15 ft/s speed limitation in liquids
15 ft/s speed limitation in liquids
(OP)
Hi all,
I have been trying to find some explanation for the commonly used figure of 15 ft/s speed for liquid streams but I can not find it. In my experience, I have found a lot of cases when we use a figure that anybody knows from where is coming and for me this looks like one of these.
Could somebody shed some light over this?
Thanks,
I have been trying to find some explanation for the commonly used figure of 15 ft/s speed for liquid streams but I can not find it. In my experience, I have found a lot of cases when we use a figure that anybody knows from where is coming and for me this looks like one of these.
Could somebody shed some light over this?
Thanks,





RE: 15 ft/s speed limitation in liquids
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
http://www.eng-tips.com/viewthread.cfm?qid=348061
http://www.amazon.com/Fluids-Through-Valves-Fittin...
RE: 15 ft/s speed limitation in liquids
I am happy to hear that answer. According to it, money is the key factor but there are not technical issues with higher velocities.
Let's talk about the case I am studying. I have an industrial fluid in the top of a tower 200 meters height. Therefore, the fluid has a huge static pressure head that I want to use to move this water to a certain point. After performing some calculations I have more energy that I need so I would like to reduce the pipe size as much as possible in order to save money.
According to your statement, I could increase the speed above 15 ft/s without any technical risk.
Am I right?
RE: 15 ft/s speed limitation in liquids
For corrosive services where erosion-corrosion is a concern, or for services containing solids which may be abrasive, you do need to concern yourself with erosion. But your problems then will be in control valve trims etc. first and in the bends/elbows of the pipe second...
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
Dejan IVANOVIC
Process Engineer, MSChE
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
The fact that this section refers specifically to lines transporting liquid from one pressure vessel to another by pressure differential and is designed to prevent vapour breakout has unfortunately become lost and in experienced engineers looking for a "code" or written limit have leapt on this figure as a do not exceed velocity. This is total rubbish and while I fully agree higher velocities above 3 to 4 m/sec present difficulties in terms of excess press drop, increased surge etc, there is no limit.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: 15 ft/s speed limitation in liquids
As the others have noted, the piping would have to be designed for the fluid, the entrained solids, and the expected fluid velocity. It may be less expected to use a lower velocity and less rigorous piping design.
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
If there are high point pockets in this downcomer, these will need to be vented out.
Is there a vortex breaker in the exit pipe at the tower top ?
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: 15 ft/s speed limitation in liquids
RE: 15 ft/s speed limitation in liquids
https://law.resource.org/pub/us/cfr/ibr/002/api.20...
There are some materials published by NFPA and others:
http://www.nfpa.org/Assets/files/AboutTheCodes/77/...
http://people.clarkson.edu/~wwilcox/Design/flamliq...
http://www.wolfsonelectrostatics.com/01_hazards/pd...
I have rarely seen any liquid lines designed above 3-4 m/sec, for many reasons.
Dejan IVANOVIC
Process Engineer, MSChE
RE: 15 ft/s speed limitation in liquids
a) Liquids with conductivities >100 pS/m, no flow velocity restrictions
b) Liquids with conductivities <100 pS/m and no immiscible components, flow velocity should be less than 7 meters per second
c) Liquids with conductivities <100 pS/m and containing immiscible components, flow velocity should be less than 1 meter per second
Locate filters and valves as far as possible from the entrance to the receiving vessel. The filters and valves must be placed upstream of the discharge point to provide at least 30 seconds of “relaxation time” prior to the liquid discharge. The relaxation time1 would depend on the liquid conductivity, flow velocity, and type of filter or valve; hence, additional relaxation may be warranted."
Dejan IVANOVIC
Process Engineer, MSChE
RE: 15 ft/s speed limitation in liquids
There is also the criteria for erosion/corrosion of the piping. Carbon steel pipe with most water flows will erode at local velocities of 18 fps, and the average velocity of 10 fps does not adequately illustrate that the local velocity at a pipe entrance ( vena contracta) will be 40% higher than the average velocity due to turbulence, and a similar effect occurs at bends and elbows. Higher grade piping can allow higher velocities before erosion becomes an issue.
Some metals erode at very low velocities ( copper and copper alloys), while others require a high velocity to prevent MIC microbiologically induced corrosion; a copper alloy condenser may not have velocities above 7 fpps, yet a stainless tube condenser should not have velocities less than 9 fps.
"Whom the gods would destroy, they first make mad "