difference between pressure surge and slug flow

[jim0210]

I am confused with some authors explanation about pressure surge and slug flow in a pipe line. Can anyone explain the difference between the two..

 

[MortenA]

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.

Best Regards

Morten

 

[25362]

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, "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".

 

[jim0210]

Thanks MortenA, thanks 25362...

 

[MortenA]

25362:

I have newer before heard a slug mentioned as "a travelling pressure wave"? I does not make sence to me. A think my definition holds: Its a "section" of liquid moving in a gas filled pipeline.

I have not heard that a slug could cause a "water hammer" or surge in it self but will not rule it out. I do however think that since a slug is travelling in a gas pipe - should it happen to hit another section of liquid i would assume that the following gas section would act as a "spring" and dampen the effect of this collision. Thgere are however problems when a slug hits a valve, shap bends etc. but this i would not call a surge or water hammer.

Best Regards

Morten

 

[neelgopc]

the difference is in the fact that one is pressure or energy change transmitted in the wave and the other is actual liquid "blob" flowing with the gas.

Surge is a change in flow pattern of gas when "slug" in a piepline changes its shape or volume in the pipe / plant(as it arrives at some valves, fittings etc; this causes a change in the flow area of the gas and results in a velocity change of the gas flow; this is expressed as "surging" in the gas plant.

 

[25362]

to MortenA, I entirely agree with you, please, read again my closing paragraph.

 

[chinwar]

"water hammer is manifest as a series of SHOCKS, sounding like hammer blows"
this the description from piping handbook by crocker king who just indicate the following reason:
"instantaneous or too rapid closing of valve in the line or by an equivalent stoppage of flow"
*******
from my understanding, shock or vibration due to the sudden direction change or flow section area change and so on r not "water hammer" defined herein.
so, it is not very hard to say which is the so-call water hammer - try to find if there is a pressure wave occured. (but remember the speed of the said pressure wave shall be the sound speed in the same piping system.)
frankly, i never considered the water hammer when i was a refinery piping design engineer for almost 8 years(maybe i was not good ). but currently i am a water treatment plant piping design engineer and my consultant asked us to consider the trust block and water hammer. it casued me lots of time to go through lots of textbooks and references, and i also needed to estimated the very big buried steel pipes by using c2.
******
in fact, water hammer is just limited for description for the result of pressure wave which caused by sudden stoppage of flow. it is a phenomenon of circulation of energe trnsformation. but lots of people also call other similar shocks as "water hammer". i think it is not strict.
the method to estimate the thrust force due to water hammer also is different from other shocks.
so, try to think if u see any big "jump" from any live pipeline during operation? while, you can find the water pipes will jump when u close the tap suddenly. it is funny that i always try to avoid the jump in my house but i always find it is very hard no matter how slow i close the tap. so i am always worring the supports for the pipes in the home amd maybe they will drop down.