Velocity Limitation in Pipe

[mech471]

What is the velocity limitation in Pipe?
If a certain Pipe say 8" A-53 SCH 20 is on Fuel Oil Service and flow rate is 210 m3/hr. Operating pressure is 6.6 Kg/cm2. Flow rate is to be increased to double or 1.5 times, how much this pipe can withstand the velocity. Obviously the pressure loss shall be increased but there is requirement of 1.4 Kg/cm2 at end point and the actual concern is the increased flow rate.
Are there any references (book article, code/standard or formula etc) in this regard?

Note
Specific Gravity of Fuel Oil 0.915
Viscoisity is 43 CP at 85 C
total pipe length is 500 meters

Waiting for your valuable comments
omer

 

[TD2K]

The concern with high velocity liquids is erosion/corrosion. API 14E recommends a maximum velocity of 100/(rho)^0.5 where rho is the bulk density in lb/ft3 (I think this equation applies to 2 phase mixtures but I've seen it frequently used for liquids also). For clean fluids, this equation gives a conservative (low) result. I've seen NACE articles on maximum velocities of clean water systems with velocities several times the above limit with no erosion problems. In Saudi Arabia, we were monitoring a pipeline downstream of a pressure reducing valve with close to 8 metre/sec velocities, again no erosion was seen but this was a clean oil service.

For your current 210 m3/hr flow rate, this is about 1.8 metre/sec or 5.9 feet/sec. 1.5 times your current flow wouldn't give me a problem. If you want to go to 2x, I'd recommend some monitoring of some elbows as a check, any erosion is going to occur where there are changes in direction. Fuel oil isn't always the cleanest material and can have catalyst particles or other solids in it that could give you an erosion problem.

Now, if you were producing oil/gas from a well, I'd be careful in pushing this limit because of potential sand production without some monitoring in place.

 

[mgithens]

According to API RP14E Section 2.2 & 2.3, they say that you should maintain a velocity between 3 and 15 feet/sec. The lower limit is to keep deposit of solids and the upper limit is to control flashing. Twice your speed would be 11.8 ft/sec, or 3.6 m/sec which is within their recommended tolerances. Trying to stay under 400 m^3/hr would be wise, pressure drop will be slightly greater and do watch out for erosion in control valves, chokes, elbows, etc.

 

[MJCronin]

omer,

TD2K's comments, as usual, are correct..... Maximum/minimum velocities depend on liquid service, piping materials selected as well as econnomics ....

Higher velocities mean increased pump rate and higher pumping costs...This is frequently a limiting factor in the system design.

The following discussion is for liquids only:

For suction lines to pumps, 3-5 ft/minute is typical for design. Pump discharge lines are 6-12 ft/second, this gives (mostly) econonomic pipe sizes and reasonable pumping costs

Many exceptions exist....

Pumping organic acids should be done only to 3-6 ft/second (erosion/corrosion accelerates above this limit)

Steam, air and especially oxyegen have thier own velocity limitations and rules of thumb......

A special economic analysis should be done for HP steam lines supplying a steam turbine. Because of the high material costs, (chrome moly steel) and because of the "special economic value" associated with steam pressure delivered at the inlet to the turbine, this situation deserves unique consideration.

Hope that this helps!!??

MJC

 

[saidsaz]

known limits for clean liquids are 3 m/s,,,,but practically and away from exact numbers ...

go for the higher flow..keep track of couple of elbows thickness frequently ...and if u start loosing thickness (how fast is how much u r lucky )then consider other alternatives...by which time u have justified other solutions...

cheers