Exceeding Pipe Velocity Limits

[snoopnoon]

Background:
I am a steam turbine engineer working on a project to add an admission line to an existing steam turbine.
Due to axial space limitations and stresses on the existing casing, i am only able to retrofit qnty 2, 6" pipes. These two pipes will connect to a manifold and 10" diameter supply line.

The supply line is 675 PSIG and under certain conditions the downstream pressure(turbine stage pressure) is 400 psiG. i am calculating the velocity to be in excess of 375 ft/sec in the 6" pipe. The 10" pipe the velocity is under our normal design limit of 300 ft/sec. My argument is that since the 6" pipe is very short in comparison with the diameter 25" long, and there is magnitude times volume in the casing for the steam to slow down that there will not be any issue.

Is there something i am not thinking about with such a high velocity in the pipe? I am getting resistance from other groups but they can not give a definitive answer to not accept it other than "this is over our design limit"

Any and all suggestions/comments is appreciated

 

[zdas04]

The real question is "what is the basis for our design limit?". So many of these "limits" started life as an absolutely arbitrary number that morphed into a regulation. If you can dig into where the limit came from you can determine if it is protecting the pipe/system against a real problem, is just trying to minimize pressure drop, or it was pulled from someone's butt. Depending on what you find you can determine the best path forward.

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.

 

[BigInch]

Assuming that "under certain conditions" means transient flow, pressure waves do not slow down. They proceed at sonic velocity to fill the entire space ahead. When they reach the end of the space the flow is filling, pressure builds against the end wall, until high enough to stop flow momentarily and then reverse the inflow back towards the source. When the reverse wave reaches the source, the inflow pressure builds higher and when high enough, turns the reversed flow forward once again. That will happen for 4 to 5 cycles over a few milliseconds if the length of transit is short before the inlet to this new piece of pipe equals 675 psig. Much more like a momentary contained explosion until steady state flow finally begins to occur.

Where there is a restriction in the line, a certain percentage of the pressure wave, equal to the ratio of inlet to outlet of the reduced areas, is reflected, while the remainder passes through to the end of the container and multiple waves over multiple paths are set up.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Drexl]

As I can see there are the following problems with high velocity:
1. Pressure drop
2. Vibrations
3. Noise
4. Erosion

Pressure drop can be calculated and will thus not be a problem, noise is perhaps not such a problem as I assume it will be very loud there anyways. Erosion is not a problem as you can't have any droplets/solids before the turbine. Vibrations are a risk however.

 

[snoopnoon]

"design conditions" are a steady state condition. For instance, at low main inlet valve flows, the pressure in the stage is lower than if it was at full load. But it is very complicated because as the pressure increases in the stage, the delta P is reduced and thus limits the flow and hence the velocity.

So the issue is really only under a small percentage of operating range, but still needs to be considered none the less.


The pressure drop is accounted for and does not pose an issue. It is more of other factors i was concerned with, or rather not personally concerned with.