I assume the slope of piping on submarines changes as the vessel navigates.  Where steam piping is installed, how is the piping designed to prevent steam hammer when the steam and condensate flow can vary from co-current to counter-current two-phase flow?  Is there a specific velocity that is not exceeded for sucessful designs?  Is there a maximum condensate depth, below which slug flow (steam hammer) will not be initiated?

I am not sure the terminology is correct.

If the mixed flow is steady state, then you might generate slug-plug flow , but steam hammer and waterhammer are usually reserved for other phenomena.

For steam hammer , you initailly have a long pipe containing high pressure steam at a high velocity. At time T=0, a stop valve closes quickly ( such as a turbine stop valve in 0.2 sec), causing all steam in the pipe to lose momentum in 0.2 sec. Tha tchange in momentum leads to a sudden pressure spike and sever piping reactions.

For one form of water hammer , as similar phenomena occurs, but is usually caused by the closure action of a check valve. Another method of causing water hammer is to casue a steam bubble to form , then have the bubble collapse at the speed of sound , causing a severe pressure pulse.

I understand further development of Baker's work by Mukherjee & Brill is often used in the petroleum industry for design of two-phase flow wet steam injection piping.  My recollection their work is primarily with respect to horizontal flow without any inclination of the pipe.

I see such tools as useful theoretical guides.  I anticipate that piping designers on ships/subs do not go to the Baker chart each time a pipe is sized.  I further anticipate that there are simplified charts or table already developed for such conditions.

Your suggestions to use tubulators or multiple pipes is interesting.  I know of the use of turbulators at low velocities, but would be concerned about erosion/corrosion at steam velocites 100 fps +/-.  I can see multiple pipes could offer some advantages...but need to think about that some more.

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