All Fixed-Loss Devices Included in Sprinkler Calc?

6/11/2024

I'm reviewing a submitted set of sprinkler plans for a partial demo and rebuild at an existing sprinklered building with an existing fire pump. They do have an existing backflow that was not included in the hydraulic calcs.

I feel, like with any fixed loss pressure device, that it should be included in the new calcs regardless if it was existing.

Am I on point with this?

Thanks in advance.

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12 Comments

Glenn Berger

6/11/2024 08:10:14 am

You are definitely on-point here. Hydraulic calculations shall include the entire system from the most hydraulically demanding area to the water supply.

6/11/2024 08:11:39 am

The backflow preventer should be on the suction side of the fire pump so if the contractor is calculating to the existing fire pump (using an annual fire pump test report) the hydraulic loss of the backflow preventer would already be taken into consideration in the FPT report numbers.

Joseph Cianfarra

6/12/2024 10:03:30 am

NK - Ya know, I think I have already lived in Florida too long where the BFP is above ground near the street. Haha

Joseph Cianfarra

6/11/2024 08:14:35 am

That would depend on where the calculation starts. If you have an existing fire pump, the most accurate flow/pressure source would be from the actual fire pump test results already past the outside supply devices. Moving the source to the street and using hydrant flow test results passing through the supply piping and devices and then adding the hypothetical fire pump boost would only serve to make the results much more squishy!

Dan Wilder

6/11/2024 08:23:19 am

Backflows are not fixed pressure loss but many times designers will just toss in the -3PSI or -5PSI (more for a RPBF) as a catch all. The curves typically start high, drop down through the normal operating range of the valve and slowly increase to their UL listed point based on GPM.

It is not uncommon to have similar pressure requirements for a small light hazard calc and an OH calc just because there is the 4-8 PSI difference for low flow vs higher flow. Also verify the backflow is listed for the flow (especially for backflows on the suction side of pumps when the pump needs to discharge 150%)....we've had to upsize, or run two in parallel, street backflows for new onsite pumps.

You are correct to require the backflow, or any device/fitting/change in hydraulics that is required.

NFPA 13 2019 - 27.2.3, 27.2.3.3, 27.2.4.8.1 all reference this.

Dan Wilder

6/11/2024 08:31:10 am

*and the exception that pops into my head...

I have used existing high demand hydraulic calc placard information and flipped it to make a supply. In that case, the backflow was not used.

Example is a high piled rack storage system where an office is added. The HPS calc plate showed a demand of 1300 GPM (I did not use hose stream) at 80 PSI at the BOR...well within a standard flow test range, so with permission of the AHJ, I utilized that as a supply (grabbed the static from the riser, used the residual and GPM from the placard) and used that for an office and an under-conveyor system. I have also used this approach on FM projects, again with prior permission.

Franck

6/11/2024 09:11:05 am

As indicated above, it has to be included.
All device/equipment that could have an effect on the friction losses need to be considered.
Hydraulic calculations are already an estimate (best guess) on what will happen. Any miss in another factor for uncertainty, that need to be avoided.

Chris Heyer

6/11/2024 09:47:48 am

Thanks to all......

Jack G

6/11/2024 10:42:49 am

As Glenn indicated the hydraulic calculations should go back to a recent ( I would want it to be within a month of the project start) city supply/ water flow test at the hydrants and include the fire pump ( most recent “tested pump curve data-“- not the factory data) . This way the 3 phases of the system can be analyzed and changes can be identified.
I do agree that calculations to the fire pump discharge flange should be accurate enough. ( sort of like an indoor flow test) however NFPA is pretty precise in what they require.

franck

6/11/2024 11:05:20 am

Note that there might be a situation where you don’t need to consider the backflow/check valve on the discharge side of the pump : if your pressure gage to measure the discharge pressure of the pump is located downstream the backflow. In that case, the fire pump curve obtained would already include the impact of the backflow and there is no need to consider it twice.

Jesse

6/11/2024 02:36:13 pm

Unless the fire pump is supplied by a suction tank then the backflow absolutely has to be included in the calcs.

Troy N

6/12/2024 11:34:03 am

It seems to me that a backflow was likely retrofitted in the system which would require calculations to include the additional friction loss per NFPA 13 2016 8.17.4.5.2 "Retroactive Installation. When backflow prevention devices are to be retroactively installed on existing systems, a thorough hydraulic analysis, including revised hydraulic calculations, new fire flow data, and all necessary system modifications to accommodate the additional friction loss, shall be completed as a part of the installation."

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All Fixed-Loss Devices Included in Sprinkler Calc?

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