Need some opinions/counsel/suggestions/insults on the upper velocity limit for saturated (two-phase) steam.

System in question is 50 psig, 65% quality, dirty steam.  Rates are 8000-45000 lbm/hr.  I am sizing the header and laterals feeding the shellsides of three heat exchangers.

My pipe sizing criterion is to select NPS's that keep the flow regime in the annular, this done to minimize dynamic effects (keep the liquid phase in suspension to prevent the liquid from slugging in the horizontal-to-vertical transitions, i.e. upstream of risers-ballvalve).  This also helps to smooth out the behavior of the exchangers and their temperature controls.  

Unfortunately I can't trap out the liquid before the steam hits the risers and send it off someplace because I have nowhere to send the condensate (there is no condensate handling system) without a WHOLE lot more work, equipment, piping, and controls.  

I am not pressure-drop-constrained; I got pressure to burn (what a luxury!).  

The annular regime requirement means the required velocities are 180-220-ish ft/sec.  

Now, API RP14E makes some suggestions as to upper velocity limit in two-phase fluids.  Using this method with the conservative value of c, i.e. c=100, the upper velocity limit for my system is 240-ish ft/sec.  So using this method, I am safe.  But...first question is, how noisy is this going to be at 220 ft/sec?

What say ye about this?  The steam is "dirty" in that there is the occasional solid particle going by.  But it is clean enough to where I think I can justify use of the c=100 factor.

The other constraint you want to look at is erosional velocity. You should try to stay within 40-70% of erosional velocity.

As SeanB has stated, erosion will likely be a real issue. Water droplets moving at steam velocity will do things like chew out the back sides of 90* els pretty quickly. I've seen installations with thermodynamic steam traps (blast discharge) with a pipe nipple then a 90* 150 malleable elbow fail within a year. Looking into one end of the el, held up to the light was like looking through a lace curtain. There were pinholes everywhere on the back side. I don't think your temp control valves on the HXs - or the HXs themselves - will like this situation either.

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