NFPA 20 : Low suction pressure & flow at fire pump

[Neel.mech]

hello all,

I working on a weird project, we have a dedicated fire pump ( 750 gpm @ 60 psi) for a foam system which is tapped off from sprinkler tree. Distance b/w fire pump & sprinkler room is about 300 meters. we are getting only 170 gpm and about 5 psi at suction and fire pump didn't kick in when tested & manufacturer rep shut it down as there is no required flow. . is this design against NFPA reliable water supply?.

incoming line from sprinkler room is 6 " and available pressure on the hydrant outside is 44 psi when tested. wondering if anyone came across something similar ? any possible solutions ?

 

[skdesigner]

Sounds like there is an almost-closed valve somewhere between the water supply and the fire pump suction. 300m is a pretty long suction pipe, if there is only 44 psi at the hydrant outside the building.

 

[LittleInch]

Pressure drop for a 6" steel pipe at 750 gpm is about 7 to 10 psi, so not negligible.

Any elevation difference from hydrant to pump inlet?

But quite possibly some small pipe or valve or valve not fully open somewhere between street and pump inlet.
Any filters in line?
You need to trace the entire line from hydrant / water connection to the main all the way through to see if there are any flow restrictions anywhere.

Is that 44 psi when the pump is flowing or trying to flow?

But clearly yes, this is not allowable.

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

 

[Neel.mech]

Thank you Guys. Just to clarify the 300 m run has a lot of elbows both horizontal and vertical, there are elevation changes within the building approx sum it up to 30 ft.

Also, another thing i have noted is that about 20 ft before entering the fire pump room, they have down sized the pipe to 4" and upsized to 6 " at FP suction side. I am not even sure if that's allowed in the code? are there references its not allowed ?

also attaching hydrant testing data.

Flow hyd - 11.5 psi

Residual Hyd - static 45 psi, residual - 30 psi

 

[skdesigner]

Was there a post about this issue a few months back? It sounds very familiar.

 

[Neel.mech]

yea, there was. you have helped me out on that one. Please don't mention the building name and location because there is security requirements i have to follow, I appreciate that.

 

[skdesigner]

My mistake. Sorry you're in this situation. The fire protection design knowledge in our area is lacking, to say the least. I would like to say that the fact that this made it all the way to the commissioning stage before anyone realized there was an issue is unbelievable... but I see it fairly regularly.

If you would like a hand offline, I'd be more than happy to offer it.

 

[Neel.mech]

i will , appreciate it . thank you.

 

[LittleInch]

Nothing attached.

Not sure how you're getting any water.

your elevation difference alone is worth 13 psi then add in the pressure losses at 750 Gpm of probably about 10-15 psi with that 4" section and all the bends and you're at close to 30 psi.

I don't get the flowing, residual and static pressures. What do they mean exactly?
At what flow is the 11.5 psi?
I'm a bit puzzled.

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

 

[skdesigner]

I am making some assumptions, but here is how I understand it:

- the 30' elevation change isn't the delta elevation between the hydrant and the fire pump, just that there is more than the 300 meters of pipe when you factor in all the rises and drops to get a 6" main through a building at ceiling level. The fire pump centerline is probably at or very near the elevation of the hydrant outside the building

- 11.5 psi likely represents the pitot pressure, which is worthless without knowing the size or shape of the flowing orifice it was measured from

- There will be a backflow preventer in the sprinkler room that is also contributing to pressure losses

Agree with the above that it would be very difficult to flow 750gpm through 6" and have it be at a reasonable positive pressure at the suction flange of the fire pump, but dropping from 45psi at the street to 5 psi at the suction flange while only flowing 170gpm is indicative of some kind of obstruction in the piping.

 

[LCREP]

Neel,

Is this a new pump or existing installation? If existing can you get the previous fire pump annual flow tests? If so get as many years as possible to see if this is something new or has always been a problem. When I was on the insurance side I always asked for 3 years of previous pump flow tests, more if I saw a problem.

As far as the fire pump not starting take a look at the fire pump and jockey pump controllers and record the stop/start settings. Compare to what NFPA 20 says as to the stop and start points. This typically is an easy fix just adjust the settings. Sometimes the fire pump representative may not also service the controller so ask who is responsible.

If this is a new installation a few things to check:

1. What is the foam system gpm & psi demands?
2. Is the fire pump sized too big ie they have a demand of 500 gpm but installed a 750 gpm.
3. Who provided the fire pump design and are they qualified?

As I always say get the insurance company loss prevention/risk consulting folks involved.

Tom

 

[Neel.mech]

Good day,

@ LittleInch : thank you, appreciate your answer, sorry I have tried attaching hydrant test data , I have done it again but if it cant be access use the link below,

https://files.engineering.com/getfile.aspx?folder=e2f24e37-302d-472e-913e-66f3f7f05478&file=619.PNG

@ skdesigner : thank you, appreciate your answer.

1. 30' elevation change : yes, you are right , hydrant and Fire Pump are in the same elevation. 30' elevation is on the 300 m pipe running up and down in the ceiling space.
2. Back flow preventer - yes there is one in the sprinkler room.

please use the link for hydrant testing data:

https://files.engineering.com/getfile.aspx?folder=e2f24e37-302d-472e-913e-66f3f7f05478&file=619.PNG

@ LCREP : thank you, appreciate your answer, this is fairly a new installation , installed 2 years ago. answers to your questions,

1. What is the foam system gpm & psi demands? : foam system flow requirement : 1864.65 l/m @ 5.23 bar , hose requirement : 946.35 l/m , so in total 742.588 gpm.
2. Is the fire pump sized too big ie they have a demand of 500 gpm but installed a 750 gpm. not sure about this .
3. Who provided the fire pump design and are they qualified? i guess so, consultant signed off design, there is P.Eng stamp on the design, we are looking at potential errors and omission.

 

[Neel.mech]

hello all, some more additional info , see pump data attached

https://files.engineering.com/getfile.aspx?folder=e651b96f-09b3-44f6-99b8-114c2be0710b&file=FP.PNG

 

[skdesigner]

Yeah, that's way too much pump. For the sprinkler system demand, the 250gpm hose allowance would be added to the hydraulic calculations at the outside hydrant, and therefore wouldn't be need to be provided by the fire pump. So, you're looking at a sprinkler demand of roughly 500 gpm. NFPA 20 requires that all fire pumps be able to flow a minimum of 150% of their rated nameplate capacity. A 400gpm pump would have done the trick for your project.

But even that is applying a band-aid solution. If the reason a fire pump was added was because the water supply to the foam system had degraded over time, then the exact same issue exists for all the other sprinkler systems that aren't boosted by the pump as well.

 

[Neel.mech]

@ skdesigner -thank you for clarifying, appreciate the answer, i will start chasing the designer. Have a great day!

 

[cdafd]

1. So the fire pump water supply is coming off the fire sprinkler system,,,, somewhere down the line???

2. Or is it in the riser room,,, and sharing the incoming underground

If option one,,, you more than likely have a problem!!!!!

 

[Neel.mech]

@ cdafd - thank you for replying.

option 2 , sharing incoming underground .

 

[cdafd]

Well number 2 is an approved method.

1. Should have asked new or existing system??

2. What do the fire risers show for pressure, before the alarm valve?

 

[LittleInch]

Neel mech,

Apologies about this, but you're really not making much sense and throwing random bits of information into the thread in different units.

So to make sense of this we need to understand what exactly is your system ( draw a simple sketch), what are the flow requirements in the same units (USgpm or pref l/min) and most importantly pressures during different events.

Then not only distance but number of bends - each one is equivalent to about 6 feet of pipe.
Up and down is not really relevant, its the elevation of the hydrant where you're taking readings and the elevation of the pump centreline

That hydrant data you posted is, for me, next to useless as there are no flows on it - what is port 1-1 (??), What does "residual" mean as opposed to "static"?

We only have one example of running flow - 5psi inlet pressure at 170 gpm. Was anything else flowing at the same time?

Whoever you talk to will demand all these things and after two years I doubt anyone will entertain you now saying it doesn't work....

It looks very much to me like your inlet line is too small and that section of 4" needs to be removed in any event. Or the pump is in the wrong location.

But the key is what is the flowing pressure of the hydrant.

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

 

[Neel.mech]

LittleInch, what do you mean by random data, thats the hydrant test flow data( canada) & Pump. Its clear that 4" doesnt work, its on NFPA 20 Table 4.28 .tested as per NFPA 291, to understnd static and dynamic & hydrant testing. its a design problem, i have a new consultant working on it, just trying to understand the problem, thank you & appeciate your answer.
i dont want to provide further details as this is military facility.