5 replies to this topic

[nybrla]

There are five LPG bullet tanks that will be arranged as shown in the attached diagram. LPG will be unloaded into the tanks from trucks using a pump and pumped out using another pump. The tank is elevated 5 ft off the ground and the fill/suction nozzles of the tanks are spaced 15 ft apart. Here are my questions:

 

1) Is it a recommended design to have a vapor equalization/return line that connects the vapor space of the bullets to the truck vapor space? I understand it expedites the unloading process by equalizing the truck pressure with the tank pressure, instead of losing pressure in the truck and building pressure in the receiving tank(s), but was wondering if all tank trucks/trailers have the capability to hook up vapor line to their tank.

 

2) I need to decide whether to fill one tank at a time or fill them simultaneously. If the tanks are filled simultaneously, would an even level maintained in the tanks? Level would be equal in the tanks prior to the start of filling, but I suspect the friction losses in the piping will force most flow into the closer tanks, and least into the furthest. But then this would be counteracted by the higher static head in the closer tanks...What am I missing?

 

3) Similarly on the pump-out side, if you take suction from the 5 tanks at the same time, will the pump pull equal amount from each bullet and the tank level will be more or less even?

 

Any insight would be appreciated.

Attached Files

•  Tanks.xlsx   14.31KB   29 downloads

Edited by nybrla, 05 June 2016 - 12:22 PM.

[Bobby Strain]

I would connect them together. And, if your delivery tankers are not in dedicated service to you, remember that part of the liquid cargo will remain in the tanker vapor. Make certain that you are getting the amount that you pay for. The best way is to weigh the tractor and tanker inbound and outbound.

 

Bobby

[nybrla]

Mr. Bobby,

 

As you recommended, weigh scales inbound and outbound will be included in the design. With the vapor equalization line, can I be assured that all 5 tanks will receive equal flow from the unloading pump and maintain an even level?  This will minimize interception from operations in selecting which tank to fill and take suction from every time a tank fills up or empties. Thank you in advance!

[Mahdi1980]

Hi,

1-The common design is using equalizing line between the tank truck and vessels to fill in them.

If your vessels are big enough, for instance above 100 m3, the amount of vapor remaining in one vessel might be noticeable and you may need to use a proper compressor for unloading the gas into vessel.

 

2- Although it may feasible to load or unload the vessels simultaneously, it is not common in industry to apply such a method.

Because you have a better monitoring and control of safety systems and alarms and isolating each vessel in due time to avoid any risks when dealing with just one vessel each time.

The level of liquid will depend on the resistance in the opposite of the flow. For example, as the inside pressure of every vessels are the same, the pump prefer to fill in the first tank as the pipeline length is shorter than the others.

So, it has the highest flow into the first vessel. It occurs for other vessels concerning the distance to the pump.

Therefore, the vessels will be full according to their hydraulic resistance, in my opinion.

 

3- similarly, for pump out it depends on the hydraulic resistance of suction line.

[nybrla]

Mahdi,

Thank you very much for such a clear explanation. These are ~230 m3 bullets; in addition to using the unloading pumps on the trucks/trailers, we are looking at the option of using a compressor for unloading (as shown in the link below). I know this method is commonly used in tank car unloading, but not certain that is typically done in trucks and trailers.

 

http://www.corken.co...id_transfer.jpg

[Mahdi1980]

It works for trucks. I have done it several times.