Fire Pump selection for a fire hydrant 2

[madvb]

Hi,
I'm trying to select a packaged diesel fire pump to provide flow to a hydrant on a site. The facility is connected to the city's fire water line, but does not have enough pressure. If a fire hydrant is required to have 1500 gpm at 20 psig, does one select a pump for 1000 gpm or 1500 gpm?

I think NFPA 20 mentions something about the fire pump being able to handle 150% of its rated capacity. Can I say that my pump "rated capacity" is 1000 gpm, and therefore, 150% of my rated capacity is 1500 gpm, and would meet the fire hydrant flow requirement? Or do I select a pump for 1500 gpm + 50% = 2250 gpm?

If the facility has a fire hydrant (1500 gpm) and 2 buildings (bldg A: 225 gpm and bldg B: 300 gpm) that require fire sprinkler system. Does one size the fire pump for a fire hydrant + bldg B = 1800 gpm, or just the worst case (1500 gpm)?

Thanks.

 

[TravisMack]

I typically see engineers specify 1000 gpm pumps for what you are doing. When a fire pump is to meet both site fire flow and sprinkler system demand (separate requirements), you typically get the gpm from the site fire flow requirements and the pressure from the sprinkler system calcs.

Example: you may only need 20 psi at 1500 gpm for the hydrant, but you need 300 gpm @ 65 psi for the sprinkler system.

You could probably get by with a 1000 gpm @ 65 psi pump, maybe even slightly less psi.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[madvb]

Travis,
The 1000 gpm for fire pump selection, is that up to the engineer then to decide what he/she wants to use? Does NFPA allow that? Any ideas what the 150% of rated capacity is meant for?

I understand what you are saying about fire hydrant flow and sprinkler system demand. So it is not a combined flow, but rather use the higher flow and make sure pressures are adequate. Does NFPA mention about using the higher flow in the code somewhere?

Thanks.

 

[NJ1]

First of all your main concern is a Fire Hydrant so lets leave the sprinkler system out side this discussion unless you are required to meet that criteria as well. However like Travis Mack said typically a 1000 GPM fire pump is the rule of thumb unless you are supplying water to high rise or you have horrible water pressure to supply both hydrant and sprinklers.
In my opinion you should contact the AHJ to ensure that the Hydrant will be the only suppression device that will be required to meet the required water flow. If there is a sprinkler system in that building you should suggest to the client to purchase a pump package that will meet both.

 

[stookeyfpe]

I'm going to ask a different question: Can your water supply pressure and demand support installing sprinklers rather than the expense of a fire pump? I ask this because it sounds like you have a fire flow problem and a code official is somewhat unwavering in their enforcement. If you sprinkler the building, I would consider waiving fire flow, with some conditions.

Or, is less expensive to install a larger diameter fire line - or - is it more reliable to install a larger diameter fire line and offset the cost with the increased reliability?

I'm just asking.

 

[madvb]

stookeyfpe,
The facility is required to have both fire sprinkler systems in the buildings and fire hydrants on site, and is connected to city water, which provides enough flow, but not enough pressure due to the difference in elevation. So fire pump is needed.

NJ1,
I just wanted to see how this 1000 gpm is determined and if NFPA has any requirements. Given enough flow, but not enough pressure at the site and that a fire pump is needed, why would one select a pump for 1000 gpm when a fire hydrant requires 1500 gpm? and in doing so, does that still comply with NFPA 20? Or maybe NFPA doesn't care as long as you can get 1500 gpm out of the fire hydrant?

The requirements here is that I have to meet NFPA 13 and 20. The problem with my facility is the pressure due to elevation difference (~100' higher than point of connection). There is enough flow. Whether the fire pump will work depends on what flow is used. If my rated flow is 1000 gpm, then per my calculations, the suction pressure at the fire pump is positive (NFPA 5.14.3.1). This works!! But if my rated flow is 1500 gpm, then I'm getting a negative suction pressure at the fire pump. So hence my question... does using 1000 gpm meet NFPA requirements, if any?

Hope that makes sense.

Reference
NFPA 20-5.14.3.1 - Unless the requirements of 5.14.3.2 are met, the size of the suction pipe for a single pump or of the suction header pipe for multiple pumps (operating together) shall be such that, with all pumps operating at 150 percent of rated capacity, the gauge pressure at the pump suction flanges shall be 0 psi (0 bar) or higher.

 

[madvb]

My other question about if the fire flows should be combined or select the higher flow was to hopefully that someone here who is more familiar with NFPA can point out where in the code dictates what to use. I thought I remember somewhere in some code or someone told me that if you have more than 1 buildings on the site, use the highest fire flow for a building as the design flow. It is assumed that you won't have more than 1 buildings on fire at the same time.

 

[TravisMack]

The use of 1000 gpm pump comes from the idea that a fire pump is required to perform at 65% of rated pressure while flowing 150% of rated capacity. Since a 1000 gpm pump can be used to get 1500 gpm, then you can go with the less expensive 1000 gpm pump. That does meet NFPA 20.

You will have to be able to test the fire pump at 1500 gpm. If you are getting negative suction pressures at 1500 gpm, then you are going to be looking at a storage tank, or putting the pump lower on the facility if possible.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[madvb]

Thanks Travis. That's good to know. Selecting a 1000 gpm pump will work at this facility, even at 150% flow, but will not work for a 1500 gpm pump when testing at 150%.

Now only if I can convince my other engineers to use the fire hydrant flow and not use combined flows of hydrant and building. You wouldn't happen to know where this verbiage is in NFPA would you?

Thanks.

 

[TravisMack]

I don't know where the exact wording is. But, you just treat them as separate systems. For example, a standpipe system is designed to meet NFPA 14 flow requirements. You don't design it to meet NFPA 13 and 14. NFPA 13 requires a hose allowance for supplemental hose streams. Do they expect you to include that when trying to add to your fire flow.

They are just simply independent systems when calculating. It was explained to me a long time ago that if you have a car fire in the parking lot and are using the hydrants, then you would not need to figure the fire sprinklers flowing. If you have a fire in the building, the sprinklers are going and you just have supplemental help from the fire dept, so you don't need to figure the full site fire flow adding to the sprinkler system. That explanation may be entirely wrong, but it at least made sense at the time.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[pipesnpumps]

I have only one more thing to add to the excellent advice posted above..

If the pump is drawing from a potable water main, make sure the pressure in the underground suction piping supplying 1500 gpm (1000 gpm pump @ 150%) does not drop below 20 psig. NFPA 20 will let it go down to 0 psig, but the environmental/AWWA regs just about anywhere in the US prohibit that. If it is less than 20 psig make darn sure you do an exhaustive code search for this location and confer with the AHJ.

 

[TravisMack]

Great addition PnP! I had just assumed he was working with 20 psi as the minimum set point. I guess we know what happens when you assume something

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[madvb]

Nooooo.... I did not know this. My civil guys did not let me know about this requirement by AWWA. The residual pressure at the POC is 57 psig @ 1500 gpm. Going through the suction piping and backflow preventer, I'm left with 1.7 psig at the fire pump.... barely meeting NFPA requirements.

Is this 20 psig pressure to ensure that the pump won't cavitate or is this to ensure that the underground piping won't get crushed?

Thanks.

 

[Arsnman4]

To protect the underground supply.

"Fire suppression is a failure in prevention"

 

[madvb]

Arsnman - Isn't the backflow preventer already doing that?

 

[madvb]

pnp - Thanks for point that out. I just asked our civil guys for the test reports, and it indicates that the agency's standard specifications prohibit water pressure below 20 psi at any location in the system.

So what does one do when we don't have the required minimum 20 psig? Install a storage tank in the suction piping? I noticed NFPA calls this a "Break Tank". It's going to be 15 minutes x 1500 gpm = 22,500 gallons.

 

[pipesnpumps]

I would try putting the backflow preventor underground as far upstream as possible.. in a pit if it has to be... And then run the calcs to show that the piping upstream of the BFP is >20 psig..

Downstream of the BFP it can be any pressure at all, as long as the suction of the pump is at 0 psig or above. In other words I think you can go negative.. but more than likely your pump will be higher than the UG line so it will always be positive.

 

[madvb]

pnp - I see what you are saying now. My civil engineer also said that the 20 psig is from city water POC to BFP, which I do have. What a relief. Well thank you ALL for your help.

On another topic, just found out yesterday that heat tape will be allowed under NFPA 2010 edition. Guess I can start specifying wet pipe system.

 

[SprinklerDesigner2]

Of course it varies from state to state but here's what rules in Georgia.

http://img34.imageshack.us/img34/1833/lowsuctionfirepumps.jpg

The city of Augusta will allow 15 psi but not sure how that squares with the state EPA.

http://ga-augusta.civicplus.com/DocumentView.aspx?DID=1796

Page 11 "3. Booster pumps installed on the service line to or within any premises, must be approved and permitted by the City of Augusta, Georgia Utilities Department. Such permitted pumps shall be equipped with a low pressure cut- off device designed to shut off the booster pump when the pressure in the suction line of the service side of the pump drops to 15 psi or below. It shall be the duty of the water consumer to maintain the low-pressure cut-off device in proper working order at all times and to certify to the City of Augusta, Georgia Utilities Department, at least once per year that the device is operable."

I do know the State Fire Marshal absolutely requires 20 psi with the pump operating at 150% measured at the pump suction downstream any backflow prevention assemblies.

From experience I know Ohio and Kentucky have the 20 psi @ 150% requirements.

I've found minimum suction pressure is nearly always addressed somewhere... it may be the state fire marshal but more likely it could be the state EPA so serious research is indicated.

 

[SprinklerDesigner2]

"On another topic, just found out yesterday that heat tape will be allowed under NFPA 2010 edition. Guess I can start specifying wet pipe system."

Read the listing for heat tape it's really not the answer. Be careful with this!

Heat tape works for very small runs line passing through an unheated space but not intended to heat entire system.