Velocity Limits in Metal Pipe 1

[AljosaM]

Howdy all,

Is there an API or any other US standard that specifies acceptable velocity limits in metal pipes? I know there are a lot of general rules of thumb that people use, and I'm familiar with them, but I'm trying to make a case for something and would like to bring up reference documentation.

If there are no API/ANSI/Hydraulic Institute standards, what are some good papers which would provide good velocity limits?

Thanks,
Al Mitrovic

 

[BigInch]

Velocity is limited indirectly by,
The costs of dealing directly with high pressures generated by water hammer effects (greater wall thicknesses, relief valves, containment tanks, etc.)
The costs associated associated with the high pressure pumping capacity and the resulting thick-walled pipe that you will need to move your fluid at the high pressures that are usually needed to generate those high velocities.

Velocity, to a certain extent, can also be limited by abrasion effects of particles carried along by the fluid, or by fluids with high molecular weight. Too high a velocity of these particles, or gases and they can erode the pipe walls. Also recall that high molecular weight gases typically have high pressure drops anyway, so high velocities are again discouraged. In pipelines and petroleum piping this is more important for production piping and field gathering systems where a lot of the sand and other impurities may still remain in the crude oils and natural gases released at the well heads. API RP17 has a velocity limitation recommendations in that regard. Occasionally velocity limits may be needed for specific types of gases, such as for oxygen, or hydrogen, where high velocities may increase chances of autoignition (O2 and H2), but those are generally rare, highly specific cases.

In most cases of clean fluids the rules of thumb work quite well in combination with the natural economic and water hammer limitations I mentioned above. Too high a velocity will not be economically justfiable in all but the shortest lengths of pipe. Unless you are transporting dirty, heavy fluids, slurries, or otherwise unusual fluids, the rules of thumb will make for a very reasonable guideline.

 

[AljosaM]

Thanks for your reply. Just to give you more details, the fluid in question is municipal water at ambient temperature, and the piping is located on a skid. Would API RP-17 apply in this case?

As I mentioned, I'm fine with using the rules of thumb, but I'm on the high end and want to back up my design with established design standards.

Also, since we're on the topic of effects of high velocities, I'd like to add to the list:
-Fatigue failure from flow-induced vibration
-Noise

Thanks again.

Aljosa Mitrovic
Mechanical Engineer

 

[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 (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

 

[rconner]

For a detailed discussion of this issue see also the third "FAQ" at

https://www.dipra.org/ductile-iron-pipe-resources/frequently-asked-questions/hydraulics.

 

[BigInch]

API RP17 was specifically developed for offshore oil & gas production platform design.

The values that bimr has shown work very well for water. You might want to use the lower ones, depending on what is going on the skid. Meters? Filters? Piping near specialized equipment may need additional considerations.

 

[AljosaM]

Thank you for your replies, but I think you misunderstand my questions. I am aware of the rules of thumb and have considered them in my primary design.

I am asking for references which specify industry standards, if there are any. From what I gather, they don't exist.

Bimr, I'll check out Crane's Technical Paper 410. Thanks you.

Regards.

Aljosa Mitrovic
Mechanical Engineer

 

[BigInch]

There are no hard limits on velocity. As I said before, water hammer problems can easily occur on most systems running over 10-15ft/s and, if the pipe is long, pressure drop will nail you at 5-7 fps, otherwise you'll be building too many pump stations, so you can only consider using high velocities 20-30fps for very short lengths of pipe anyway. If you try to use velocities below 3 fps then pipe is too big a diameter and it costs too much, so all of that pushes you back into 3-10 fps range.

With naturally inherent design limits like that, we don't need (or want) any API telling us what to do. Reference the Crane Manual if you need to, but in any case, if you do your design right, you'll never be able to use anything different than what Crane says, unless the costs of steel, pumps, pressure control valves and power suddenly become 0. Occasionally, factors such as cheap steel, or cheap energy, or both might favor one end or the other of the 3-10 fps range, but it will seldom, if ever, push you out of it. You might consider higher velocities for very short lengths of pipes, usually inside a pump station, but that's about it.

 

[AljosaM]

With naturally inherent design limits like that, we don't need (or want) any API telling us what to do.

Amen to that.

Aljosa Mitrovic
Mechanical Engineer

 

[BigInch]

I always think those are for people that don't know what to do, although they can be helpful remiders to make sure you didn't forget something.
We're all (still) human ... right?
[ ] tick the box if you're not a bot.

 

[LittleInch]

I agree with all above - your only issue on which to hang anything is erosion for short, skid piping.

See posts like this

http://www.eng-tips.com/viewthread.cfm?qid=363403

and

http://www.eng-tips.com/viewthread.cfm?qid=191447

and

http://www.eng-tips.com/viewthread.cfm?qid=152583

I wouldn't like to see much more than about 10m/sec, absolute max of 15m/sec in any one location and avoid going from a branch into a tee or a LR elbow at that velocity, even with clean single phase liquid

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

 

[EdStainless]

In metals that are softer and erosion sensitive (Cu and brass) even clean pure water will cause erosion when you get over 6ft/sec.
These materials have a very soft surface film and some metal is always being lost into the system.
I have seen tests done on high alloy stainless with jet impingement, no measurable erosion at 134ft/sec.
The highest velocity that I have ever seen in service was in a heat exchanger, they were running 85dt/sec in 3/4" tubes. The noise was incredible, but so was the heat transfer.

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P.E. Metallurgy, Plymouth Tube