Steam Velocity and Erosion 1

There's a correlation between pressure DROP and velocity. If you have a 125 PSIG supply, you can take a 10 or 15 PSI drop on a peak. If you only have a 10 PSIG supply and you're taking a 10 PSI drop, there will be problems Unless you have wet steam, excessive pressure drops and/or noise will be a problem first. For relatively short runs, I usually size process lines for 200 feet per minute on a peak. If you try to carry that velocity over 1,000 feet or more of pipe, the pressure drops get you. A 125 PSIG supply turns into (maybe) 75 PSIG at the other end.

TBP, I think you meant to say 200 ft per SECOND, didn't you? That would be in line with the rule of thumb to keep the velocity between 6,000 and 12,000 ft per minute.

Yes I did, thanks. 200 feet per SECOND would be correct.

To all,

I wrote an FAQ that partially answers this question.

The design velocity depends on the dsteam conditions(saturated or superheated) an a method is contained in Crane #410 and the "Piping Handbook" by NAYYAR

MJC

Gentlemen,

Thank you for your posts.

I am looking for more of an "educational" answer to my questions. I know absolutely nothing about Piping and Fluid Mechanics. Or the design requirements for steam piping or tubing. I have seen examples of steam erosion in my career as a welder, but I don't know very much about the root causes for the erosion.
Is there a pressure that, above which, erosion is likely to occur? Or is it strictly a velocity issue?
Are suspended particulates in the steam, such as exfoliation of the steam-side scale in a boiler tube, necessary for erosion to occur? Is a physical impact required for erosion to occur?
Is the temperature of the steam a factor?
I have seen, and repaired, examples of "particulate" erosion(is there a special term for this?.
By saying "particulates", I mean dirt, lime, wood chips, etc... .
I don't know if the examples of steam erosion I've seen had particulates in it or not.

"Rules of Thumb" would be great.

Thank You

Dale Simonds

Dale - I hate to say this but - without having the engineering coursework background, it's pretty hard to explain this stuff and to adequately explain what's going on here without taking up a WHOLE lot of space here or boring you to tears, or both. Now, having said that...

The rule of thumb we use around here for max velocity in saturated steam (less than 100% quality, or, what some people call 'wet steam') piping is - the velocity should be less than about 90 feet per second for CLEAN saturated steam. Velocities greater than this CAN, but not always, cause erosion at direction changes (ells and tees). If you have particulates, e.g. boiler scale, rust, red rags, beanie weenie cans, then the erosional velocity will be smaller. In our business (steam distribution for enhanced oil recovery) we use a target design velocity of about 60 ft/sec because that gives a reasonable pressure drop and reduced energy loss for a given pipe size and length, it gives decent flowsplitting (i.e. you don't get all liquid down one leg of the tee flowsplit and all vapor down the other leg). Some owners will want a max velocity lower than 90 ft/sec, some will allow greater, but 90 ft/sec is a good conservative starting point.

Erosion is strictly a velocity issue, and velocity is based on pipe diameter, flowrate, and pressure drop.

A pretty good reference for this is "Crane Technical Paper 410". It is relatively non-technical. Hope this helps! Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com