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Limiting factors for sonic velocity with LOW K values.

Limiting factors for sonic velocity with LOW K values.

Limiting factors for sonic velocity with LOW K values.

(OP)
Hello,


   Crane's table on page A-22 shows the limiting factors for sonic velocity, but the table ends at low K values < 1.2. Is it accurate if I extrapolate values of K, Y and DP/P?

  Thank you.

   - Dan

RE: Limiting factors for sonic velocity with LOW K values.

The basis of A-22 is a pipe that ends into a large flow area, so there will always be a sudden expansion (K = 1) in the problem.  With that included, do you still need to extrapolate, remaining K < 0.2?  If so, I don't see much harm in doing this.  It's not going to make much difference.

Good luck,
Latexman

RE: Limiting factors for sonic velocity with LOW K values.

(OP)
Hello,


   I would like to extrapolate low K values because I want to do a case study for individual components.

Elbows
Valves
Short Pipe

I want to perform hand calculations with a pressure ratio across each single component and then perform a Flowmaster analysis on the component to see the similarity.

For me to perform hand calcs on a single component with a low K value, I need the limiting sonic factor Y for sub K=1.2 components.

So you are saying that extrapolating is ok?

  Thanks!

RE: Limiting factors for sonic velocity with LOW K values.

I think you could build a model in Flowmaster that is close enough to the basis of A-22 such that extrapolation to K < 1.2 would be reasonable.

Good luck,
Latexman

RE: Limiting factors for sonic velocity with LOW K values.

As Latexman stated, what you would be extrapolating is between the values of 1.0 and 1.2. Extrapolating between these numbers would be ok.  Trying to extrapolate below 1.0 would not, as you would no longer be within the assumptions on which the table was generated.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: Limiting factors for sonic velocity with LOW K values.

(OP)
Thank you for the reply.

  Than what should I do if I am faced with flow across a component with a K factor that is less than one but a pressure ratio that is large? How would I determine limiting sonic Y?

  Thank you again!!

 

RE: Limiting factors for sonic velocity with LOW K values.

Why do you believe you will have a sonic velocity across the single component? Based on the information on page 1-9 (Limiting flow of gases and vapors) and the examples worked on pages 4-13 and 4-14, you add the K values for all components (piping, valves, bends etc.) before calculating the ΔP/P' value and then using the chart to determine Y.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: Limiting factors for sonic velocity with LOW K values.

(OP)
I will have sonic velocity across the component because I am doing a case study on a single component at a time in flowmaster with a known delta P.

If my upstream pressure is 100 psia and downstream is atmosphere then my DP/P is 0.853. If I am applying this to a single 1" diameter elbow with r/d of 3 than my K will be K=12*(0.02) = 0.24.

There is no K value low enough on the table to look up the limiting sonic velocity, but I'm pretty sure it will be sonic because of the small diameter and barely any losses.

Even with a K value of 1.2, the limiting DP/P is 0.552, if I am at 0.853 i would need a K of more than 20 keep it sub sonic.

I just need help calculating the flow rate across this component, and if I cannot find Y, than i cannot use equation 3-20 of crane's.

Thank you again.

RE: Limiting factors for sonic velocity with LOW K values.

I think a model must mimic reality.  In the real, physical world, there will probably be a source (cylinder, tank, pipeline, etc.) of gas at some T and P.  This may be piped to an area where these components could be tested.  What's downstream of these components?  Open to atmosphere?  Piped to a safe location?  You need to build a model of this entire system between the initial and final conditions that you know and then see the difference changing each of these components make.

Doing a flow model of a valve hanging in mid-air (no inlet or outlet pipe) doesn't do much for me.  Seeing the estimated differences these components make in the flow of a real system, does.

Good luck,
Latexman

RE: Limiting factors for sonic velocity with LOW K values.

(OP)
Latexman I understand what you mean.

while you were replying I have done some calculation of an elbow attached to a segment of 10 foot pipe. At that point, it is safe to say that the K value would be > 1.2.

I am not trying to build a real model at this point, i just want to see if the flowmaster theory is close to that of crane's. The only way for me to do this is test a single component at a time between two known pressure sources, and do that calculations by hand to verify.

The reason I am doing this is because I am building my own flow system solver for a specific system.

I want to compare my system to flowmaster v6.5.2. It is just being used to validate, so the relations that flowmaster uses must be the same ones i use or else it will validate nothing for me.

I appreciate all your help. I am slowly understanding what to do now.

We have to think in terms of theory, imagine at this point there is no 'system'. Lets just study each component case by case.

If you must include reality, I can build a scenario that satisfies my cases.

Imagine a huge tank of nitrogen at 100 psia. There is an elbow welded directly to the tank and a valve attached to the elbow. A truck accidentally crashes into the valve and breaks it off, the elbow is now discharging 100 psia nitrogen to the atmosphere.

The flow rate through this elbow can be calculated with equation 3-20. But how do you calculate the limiting parameter Y if the K value of this elbow is 0.24?


Am I missing something?


Please bare with me.

I am a fast learner, I just need more time :)

RE: Limiting factors for sonic velocity with LOW K values.

I understand what you're wanting to do, however, it seems to be beyond what the Crane handbook was designed for. You may need to talk to Crane directly.  They have a website and have been helpful when I've called them before.

 

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: Limiting factors for sonic velocity with LOW K values.

How about this - calculate K for the elbow, set C = beta, use the equation on the bottom left on A-20 to back-calculate beta, use the graphs for an orifice on A-21 to get Y, d1 = inside diameter of elbow, and use equation 3-22 to calculate flow.

Good luck,
Latexman

RE: Limiting factors for sonic velocity with LOW K values.

Btw, the C = beta was only meant as an initial guess.  This approximation depends on Reynolds Number and Beta.  You'll need to check this assumption after the first calculation and use the graph for orifices on A-20 to get a better assumption for the next iteration.

IMHO, a single elbow would act more like an orifice than a pipe in your application anyway.

Good luck,
Latexman

RE: Limiting factors for sonic velocity with LOW K values.

(OP)
Hello,

 Thank you for the info.

Does anyone happen to have the correlation that the table on A-22 is built upon? I want to curve fit the data points but I would much rather have the real correlation.


I need the continuous solution so don't have to use a lookup table.

   Thank you!!

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