Water Service Sizing and velocities of 8

[moberschmidt]

I am having to do more and more water service sizing and am looking for some insight into the maximum velocity that should be used in sizing water services for multifamily. In general a 3/4" service is adequate for a single family and possibly a duplex. Where does one draw the line at velocity and go up a size. Any resources would also be appreciated

Thanks
Mark Oberschmidt

 

[RWF7437]

Get a copy of AWWA Manual of Practice on Water Service sizing. Don't have the number but you can find it at the website :

http://www.awwa.org.

Also, the Uniform Plumbing Code has a method called the "demand fixture unit method" which is very conservative but sometimes required.

If you don't have either reference check with your local plumbing inspector and with the water service provider. They should be willing to let you use their copies for a short time at their offices.

Good luck

 

[dicksewerrat]

Best bet is to contact the water supplier or Bldg. Insp. Dept. as RWF7437 says. If you say 1" tap and 1 1/4" line and owner gets pricing, then water util. says 2" line the owner wil be very upset.

 

[canman]

You may want to take a look at the following link for more pipe capacity and flow data, it may be helpful for future projects.

http://www.muellercompany.com/test/download/pdf_down/16-28-29.pdf

Canman

 

[KRSServices]

Maximum velocities should not be the sole parameter. The key should be the headloss through the line and the existing line pressure at the main. With minimal pressure, say 40 psi at the main and a hill with a typical two storied home, with a sprinkler system...the overiding concern is going to be the headloss at maximum flow. Of course with too small a line, the velocities will be up there...but so will the headloss. For me, that is usually the warning that the service line may require to be larger.

KRS Services

http://www.krs-services.com

 

[m718]

Hi,
Basic question,
Flow of liquid depends on the velocity and area. If I use 4" sch. 40 C/S Pipe, and keep increasing the velocity (just assume), flow will also increase. Can anyone tell me what is the maximum flow can be achieved from 4" pipe and why there is a limit of maximum flow.

Thanks

 

[RWF7437]

m718,

There is no theoretical limit to the flow. The real limits are: 1 the strength of the pipe material and 2 the energy required to achieve the higher velocity. In the water business the test is the practical one of cost. If you use a small pipe then pumping (energy) costs will be higher. If you use a larger, more expensive, pipe then pipe costs are higher but pumping costs are lower. The "break even" point usually occurs around a velocity between 4 to 6 feet per second. This is a rough rule of thumb but is widely used and has been for many years.

good luck

 

[bimr]

For practical purposes, it can be assumed that the head loss due to the fluid flow varies as the square of the velocity. The maximum flow will be when the head loss is equal to the available pumping head.

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

 

[bimr]

There is a maximum flow in m718's example. It is when the headloss is the same as the maximum allowable pressure of sch. 40 pipe. For a 4" dia. sch. 40 A106 Grade B carbon steel pipe, that would be 1425 psig @ 100 Deg F.

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

 

[LHA]

bimr:
Can the joints, even restrained ones, endure 1425 psi? I don't know, I'm just asking, but can't see how.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[katmar]

I agree with KRS Services - it is essential to look at the pressure drop in addition to the flow velocity.

Be aware that for a given velocity the pressure drop is higher in small pipes than in large ones. This means you can use higher velocities in larger pipes for the same pressure drop.

For example, a velocity of 6 ft/s for water in a 3/4" pipe will give a pressure drop of 14 PSI over 100 ft of pipe. However, in a 4" pipe the same velocity of 6 ft/s gives a pressure drop of only 1.9 PSI over 100 ft. In the 4" pipe you would have to increase the velocity to 16.5 ft/s to get a pressure drop of 14 PSI.

regards
Katmar

 

[bimr]

lha,

Maximum allowable pressure ratings of fittings is equal to that computed for straight seamless pipe of equivalent material. Maximum allowable pressure varies directly with the allowable stress. The design pressure is the maximum sustained pressure that a piping system must contain without exceeding its code-defined allowable stress limits.