Maximum Velocity for Water in Hydraulic Calcs?

2/20/2019

Does NFPA 13 limit the friction loss per foot of pipe? What is good practice to follow in calculation process?

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13 Comments

Naveen Kumar S link

2/20/2019 07:52:46 am

it's 15FPS.

John P Malescio

2/20/2019 10:08:32 am

NFPA-13 does not have any restrictions on piping velocities, but NFPA-20 does have restrictions.

If you look at the history of the Hazen-Williams Equation, you'll find the velocities higher than 25 FPS start to break down the equation due to the extra forces in the velocity pressures.

I always try to keep them sub 25 FPS optimally sub 20 FPS

Anon

2/20/2019 10:47:10 am

There are no velocity limits in NFPA 13.
I have long heard of the limitations of the H-W at higher velocities, and have read Ken Isman's NFSA article comparing the H-W to the D-W in terms of velocities. Can you elaborate on how/where the breakdown with H-W occurs? I have not been able to find this information, nor has anyone been able to walk me through it.

James

2/20/2019 10:30:17 am

While accurate, this is a baseless response. There is no maximum requirement in NFPA 13 or in building codes for hydraulic calculations. However, NFPA handbooks tell you to use velocities between 10-20 ft/s. The reason is that the values of the friction loss coefficients used are accurate only if the flow velocity is close to that at which the value of C was measured.

James

2/20/2019 10:32:46 am

To be specific, this is for the hazen-williams equation.

Anon

2/20/2019 10:51:06 am

John, that is consistent with what I have read.

PETE

2/20/2019 10:44:55 am

I set a warning threshold at 32 fps in hydracalc, and don't really even pay attention to that if it is only in the node segment supplying the most hydraulically remote sprinkler (nearest sprinkler on the furthest line). Velocity pressure only accounts for a small percentage of the total pressure, and any deviation from the Hazen-Williams formula can be correct for with all of the other safety factors in the NFPA 13 standard. Velocity pressures DO need to be accounted for in other types of systems, but if you're concerned about it, AutoSprink calculations automatically include the velocity pressure in the calcs.

Franck Orset

2/20/2019 11:02:37 am

No restriction from NFPA

When I was working for an Insurance company reviewing hydraulic calculations for our clients, we were using the Factory Mutual requirement (20 ft/s or 6 m/s in metric) for gridded systems.
At that time, the French standard had a limitation at 7 m/s (23 ft/s). Not sure if it is still the case...

It is anyway advisable trying to keep a velocity below 20 ft/s, especially in the area where the sprinkler heads are operating.
The reason is that if the velocity is high where water is flowing through a sprinkler head, the distribution pattern could be problematic. The high velocity in this area is an indication of a possible issue.
If the high velocity is on the feed main, away from the calculated remote area, this is less of a concern.

Franck Orset

2/20/2019 11:09:22 am

I suddenly realized, by reading again the question, that the answer is not to the question (as well as all the answer above).
The question is not regarding velocity limitation in pipes, but about maximum friction loss per foot of pipe.
It is true that high friction loss is also related to velocity.

Again, the answer is no. There is no limitation. Just common sense. If you have huge friction losses, you will need a higher pressure from your water supply and it is never advisable.
The art is to find the equilibrium between the efficiency (limited friction loss = larger pipe) and the cost (larger pipe = larger cost and possible obstruction issues)
But if your water supply is able to compensate your high friction loss in a small portion of pipe... no big deal. It just means that your system has not been optimized and you're a poor designer :)

Anon

2/20/2019 12:02:16 pm

[sheepishly] I also did not read the original post correctly, and went straight down the old velocity rabbit hole. I concur with Mr. Orset.

Patrick Drumm

2/21/2019 07:15:16 am

I agree that there is no requirement regarding limits of pressure loss per foot in NFPA #13. It can be important to keep in mind high pipe velocities and high pressure loss when designing systems, because if changes are needed to be made in the field in these areass due to any number of issues (coordination, lighting location changes), additional pipe and fittings in these areas can cause signifigant increases in pressure requirments due to the pressure loss per foot previously identified and the added equivalent lenght of pipe you would be adding.

Piero Piemonte

2/21/2019 01:45:02 am

European norm UN EN12845:2015 indicates maximum velocity and not the limit of friction losses.
Those limits are:
6 m/s (19,68 ft/s) through Control Valves and Flow Switches
10 m/s (32,80 ft/s) in any other net part

Ivonn link

2/20/2019 12:18:07 pm

In NFPA 13 Ed.2019 include a new statement in the section 27.2.1.4 says : the velocity of water flow shall not be limited when hydraulic calculations are performed using the Hazen - Williams.

Is not a ilimitation but you need to keep in mind friction loss and that are related to velocity, you need to take care of the friction loss but if you system have a good cushion you don't need to worry about the velocity.

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