Fluid velocity

[sheenders]

Typical target I have used for fluid velocity in piping design is 7-10 ft/sec. What are the implications of higer and lower values? I realize the obvious, like friction losses and the related pump sizing, but am not sure about performance.

If a lower value is used, say 1-2 ft/sec to gain lower pressure drop what are the implications in performance? Any references to literature would be helpful.

Thanks

 

[Chazadd]

sheenders,

Could you be a bit more specific about what you mean by performance?

And also the service required.

If you want to size a pipe then take a look at API 14E, page 21 (i think).
it helps you size a pipe taking into consideration erosional velocity. For a set of inlet conditions you determine an erosion velocity and you then change your pipe diameter to ensure that the actual velocity is less than the erosion velocity. This is just an estimate though, and is highly dependent on the choice of a particular constant. And if the line contains sand then choice of flowing velocity is complicated more.

You also have to size the pipe wrt pressure drop, unless you want to reduce pressure i would try to minimise pressure drop as much as possible. For example: if you have a high pressure drop in a line feeding a pump or compressor, you have just wasted energy because you now have to do more work to pressurise up to a set value.
Evaluate the expense of using larger bore pipe against the increase in energy costs of pumping/compression.

Could you be a bit more specific on what you mean by performance?

Hope this gets you thinking.

Chaz

 

[pmureiko]

This could hurt the performance of a heat exchanger if one is in your process. Reducing the velocity will lower the heat transfer coefficient and exchanger duty.

Lowering the velocity decreases your operating costs to overcome friction and increases your installation cost through bigger pipes. There are economic optimization formulas to determine what size pipe to use, but usually the shortcut guidance is an acceptable velocity range such as my company's of 5 to 7 ft/s .

 

[sheenders]

Thanks for the reply.

By performance I meant some of the things you mentioned, pressure drop, size and cost of piping installation, pump hp., and your comment about erosion of the piping, valves and fittings. In addition, for velocities above the norm I was thinking about possible turbulent flow creating flow problems through valves and fittings, but I supppose that will manifest itself through the pressure drop calculations. For velocities lower than the norm, I suppose for suspensions you run the risk of solids dropping out but I really couldn't think of another negative to lower velocities.

Anyway, my fluid is water so some of this doesn't even apply. Do you agree with the rule of thumb of 7-10 ft/sec fluid velocity?

Thanks again for your answer it was very helpful.

 

[lilliput1]

In addition you should consider noise & future expansion. It may be wise to size the piping to accept about 15% more gpm in the future.

For noise concern keep velocity in the 4 to 6 FPS range. In Mech Rms or buried pipes use up to 8.6 FPS for acceptable noise & for up to 5000 operating hours per year for pipe erosion concern.

For piping operating 2000 hours/ year less operate at up to 12 FPS.