Is there a maximum to flow velocity in a pipe due to physical reasons? 5

[Yobbo]

I have been confronted with a principal question abouw flow velocity in a pipe. This question is not about the economical flow velocity in a pipe, that one can find in some textbooks, but about whether there is a limit flow velocity, if being exceeded, will ultimately lead to the degeneration of the pipe material. Cavitation due to a pressure lower than the local saturation pressure is a known factor, but should not be considered here.
It would be interesting to hear your comments on this.

 

[zekeman]

I think the limitation would be more in the line of having a pressure so great as to blow up the pipe.

 

[BigInch]

Yes. Practically speaking, I refuse to split hairs any longer, with a fixed inlet pressure the theoretical maximum is near the velocity of sound in the fluid. More pressure drop will not increase mass flow.

Here's a basic discussion,

http://en.wikipedia.org/wiki/Choked_flow

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[dcasto]

its called the errosional velocity.

 

[BigInch]

Excellent answer dcasto, well... except for the extra "r" . I totally went down the wrong track on that one. I should have paid attention to the OP, rather than get sidetracked by the first response.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[JohnGP]

API RP 14E provides some guidance on erosional velocity considerations for two phase gas/liquid piping on offshore platforms. It presents an empirical relationship
V_e=c/[√](pm)
V_e is erosional velocity
c= empirical constant (=100 continuous duty, =125 intermittent, and for no solids in the fluid stream)
pm = gas/liquid mixture density lbs/ft³

Higher values of "c" can be used if corrosion is not a factor.

Cheers,
John

 

[ione]

I think that talking about “erosional velocity” is meaningful only for some fluid. Just to clarify, IMO speaking of erosional velocity for water (pure, drinkable water without particles or fines suspended) is kind of a nonsense.

 

[cvg]

unless the fluid contains meaningfull amounts of sediment or other particles, rational design and operation of most facilities will rarely if ever produce flow velocities in pipes that creates significant amounts of erosion during the design lifetime. That said, I have seen very damaging erosion in gravity flow drains that are not even flowing full and at relatively low velocities but are carrying a significant bed load of sand and gravel.

 

[IRstuff]

interesting reading on the subject:

http://www.mms.gov/tarprojects/131/131AE.PDF

TTFN

FAQ731-376

 

[BigInch]

You sand coming out of a gas well will cut through anything.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[IRstuff]

Water, alone, can also do cutting:

http://en.wikipedia.org/wiki/Water_jet_cutter

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FAQ731-376

 

[rmw]

If you rule out "erosional wear rate" (which I do not), then there is the issue of having to go rob a bank to buy enough pump to push the fluid through the pipe.

rmw

 

[cvg]

water jet cutting:
Uses a high velocity stream of abrasive particles suspended in a stream of Ultra High Pressure Water (30,000 - 90,000 psi) which is produced by a water jet intensifier pump.

I would like to see any ordinary pipeline exposed to these pressures. This is a specialty and is far beyond the limits of the typical design. And according to wikipedia, it is done with abrasive particles.

 

[ione]

The rate of erosion wear is proportional to the velocity of the fluid, to the flow rate, and to both
size and shape of abrasive particles in the flowing medium. So the “quality” of the fluid plays a very big role here.
A cutting process with water jets could be made even with pure water, but jet impacts the surface to be cut orthogonally, that is in a way a bit different from what a fluid does when flowing in a pipe.

 

[davefitz]

This is normally addressed at the outlet of pumps. For carbon steel piping, the max velocity ( considering flow unbalances at elbows and expansions) is 20 fps, with a typical design max average of 15 fps. For 1% chrome alloy pipe that 20 fps may be raised to 30 fps. For incolnel and stainless steel ( in zero chlorine environment) maybe 70 fps is the max for liquids.Capillary tubes may use SS or incolnel at high velocities to frictionally choke the fluid in high pressure drop applications .

Also, in the case of pump discharges, one must provide at least 7 diameters straight run prior to an elbow or to an expanded transition to larger diameter carbon steel piping.

 

[ione]

Yobbo,

If you are interested in further info concerning erosion related issue, you can take a glance at the attached paper.

I also insist that for pure water (no fines or abrasive particles in) erosion velocity is meaningless. Cavitation phenomena, induced in areas where the liquid pressure drops below the vapor pressure, are by far more worrying.

 

[vpl]

From a practical matter, I have seen heat exchanger tubes that have been eroded through-wall due to high velocity water running through them. No expensive pump needed ... purer water than most of us drink ... took about 3 years. Water velocity, if I remember correctly, was in the 17 to 20 foot per second range (5 to 6 m/sec).

Granted, they were thin-wall "tubes" versus a pipe.

Patricia Lougheed

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[davefitz]

ione-

pure water ( demineralized holy water) will erode feedwater heater tubes quickly immediately downstream of the tube inlet ( tube + tubesheet design ) for the case of AVT all volatile feedwater treatment with hydrazine type oxygen inhibitors, if the average water velocity exceeds about 10 fps ( vena contracta velocity above about 15 fps)and you are using CS tubes.

2 reasons for this- first, the fluid streamlines at the entrance cause a pronounced vena contracta increase in peak velocity ( which is also reflected in a high recirculation eddy velocity in the boundary layer at the wall) , second, the type of oxide layer formed is magnetite, which is weakly attached and will spall off easily. A better solution is to use the combined oxygenated feedwater treatment system to 250 ppb O2, and this causes the oxide scale to be a tough hematite.

 

[ione]

Davefitz,

Star for you.

Anyway are these phenomena imputable to erosion for sure? What I mean is: with heater tubes (high temperature – lower vapour pressure) and with high fluid velocity, I think cavitation plays a role and so I am not sure we can speak of simple erosion.


By the way is really hydrazine still used as oxygen scavenger? In Italy it has been banned due to carcinogen issues.

 

[IRstuff]

18Mohm deionized water, which is ostensibly "pure" actually tends to get acidic over a short period of time, due to the absorption of CO2 from the air, resulting in carbonic acid.

TTFN

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