Exact Source Point for Hydraulic Calculations?

1/17/2020

Where exactly does a hydraulic calculation need to end?

When we run a hydrant flow test, there's a static/residual hydrant and a flow hydrant. I regularly run my hydraulic calculations from the building, through the service main, to a street main, and then up the tee'd branch that serves a hydrant so that my source point is exactly at the elevation of the static/residual hydrant. I do this so that I make sure to account for the proper elevation of the water supply.

Most other hydraulic calculations I see will end wherever the building's service main intersects with the street's supply. Is this correct? Wouldn't that place the source at an elevation lower than what the hydrant indicated is available?

Wondering what the proper way is that I should be precisely calculating systems. Thanks in advance.

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7 Comments

Brian Gerdwagen FPE

1/17/2020 10:22:05 am

The source of the hydraulic calculations should coincide with the flow test. If the public main was flowed, and it is circulating, ending the calc at the public main is fine. Otherwise, you need to prove back to the hydrant where you had the gauge. That is the point at which you measured the residual pressure at the flow rate from the flow hydrant.

Colin link

1/17/2020 10:59:44 am

I agree this is good practice, with one clarification; if you are in a situation that requires you to prove flow back to the flow test hydrants, I would place the hydraulic calculation source at the point where the static/residual pressure hydrant underground lateral connects with the larger underground supply main, and adjust the static and residual pressures for the elevation difference between the gauge and the ug main.

For example, if an 8" ug main supplies a test pressure hydrant via a 6" lateral pipe, I would end the calculation at the point where the 6" hydrant lateral tee's off the 8" main. It doesn't make sense to me to carry the calculation all the way back to the gauge at the hydrant outlet, as that is not an accurate representation of how the system is actually flowing....there is no water flowing through the hydrant lateral, the flow is through the 8" main.

Would you agree? Interested to hear thoughts on this....

Wayne Ammons

1/17/2020 11:06:11 am

I think we posted at the exact same time Colin! I believe your good explanation is in line with what I was trying to say.

Colin link

1/17/2020 11:19:13 am

Yep! Same exact idea Wayne; agreed!

Alex

1/17/2020 10:36:23 am

I think your rationale is defensible. For example, if the tee to the test hydrant is at sea level and the point of connection to the site is 10 ft above sea level, the calculation would calculation would be neglecting 4.33 pounds of elevation loss.

However, the friction losses through large diameter city mains are negligible over relatively short distances, so the elevation for the hydrant connection could be inputted at the site connection point to account for the more noteworthy pressure loss.

Wayne Ammons

1/17/2020 10:59:44 am

I don’t have the opportunity today to look through some reference material to see if there is guidance in black and white, but for the purposes of calculating the sprinkler system, the flows and pressures available at the hydrant outlet itself will be available at the underground connection point for the hydrant, correct? To calculate all the way through the piping and fittings for the hydrant itself would be a more conservative approach, but unnecessary since the water for the sprinkler system is not being drawn from the hydrant outlet, it’s being drawn from the water main the hydrant connects to.

You do have to account for the difference in elevation from the test hydrant to the site, but I input that elevation at the water main connection for the hydrant. As Brian stated above, you also need to know if the water main is circulating or dead-end to ensure you locate the flow test effective point properly.

Colin link

1/17/2020 11:02:01 am

Agree 100% with this method Wayne...no need account for looses through the hydrant lateral as that is not how the system will actually flow.

Exact Source Point for Hydraulic Calculations?

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