Diesel fuel flow through a vertical pipe 12

[jreng1]

I am working on a fuel delivery pipe line to deliver diesel fuel to an underground mine. The system is comprised of an open tank on surface with a valve, a 1 ½” vertical pipe line, and an open tank underground.

My question is; Will the flow be uniform and fill the entire pipe to push the air out to the bottom, or will the air tend to make its way up the pipe causing air bubbles.

 

[quark]

What do you mean by open underground tank?(do you mean which is at atmospheric pressure?) You sure will have problems if the level of your underground tank vent is not above that of above ground tank.

Regards,

 

[jreng1]

Thanks for your input quark,

Both tanks are at atmospheric pressure, and the underground tank will have a volume greater than the volume of diesel fuel being delivered; therefore the underground tank will not overflow.

 

[vpl]

jreng1

If you're in the US, there are some fire protection codes which specify location and sizing requirements for underground tanks holding diesel fuel. Specifically go find yourself a copy of NFPA 30-1977 (or later.)

Patricia Lougheed

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

 

[quark]

Jreng!

I am not talking about overflow issues. I suggest you to keep the vent pipe of your underground tank above the vent pipe level of above ground storage tank so that thes will become the highest point in the system. If you don't have proper venting arrangement, air will definitely pass through the vertical pipe and will obstruct the diesel flow.

Regards,

 

[jreng1]

quark,

Can you please elaborate on your theory. thanks.

 

[quark]

When you are filling the underground tank, diesel displaces the air. In the absence of underground tank vent, air passes through the discharge pipe and through the vent of above ground tank.

If you provide vent to the underground tank taking to a height higher than that of atmospheric tank vent, then air will pass through this and you won't have any problems.(simply what I mean is that the underground tank vent should be the highest point in the system) Infact I tried to draw it here but not getting it right. Give me your e-mail id and I will send you the schematic.

You should also check the safety codes as suggested by VPL.

Regards,

 

[CCA]

At my opinion, even a short vent on the underground tank will do. The air will escape preferably by the vent than by the dieselline and therefore no air will escape by the dieselline. The only reason I see for extending the vent untill above ground level is for removal of the diesel vapors that will escape trough the vent together with air. Although this is a minor healty and oder problem.

Michel

 

[jreng1]

Thanks quark, but I have to agree with Michel (CCA).

 

[PUMPDESIGNER]

I certainly have no experience with mines or deisel.
Vent problem is interesting, no comment from me on that.

However with any fluid I would not let the "drop" pipe ever empty. If the "drop" pipe empties each time you will have a lot of fun predicting how the air in the drop line will react to the fuel coming down again, could be interesting, a research paper perhaps?

Avoid that mess. Install a valve at the bottom of the drop line, link that valve to a float on the top tank. when the top tank is almost empty, as fuel is flowing, shut the valve off just before exposing the inlet of the drop pipe, and keep that drop line full.

PUMPDESIGNER

 

[vpl]

jreng1

As I stated before, you need to check the appropriate standards. I'm taking a guess from your handle that you are a "junior engineer". If that's the case, then learn to go check out the requirements, don't just take someone's word -- and if you get differing opinions, that means that you need to do more research to find out which is right.

CCA

With all due respect, I sincerely hope you aren't designing underground diesel tanks. There are requirements for venting of such tanks and those requirements are not there just to make your life miserable. I'm not pretending to be an expert - but I do know enough to go look up the standards.

I'll get off my soapbox now and probably should duck under it!

Patricia Lougheed

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

 

[quark]

How will you open a vent underground?

Moreover, if the vent height of underground tank is morethan that of above ground tank overflow never occurs in the underground tank. (let us assume a condition where your underground tank is half filled previously and then you are filling it again)

Just keep in mind that air floats up and tries to go out via least resistance path. By providing higher vent (i.e by providing greater pressure differential) you are creating least resistance path. But one thing for sure, it is easy for air to have a cake walk through liquids and this caused many a headache for me in the past.

PS: What does the local code say about venting?

Regards,

 

[lilliput1]

Vent height should be such that if totally filled, the pressure excerted by the fluid would be less than the tank design pressure.

Content of both tanks should be monitored to avoid overfilling. Both tanks should have rupture basin (or be double wall) with leak detector.

Double wall flexpipe from 1 1/2" to 2 1/2" are available, rated up to 100 psig. Low point should terminate in a sump where leak detector can be located. Use float type since discriminating sump type sensors by Veeder-Root or Tracetek takes 2 to 12 hours to detect diesel leak. Veeder-Root ir interstitial sensor however is almost instanteneous.

The fill pipe in the tank should terminate near the bottom of the tank. The area opposite the discharge is reinforced with welded plate or FRP if thank is FRP.

There will be air in the fill line as the fuel flows. Avoid too high a flow rate to minimize splashing.

 

[katmar]

Although vertical downflow seems to be the easiest type of pipeflow to deal with, there are potential dangers and you have wisely foreseen the potential problem with removing the air.

You need to analize what the diesel flowrate is expected to be in your 1.5" pipe, and then from the Froude number you can predict whether you will have a syphon (ie air is drawn down with the diesel) or you have a self-venting system (ie air bubbles up through the diesel to the top tank).

There was a good article on this in Chemical Engineering magazine, way back on June 17, 1968 - maybe your library has it?

Otherwise have a look in Perry's Chemical Engineers Handbook - I'm sure you will find something on syphons and self-venting flow there.

I have seen expensive equipment destroyed through this simple analysis being overlooked, and piping deigners should always keep this at the back of their minds.

regards
Harvey

 

[jreng1]

VPL

This discussion has merely turned into a discussion on vent positioning and tank design. My original question is on air entrainment within the fluid as it falls down the pipe.

To simplify my question, lets assume I am pouring a fluid from a tank down a pipe into an open tank. Will air travel up the pipe, or will it be pushed to the bottom.

Thanks

 

[quark]

Jreng!

For sure, air always travels up.

Lilliput!

It's general practice to flow in the liquids through a dip pipe(with holes on it's periphery above liquid level)and impingement plate arrangement. However in this particular case I think this worsens the situation.

Regarding the air vent height, yes, ideally it should take care about the tank pressurisation. But again with this particular case, a totally filled underground tank is already pressurised by the liquid in the above ground tank and the supply pipe. Moreover, liquid level in the underground tank vent pipe is just equal to the liquid level in the above ground tank.

Regards,

 

[PUMPDESIGNER]

jreng1,
My post above ignored the vent situation entirely,
was meant to deal with your exact question you just restated.

PUMPDESIGNER

 

[smckennz]

In answer to your original question, it depends how fast you pump the diesel down the pipe and how fast the bubbles rise. If the diesel downward velocity exceeds the bubble rising velocity the no bubbles will come out the top.
If the converse, then yes they will.

Do the sums.

Cheers

Steve

 

[lilliput1]

If you let the piping completely fill up with diesel (locate a shutoff valve at the bottom & open this last)first, air will naturally rise to the top, & providing there are no air pockets (piping drops continuously), there should be no air in the piping.

 

[CanuckMiner]

A few clarifications are in order here methinks.

In an underground mine, it is common practise, indeed a requirement, to use a "dry line" (i.e. you are not allowed to fill the pipe with fuel and then open a valve at the bottom for delivery). You must keep the transfer pipe empty except during the immediate transfer. This is for safety and environmental (risk of spill) reasons.

In essence what we have is a batch tank (Tank A) on surface (say 2000 litres). There is an outlet on the bottom of the tank, with a valve. Below this is a 1.5" dia. pipe that drains to the underground tank (Tank B) located far below the surface of the earth. We open the valve at the bottom of Tank A and the fuel starts to flow down the 1.5" dia. pipe. It is not pumped, it is gravity flow.

The underground tank that jreng1 is referring to is not an UST (Underground Storage Tank). It is an AST (Aboveground Storage Tank) installed several thousand feet below surface, located in an open cavern in hard rock (read granite or dolomite...not soil). The tank will be vented to the local ambient atmosphere. Yes the local ambient atmospheric pressure at depth will be higher than on surface (say 18 psi versus 15 psi), but it shouldn't be significant. A vent pipe back to surface is out of the question, and indeed is not installed on other similar installations. The diesel fuel storage area underground will be ventilated, with the fumes being directed to a nearby Return Air Raise (i.e. ultimately exhausted via a big "rock chimney" some 18-feet in diameter to surface).

Standards and codes have been consulted (what little there is that is applicable - NFPA 30, NFPA 122, CSA standards - in Canada, etc.), but there is nothing that describes the "science" part of these things, only the health and safety aspects which are obviously important, but do not answer the questions.

Putting to one side the diesel fuel and the regulations, codes, and standards, there remains the basic physics of the system: a liquid free flowing down a pipe. You do flow calculations based on Bernoulli's equation and you come up with some anticipated flow rate (in fact the fluid reaches a "terminal velocity" based on the falling fluid being impeded by friction in the pipe). However, the equation ASSUMES a full pipe (i.e. filled with the liquid). The question remains: is this a valid assumption? Does the air get pushed down the pipe into the tank and out the vent? Or does the air bubble back up through the liquid, and out through the vent in the upper tank?

The has come up because of what some other mines with similar systems report, namely that when the valve is opened, the line "bubbles and gurgles" and drains slowly, then after a period of several minutes the flow picks up and it drains much more rapidly. We are trying to establish the science/physics of what is going on.

This seems like an elementary problem at first glance, but oddly enough, calculations, discussions, research, consulting of fluids texts, etc. have not produced answers.

We have contacted Chemical Engineering magazine to locate the article katmar describes (Thanks!) and we're trying to "get back" our copy of the Chemical Engineer's Handbook (having had it pilfered from our library).

Hope I didn't bore you all with these details, just trying to outline the background for the simple question asked by my colleague.

Cheers,
CanuckMiner