Can increasing backpressure increase the flow of a flashing liquid? 3

[JKoenders]

Suppose there's a pipe connecting two vessels that is very long relative to its diameter, and there is a pressure differential between the tanks driving the flow of liquid through the pipe. Now suppose that somewhere along the length of this pipe the pressure-drop causes the fluid to flash, creating a two phase flow regime.

My understanding is that the two phase flow will cause a greater pressure drop along the pipe than if the material remained in liquid phase. So, conceptually speaking, could adding a restriction (such as by throttling a valve, or adding an orifice) at the downstream end of the pipe increase flow through the pipe by forcing the material to remain in the liquid phase until it gets to the end of the pipe?

This logic, while counter-intuitive, seems sensible to me. Intuitively, though, I would think that adding any restriction must decrease the flow, and removing a restriction must increase the flow.

Is there any truth to this theory, or is my intuition correct?

 

[BigInch]

OK, now supercool the wires. What happens? Is that the analogy to supercritical fluid flow?

**********************
"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/

 

[FeX32]

Hmm. Then it would not apply.
Unless maybe we are in another dimension.


Fe

 

[JKoenders]

The problem with the electrical analogy is that the addition of the resistor does not change the inherant characteristics of the electricity itself. The concept behind the orifice scenario is that there is a change in the phase, thus changing the "electricity" itself to something that behaves differently than it did before. I think the electrical analogy only applies if we're talking about a single phase thorughout the system.

In my original post, I should have said "My understanding is that the two phase flow will cause greater resistance in the pipe than if the material remained in liquid phase." The total pressure drop will remain the same between the two tanks regardless of where the orifice is or isn't, it's just a matter of where that pressure drop occurs and what it does to the fluid in the pipe. The theory is that if the major component of the pressure drop occurs at the end of the pipe, the liquid is allowed to expand to a vapor where there are no hinderences, as opposed to flashing mid-way along the pipe and having high-resistance two-phase flow along the rest of the pipe. Can the resistance added by an orifice that forces liquid-phase flow be less than the resistance of two-phase flow along the pipe?

BigInch, you have my permission to solicit input from wherever you like, but I'd prefer that the conversation doesn't stray too far from the original constraints of the problem (flashing liquid, constant tank pressures / available dP, fixed system geometry).

An alternative consideration to adding the orifice to delay flashing, however, would be to increase the downstream tank pressure, which would effectively create the same scenario within the pipe. The dP from the front end of the pipe to the front of the orifice (orifice scenario) or to the end of the pipe (increased downstream pressure scenario) are identical if flashing is occuring right at the end of the pipe in both scenarios.

Thus, another way to pose the question is, can a saturated liquid flow at a greater mass flow rate with less available dP than a two-phase system in the same pipe with a higher available dP? Keep in mind that when I say dP here, I mean the available driving force to push the flow; i.e. the difference in the tank pressures.

 

[BigInch]

Right, all you really have to do is increase the operating pressure of the whole pipeline, until your fluid finds it possible to flow at a lower friction factor.

In the case of CO2, if you have your upstream pressure at let's say 2000 and the outlet tank at 1500 psia (I hope both of those pressures really are in the supercritical phase) you could get a higher mass throughput than if you had the inlet pressure at 1000 and the outlet tank pressure at 500 psia, where (I think) it would be flowing as a liquid. And that would probably be more mass throughput than if it was at 550 and 50, where let's say for argument's sake that it would probably be 2phase flow.

And these things really don't have to be supercritical flow either, say if you were flowing CokaCola. If you kept the pressure above your solution pressure for the quantity of CO2 in the flowstream, you'd have 1 phase flow, or a water line with air near the soluable limits, if you kept your pressure above the solution pressure for that quantity of air, 1 phase flow. Or just water alone, if its above the vapor pressure, 1 phase flow, below the vapor pressure its all vapor, or in and out of 2 phase, if the pressure varies enough at the high low points.

**********************
"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/

 

[dcasto]

so, whats the absolute difference between an orifice and increasing the lenght of the pipe such that the DP is the same? So why noy just put 100 feet of pipe in a coil at the end.

NOPE, my simulator can't find a solution where that will happen.

 

[BigInch]

Your simulator is busted.

If your inlet pressure is over supercritical pressure, you'll be flowing sc for a little while, no. Whether you'll do that for the entire length of the pipeline or not will depend on the backpressure you're holding.

**********************
"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/

 

[ginsoakedboy]

BigInch,

Compositepro and dcasto have really summed up the scenarios nicely.

If I understand correctly, your recommendation is to increase the system inlet and outlet pressures so that the cumulative losses due to friction and orifice plate do not drop the pressure to a point where flashing occurs. Essentially, shifting the entire transport process above the critical point.

In the more general case, where you may not be able to freely change inlet and outlet pressures beyond a limit, you may not be able to shift the process into supercritical region.

IMHO adding the orifice plate in that case will only suppress the flow rate.

 

[BigInch]

Yes, quite true, but that's why I asked the op if his intent was to allow those kinds of scenarios.

**********************
"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/

 

[JKoenders]

It is apparent that given a fixed system geometry, and a constant available pressure differential, that the mass flow rate will be greater if the whole system is at a high pressure that keeps the material in a liquid (or supercritical) phase, vs a lower pressure where two phase, or vapor only exists.

That is not within the confines of the problem statement, however. The upstream pressure is fixed, and the fluid will flash by time it reaches the downstream pressure.

I think the pipe extension scenario clears it up. To reastae the idea: keeping the material a liquid (or supercritical fluid) until the end of the pipe, by increasing backpressure (however that might be done), will increase flow. It is obvious, however, that in the situation where the pressure profile (and therfor phase profile) is shifted downstream by adding pipe on the end, that the flow rate upstream of the flash point will not increase, but rather decrease. How could more pipe, with two phase flow at the end, flow more than the shorter pipe? If you include the proposed end-of-line orifice on the short pipe, and compare to the upstream-of-flash-point portion of the elongated pipe, the pressure profile, phase, and length of pipe, upstream of the flash point in both situations is identical, and therefor the flow must be the same. So, if it's obvious that a longer pipe must decrease flow, then the addition of the orifice must also decrease flow. Agreed?

 

[rneill]

I think I'm getting a headache, and forgive me if I err since this is definitely not an area of expertise for me.

Regarding the discussion around supercritical CO2 pipelines (and LNG lines), is it possible that rather than maintain the same inlet pressure and using an orifice to hold backpressure that what is really going on is that we are jacking the inlet pressure and then we have to install an orifice on the outlet to take away the extra pressure we put into the system in order to hold everything supercritical (or in the case of LNG as a liquid). In this case, we would trade off the cost of generating the extra pressure against the savings in pipe size associated with gaseous or 2-phase flow but we wouldn't be getting something for nothing as we had to invest extra capital in additional equipment in order to generate the additional pressure. We just made a determination that for a given length of pipeline, the extra pressure paid off.

This is a different argument than suggesting that we are providing the same inlet pressure and adding pressure drop (e.g., restriction orifice) to gain savings in friction which would amount to getting something for nothing - all we had was the cost of an orifice and we increased mass flow?

Again, I'm just trying to wrap my head around it as the arguments proposed on both sides of the debate seemed on their faces to make sense. This is the only way I can rationalize the fact that CO2, LNG, etc are definitely transported in SC or liquid states against the analogy presented by Wisepeppy which also appears to make complete sense.

 

[BigInch]

Yes, you are correct in one sense. You have recognized the difference in absolute inlet and outlet pressures as opposed to differential pressure.

In your method you only include the cost of compression. You also have to realize that the cost of generating the pressure is only one cost of many. Actual transportation cost is not a linear function of increasing the operating pressure, which can be balanced against the cost of generating that pressure. You still need to include the costs of minimizing the loss of that pressure as you try to meet the final objective, transport to point B. For typical fluids that is basically only pipe diameter. The larger diameter, the lesser the friction cost. But for some fluids, ...

again considering CO2, after compression to a liquid, the cost of further compression to supercritical regions I would guess is nearly linear with pressure, but the cost of friction when in that critical phase will be 1/100th of what it is in the liquid phase, so assuming that compression all the way to SCritical doesn't cost 100 X compression only to a liquid, its a good deal.

**********************
"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/