How to Create Flow Test "Artificial Drop" of 10%?

6/26/2024

I am coordinating a flow test with a contractor.

NFPA 291 Section 4.4.6 states "To obtain satisfactory test results of theoretical calculation of expected flows or rated capacities, sufficient discharge should be achieved to cause a drop in pressure at the residual hydrant of at least 10 percent. In water supply systems where additional municipal pumps increase the flow and pressure as additional test hydrants are opened, it might be necessary to declare an artificial drop in the static pressure of 10 percent to create a theoretical water supply curve."

The contractor did not achieve the 10% pressure drop and asked if the "artificial pressure drop" option is a possibility here. I do not see anywhere in NFPA 291 or elsewhere explaining how to do this.

Any recommendations on what to advise the contractor / how to even calculate the artificial pressure drop?

Thanks in advance.

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14 Comments

Pete H

6/26/2024 06:51:27 am

I can be entirely wrong about this, but I'm pretty sure you just make sure your residual is 10% less than your static.

So say your static on the flow test comes out at 90, but your residual came out at 85. That's not a full 10% drop in pressure So you'd run your calc of an artificially adjusted flow test of 90/81.

Once again, I could be completely misinterpreting the question.

Glenn Berger

6/26/2024 08:12:16 am

Do not attempt to "create" an artificial pressure drop.

Try to find hydrants close to the project site where two or more hydrants are located on a dead-end line. On many occasions, I have had to shut off sectional valves in order to create the dead-end conditions. This makes your flow test more conservative.

As another means, you can open additional outlets, which cause additional water flow and additional pressure drop. NFPA 291 has diagrams in how to pick test and flow hydrants for these conditions.

Municipalities with gridded or looped water systems are very difficult to obtain required pressure drops.

Eric R

6/26/2024 08:18:36 am

Glenn,

I don't disagree with your approach here from an engineering point of view.

But in my neck of the woods the water authority would just give a big belly laugh if I told them I wanted to shut off sectional valves or flow excessive amounts of water to get a more accurate test.

At the end of the day water authorities have pretty much carte blanche to limit your tests in any way they feel like without much option for pushback, so having to artificially reduce the flowtest data you get to be more conservative is sometimes the only option you have.

Dan Wilder

6/26/2024 08:12:48 am

As long as the discharge flow meets or exceeds the demand of the system being installed, Pete has it correct by dropping the residual by the needed 10%. Also, this 10% reduction should be independent of any additional safety factor required/requested for the system calculations.

Remember that 291 is a "Recommended Practice" not a "Standard" so there is more latitude as to available options hence the word "Shall" is not used anywhere within 291, only "should".

6/26/2024 08:13:14 am

Pete is correct on this.

If your test data comes back as 90 static and 85 residual at 1200gpm you would use 90/81 at 1200gpm as the water source data in your hydraulic calculations.

As an extra caveat, depending on the scale of the raw data you might want to use a specific psi value drop as opposed to using 10% to make sure you are providing an appropriate level of safety factor. For example if you get 45/40 as your raw data I would probably drop that to 45/35 simply because the initial spread is so close that gauge inaccuracy could be a confounding factor.

Joe

6/26/2024 08:14:55 am

Such an interesting way of going about it - the gauge inaccuracy is a huge compounding factor here.

Learned something new today. Well said.

Bob

6/26/2024 08:15:58 am

To get the artificial 10% pressure drop you would use the same formula that you do for the 20psi calculation. Calculation is found in 291 4.13.1.2

Jesse

6/26/2024 09:55:11 am

291 is an interesting document because its a recommended process.

In my area, one AHJ mandates we flow multiple outlets until we get that pressure drop or, attain our max fire flow.

The max fire flow is pretty easy in most cases.

Jack G

6/26/2024 11:01:18 am

I m with Dan and Glenn on this.
First of all, the required flow plus hose streams must be between the static and residual points .
Never extrapolate beyond the residual point for the required flow.
A 4.5 inch outlet would get a good flow. However water companies may “ lock you up “ if you tried that.
Dropping the curve 10% is what I d do. Then any safety factors also, so in this case 20 or more psi.

Casey Milhorn

6/26/2024 05:30:38 pm

I completely disagree with this "suggestion" in NFPA 291. I think it should be replaced with "your gpm achieved during the flow test must match, at minimum, the gpm required in the most demanding calculation". I'm more worried about achieving enough flow than achieving a certain "drop" in pressure. Too many times designers are designing fire pumps based on points out on the curve where actual results weren't achieved.

6/27/2024 07:09:39 am

Casey,

The section regarding artificial pressure drop doesn't conflict with the requirement that you flow enough water to hit your system demand.

The issue here is that if you hit that flow number before you get a 10% drop from your static pressure you need to add an additional pressure loss to ensure your flow data is conservative.

This is especially important when you have strong water supply while also designing for a high hazard system. If your flow curve is to flat the math breaks down and you start getting crazy numbers of available theoretical flow. The artificial drop steepens the curve to prevent designers from being to liberal in reducing pipe sizes that may cause the system to fail in the future if the water supply weakens a bit.

6/27/2024 07:29:52 am

Thanks Eric. Yeah, I understand the reasoning for this "safety margin" but I don't agree with the approach. Most AHJs and/or engineers specs we see, ask for a 5% or 10% (or even more) safety margin. Even when this is not required, I've taught all of my designers to never design right to the edge of the supply. Factoring in a safety margin across the curve makes more sense. In a light hazard scenario, we are typically working right up against the static pressure and regardless of the 10% drop across the flow at residual, you don't get any safety margin benefit up top. If you've ever taken flow tests at different times of day, or on different days, you can see a drastic change in static pressure in some city systems, which usually has a drastic change on the residual side as well. Flow of course has some correlation to static and residual pressure, but also with overall system capacity. That being said, I worry more about applying an overall safety margin across the curve and also ensuring that the actual water flowed during the test is more than the anticipated system demand and the designer isn't working in "theoretical" residuals and flows. Achieving a 10% drop on the residual side of the curve makes no sense unless you are working out past your residual point in anticipated flow demand.

6/26/2024 07:53:22 pm

Can you do a 24 Hour Static Test instead?

Mark Harris

6/29/2024 08:00:03 am

Fire Marshal Association in my market requires a 20% safety factor but need to take the static adjustment to both static and residual so the available supply curve is still parallel to the flow test. For example a 100 static 80 residual 10,000 GPM actual flow test would be 84 static and 64 residual.

How to Create Flow Test "Artificial Drop" of 10%?

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