Water Distribution Pipeline Velocity Criteria. 5

[SSU_PE]

Dear all,

Currently, I am working on the pipeline project for water distribution.
We are using carbon steel pipeline with cement lining.
However, for this velocity of the water should be maintained below 20 ft/s(6.1 m/s) as per AWWA-C205.

Could anyone of the community has come across or experienced the water pipeline distribution with velocity criteria as high as 19.7 ft/s (6 m/s)?

Thanks,
Subodh.

 

[bimr]

Fluid velocity is determined by the economics of pumping, not some arbitrary factor. A fluid velocity higher than 15 ft/s is possible. However, you can be sure that high fluid velocities and long pipe runs will not be economical.

The velocity that you reference is to keep the pipe from eroding, not for economical piping. At that extreme velocity, you would have an estimated headloss of 60 psi per 1000 feet. That would mean installing a booster pump station every mile along your pipeline, which will not be economical in both capital costs and operating costs.

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 (from Cranes Technical Paper 410):

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

Go to a basic hydraulics book. Try Cranes Technical Paper 410 as a reference for the above velocities.

http://www.eng-tips.com/viewthread.cfm?qid=111206

http://www.eng-tips.com/viewthread.cfm?qid=348061

Crane

Link

 

[LittleInch]

It's a bit high, but not extraordinary.

Your pressure drop will be quite high and you risk surge events if you can stop the flow, but if it needs to happen either for an emergency or over a short length then it shows there isn't a problem.

You would normally aim for 1 5 to 2.5 m/sec for economic flow.

I agree with the post above.

Any more background?

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

 

[1503-44]

You should not use a velocity higher than 4m/s, unless you prove by dynamic analysis that surge pressures do not exceeded pipe allowable pressures.

 

[Joe591]

When I was still in technikon they taught us that piping for pumped systems had to be sized for a 1meter pressure drop for every 100meters length of piping. We were taught that this was the most economic trade off between initial installation and running costs of the plant. Obviously this was about 13 years ago, so I'm not sure if the number has changed since then or how this ties in with code requirements, but it's just food for thought. That's, of course, assuming that this is a pumped system.

 

[LittleInch]

Therein lies a key difference between piping and pipelines. 1 bar per km is more like it for pipelines due to their much longer length.

But piping with much shorter lengths can afford the pressure drop. Probably close to 5m/sec, but very fluid dependant.

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

 

[bimr]

I don't know of any application in a water distribution system where you would use 5m/sec.

Municipal systems are required to have a minimum pressure of 20 psi. A velocity of 5m/sec would have a pressure drop of approximately 20 psi in just 50 feet. A velocity of 5m/sec would just not work in a water distribution system.

Fire flows in municipal water distribution systems are typically maxed out at 3m/sec. With that high of a water velocity, the fire department is required to slowly close the fire hydrants. Otherwise, the water distribution system would have water main breaks from the resulting water hammer if the water hydrants were closed quickly.

There is nothing new here. The Crane Technical Paper 410 was first published in May 1942.

 

[fel3]

bimr...

Right idea, and I agree with the criteria in your initial post, but I think 20 psi in just 50 feet is far too much unit head loss. Here are the results of three trials I made, with everything converted to USCS because that's what my calculator program for this stuff uses:

Data:
V = 5 m/s = 16.4 ft/s
dP = 20 psi = 46.1 ft of head

Using the Continuity Equation:
For d = 6 in, Q = 1,446 gpm
For d = 12 in, Q = 5,783 gpm
For d = 18 in, Q = 13,011 gpm

Using the Hazen Williams Equation:
For dP = 46.1 ft, C = 130, d = 6 in, and Q = 1,446 gpm, L = 314 ft
For dP = 46.1 ft, C = 130, d = 12 in, and Q = 5,783 gpm, L = 705 ft
For dP = 46.1 ft, C = 130, d = 18 in, and Q = 13,011 gpm, L = 1,132 ft



============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[dik]

Can high velocities contribute to cavitation for any irregularities?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[SSU_PE]

Dear all, as expected i have found some good discussion & quick response over the thread in this forum, thanks!

bimr: I understands the concern raised by you, however, it seems that its because i have not explained the issue completely. Please excuse me for that. Moreover, the references to threads & paper provided by you are helpful & improves practical understanding in this case. Could you please add any more referring to the below provided information.

LittleInch: The problem starts at emergency & short duration of run. Please refer below info.

As per AWWA-C205 water velocity should be maintained in normal ranges (not specified) for best performance of lining. It also states that velocity above 6.1 m/s (20 ft/s) will require special study (to understand the lining performance).

In our case, we are constructing redundant pipeline(CS with cement lining) to the existing GRP pipelines(#2) for the short distance (less than 2 km), where failure in GRP line were experienced in the past. During normal operation velocity are within the range of 3-4.5 m/s. When both GRP lines fails, the new designed CS pipeline should be capable for the complete flow (total flow in #2 GRP pipelines). However, this is considered as emergency situation & for short duration. With above consideration, we would like to have pipeline (65 inch) velocity at or below 6m/s in emergency case. We have evaluated that frictional losses, cavitation & surge pressure are within the design limit because of this high velocity by performing steady & transient hydraulic analysis.

However, we have more concerns about performance of cement lining considering erosion and damage at this emergency situation.

In this concern, client wants to understand/quantify for how much short duration (specifically like in days) he can run the system without any damage to cement lining.

We have communicated with AWWA technical committee, to understand if this velocity limit (6.1 m/s) is based on some practical experiments or on field experience. However, it looks that it is based on some other reference standard.

Thus, i am looking at, if the above design consideration is unusual or still there are water systems that are designed or run at water pipeline velocity as high as 6 m/s. From where, i can get idea/information about allowable duration of the run.
I hope the problem is more clear now.

Thanks, Subodh.

 

[bimr]

Subodh,

Project engineering design requires estimation of capital and operating costs, not just a theoretical evaluation of maximum pipeline velocities. The maximum velocity is limited only by what can be practically pumped through a pipeline. The design engineer optimizes the size of the pipe based on the life-cycle cost, including power and capital costs. A practical maximum velocity is about 2.4 m/s (8 ft/s). Higher flow results in greater headlosses and may result in excessive water hammer. (Garr Jones Pump Station Design, 3rd edition)

Ductile iron pipe is cement lined as well. Here is the Ductile Iron industry states: "Although there are differing opinions on this subject, a conservative maximum velocity for design purposes is 7 fps (feet per second).

The AWWA (American Water Works Association) standard for thickness design of ductile iron pipe is C150. The exercise for calculating the required thickness based on internal pressure includes a 100 psi allowance for surge pressure and a 2:1 safety factor. The surge pressure allowance is based on a 50 psi pressure rise for each foot of extinguished velocity, and the fact that most domestic water systems operate at approximately 2 fps.

Ductile Iron pipe may be rated as high as 350 psi service. A pipeline operating at 7 fps velocity could account for a 350 psi pressure surge (7fps X 50 psi/fps). Adding a potential surge pressure equal to the pressure rating of the pipe encroaches significantly on the safety factor. Exceeding 7 fps velocity could produce potentially damaging surge pressure."

The maximum velocity in force mains is about 2.4 m/s (8 ft/s), although for these small pumping stations a practical maximum might be 1.8 m/s (6 ft/s) to allow for future growth) (and also to reduce the severity of water hammer as a pump is activated. (Garr Jones Pump Station Design, 3rd edition)

The issue is not the theoretical maximum velocity possible without damaging the pipeline through erosion.

The issue is the project feasibility of capital and operating costs.

Operating a water pipeline at 20 ft/s (6.1 m/s) would require installing a booster pump station every mile along your pipeline route, which will be tremendously expensive to install and to operate.

Right idea, and I agree with the criteria in your initial post, but I think 20 psi in just 50 feet is far too much unit head loss.

Yes, everything was just a guess until the poster described his project.

Now he states a 65 Inch pipeline and 2 km length. That would be about 30 psi headloss st 20 ft/sec. That velocity results in an estimated pumping cost of $8,000 per day as compared to $1,200 per day when operating at 7 ft/sec.

The post raises some interesting questions on how a pumping system can have a practical design when operating at the different velocities.

 

[1503-44]

Practicality or good economics is not a reasonable criteria when operating outside normal conditions at velocities 3X higher. For extreme conditions you should look at only if the possibility exists to operate at such a high flow rate and pressure within acceptable risk levels. You are not overly concerned with the high pressure and surge, so why are you getting so concerned about erosion? You said this will not be a long term condition.

You design everything for normal operating ranges. Then you analyse that design to find out what your extreme performance limitations are. You should only revise the design further when extreme performance limits of the normal condition design still cannot be accepted.

Designing for 3X extreme conditions and expecting practicality and economics at all ranges is not realistic.

 

[HTURKAK]

As far as i understand , you have 2X 65 in GRP lines which failed in the past and you want to construct a redundant pipeline CS with cement lining . The new CS pipeline should be capable for the complete flow (total flow in #2 GRP pipelines) and will be in service only for emergency case . The set up would be ; 2X65 in GRP pl DUTY + 1X65 in STAND BY..
Is this correct ?

I just screened the previous posts and hope my post is not repetion of others. The relevant standard and literature ;

AWWA C205,

- II.C. Flow velocity. Cement–mortar linings perform best when flow velocities are in normal ranges. When the flow velocity exceeds approximately 20 ft/sec (6.1 m/sec), special studies may be required to determine the suitability of this type
of lining material.

- II.B. Intermittent operations. Cement–mortar linings are best suited for pipelines that are continuously filled with water. When cement–mortar-lined pipelines are operated under prolonged empty conditions, special precautions may have to be taken to prevent excessive drying out of the cement–mortar lining.

DIPRA CEMENT-MORTAR LININGS FOR DI PIPE,

.....the available literature lists satisfactory performance for cement/cement-mortar linings for potable water with velocities of 20 to 40 fps.... Different installations will have different configurations, bend angles, flow characteristics, amount and shape of solids content in the water, etc. Using a velocity of 20 fps and applying a safety factor of 2,
remembering that the kinetic energy of a particle is a function of the square of the velocity, will result in a velocity of 14 fps.


My concerns for the proposed set up,

- If the service is not continuous , the cement lining would get dry and cracks and spalling could develop,

- If the PL is buried , the cement lining has poor flexibility and again easily spall with deflection of PL under backfill loads , and traffic impact loads..


IMO ;

- The three pipes could operate continuous ( in this case the during normal operation velocity would be within the range of 2.0-2.5 m/s. and CS line with cement mortar, would be OK for intermittent velocity 6.0 m/sec.)

- If the deflection of the CS line under surcharge loads and traffic less than 2 in, it is fine..

- If i were with your shoes , i would consider also CS with epoxy coating or DI with PU coating..

Just my thoughts ...