Fire Flow Test and Connection Point Modeling

[CPENG78]

I have a single point of connection and I received fire flow test data from an existing hydrant nearest to the connection point.

I am setting up a model in WaterNET and want to make sure my thinking is correct. The existing hydrant nozzle where the test was taken in approximately 8 feet above from where my connection point will take place.

I am modeling this with a reservoir and pump at my connection point. Based on discussions with Boss Int, the reservoir must be set at the same elevation as your connection point and thereby making the pump do all the work to throttle (at the rate as determined by the fire flow test)the water into my proposed system.

Due to the elevation difference, I am thinking that I have to adjust the static head (only the static head) from the fire flow test to account for the 8 feet elevation difference. Is this correct?

I did so and ran the model without an actual fire flow demand and the expected static pressure was obtained at both the node where the fire flow reading was taken and at the node representing my connection point.

Any input is greatly appreciated. Thanks.

 

[RWF7437]

It may not be quite that simple.

You need to model the flow at the most critical point in your proposed system. This is not necessarily the point of connection. It will probably be at the most hydraulically remote point which may or may not be at the highest point.

You need to model the required fire flow at that most critical point, not at the flow "at the rate as determined by the fire flow test". This will be the flow rate determined probably by the Fire Chief or Fire Marshall.

Also, it is not clear whether you are modeling the pumpa at a single point on the pump curve or are using a pump curve with multiple ( at least three ) points.

good luck

 

[CPENG78]

RWF7437,
Thanks for your quick input and let me provide some clarification. Yes I need to model the most critical point(s), in this case, 2 proposed hydrants and 1 existing hydrant near my proposed bulding. In my case per CFC 2007, the required flow is 3250 gpm. However the fire marshal has allowed a 50% reduction in this flow since my building has a sprinkler system. Therefore there is a demand of 1625 gpm split between two hydrants (Or any combination of the three listed above).

The question I had on my original post, refered to my thinking regarding the elevation difference between the point where the flow was measured (existing hydrant nozzle) and point of connection [where the proposed system will tee off (invert of pipe) into the existing pipe]. In a way my question has more to do with the initial set up of the model before the actual demands are applied at the nodes of interest.

By the way, the reason I believe I need an adjustment is because my point of connection has that elevation difference from where the data was collected. Therefore the pump that is throttling the water into my proposed system has a 3 point curve definition based on the fire flow test data that was collected with the slight adjustment to the static head of the reading due to the 8 ft elevation difference. I hope this provides more information to clarify my situation. Thanks.

 

[RWF7437]

Yes, you do need to assign realistic elevations to all points in the system. I do not use the method you have outlined to EPANet analyses so I don't understand how it works but pressure, energy and flow of water in any system depend on elevation. You may be able to find an example in the EPANet documentation that will help. If not there, try BOSS again since they seem willing to help. If not there, have you no experienced colleagues to ask ? If not there. there are many books listed on the internet and elsewhere in these fora you can read.

good luck

 

[RWF7437]

http://www.watersanitationhygiene.org/forum/phpBB3/viewtopic.php?f=12&t=49

 

[Rookie2]

The way I've always modeled the pump and reservoir is by placing the pump at the location of residual hydrant (gauged hydrant)not at the connection point. The reservoir is set at the static elevation given at the residual hydrant-Basically I assume that the residual hydrant is the connection point. Using this method the elevation of the connection point is irrelevant unless you happen to have a hydrant there.

If you are modeling the pump at the connection point how would you know the residual pressure there to calculate the 3 point pump curve? -Unless your connection is very close to the residual hydrant (or the pipe is very large) and you are comfortable assuming that the friction losses between the residual hydrant and the connection point are neglible.

Changing the static pressure and not the residual pressure will change the shape of the pump curve - it won't match what was given by the flow test. Static pressures will be the same because they are based on elevation only.

 

[CPENG78]

Rookie2,
Yes I agree that it will change the shape of the pump curve, in my case actually reducing the residual flow available at the 20 psi mark. So I suppose that if I wanted to transfer the data collected at the gauge to the true connection point (tapping location of the existing line) I would have to go through a whole energy and loss calculation exercise from one place to the other? what are your thoughts on this?

 

[Rookie2]

CPENG78,
Yes that's basically what I'm saying. Also, If you are adjusting the static pressure given by the flow test for the difference in elevation at your connection point, why wouldn't you adjust the residual pressure also? The residual static pressure also varies with elevation and was measured at the same outlet the static pressure was. I have questions about this as well (see my post on this forum) so please correct me if I'm wrong.