Sizing distribution main for fire pumps 2

[chileheadcraig]

We have a maximum velocity limit of 10 ft/s used as guidance for sizing water mains in our system, which includes fire flows. It seems that when this topic is discussed, its with regard to the design fire demand. My question/concern is with regard to if a building needs a fire pump. Per NFPA 25, annual testing requires the pump be run at 150% of the pump rated capacity. If piping is designed for design fire flow, then maximum allowable velocity will likely be exceeded [at least] annually during the fire pump test.

How do other utilities approach pipe sizing when a fire pump is involved? Without much reference to go on, I've tentatively set a rule for 10 ft/s at design flow or 12 ft/s at pump runout (150%), whichever is more stringent (not everyone needs a fire pump). I'm curious if others feel this is too stringent (should base on design flow only?) or even maybe too lax (10 ft/s max, even at runout?)?

We had to ask someone to make a costly 12-in extension as opposed to an 8-in for a 1500 gpm fire flow, but I wasn't comfortable with an 8-in knowing they would be flowing at 2,250 gpm (14+ ft/s) at least once a year.

Any thoughts?

 

[bimr]

Reasonable pipe velocities depend on the application. There is no correct velocity for all applications. Here is a general guideline.

Reasonable Velocities for the Flow of Water through Pipe:

Boiler Feed.............8 to 15 ft/sec
Pump Suction ............4 to 7 ft/sec
General Service.........4 to 10 ft/sec
City.......................to 7 ft/sec
Transmission Pipelines...3 to 5 ft/sec

This is published in Crane's technical manual 410.

The general service recommendation of 4 to 10 ft/sec velocity is appropriate in buildings. If your velocity is above 10 ft/sec, it would be a general recommendation that the pipe supports be evaluated for the increased forces with higher velocities.

What you question is missing is the length of pipe and pressure drop. For practical purposes, it can be assumed that the head loss due to the fluid flow varies as the square of the velocity.

Use the Darcy equation:

h= (fL/D) v*2/2g

where h= head in feet

f= pipe friction factor, assume 0.17
L= pipe length, feet
D= pipe diameter, feet
v= fluid velocity, ft/sec
g= 32.2 ft per sec per sec

A flow velocity of 10 ft/sec may be acceptable in buildings and short pipe runs, but is not a practical velocity for transmission mains as that velocity would result in a pressure drop of 60 psi per mile. A flow velocity of 5 ft/sec is a more practical velocity for transmission mains as that velocity would result in a pressure drop of 15 psi per mile.

 

[chileheadcraig]

I can evaluate pressure drop and our system has no problem providing, in most cases, while maintaining 20 psi residual. The main concern was the potential for water hammer in our system when stopping their fire pumps. Also, this is for fire flow, not typical system demands. Thanks for the feedback.

 

[bimr]

Water hammer considerations is a secondary consideration with flow rates. That is also the reason that the valves are made to operate slowly with multiple handle turns.

 

[KoachCSR]

Depending on your system configuration, water hammer is not typically a concern with respect to velocity in the pipe with a fire protection system - as bimr stated - so long as you don't have a quarter-turn valve that will "instantly" stop flow after a long, straight run, for example.

The only real effect that's usually encountered with increased velocities is a vibrating or humming in the pipe during operation (rattling or banging at excessive velocities, as well as increased pressure drop). In the industrial world, we typically see velocities around 15 ft/s (as bimr noted) and in certain cases, may allow the velocity to approach upwards of 20 ft/s (for "upset" scenarios, for example).

 

[chileheadcraig]

Thanks for your input, KoackCSR! In particular I was more concerned with pump shutdown. In other instances, we have experienced severe hammer and customer property damage from fire hydrants being closed too quickly, so its not beyond the realm of possibility based on what we've seen.

 

[bimr]

Thanks for your input, KoackCSR! In particular I was more concerned with pump shutdown. In other instances, we have experienced severe hammer and customer property damage from fire hydrants being closed too quickly, so its not beyond the realm of possibility based on what we've seen.

To minimize water hammer, especially for plastic (PVC) pipe, water velocities should generally be limited to 7 ft/sec unless the water system is evaluated for water hammer. Most agree that the velocity should never exceed 10 ft/sec.

Fire fighter training includes a section on water hammer although some may not be paying attention. I had a water department manager show me his water pressure chart which showed that shutting off the fire truck valves too fast caused a main break.

 

[chileheadcraig]

Thanks!

 

[cvg]

I spent years evaluating municipal water systems and never did an analysis for a fire pump on private property. the analysis stopped at the curb. anything extending into the private property beyond the city right of way was not considered. It would be up to the developer or owner of the property to prove to the fire marshall that he has sufficient pressure and flow to handle fire flow requirements whether that is by fire hydrants or tanks and fire pumps. that said, I agree with the previous advice that your system can probably handle a brief, higher than expected demand. and if you are setting requirements for pipe sizes within private property, than another matter entirely.

 

[chileheadcraig]

cvg, in this case a water main extension is required which they will pay for and deed over to us, so in this case, our design standards affect the water main extension cost (to them). If we allow them to install an 8-in main knowing that their pump would create velocities of 12-15 ft/s in our main, would you not be concerned about hammer when they cut their fire pump off?

 

[cvg]

well if they are going to construct a watermain for you, than make it larger, 10 inch would be adequate for pump testing. although, if this is only happening once a year, your risk is already fairly low. most agencies set velocity limits for design flows, not for fire flows. Pressure of 20 psi is generally the controlling factor for fire.

 

[chileheadcraig]

I agree, but we don't allow oddball sizes, so we go from 8-in to 12-in. Thanks!

 

[LittleInch]

10 inch oddball????

14 maybe but not 10.

However the point is that even if its only once a year, if the impact of the flow is damage to the rest of your system from surge then that becomes the design flow. IMHO.

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

 

[chileheadcraig]

LittleInch, in these parts 10-in is an oddball! I tend to agree with your thought on this. Thanks!

 

[77JQX]

Our agency allows for up to 18 fps during fire flows. While I've seen lots of surge issues with pump trips, I don't recall one on the suction side - always the discharge side on pump trips. We would allow the 8-inch pipe.

 

[chileheadcraig]

77JQX, thanks for the feedback!