevation = 53 psi/0.4333 = 122 ft) so this is the elevation on my reservoir and then i set the elevation at the junctions on the proposed water line 3 feet below the finish grade.
Second question - i know that the line i am tie in is a pressurized line and that the pressure at the existing line is 53 psi, what do I use as far as data for the pump?
Third question - i calculated that the needed fire floe to be 1,500 gpm and need 3 hydrants, do I add the demand of 500 gpm in the three hydrants?

Thanks if anybody can help

 

[CPENG78]

Hi Ximba1,
You are going to need a bit more information than that. There are a few ways to model it but you need more information. Typically a hydrant flow test or fire flow test is performed either by the fire department or the water agency. Or you can be given the information based on historical records, say a recent fire flow test that was done. In some cases, the water agency itself may have its own hydraulic model and they just give you the information at the location of interest. The information you will need is the static pressure (appears they gave you that as 53 psi), and then residual pressure with the flow rate at the residual pressure.

So there are a few ways to model it.

First Option: Yes, the static pressure can be set as the elevation of your reservoir but you need to be able to calibrate your model by knowing the residual flow and pressure. Once you set your model, you would apply that residual flow at the node where the measured flow rate was taken and then you would calibrate the model until you get the same residual pressure at that node. Calibration can be done by way of adjusting your pipe lengths and adjusting your Hazen Williams coefficients to have the model mimic your readings. There would not be a need for a pump in this option because your reservoir would be higher than your site (I'm assuming thats the case) and so when you run the model everything will run under pressure.

Second option: You will still need the information as described. However in this case you can set the reservoir at the same elevation as the lowest node in your system and then link your reservoir to your model with a pump. In this case, you wold use the results from the hydrant flow test to develop your pump curve. Again calibration would be carried out until you get the desired values.

Once calibration is set than you can apply your required fire flow out of the required number of hydrants. In response to your third question yes, you would divide the required fire flow by the number of hydrants so if its 1500 gpm, it would be 500 each. However, I do want to ask how did you determine the fire flow and that you need 3 hydrants. It seems too many hydrants for that required fire flow. Unless the site has an odd geometry that requires you to add additional hydrants. Hope this helps.

 

[chileheadcraig]

I would add that if your needed fire flow is 1500 gpm, you will likely need to have the ability to deliver that flow to each of the hydrants with a minimum residual pressure of 20 psi. You may want to shoot for 25 psi to take into account the degree of error in your model (depending on the level of calibration that you do). There is a fire flow calculation option that you can run that will evaluate each hydrant for the available fire flow given the parameters and constraints that you input (minimum residual pressure at the node and for the zone, needed fire flow, velocity upper limit, etc).

 

[BenJohnson]

The entire city water system is a network of pipes. The design of the whole system should be designed for fire flow at certain points.

Trying to design your small system requires that you have more than just water pressure at one point. The City Engineer should have a lot more information (hopefully in computer format). If you are going to be pulling water out of the city water system with a pump, it is impossible to answer the questions you are asking without a understanding of the water network.

You can't just throw in 3 fire hydrants because you need 1500 gpm. You must be sure the network of pipes serving your project are designed to deliver the quantity of water you need for the project. And if you are using a pump, that could affect the water pressure in the main water system all over the neighborhood.

Just plugging in numbers in a CAD program is not the right approach if you don't understand the entire water system (or the basic concepts).

 

[ximba1]

Thanks for the replies.
CPENG78
Actually the 53 psi is a residual pressure. I have seen engineers doing this
(53 psi/0.4333) + finish ground elevation say 60' = 182' and use this as the elevation at the reservoir. IS this correct?

 

[CPENG78]

Ximba1,
I agree with both chileheadcraig and BenJohnson's points. My previous response was to more to get you started,

In terms of Chileheadcraig response:
I was wondering the same as it is not clear how the fire flow and number of hydrants was determined. If it was determined that 1500 gpm was needed on site and three hydrants then I stand by my answer, the flow would be divided into 3 hydrants. For example in California, the fire code has a required fire flow based on building square footage (fire flow area) and type of construction. The code then allows for reductions with the approval of the fire marshal if there is sprinkler system provided for the buildings. I also agree that the minimum pressure of 20 psi needs to be achieved at the hydrants but from dealing with several fire agencies, the 20 psi is not just at the hydrant but throughout the entire system that you are designing. This is to prevent creating near 0 or negative pressures in other nodes in the system that would create pipe collapse as a vacuum would get formed.

In response to BenJohnson's response:
Unfortunately sometimes the city, county or water agency does not have all the information. That is why I check and recheck information that is give to me as well as adding disclaimers in my final report as necessary. In understanding the city system, Ximba1, you should also be asking if there are pumps, pressure reducing stations, is there more than one pressure zone near your project, etc. As Ben implies, sometimes is not as simple as just plugging in hydrants and pressures in the software hoping to get the results. The model is only as good as the data that you have.

Going back to your last question:
The straight conversion from pressure to an elevation only works for the static pressure. If the pressure given to you was the residual pressure then it implies that there was a flow associated with it. They would have to give you this as well. You would use both the residual pressure and flow along with your static pressure to develop a system curve similar to pump curve of how water is delivered to your tie in location. I don't mean to make this more complicated but I don't think I have ever heard of the term tie in pressure. So maybe the information from the county is not been provided to you correctly.

Sorry for the delay in response.