Pipeline modeling can either be the easiest thing you do today, or the most difficult.

In fact, did you know that AspenTech has *at least *four different tools to model the pressure-flow profile of a fluid through pipes? Aspen HYSYS^{®} Pipe Segment, Aspen HYSYS Compressible Gas Pipe, Aspen HYSYS Hydraulics (AHH) and Aspen Flare System Analyzer (AFSA) — to name a few.

For the sake of this article, I’m going to focus on steady-state tools in Aspen HYSYS (Pipe Segment, Compressible Gas Pipe and Aspen HYSYS Hydraulics). We’ll leave the dynamic discussion to a later article.

Now, there's a variety of reasons we don't combine these tools (organizational inertia is certainly one of them, though not the primary one). But the *main* reason is that, fundamentally, different problems require different solutions.

Are you modeling connected to plant data? Then solve time and performance may be important.

Are you modeling two-phase flow with risk of slugging? Then more rigorous two-phase models are essential.

In the AspenTech ecosystem, some of our models solve faster than others (e.g., Aspen HYSYS Pipe Segment), while others provide more rigor and fewer assumptions (AHH). Some models have correlations and assumptions tailored primarily for vapor-filled, high-velocity fluid flow (AFSA), while others are intended for multi-phase flow (AHH).

OK, you might say, but why not create one tool with full rigor and perfect performance? Why not one piping tool to rule all others?

As you may know, the Navier-Stokes existence and smoothness problem is one of seven Millenium Prize problems in mathematics, because no solution yet exists on how to fully model fluid flow. Without a solution to this equation, no perfect model exists. All fluid flow models are empirical, which means that they have ranges of applicability — or they are mechanistic with some major assumptions involved in their derivation. Moreover, computational power is not infinite, so full rigor (even in today’s sense) means that you must sacrifice performance.

We live in a world of trade-offs. But hope is not lost!

As engineers, we believe in being pragmatic to obtain a “good enough” solution if an exact solution is not possible. By understanding the key advantages and disadvantages of the different piping models, it will become much easier to decide which tool to use, and what you are sacrificing in the analysis.

First, let’s examine the key assumptions in each of the pipe modeling tools AspenTech provides.

**Aspen HYSYS Pipe Segment:**

- Uses Darcy-Weisbach for single-phase flow (incompressible flow assumed).
- For single-phase vapor flow, the acceleration term in the pressure drop equation is ignored, so error is introduced at high Mach number flow.
- Ignores the kinetic energy term of the energy balance equation for all phases.
- Acceleration is only included for a subset of the two-phase flow correlations.
- Slip is not accounted for, even if the two-phase flow model provides a hold-up model to account for slip.

** **

**Aspen**** HYSYS Compressible Gas Pipe:**

**Applicable for single-phase flow only**— uses a single density for calculations.- Solves the mass, momentum and energy balance equations simultaneously (including all terms of these equations according to equations outlined in F1 help).
- Intended for dynamic modeling.
**Do not use the interpolation mode**unless you carefully assess the properties.

** **

**Aspen HYSYS Hydraulics:**

- Solves the mass, momentum and energy balance equations simultaneously on each phase present.
- Provides rigorous two-phase flow models with slip included.
- Equation-oriented solution for the entire flowsheet (so it handles networked piping).

I went into much more detail on these tools in our recent on-demand webinar, so please watch it if you are interested in learning more! For now, I’ll provide a “cheat sheet” for which tools to use for some common systems.

**Utility Header Systems**

- For these systems, you really want to use Aspen HYSYS Hydraulics. This is because header systems involve complex piping networks, often with pressure-only boundary specifications.
- For single-phase systems, you should be ensuring that the system is sub-cooled or super-heated at all times. Otherwise, you’ll run into convergence issues at the phase discontinuity (especially for pressure-boundary systems).

**Safety Piping**

- Use Aspen Flare System Analyzer for analysis of safety piping.
- For complex inlet piping configurations, use Aspen HYSYS Compressible Gas Pipe for 100-percent vapor systems.
- If you have two-phase flow, use Aspen HYSYS
^{ }Pipe Segment. - If you want to use Aspen HYSYS Hydraulics, you’ll need an additional macro to exclude the acceleration pressure drop across swages (which can be significant).

**Single Pipe Line Sizing**

- Use the Pipe Sizing utility in Aspen HYSYS for quick analysis of KPIs.
- Aspen HYSYS Hydraulics is recommended for rating or revalidation work.

**Pipeline Network Flow Assurance in Steady State**

- Use Aspen HYSYS Hydraulics for erosion, CO
_{2}corrosion, hydrates and slug analysis. - For wax deposition analysis, you’ll need to use Aspen HYSYS
^{ }Pipe Segment — but you will lose some accuracy on the pressure profile.

That concludes our discussion on steady-state modeling of pipes and pipe networks in Aspen HYSYS. Please comment below if you have suggestions to enhance this article or ideas for future articles.

*To learn more about how to effectively use pipe models in the aspenONE ^{®} Engineering suite, view our on-demand webinar. *

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