Velocity limitation in water pipes 2

[Ktracid]

Hello all,

My hydraulic analysis indicate that due to high available static heads, the water velocity in a relatively short (~50 m) 8" pipe occasionally reaches velocities as high as 15 - 20 m/s. My understanding is that such high velocities could lead to excessive noise, high erosion rates, and even large piping loads; therefore, have to be limited but I do not know to what value. I was wondering if someone could help me out with this issue with reference to a code, standard, or a guideline.

For your information, so far, my research has indicated that according to section 2.3 of API RP14E and page 33.3 of 1997 ASHRAE Fundamentals Handbook , in order to minimize cavitation and erosion, the flow velocity should not exceed 15 ft/s.

Thanks in advance,

Pirooz

 

[davefitz]

general guidelines that I had been familiar with were:

Based on erosion/ corrosion issues for CS pipe , max steady, local velocity should not exceed 20 fps incuding effect of vena contracta at the inlet or near orifces. That could be increased to 30 fps with 1% chrome alloy pipe, and 75 fps for incolnel or SS pipe.

The "economic pipe size" for process costs that include pump power consumption is roughly 10 fps average or nominal velocity at full load for water systems .

For pipes feeding pumps with NPSH issues ( saturated liquid), a 5 fps design velocity may be used, but that may be increased to account for dynamic effects to avoid flashing in the downcomer during pressure transients.


2 phase flow , compressible flow, waterhammer cases involve other criteria.

 

[Ktracid]

Hi Dave,

Thanks for your reply. Do you know of any references for the first part of your response?

Piroz

 

[BigInch]

Waterhammer can become an issue at 10 fps or over, but depends heavily on the attached equipment, size of pumps, speed of valve closure, sensitive stuff like meters, how fast control valves react and pipe configuration. In pump stations and plants 30 fps is a typical max for short sections. Over that the pressure loss usually becomes problematic. 60 fps is pretty fast and should only be allowed for limited time upset conditions, never an operating range flowrate. It also depends heavily on quantity and hardness of solids that may be in the flow stream. There you would probably want to stay under (maybe well under) 10 fps.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[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

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.amazon.com/Fluids-Throug...542&sr=8-3&keywords=crane+technical+paper+410

 

[zdas04]

The "errosional velocity" calculation is described in one of the ASME standards, but I can't remember which one. I also don't know how to convert it to metric on input, so here is what I use:

v(max)=100 ft/sec / (&rho^0.5

With ρ in lbm/ft^3. I know it is ugly and that the units don't work out, but that is the published empirical equation. For water (ρ=62.4 lbm/ft^3) it works out to 12.6 ft/sec (3.9 m/sec). A lot of people round that up to 15 ft/sec. There is so much safety factor built into the constant that rounding it up to 20 ft/sec would be unlikely to lead to an increased incidence of failure.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[vpl]

A lot depends on whether your 50m of piping is all straight or whether it has elbows, valves, pumps or other things in it. It also depends on the piping material used.

I agree with bimr: Crane's Technical Paper 410, Flow of Fluids is relatively inexpensive and a good place to start. If you're going to be working with water piping, it's worth buying a copy. Besides from Amazon, you can get it direct from Crane (or their distributor):

http://www.flowoffluids.com/

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Ktracid]

Thanks everyones for your replies.

I would like to add that I am dealing with an intermittent service; something that is barely touched in any code, standard, or guideline.

Is 15 m/s fine for such services? That is what I am not sure of.

Cheers,
Pirooz

 

[bimr]

It is probably unrealistic to expect a water flow of 7,750 gallons per minute in an 8-Inch pipe (velocity 50ft/sec). That equates to aproximately 41 psi head loss in 50 meters.

You should be able to get about 2300 gallons per minute (velocity 15 ft/sec or 5 m/sec). As a point of reference, an 8-Inch fire hydrant puts out approximately 1500 gallons per minute.

 

[bimr]

Crane's Technical Paper 410, Flow of Fluids shows that you can get 3100 gallons per minute (velocity 20 ft/sec or 6 m/sec) with about 40 ft of head loss. However, no one will recommend operating at that velocity.

 

[BigInch]

Disregard my post above. 10 fps max for waterhammer is ok, but somehow ... I managed to post the other max velocity limits for gases.

If it ain't broke, don't fix it. If it's not safe ... make it that way.