Add more water volumen to a fire sprinklers system. 1

[Sergiosldv]

Hello, good day.

I have some questions that confusesme.

1- A couple of engineers toldme that they always use 500 gpm vs XXX psi and the system always is "right".
I have seen that when you calculate your design, and you have something like 250 gpm vs XXX psi, then for any reason they add 250 gpm to the design, i mean, i know that the code says that you should add 250 gpm in 60-90 min (ordinary risk) if you have external and internal hidrants, but if i calculate my pipe's diameter for 250 gpm (which are going to go to the sprinklers), what do i do with the other 250 gpm? where are they going? are they going to flow to the last sprinkler? are those gpm going to change the preassure in every sprinklers of my design area?. do i have to resize the pipes?


2- When i calculate a sprinklers system by the area/density method, and i want to add a hose flow to the main pipe, the code says that a hose should have a minimun residual preassure of 65psi. I always think that i will add 65 psi to the preassure loss in the system and obviously the 50 gpm of the hose, i dont know if that way is correct. ie: my design area gives me 225gpm vs 20 psi until the connection with the hose, so should i just add 50 gpm to the water flow, and 65 psi to my total preassure lost until there? a work the rest of the pipe with 275 gpm with 85 psi and add the rest of the preassure lost in the rest of the pipe until the pump.

3- some examples and calculations that i have seen never includes the preassure lost in the pump's suction area, is that correct? i havent finded anything in the codes about considerations of suction.

Thank you

 

[SprinklerDesigner2]

Everything I refer to is per current NFPA standards and how things are usually done in the US.

First thing to recognize is there is a difference between codes and standards as codes tell us where sprinklers are required while standards tell us how to do it. All NFPA documents are standards and not codes.

1- A couple of engineers told me that they always use 500 gpm vs XXX psi and the system always is "right".

Your engineers are wrong. I learned a long time ago never to use the word "always" and as an example I present Section 18.3 of the standard that requires 750 gpm hose stream for some rubber tire storage:

18.3 Water Supplies
Total water supplies shall be in accordance with the following options:
(1) A minimum of not less than 750 gpm (2835 L/min) for hose streams in addition to that required for automatic sprinklers and foam systems. Water supplies shall be capable of supplying the demand for sprinkler systems and hose streams for not less than 3 hours.

So much for the always right.

For Light Hazard Occupancies the combined inside and outside hose stream demand is 100 gpm.

For Ordinary Hazard Occupancies the combined inside and outside hose stream demand is 250 gpm.

For Extra Hazard Occupancies the combined inside and outside hose stream demand is 500 gpm.

For most, but not all, storage occupancies the combined inside and outside hose stream demand usually 500 gpm except for most ESFR systems the combined inside and outside hose stream demand is 250 gpm and, depending on the storage, hose stream can be more than 500 gpm.

When it comes to fire sprinkler systems the 65 psi pressure for hose streams are irrelevant and not applicable. I think you are getting the 65 psi pressure from standpipe systems.

Consider the following three figures:

In Figure #1 you would calculate the overhead sprinkler and carry that water all the way to the source where you would then add your hose stream. If the system were OH-2 requiring 374.6 gpm at the base of the riser then you would flow 374.6 gpm through both the 6" and 8" underground finally adding 250 gpm hose at the source connection.

In Figure #2 you would add your hose stream at reference point T1. Assume OH-2 again requiring 374.6 gpm you would flow 374.6 gpm to reference point T1 where you would add 250 gpm for hose stream for a total water demand of 624.6 gpm. From reference point T1 to the Source you would flow 624.6 gpm.

In Figure #3 we have an ancient and out of date 1 1/2" hose rack unit for occupant use. Assume again OH-2 where the overhead demand was 374.6 gpm you would add 50.0 gpm at the point where the hose rack tied into the system for a total of 424.6 gpm carrying that out to reference point T1 where you would add 200 gpm more for the combined inside and outside hose stream demand of 250 gpm. From reference point T1 you would calculate the head loss flowing 624.6 gpm to the source.

Adding hose streams seldom, I say seldom because I never use the word "never", has anything to do with pressures other than adding head loss through the underground.

A side note on inside hose streams in my home state of Georgia. We don't have any; in the past ten years I have seen only one 1 1/2" hose unit for occupant use and that is only because someone missed taking it out. I don't think a single 1 1/2" hose rack has been sold in Georgia for 15 years... they are all gone.

If a company wishes to have hose rack units they can do so but only if all, as in 100%, of all company employees are trained in the proper use of hose streams. For this training all employees can go to the state fire marshals academy where they can spend a week learning how to use hose streams. When a new employee shows up before he can start work he need to spend a week at the academy... payroll for a week, food, transportation... yeah, this is why we don't have any 1 1/2" hose rack units in Georgia anymore.

The deletion of occupant use hose racks from buildings make sense from an lawyer standpoint. If a fire gets so big it can not be extinguished using a simple fire extinguisher the occupants simply need to evacuate the building leaving the fire fighting to trained professionals.

 

[SprinklerDesigner2]

Over the years a good number of beginning students had difficulty in figuring out where hose streams should be added and where is the source.

In the four examples below where would we add the hose stream demand if we were dealing with an OH-2 Occupancy?

In addition to hose stream exactly where is the source in each of the following figures?