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.

 

[BigInch]

Does reducing the velocity help, and if it did, would that be due to a lesser volume of water flowing past, or actually due to the lower velocity?

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

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

 

[IRstuff]

For a strictly kinetic erosion, net mass flow would seem to be the key. Ion milling and sputtering are gas phase versions of that.

For cavitation, it's mostly velocity, since that's what causes the cavitation bubbles whose collapse and subsequent shockwave supposedly results in erosion.

TTFN

FAQ731-376

 

[ione]

It is not unusual to take cavitation induced damages for erosion related issues. In effect the results can be the same, but I am/was more prone to speak about erosion, if abrasive media are dispersed into the fluid.

 

[BigInch]

OK. So you were speaking about the effects of two mechanisms together. Erosion by water contact-impact, without help from sand, and material loss from cavitation.

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

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

 

[ione]

BigInch,

Yeah. I have some difficulties (my fault) to accept the concept of erosion when dealing with a fluid which doesn't have abrasive particles or fines in.

 

[BigInch]

We must remember that molecules, atoms, electrons, photons, quarks, etc. can be "particles", at least when some of them are not acting like waves.

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

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

 

[rconner]

While perhaps most popularly perceived or assumed, “erosional” flow velocity is actually not the only “physical” consideration [you may wish to read the information/links etc. in e.g. many past threads accessible with “Advanced Search” such as

http://eng-tips.com/viewthread.cfm?qid=252110

,

http://www.eng-tips.com/viewthread.cfm?qid=167516&page=9,

http://eng-tips.com/viewthread.cfm?qid=241013

,

http://eng-tips.com/viewthread.cfm?qid=212461,

http://eng-tips.com/viewthread.cfm?qid=256160

and probably some others.]

 

[KernOily]

Just a word of caution on the use of API RP 14E guidelines. They are just that, guidelines. A lot of folks take them as the gospel truth and that is a capital mistake. Use them as a starting point and go from there. Actual field experience is the best gauge of the upper limit erosional design velocity.

Like BigInch, I too have seen pipe cut through very quickly. It doesn't take much. We used to use 2" sch 80 CS elbows at the first 90 downstream of the wellhead on gas well connections. Those lasted about two months. This was gas service at 60 ft/sec, 200 deg F, and "some" sand. We switched the elbow to a tee and voila - no more cut-out problems.

 

[cvg]

is the tee thicker or stronger (or harder) than the elbow?

 

[KernOily]

No - Same material. The tee redirects the velocity of the stream away from the wall and into the open space end of the tee. The part of the stream that was moving along the elbow ID is now busted up due to the empty space presented by the open tee.

The downside of this scheme is the tee creates a very short dead leg on the other end of the tee because the other end is plugged or capped. If you can't tolerate any dead legs, e.g. corrosion cells forming inside the tee, then this idea doesn't work.

 

[rconner]

I have seen that same principle (removeable plate or "wear plate"), with dead pocket etc. has been used with lesser bend angles as well, e.g. using a wye or lateral for 45 pipeline bends etc., such as shown at

http://www.iac-intl.com/pdf/drawings/elbows - Pro Hard and Pro Wear/MD118-07_PDF.pdf.

 

[mauricestoker]

Erosional effects of air in water are addressed by ASHRAE HOF. Erosion effects of air would depend on whether you have air separators. I'm not sure how it would be different between pump or piping, and I've definitely seen scoured elbows.

 

[itdepends]

I've seen water "erode" domestic copper water pipe. It appeared that the water velocity was sufficient in areas of high turbulence (bends/restrictions) to strip the protective oxide coating off. The piping corroded away in these areas- but the root cause was the high water velocity.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[DSB123]

itdepends,
You say "The piping corroded away in these areas- but the root cause was the high water velocity." but I think you meant to say "the piping eroded away..." since water does not corrode copper pipe!!