Use Main Drain Test for Base of Riser Calculation?

2/25/2021

Is there a method to use a main drain test to calculate the hydraulic curve at the riser itself?

I haven't done it myself, but using a static pressure at the riser, with a residual pressure at the riser, and then to gather the flow a pitot reading on the main drain discharge to measure how much flow?

If the flow discharge outside was greater than the demand of the system, conceivably a hydraulic calculation could be made with the riser as the source point, and with more accuracy than a hydrant flow test somewhere near the site.

Obviously if a backflow test header or hose valve were supplied, a higher flow could be obtained with a better curve.

I heard this mentioned recently so it's just theory to me, but for small remodels having a setup like this could be far simpler (and potentially more accurate) to know the available water supply at the system.

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16 Comments

Dan Wilder

2/25/2021 08:54:25 am

While technically accurate to create a "micro" water supply curve (assuming the discharge pipe can be routed into a known orifice & pitot readings are accurate), it does not create an accurate water supply for the building, just a baseline to compare future results to. Gauge placement becomes very important for the static/residual readings (think performing a hydrant static/residual/flow test from a single hydrant....it can be done but the numbers are skewed). Outside hose also is a question.

You may exceed the water demand requirements of the system, but I would question if your residual pressure would still be acceptable. Once graphed out, that steep of a curve with added hose streams would quickly exceed the supply curve.

However, I have used existing hydraulic placards for small, lesser hazard TI's when I was unable to perform a flow test outside the building without an issue (AHJ preapproval was requested).

2/25/2021 08:57:56 am

Dan - agreed on all points.

I haven't used placards myself, but typically those demands are for a specific design flow and design pressure... the placards don't typically give a static pressure or flow test information.

Do you just assume no increase on static and design below that "line" to be conservative?

I don't run into this situation often (typically just larger new work), but appreciate the insight.

2/25/2021 09:28:17 am

I'll take the current static from the system (bleed off the pressure above the check valve or past the backflow so I don't get an artificially high static), then use the residual required flow & pressure. It is super conservative as those numbers are from a demand calc (not an actual supply test) but again, I've only used it for lesser hazard additions as I can prove the new area demand is less than the existing area demand requirements.

I have not applied a safety factor to those calculations because the numbers being used (assumed) are already below the water supply + safety factor. I also do not "add" in any shown safety factor on the plate...again, super conservative.

Jesse

2/25/2021 09:08:49 am

Agree with Dan's comments about it really just being a benchmark for future comparative analysis. We need to be accurate in our calculations, and that accuracy begins with the flow test we use as the basis of design. I mandate all of our field staff to strictly adhere to NFPA 291 Ch. 4 when obtaining flow values for our design calcs.

Dennis R.

2/25/2021 10:44:01 am

Agreed! Accuracy is of the utmost importance when performing a flow test. The results form the basis of the whole calculation process!

2/25/2021 09:21:16 am

I was interested to see the replies to this comment. I read the old GAPS Guideline on this very thing. Although I can't find it online, I did save a copy to my computer. It makes a lot of sense, but obviously isn't within the intent of NFPA13.

Also, curious how you would extrapolate the data from a placard for calculations?

Matt

2/25/2021 10:20:08 am

Typically I would use the Residual flow and pressure from the base of riser placard, and use the reading of the city pressure gauge for my static pressure. As mentioned before, this is NOT an accurate model of the water supply, but depending on the age of the system (keeping in mind that water supplies can change drastically over time) this can provide reasonable constraints to use when modeling changes/modifications in an existing system.

Jeff link

2/25/2021 10:42:56 am

Please keep in mind that not all Hydraulic Placards are
installed or populated with data from the installing company.

They may be calculated by an inspection company who has NO
idea what the water supply is capable of providing -
(It is a "demand calc")

jay richardson

2/25/2021 10:48:48 am

Yes you can do this to get a relatively accurate available water supply at the BOR, but you don't know the coefficient of discharge to use for the main drain. I would connect a hose to the main drain pipe and use a hose monster nozzle (known Cd) to determine the flow.
Besides the main drain, NFPA 13 requires a means to forward flow test the backflow preventer and suggests a bypass pipe with control valve around the FDC check valve to accomplish this. In this case you can connect a length of hose to the FDC and use the hose monster.
The outside hose allowance is added at the connection to the city main or hydrant, which will be minimum 6 to 8 inch pipe, and 100-250gpm added at this point will have minimal effect on the residual pressure at BOR. A greater effect on the residual pressure will be the daily and seasonal fluctuations that usually aren't accounted for in the flow test results.
If the General Information Sign is installed as required by NFPA 13, that has the original flow test and main drain test results on it. But if you're using the flow test to hydraulically calculate a system, NFPA 13 requires the flow test to have been performed in the last 12 months. The General Info sign main drain test data is best used to recognize a decrease in municipal water supply since the system was first installed, which is very common in my area where there has been extensive growth/construction over the last 30 years with no upgrades to the water supply.

Franck

2/25/2021 11:13:24 am

I like the conservative approach of Dan to have a rough idea on the possible available water supply on a system.
The only limitation is that you can use it to validate lesser demand calculations.
And hoping that the city water supply did not fluctuate in the meantime (and that the previous contractor did a good job, i.e. ensure that the existing water supply was above that demand point at the base of riser).
But if you have nothing, it is better than nothing.

2/25/2021 11:19:03 am

To come back to the initial question, this is not something I would do for a new system.

One of the main reason is the limited flow you will have through the main drain. This means that you will have a point with a huge uncertainty to plot on your Q1.85 graph. If your hydraulic demand has a much greater flow, you will definitely be in the grey area...

Remember that the size of the main drains are only:
- For riser main size up to 2 in.: 3/4 in.
- For riser main size from 2-1/2 in. to 3-1/2 in.: 1-1/4 in.
- For riser main size 4 in. and larger: 2 in.

This is far less that what you will flow from a hydrant.

Dennis Randolph

2/25/2021 11:32:54 am

Here is the paper I was referring to earlier, if anybody is interested. I couldn't find it online again, so I stuck it in Dropbox.

https://www.dropbox.com/sh/13k51zc7lf2w2e5/AAAQCopjITeuHnGA7DHJw4Jya?dl=0

Jonathan Sullivan

2/26/2021 09:13:56 am

Thanks Dennis. Why do you think the recommendation is that the gauge must be tapped into the riser rather than the hose?

2/26/2021 09:40:17 am

I would think it is the same principal as NFPA13 (213 Ed.) A.8.16.2.4 in that you would not record a true residual reading if the gauge is on the drain.

Also, I had seen this paper and thought it was an interesting conversation point. I would never design a system using data collected this way.

James Richardson

2/26/2021 11:44:47 am

Thanks for that paper.
Very interesting!

2/26/2021 10:49:43 am

I would consider this method to be primarily a starting point used when questioning if the incoming water can meet the numbers on the placard.
On the other hand it is very common to use the data from the annual pump test report to run calcs for tenant improvements, modifications, or change in hazard.

Use Main Drain Test for Base of Riser Calculation?

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