LPG long transfer line

[barqueiroenmarte]

Hi!

I am designing a transfer system from three propane storage bullets (at atmospheric temperature, 400 m3 each, towards trucks loading facilities using a pump near the bullets and a vapor return line from the trucks to the bullets.

The distance from the bullets to the trucks loading point is almost 1 km.

I have read some recommendations about making the loading line as short as possible, but in our case is quite complicated to get it. Do you know of the possible problems that can arise with such a long loading line? If
I know them in advance perhaps I can take any measure to avoid.

I can think of water hammer problems, and obvious increased pumping power, but they do not seem to me critical.

Best regards and thank you for any help

 

[MortenA]

The surge could be quite serious! Have you done a check (at least Joukowsky)?

Then column separation could also be a problem since the liquid is at its boiling point.

Best regards

Morten

 

[dcasto]

You'll just need more head from your pump that what a typical loading pump is capable of doing. Size your liquid and vapor line and realize that the loading pressure will be higher than the tank by the amount of vapor return and the pressure drop to get the vapors back. Propane just won't water hammer unless you are well over 600 psig.

If you are loading newer model trucks with spray bars, you may not even need a vapor return line! Just a loading pump rated at about a 50 psig differential at the load rate you want.

 

[BigInch]

dcastro, I think there is a possibility of hammer if you happen to cross and recross the phase line. Operating pressures of 600 psi wern't mentioned. Are you saying that that would not the case at lower operating pressures?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

And, IF it is the case, and since it is not hammer associated with valve opening or closure, but collapsing vapor pockets where local velocities could be quite high, could not an even higher "hammer" pressure than usual be expected?

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

If you cross the phase line while loading you'll flood back so much propane vapor that the pressure drop will always back the system pressure to above the vapor pressure point. That is why he needs a pump with enough deltaP to keep it all one phase. By the time the liquid gets to the tanker, the vapor pocket will also be a the biggest shock absorber around.

I've never heard of hammering on a propane loading rack, even when the rail cars were a 1/2 mile away. My reference to 600 psi said propane won't hammer unless you are over 600 psi ie, you've compressed it up. Propane has some compressibility at loading pressures near its bubble point.

 

[BigInch]

Yes, but I was referring to exactly what happens when the vapor pocket does collapse on pressure build up. Apparently what you're saying is that SG is too light to cause any significant hammering, as long as the press build up is not to 600+ psig. No need to respond if that's it.

Thanks.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

I'm just thinking out loud here:

1) Is there any concerns with temperature effects on the pipeline itself? It is 1 km long. Will it heat up or cool down?

2) Is there any concerns with leakage? With a 1 km line, how many possbile leak points are there?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

Good thoughts Ashereng, as a pipeline guy I assume a pipeline is buried. If the line is all above ground, then there may be radiation heat causing a presssure build up and vapors in the line that will be pushed into the truck and back to the tank through the vapor return system. Further more, what kind of controls are at the truck end. Will the controls and shutdowns respond correctly if they are in a vapor phase for a few seconds or minutes?

As for leak points, nothing special I can think of over any other line operating at under 300 psig. Internally, propane is very nice to work with, no corrosion.

 

[barqueiroenmarte]

Hello and thank you all for your answers.

As a summary, the possible disadvantages of installing such as a long loading line are:

- Surge and column separation: I have performed a Joukowsky first preliminary checking and the pressure during the transient can be lower than vapor pressure so column separation may occur. Don't I have to worry about because the pressure is lower than 600 psig as commented in previous posts? How to avoid it? I am sorry, but I haven't understood that explanation very well...

-Line heating up: As a clarification the storage bullets and the loading line are both above ground and at atmospheric temperature so the heat gain of the LPG during loading should be minimum (perhaps the pipe is a bit warmer than the LPG in the bullets but only few degrees) so in my case it shouldn't be a matter to concern.

Regards

 

[BigInch]

You must keep the pressure higher than the vapor pressure at all times, or design for the transient spikes associated with the collapsing column.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[barqueiroenmarte]

Hello again!

In a previous thread Dcasto said that "the loading pressure will be higher than the tank by the amount of vapor return and the pressure drop to get the vapors back". In my case the vapor return line provokes a head loss of 2 bar approx. So, in the LPG bullets the LPG is at ambient temperature and at its vapor pressure and if I have to apply a pressure at the vapor return line of 2 bar over the bullet pressure (=vapor pressure), won't LPG condense in the vapor return line? Is it usually taken any measure to avoid that condensation?

Regards

 

[barqueiroenmarte]

Any experience about this LPG pumping service? Is condensation in the vapor return line a problem? Any criteria for sizing that pumps?
Thank you for any help...

 

[BigInch]

Have a look at the technical publications and training manuals available here,

http://www.corken.com/publications/index.html

http://virtualpipeline.spaces.msn.com

 

[dcasto]

Your tank is big enough to pump at rates of 300 gpm which is a typical truck loading rate (200 to about 350 gallons/minute).

At the loading dock you need the LPG be at a pressure about 30% over the tank pressure if you have a loading meter. If no loading meter, then you only need to be about 10% over vapor pressure. So you can select the combination pump head versus line size. Its normally for propane to be a 3 inch line, but a 4" is better especially for your line and about a 50 psig differential head, but a 60 psig for yours is better (assuming propane and no loading meter at 100Deg F). If you have a loading meter, then you will need about 90 psig differential head. The vapor return line works out to be the next size smaller than the liquid line. A smaller line could be used, but you don't have a steady state process and you get a large amout of vapor return during the first few minutes of loading.

The pump will be a 15 to 30 HP depending on your presssure and efficency. NPSH is a problem, I like vertical can pumps for the high pressure loading and if you went with a 6" line and a low pressure loading casr, a Blackmer vane pump is good.

Yes some vapors will condense on the way back, just don't put a huge "pocket" in the return line, in other words, there may be an extra 10 psi pressure drop in the vapor return line to push the condensed LPG up into the storage tank, so return the vapor line should never go higher than the top of the thank plus fittings.

 

[barqueiroenmarte]

Thank you again!

I have reviewing Corken documentation and in their pumps manual it says that "the equalization line assures that both tanks remain at the same pressure" but I think it can't be truth since during loading there is a fluid flow from loading truck towards the LPG storage vessel, so the pressure at the truck must be higher, isn't it?

Dcasto, you mention some typical differential pressures with/without metering. In order to check them in my case, I suppose that I have to calculate the head loss in the loading liquid line (including metering and flow control valve at the loading point) and add the head losses in the vapor return. Moreover I understand that in your opinion other extra 10 psi must be added to such head losses in order to take into account the frictional losses due to condensate, is it so?

Finally, I am a bit concerned about the condensate formation in the vapor return. In my case it is not possible to design the line completely upwards from the truck towards the LPG storage tank but I will have a pocket (200 ft approx). Is it usual to install in those cases drain pots or something similar?

Regards

 

[BigInch]

From what I've looked at, I doubt they're wrong. Are you looking at diagram showing loading from a tank without using a pump?

I think, if you use their pump, the pressure at the return line inlet near the truck would be higher than at the tank and you return using that excess pressure.

http://virtualpipeline.spaces.msn.com

 

[barqueiroenmarte]

I thomk that we are both saying the same: the pressure at the return line inlet near the truck would be higher than at the tank in order to establish a fluid flow. As in the tank the pressure is supposed to be the vapor pressure, near the tank there must be some condensation.