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# Understanding Hydraulic Calculations Using Hysys Different Tools

fluid pipe hydraulics

4 replies to this topic
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### #1 IonCube

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Posted 30 March 2018 - 12:06 PM

First the above posts cleared many of my questions regarding fluid modelling in HYSYS, so thanks

My questions relates to different tools provided in HYSYS for doing fluid flow calculations. The above recent article summarizes these tools & the pros n cons as well for each method. I need some clarifications:

1] Referring to Pipe Segment author says that Slip is not accounted for What does that mean? This is also referred here at page 5/5 which states The pipe segment in dynamics does NOT model phase slip, so it is not suited to predict slug volumes through dynamics modeling.

2] Again for Pipe Segment author says acceleration term in the pressure drop equation ...does that mean pipe segment shouldn't be used when there is say a JT Valve (in refrigeration cycles ...too muh pressure drop means excessive density decrements & excessive velocities!)

3] Again for Pipe Segment author says Ignores the kinetic energy term of the energy balance equation for all phases ...??

4] For Compressible Gas Pipe quoting Do not use the interpolation mode ...??

So I cannot understand these x4 statements. Please clarify

### #2 serra

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Posted 30 March 2018 - 02:56 PM

I am not an expert with Hysys but to model pipelines (two phases / multiphase) you would need a procedure which solves phase equilibria plus energy (if required taking in account the heat transfer or with adiabatic model) , normally such procedures divide the pipe unit in several segments which are then solved in sequence, there are several correlations which can account (or not) for vapor/liquid slip,

these procedures are available in many applications (I use that available in Prode Properties) so I am quite sure there is an equivalent in Hysys, you need only to select a suitable unit...

### #3 Technical Bard

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Posted 30 March 2018 - 11:42 PM   Best Answer

The hydraulic tools in thermodynamic simulators are generally there to give you a general idea of an answer, not a final answer:

1. In two (and three) phase flow, the phases do not move at the same velocity through the pipe.  The difference in velocity is the "slip", as the vapour phase slips past the liquid.  It is critical in two-phase systems to model this, especially if you are trying to do transient modeling to predict slug formation.
2. Ignoring the acceleration and kinetic energy term (velocity head) means it won't predict dynamic pressures accurately if the fluid slows or accelerates through a change in line size.  It also don't handle the kinetic energy in two phase systems (which is what drives the "slip" in item 1)
3. The interpolation mode for the gas pipe is guessing at intermediate conditions and sometimes isn't very good.  See the later comments in that cheresources forum for more on the subject.

Although Aspentech says otherwise, I wouldn't use the simulator to do hydraulic calculations.

### #4 IonCube

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Posted 31 March 2018 - 08:16 AM

Thanks Bard & serra ...Your comments regarding pipe segment & two-phase ...In HYSYS in the unit operation namely Pipe Segment as well as PIPESYS extension there exists dedicated calculation methods for two phase & if they exists; slip considerations should be included as well? Are you saying that even when using those dedicated 2/3-phase correlations Pipe Segment will neglect acceleration losses?

Secondly regarding Compressible Pipe Segment ....if I include may many increments** in normal Pipe Segment UO (unit operation) can it be modelled as something equal to Compressible Pipe Segment UO?

**By many increments I mean count of increments such that there is always less than 5% density difference within any increment

### #5 serra

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Posted 03 April 2018 - 03:06 AM

the procedure divides the pipe unit in several segments in order to obtain about constant properties (VLE, density, viscosity...) for the calculated segment...
for multiphase (the normal area of application for these procedures) acceleration gradient is usually a small term and is frequently neglected (for the details of the model you may wish to contact Aspen technical assistance)
the software which I use (Prode) has different options and I am not sure whether procedures can be compared...