New FAQ

[zdas04]

Site management asked me to highlight faq378-1864 in this forum. Take a look at it if you are interested in the topic and give me some feedback either in this thread or in the e-mail feature.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[BigInch]

What's a 1.5 inch "irregular" hole.

 

[zdas04]

I took a piece of graph paper, laid it over the hole and outlined it, then counted the squares and converted that to the equivalent of a circle. That circle would have had a diameter of 1.5 inches. If you can think of a clean way to say that, I'll modify the FAQ.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

It's clear to me that the flow is dynamic. It proceeds at a rate in the upper right side along the red line(s) to the lower left side. It may not be clear to some. Maybe some directional arrows on the red line or something? Or, "Initial Flow" near line in upper right?

Obviously, the example is a case where the surrounding pressure at the 1.5" hole is sub-atmospheric. I'm not that familiar with pipe blow downs, but isn't blowing down to atmosphere the norm? This may puzzle some folks.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

It is at atmospheric pressure at 5300 ft elevation. I work in the Rockies and tend to get tired of all the examples people use being at sea level.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[BigInch]

If you prefer to use non-standard sea level as a reference, you should at least state what you are using. I was going to ask the temperature before, but did not. Now I feel obligated to do so. Base pressure and temperature conditions must always be stated if they are not "STP". In fact, even if they are.

There is a bit of theoretical difficulty in converting an irregular hole, or crack, to a regular circular hole. There should be some kind of a Cd coefficient, or did you include Cd for an irregular hole and its equivalent in "irregular" dimensions, converted to an equivalent circular hole, or is that ignored. As you know, even a round hole has a Cd coefficient that depends on the edge type as well. What edge shape did you use?

 

[LittleInch]

If the question being asked is "what does the flow profile look like for a gas pipeline venting to atmosphere" then some of this data would seem to be simply getting in the way.

Surely what is being given is the basic shape of the graph. The actual number will vary with atmospheric pressure, temperature etc so perhaps just remove the numbers and put in a marker like critical pressure, 90% of Pcrit and atmospheric pressure at the end and be done with it.

Usually on a blowdown the big issue is temperature, either local at the hole (minus xxx) or the gas temperature along the line, especially as the pressure gets low and hence expansion rate is highest.

usually the question is "how long will it take" or "how fast can I blow it down".

It's a good start, but maybe just a bit too precise when only an example can be given. volume to be blown down clearly affects time and temperature fall.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

LittleInch,
That is a great point. I've changed the graphic to get rid of the numbers and added the equations (without adding nomenclature, or explaining exactly how often you need to iterate, or the importance of recalculating friction factor at every time step--I knew there was a reason that I didn't include the equations to start with). That gets rid of a lot of the irrelevancies.

I've done this calc for a lot of blowdowns, and the end of choked flow is really easy to see in the field, and the elapsed time is not dependent on local ambient temperature. The temperature I use (for buried pipelines) is a ground temp of 65F and the gas at ground temp. I've seen the drop out of choked flow within seconds of when I predicted on blowdowns that took hours.

Now predicting the duration of the transonic region is a different kettle of fish, and I've never been close to predicting that one, consequently I've never come close to predicting the time to zero.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

That's better!

However, on my laptop tonight the graphic is about 2X the size of my screen. Is anyone else seeing that? If weather permits, w ill look tomorrow on my big flat screen at work.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[BigInch]

Ya. I had to put my zoom at 25%

 

[europipe]

Latexman, You can always use the ctrl button + scrollwheel to change the view of Your screen

 

[zdas04]

The img command doesn't have any parameters and I created it at 72 dpi. When I went coarser than that the equations pixilated out. I'm not sure how to make it any smaller. O[en to any ideas.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

europipe, I don't think I have a scroll wheel on my laptop?. I use "ctrl and shift and +"

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[europipe]

Maybe use a mouse?

 

[Latexman]

I have one for my laptop somewhere deep in the bowels of my backpack. I use the finger mouse button mainly, sometimes the touch pad, but hardly ever the mouse.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

I shrunk it from 1103 X ???? to 840 X ???. See if it fits better for you now.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[IRstuff]

Interestingly, your original image, extracted from the page, has 2200 x 1700 resolution. Even more interestingly, it now results in 1576 x 1216, but in Firefox the image is actually displayed on my same screen as larger by about 20% on the screen at 100% mag.


TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[zdas04]

Any ideas gratefully accepted. This is my first attempt to put an image into a post and it is less fun than I expected.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[Latexman]

Better, but still to big. Maybe about a 640x???

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[zdas04]

I got it down to 607 X ???. Looks smaller to me, but how do you tell?

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist