Calculation Method for Wet, Manual Standpipe?

10/30/2020

We are designing a residential building that will have a manual wet standpipe in a fully-sprinklered building. The building itself is less than 75-feet high.

I need to calculate the wet manual standpipe but would like to check my approach.

Do I need to calculate the remote standpipe at the two most remote hose valves at 100 psi, 250 gpm each, plus one more hose valve closer to the source at 250 gpm all the way back from a supply at the fire truck?

Thanks in advance for any feedback.

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8 Comments

Mike

10/30/2020 10:08:12 am

You're performing a demand calculation. Usually will apply a placard to the FDC stating required flow and pressure.

You're correct if you mean the next, 3rd, most remote hose valve.

The appendix of NFPA 14 has some good examples to follow.

Also, verify with the fire department the maximum pressure they can supply. Usually in my experience, it's been 150psi. Usually this nessitates 6" t0 supply 3 FDC's. 4" for 2.

Hope this helps!

Casey Milhorn

10/30/2020 10:13:34 am

Mike pretty much covered it. 150 psi is the typical pressure that fire departments will supply as a default at a manual wet standpipe, but confirm with your local AHJ what their standard procedure is for manual wet standpipes. Size your standpipe feed mains and standpipes based on this. If you have more than one FDC location, calc to the most remote. But yes, 250 gpm at the top two remote hose valves (for 500 gpm total) plus 250 gpm for each additional standpipe up to a total of 1000 gpm for a fully sprinklered building. 1250 gpm in an unsprinklered building or in certain sprinklered buildings with a footprint that exceeds 80,000 sqft.

Josh Gilliam

10/30/2020 10:25:24 am

Good to see you on here too. This is a nice resource.

sean

10/30/2020 10:17:31 am

Yep, just like the automatic system but you use the fire pump specs.

Taylor S.

10/30/2020 10:20:30 am

If you have one standpipe riser, you would calculate the most remote hose valve at 250 GPM and 100 PSI as well as the one on the floor below it to the same demand (500 GPM). If you have two standpipe risers, you would do the same calculation with an additional hose valve on the other standpipe riser (750 GPM). For three and more standpipe risers, add one more hose valve to the calculation (1000 GPM). I have calculated these two ways; all the way back through the system, and I have also calculated these to the FDC using the pumper truck as the supply.

Reilly

10/30/2020 11:49:38 am

So how many FDC inlets on a combination sprk/standpipe System with two risers- typical residential bldg not high rise no pump?

sean

10/30/2020 12:58:38 pm

the code addresses this one 2.5" inlet for every 250 gpm calculated

Pete D.

10/30/2020 02:59:14 pm

On a side note, wet manual standpipes in my experience are on a city supply, but the height of the building exceeds the available pressure from that supply to meet the NFPA 14 requirements. 14 also has max pressure requirements for making the hoses manageable. The placard is to let the pumper operator know how to throttle the pump. Using hydraCalc, I have found the equivalent loss difference in terms of pipe length, diameter & fittings between the FDC node and junction with the existing city supply node path, and added it in. For example, if the FDC meets the junction of the supply path with 20 ft less of 4" pipe with 2 fewer 4" 90s, I would just subtract that from the run between the supply node and that junction. Then, run the calc and let it fail. Whatever negative number is displayed, multiply by - 1 and that # goes on your placard.

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