DP METHOD: Compressible flow Darcy or Bernoulli & in aspen ? (most accurate valid)

[jamesbanda]

What is other users experience for gas phase pressure drop in Aspen using the pipe block. And, under deep vacuum ?

We have a system with HX's and Pipework under deep vacuum so the condensation thermo impacts the flow rates etc.. so using aspen is very attrative.

Aspen users the darcy method wherease i'm used to using Bernoulli for relief valve systems in excel..

So, what is users experience with the methods?

i seem to have read that darcy is a modified bernulli and therefore can i assume it is a reasonable method.

 

[zdas04]

Bernoulli is only valid when the density at point 2 > 98% of the density at Point 1 (it is far more sensitive to this parameter than Darcy, Isothermal Gas Flow, or AGA Fully Turbulent). You never have that density consistency in a gas flow in vacuum conditions (starting density is so low that very small changes are too big for Bernoulli to work. I've never gotten an incompressible equation (like both Darcy and Bernoulli) to match reality in vacuum flows. You have to find a compressible flow equation. Spitzglass is kind of crappy, but it gets answers very close to Olga output for a lot less money (and neither of them ever matches reality very well). I'm not confident that Colebrook is valid for compressible flow, but I've never delved into its derivation to look at the assumptions.

You have to keep in mind that most of your standard tools start with an "incompressible flow" assumption. In vacuum operations you really have to start with a "compressible flow" assumption which puts you in the realm of some very difficult and problematic arithmetic.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[KernOily]

The Bernoulli equation is just an energy balance, valid in general for any fluid flow problem. The Darcy equation is used for the head loss term in Bernoulli for incompressible flow.

As David says, but I'll gently disagree with the %, I have found that you can reasonably use Darcy if the outlet density is no more than about 10% different that the inlet density for MOST compressible flow problems. That is usually good enough.

But... Your situation is different. If I was in your shoes I would use a compressible flow simulator that uses a generalized energy equation and a proper EOS for your particular gas at vacuum conditions. I do not know if Aspen does that but I bet it does given that Aspen costs dang near more than my first house. The documentation will tell you. You might have to use a fancy simulator like one of the NASA ones. Just make sure Aspen and the EOS are applicable to your situation; it may not be.

 

[zdas04]

The percentage I mentioned was about Bernoulli, not D'Arcy. I've used D'Arcy with more than 10% pressure drop and gotten numbers that led me to a good decision (reality was pretty far from the simulator, but not so far as to change the decision).

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[hacksaw]

what are you classifying as "deep" vacuum?