Greetings, I have a high rise ho

[AntoineCham]

Greetings,

I have a high rise hotel where fire flow is requested to be 1500 gpm while standpipe demand is 1000 gpm. The required minimum pressure for the hydrants (1500 gpm) is 20 psi and for the stanpipe is 225 psi. The designer selected a fire pump of 1500 gpm at 225 psi. I guess this system is oversized. My proposal is to change the pump into 1000 gpm @ 225 psi. Also the water tanks if sized at 1500 gpm for 2 hours is giving 680 cu.m. Figures are huge. Any advice...
Regards,

 

[UFT12]

The 20 psi refer to the hydrant while the 225 surely not on the connection rather to the pump. So these do not seem comparable.

Anyway, although your though appears reasonable to me, you are pushing your pump to its limit (+50% from rating point, 1500 gpm). That may not be the best idea. Consider factors of deterioration over time that could introduce a greater pressure drop.
A quick advice would be to either specify two separate pump sets, one for low pressure and one for high pressure or specify one high pressure pump set only, with the branch taking off for the hydrants equipped with a pressure reducing valve. You would have to address that to your AHJ and see if it is acceptable by them.

 

[MGXFP]

Your fire pump size should be 90%-140% of the fire water (standpipe) demand per NFPA 20. The 1,500 gpm pump is too big. This is assuming the fire pump is inside the building and supplying the fire sprinklers and standpipes inside the building. That also means the city water supply gives you the 1,500 gpm fire flow demand. Your water tanks need to be sized to match your biggest fire water demand x duration.

 

[LittleInch]

Antoine,

There isn't a lot to go on here and the data is not very clear.

"High rise"? 5 stories? 50 stories?

What is "fire flow"? Is this extra to the stand pipe or included?

Do you have a municipal supply?

What has this tank got to do with anything?

Where are the hydrants? On each floor or only at ground, level?

Where is the standpipe pressure required?

Draw this out a bit and fill in some missing crucial details and we might get somewhere.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[AntoineCham]

Dear littleinch,

The building is 15 storey with 55 meters high. The fire flow is what NFPA 1 stipulates for hydrants flow. The standpipe requires 1000 GPM at 15 Bar whereas the hydrants require 1500 GPM @ 1.4 Bar.The scenario is simple but the outcome is an 1500 GPM @ 15 Bar.I need to know whether I can use a smaller pump? 1000 @ 15.
No available municipality flow or pressure.
Regards,

 

[TravisMack]

Many times when a fire pump is supplying yard hydrants and a fire sprinkler or standpipe system, you end up having a larger flow from the fire flow and a larger pressure from the standpipe / sprinklers. So, the capacity (GPM) is determined generally by the highest flow demand. In the case of a 1500 gpm hydrant demand, I would have likely selected a 1250 gpm fire pump. The rating (psi) of the fire pump is determined by the sprinkler pressure demand or the standpipe demand. You stated you need a 225 psi fire pump, so your final pump sizing would be 1250 gpm @ 225 psi. The 1000 gpm pump would work, but I typically don't like to run a fire pump all the way out to 150%. However, you are permitted to do so by NFPA 20 sizing.

Travis Mack, SET, RME-G,
MEPCad, Inc
AutoSPRINK | AutoSPRINK FAB | AutoSPRINK RVT

 

[AntoineCham]

Thank you dear for the advice regarding selecting 1250 HPM @ 15 Bar. The thing is that some suppliers such as XYLEM don't manufacture sump flow at 225 psi head. They have it at 215 psi max. In case NFPA 20 covers the selection, I guess optimizing the pump would have greater benefits such as reducing pipe sizes, having a flatter curve with acceptable churn pressure, etc... what remains high is the water storage capacity of 680 cu.m for 2 hr duration for the 1500 GPM. It looks like if someone diggs further in any code, he can deviate sasily...
Regards,