NFPA 13 doesn't contain velocity limits because the Committee on Automatic Sprinklers has long felt that piping velocities are self limiting. The pressure losses increase exponentially as velocities increase, so pipe sizes must be increased to make use of available water supply pressures. The committee also believes that hydraulic calculation inaccuracies using Hazen-Williams are minor compared to the inaccuracies associated with assuming a long-term pipe roughness factor. In one 1968 study of actual roughness of aged steel pipe, the measured C values ranged from 82 to 135.
You still can get a sense of the velocities on the sprinkler lines by estimating the number of heads going off during a fire, the "K" factor of the sprinkler heads, sprinkler orifice size, line ID's serving the sprinkler heads going off, the line pressures at the time of flow and the flow equation.
Some specs allow certain velocities. A certain large insurance company only approves velocities not in excess of 20fps.
I've been told 32 fps is the upper limit to avoid water hammer. I'm skeptical as to the credulity of this statement and haven't been able to find a proof sufficient for me to make the statement myself. Nonetheless no calcs leave here exceeding 32 fps. Might be an old wives' tale.
Which large insurance co. are you referring to? I was told by a senior engineer with FM Global that they dropped their velocity restrictions several years ago.
While I am not an expert in this standard nor its intent, I guess I was some surprised by some information and comments of this thread. I do understand NFPA 13 now mentions pipes other than metal (e.g. plastics like “cpvc”). Based on the flow velocities mentioned in this thread it thus seems odd to be eliminating any mention/concerns? of flow velocity in pipelines, while at virtually the same time it appears bringing e.g. weaker plastic pipes into the picture.
Irrespective of such issues as survivability of exposed pipes or increased toxics or dangers in combustion etc., I believe many other authorities, manuals and specifications etc. do suggest limitation of flow velocity in at least plastic pipes (see even article it appears promoting some use of plastic pipes at
I suspect based on experience, and/or suspected response in maybe particularly “water hammer” or other damages).
While I guess I also understand the premise of the tradeoff, “…damage a piece of equipment, but put out the fire - I will accept that”, it would appear that a delivery pipeline splitting near immediately due to shock in a “water hammer” event would probably not help much with the latter objective.
If I was using plastic piping for my fire suppression - I would definitely look at the characteristics of the piping.
I have yet to use it on a commercial project. Not many owners have indicated a preference for it.
Water hammer is caused by either a rapid stopping or rapid starting of water floo.
If a sprinkler system is on "city pressure" - I don't see a rapid flow start. A sprinkler or two goes off and you have 20-30 gpm starting to flow. At the larger flow in a 1" pipe, that would be ~14 fps - not excessive.
If the system is on a fire pump system - and the pump does not have a soft start or ramp up starting mode - then you could have excessive speeds and potentially water hammer.
I don't understand why fire pumps in new construction are specified without a ramp up function.
Where I have seen high flow velocities or others have seen them is,
1. When the sprinkler contractor is just putting numbers in his software program without understanding the real situation (i.e. trying to put 700 gpm through a 2.5" pressure regulating valve off a 6" standpipe because that is the only size the valve comes in)
2. When the sprinkler contractor is trying to use the smallest piping he can to keep his cost down - even though the velocities will be in the 30-40 fps range.
The plumbing code (ie IPC) limits the velocity of piping with a maximum velocity. We always ask for it on shop drawings/calculations in additon to sprinkler calcs.
The IPC has maximum velocities:
all piping 1/2 inch or smaller = 5 feet/sec.
copper, 5/8" - 1-1/4" = 5 feet/sec.
CPVC, 5/8" - 1" = 8 feet/sec.
Galvanized, 5/8" - 1-1/2" = 8 feet/sec.
Polyethylene, 5/8" or larger 8 feet/sec.
Polybutylene, 5/8" - 1" = 8 feet/sec.
PVC, 5/8" - 1" = 8 feet/sec.
Right, if sprinkler systems were limited to such velocities I can already tell that pipe sizing would be unreasonably large.
There's a bit of a fallacy in that reasoning however. Specifically a sprinkler designer doesn't know how many sprinklers will activate should a fire start. If all sprinklers in an auditorium open (just thinking here) then the system's effectiveness would be little on a fire with sufficient fuel loads. If only a single sprinkler opens in a small compartment, statistical data suggests one sprinkler can often extinguish a fire in a small compartment considered light hazard or residential. Designing for the upper limit makes sense in a worst case scenario (such as a fire is anyways).
Why fire pumps are not designed to ramp up pressure is beyond me and I haven't found a reason. I have seen underground loops blown out by water hammer from testing of fire pumps. Also mains on the suction side have been collapsed, which may have been avoided if the pump ramped up and gave more time for pressures to equalize.
To PEDDARIN2, the advantage of using CPVC piping is not necessarily its friction or mechanical properties relating to calculations but to its being able to be cut and fitted in the field. Residential construction (homes, motels, etc) and most stick frame tends to be very sloppy if one is accustomed to steel buildings. If such construction would be protected by steel piping then even a very good field survey would require many hours of cutting and threading steel pipe on the job site to make it fit properly. The greatest downside is when a fitter omits to glue a joint and it blows out six months or a year after the job is finaled.
If we took our standard 3 story stick built motel but used steel inlieu of plastic our price would triple or more.
Instead of $40,000 we'd easily be $120,000 and that wouldn't be the cost of materials, I am not even sure material would go up and even if it did it wouldn't be all that significant, but labor would go through the roof. Owners are watching price now more than ever.
I don't deal much with residential construction so I cannot address that.
As stated, I haven't done any plastic sprikler piping - yet.
Some of it is the owners haven't requested/required it. Another is that there is some quirkiness in the 25/50 smoke and flame spread rules that keeps it out in a plenum area.
On my projects, there is no field cutting of the piping. I get a sprinkler layout drawing to review. From that I can check various things about the proposed layout. Once this is reviewed (by me and the AHJ) and approved (by the AHJ) - piping is cut to match the plans and sent out to the field so the installer just has to put the pieces together in order.
When you mention a glue joint failing months or a year after completion - isn't there some requirement for a pressure test on the system?
It might not be required on residential but I require it on my projects. A joint that is not good or a weld that is incomplete shows up during this test - so I don't have to worry about it failing if there is a fire.
You can pretty much bet there is field cutting going on. It is very rare for a job to go in just like a puzzle without having to cut some pieces to fit. What happens when the duct guy moves his trunk line? There is going to be some field cutting? If a plumber had to reroute a drain for something and it is going to interfer with the branch piping, there will be some field cutting.
Sure, good designers will minimize the amount of field cutting, but it is rare to see any job where the Rigid 300 does not get turned on.
I don't doubt there is some field cutting. There are always field changes and the good coordination is done by good contractors.
But, if there is a lot of it, the permit drawings (showing lengths of piping, elevations, etc.) the sprinkler contractor is supposed to be building to have to be modified and the permit has to be reapplied.
After a couple of those, the HVAC or the plumbing contractor has to make it work to get around the sprinkler piping.
I have seen fittings hold during a test, and after that were not glued. After they blew apart it was discovered that just a bit of over applied cement from the opposite side of the fitting had been holding it in place.
