Maximum Velocity in Piping Systems 3

[Clamont]

Dear All,

I would like to find out where I can find the maximum allowable velocity's in piping sytems in an ASME or API standard. It has always been stated that a maximum allowable velocity for product piping is 1 m/s and water piping 3 m/s however I would like to know where this is stated.

Thanks

 

[LittleInch]

Clamont,

I think you'll be looking a long time... The only max velocity calulation is erosional velocity which I think is API RP 14E?. Hover this will give you enormous velocities for clean fluid and is realy designed for gas and fluids with grit or sand in them.

"It has always been stated.." - Err by whom?, where? You've clearly got this into your head from somewhere.

There are guidelines in some companies which then sometimes get confused with rules and "cannot exceed", but they are just guidelines. 2.5 to 3m/sec for any liquid, product or water, is a reasonable figure to start with for a long ish pipe or pipeline as it minimises pipe diameter whilst not having pressure drop too high. However shorter lines I've seen up to 7-8 m/sec for liquids, but then you run into big issues about static electricity and very high surge pressures (water hammer).

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

You cannot reach an errosional velocity in gas without doing some really specific things (I get the same fluid momentum in gas around Mach 1.5 as I get with water using the API equation). Mach 1.5 is somewhere around 500 m/s.

The upper limit in gas is purely economic--higher velocities result in a higher friction loss that has to be made up in compression. The question is "how much loss is it economic to replace?" The number that most people come up with is somewhere around 100 ft/s (30 m/s). Going higher doesn't break anything or approach an inflection where small increases in velocity will result in huge increases pressure drop. It is a very arbitrary (and often used) number.

For liquids, the API limit (around 12.4 ft/s or 3.8 m/s for pure water, higher for hydrocarbon products) minimizes scouring off the passivation layer, Beyond that you are concerned about replacing lost energy with pumps. The 3 m/s (10 ft/s) number is often thrown around, but I think it's genesis is pretty arbitrary.

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.
The plural of anecdote is not "data"

 

[davefitz]

Clamont;

Each process or system has different velocity limits. Typical paramters to be evaluated include:
a) economics- power cost of pumping vs capital cost of piping, valves, supports
b)erosion and erosion /corrosion- affected by alloy selection and local geometry as well
c)noise and vibration==> mach number limits
d) rule of thumb gurus==>office politics

"Nobody expects the Spanish Inquisition! "

 

[MJCronin]

Clamont:

davefitz has it correct...

Velocity limits of piping systems can vary considerably and may be a function of several aspects of the system design.

For example: concentrated sulfuric acid piping systems should be no more than 2-3 ft/sec in carbon steel piping.

Pump suction piping design velocities will depend on the design pressure of the system, the configuration and the materials selected.

Reasonable boiler feedwater piping velocities may be quite high (12 - 20 ft/second)

Heat exchanger tubing has a different set of rules and much higher or lower velocities may be correct...!

 

[rconner]

This is an FAQ, for which you should find many past discussions with the advanced search feature and a few keywords in the archives etc. While it may be tempting to perceive that flow erosion is the limiting factor of high velocity flow, that may not necessarily be the most significant consequence pipelines face. It has also long been reported that high flow velocities also increase the likelihood of "transient" events (like water hammer, water column separation and vacuum/collapse etc.) Couple high flow velocities and significant transient events with particularly some relatively weak pipe materials, and there is a real potential for problems.

 

[DRWeig]

Hi Clamont,

I can speak for water in building hydronic systems. As davefitz pointed out, noise and cost are big factors. Inside buildings, it is the guiding factor in most cases.

4 ft/s (1.2 m/s) = quiet enough for a pipe passing above an office or conference room

5-6 ft/s (1.5 to 1.9 m/s) = quiet enough for a pipe passing above a corridor ceiling, if separated from offices

7-9 ft/s (2.2 - 2.8 m/s) = keep it in mechanical rooms.

Above about 10 ft/s (3 m/s), the pumping energy cost outweighs the savings in pipe size (in general, in buildings) by enough to make one reconsider. The pressure drops can affect control valve authority too.

I know nothing about pipe erosion or other stuff outside of buildings, so others here will be

Best to you,

Goober Dave

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[zdas04]

DRWeig,
That is useful information. I'm assuming that those numbers are for a single-phase liquid like potable water, not a potentially gassy liquid like condensate (in the steam plant sense, not the Oil & Gas sense)?

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.
The plural of anecdote is not "data"

 

[DRWeig]

Yes, David -- water only.

Best to you,

Goober Dave

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[EnergyMix]

I will chime in on water in terms of erosion and corrosion, espeically on elbows and thin walled tubing (heat exchangers). Generally, my experience from nearly 30 years in the nuclear industry has been that you want to keep water between 3 - 7 feet per second if you have a lot of elbows or very thin tubing, unless you would like to replace it frequently (or can do with plugging tubes). Nuclear plants often look for 25+ years piping replacement times, but with increasing velocities, I've seen where replacements have occurred in as little as 5 years. Worst case was less than a year (and, no I can't give specifics, sorry.) That gets expensive quickly.

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[danielsonx]

@EnergyMix

I wonder what importance does the water quality have. If we're talking about de-mineralised water (boiler feed water) in power generation application I would imagine that permissible velocity should be higher than for "raw' water.

I'm currently looking at a possibility of increasing flow-rate in ABS piping that convey demin water from 1.5 m/s to 2.5 m/s. Unfortunately standards (including BS 5391-1:2006 "crylonitrile-butadiene-styrene (ABS) pressure pipe. Specification") do not seem to have any guidance on the subject.

Would 2.5 m/s be an acceptable velocity in a system that deals with demin water where noise isn't much of an issue?
As the system have plenty of 90 degree elbows I'm a bit concerned about the risk of erosion as indicated by @EnergyMix.

 

[rconner]

As long as we are listing potential problems on this thread, I guess I could go ahead as well and mention that high flow rates have also been reported a factor in vibration and resonance problems in at least some rather flimsy (or relatively "thin", as another has mentioned?) piping/support systems. See e.g. that the site at

http://www.engdyn.com/images/upload...ting_of_existing_piping_systems_-_drs&jcw.pdf

explains some piping can be "...accoustically excited by high flow rates." I suspect localities near supports (to weak shells) may be particularly vulnerable to such.

 

[snoopnoon]

Our target velocity for SH Steam is around 300 ft/s

for extractions near the wilson line we drop down to 150 ft/s

 

[BigInch]

300 inches/sec is more than enough for me.

Independent events are seldomly independent.

 

[Drexl]

EnergyMix, might this experience come from BWR? If so it's quite a BWR-specific FAC problem that occurs only with neutral or low pH. In convetional power plants the pH of feed water/steam is much higher and there is basically no FAC.

 

[EnergyMix]

Drexl, No it's from both BWRs and PWRs, various systems. One plant eventually opted for stainless steel piping because they were replacing the carbon steel so frequently. All I can say is that water doesn't like to change direction rapidly.

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