Surge is also called waterhammer and is a a liquidfilled pipeline.

Surge can occur when flow is change abruptly e.g. if a pump is tripped or a valve is closed. Ist a fairly well know science. Surge prediction can be accomplished by relatively simple formulas. Surge prevention is more complicated.


Slug (flow) is in a "slug" og liquid moving in a gas / two phase pipepline.

A slug can be formed in many circumstances. It could be problematic because it travels at relatively high speed (the same as the gas) and when it arrives at e.g. a separator it can "fill up" the vessel. It will also cause trancients in pressure before and after arrival.

Slug prediction / two phase flow is still a fairly new science. There are a lot of different theories and formulas that each may have strong and weak sides.

Jim0210, I wish that the subject would be clear-cut, but unfortunately it is not. The confusion, I think, results from a mix up of causes and effects.
 
Both, a travelling pressure wave as well as a travelling slug of liquid can produce pressure surges resulting in water hammer.
This pipe pounding is actually initiated by the rapid stoppage of an incompressible flowing liquid and it can also occur in two-phase flows.

Conditions causing "water hammer" can be "hydraulic shocks" as mentioned by MortenA as from a sudden closing of a valve(Resilient valve), "thermal shocks" and
"differential shocks". Thermal shocks generally are the result of a sudden condensation and collapse of steam bubbles trapped by pools of condensate below saturation. The ensuing drop in pressure accelerates the liquid condensate which impacts containing surfaces, sometimes chipping away protective oxide layers resulting in accelerated corrosion. I remember having seen this subject treated in another thread.

"Differential shocks" are the case in question here. When steam and condensate travel in parallel, steam usually has a much higher velocity. Liquid waves start to form, sometimes as an effect of thermal shock, and rise until they form a seal, in fact a slug aka piston with pressure behind. This slug picks up more condensate increasing its mass on its way, at progressively increasing velocities. On changing directions, such as when encountering elbows, tees, or other fittings, the momentum is discharged and great damage can result. It can also happen when, in a vertical line discharging condensate from elevated heat exchangers, a sudden steam condensation occurs downstream because of the lack of insulation. This sudden pressure reduction plus gravity can result in an accelerated vertical slug of condensate carrying sufficient force to damage traps and other pipe appurtenances.  

"Slug flow" biphase liquid-liquid or gas-liquid flow regimes not always cause water hammer. However, when two-phase slug flows are indeed the cause of a water hammer pressure surging, it wrongly appears that both expressions could equally apply.

When dealing with flows in pipes, it is preferable -as MortenA says- to reserve the surge wording for travelling pressure waves, and keep the slug flow expression for travelling slugs of liquid in biphase flow regimes, albeit their pounding effects are both usually included under the category of "water hammer".

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