Calculating NPSHA 3

[Smackavelly]

On a stripper tower, does the vessel pressure equal to the vapor pressure of the process?

 

[GaTechTheron]

I do not know your particular application, but with any static fluid in a vessel, and vessel pressure is the vapor pressure.

 

[rmw]

That is not true of water if the water in the vessel is at a temperature less than saturation temperature.

rmw

 

[BigInch]

rmw, I'm no chemE, but I believe it is true for water. If water in the vessel is at a temperature less than saturation temperature, the vapor pressure of that water is whatever it is at whatever temperature is happens to be at. Just that the vapor pressure does not equal the saturation pressure. If there is only one chemical or compound in that vessel, then the vapor pressure of that one chemical or compound is the sumation of all partial pressures of all compounds and the vessel pressure must equal that sum. The vapor pressure happens to be lower than the saturation pressure, but its still the vapor pressure. Even if you lower the temperature below the freeze point, you're still left with the vapor pressure of the ice remaining in the container.

Least that's how I understand it.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[bk19702]

That's how I understood it also. Star for BigInch.

 

[25362]

See thread798-129328 for a reasonably good discussion.

 

[BigInch]

Hey 25362, we've been missing you here.

To spare me the agony of reading (and actually trying to understand) the 50 some odd responses to that thread, would you mind just nodding your head one way or another here? Just this once. If I'm wrong, I promise I'll read, reread and understand every response in your ref.

If I don't deserve the star, I don't want it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[25362]

To BigInch, you're not wrong.

One of the items I recall in that thread refers to the fact that saturated water at a higher temperature (and pressure) with about the same NPSHA causes less cavitation damage than cold water.

The proposed reason can be exemplified when comparing water at 10^oC vs water at 90^oC. It is a bit exaggerated, anyhow...

The specific volume of saturated steam at 10^oC is 45 times larger than that at 90^oC. The latent heat at 90^oC is just about 92% of that at 10^oC.

Therefore to produce at 90^oC the same volume of vapor bubbles than at 10^oC one needs to add about 41 times the heat.

Since the residence time of the liquid at the lower pressure zone inside the pump is very short, the net resulting effect is a lower cavitation intensity at 90^oC due to a lower volume of imploding bubbles.

The other point was that raised by Mr Montemayor on adding an inert (hardly dissolvable) gas on top of a liquified petroleum gas (also in equilibrium, e.g., saturated) to improve the normally very low NPSHA values.

Plus a lot of good advice by others...

I hope this short message saves you time in reading the whole thread.

 

[rmw]

BigINch, It was past my bedtime when I typed that last evening. What was I thinking? I'm throwing in the towel, don't let it hit you.

rmw

 

[BigInch]

rmw, Actually you had me pretty worried about this. I respect your knowledge quite a lot. So much so that I really questioned my own and had bad dreams about whether I should post my comment or not... and I was still unsure, even after bk19702 chimed in to back me up. I just find it hard to totally trust MEs on these kind of topics, you know. I did feel better when 25362 Chemical showed up and didn't flame me. He really knows his stuff too, but if you notice, he still didn't say I was "right". "You're not wrong", just wasn't what I needed to hear right then. And he referenced that long thread, which I also really didn not want to decipher either 'cause I thought I would be more confused afterwards than I already was and was afraid I might have to reboot from a CD. Well, now its resolved. Hey.. happens to me sometimes too.
BTW, missed with the towel.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rmw]

BigInch, earlier that evening I had had more excitement than a man should have all in one day. I found a local store that sells Rioja. I was fearful that I was going to wake up in the morning and think I had been dreaming. After that discovery I was trying to think about vapor pressures??????? I should have known better.

rmw

 

[25362]

The question posted by Smackavelly didn't refer to any particular fluid. And the answer should be no. When calculating the NPSHA, the vessel pressure is generally higher than the vapor pressure of the stripped liquid to be pumped.

I said generally, since there are occasions when chemical reactions may produce "volatiles" in the stripped liquid e.g., thermal cracking, obliging to quench the "bottom" liquid to a lower "safer" temperature level.

In fact, the very reason to use stripping, i.e, to inject a gas (steam, nitrogen, etc.) is to reduce the pressure (at a given temperature) against which one wishes to eliminate -the more vaporizable- fractions originally present in the liquid.

 

[BigInch]

25362 Thanks very much for that additional note. Clears up a lot.

rmw, be sure to let me know if the local supply runs out. In the meantime have all you want... and enjoy. If they have white Rioja, grill a lobster.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[katmar]

The vessel pressure is only equal to the vapor pressure when the liquid is boiling. This is important because there are different types of strippers. In some strippers, like that descibed by 25362 the liquid is cooled and then the volatiles are stripped using a gas. However, you also get reboiled strippers where the fluid is boiling.

In the first case the vessel pressure is higher than the vapor pressure, but in the second case the vessel pressure and the vapor pressure are the same.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

What katmar has added regarding reboiled strippers is absolutely correct. It merits a star. The question still remains what was Smackavelly's intention when asking about stripping.

The only comment I have (following the OP's question) is that the "vessel pressure" which is equal to the "vapor pressure of the liquid" is not necessarily the column's top pressure but the one at the reboiler. The difference is generally due to the [Δ]P caused by the column internals (packing, trays, etc.).

 

[BigInch]

I don't know about anybody else, but I'm clear enough for my purposes. I understand how pressures (and temperatures) can vary within a column and what you'd need to do would vary with if you were stripping, or distilling etc. and taking diesel, kero and gasoline off at different levels, or removing water from a glycol dehy, but you guys go ahead and slug it out until everybody's happy.

Smackavelly are you happy?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[21121956]

Hello everybody:

Thanks to all for your interesting comments. I wonder, Smackavelly, are you there? I can not hear you.

 

[Artisi]

No further contact from the OP is not that unusual, luckily we are big hearted and we can handle the rejection - plus it keeps some of us occupied during the day

 

[BigInch]

And night.

How do you deal with the rejection?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Put it down to life's experience