5 replies to this topic

[JEBradley]

Im sure this is an exceedingly simple question for anyone involved in manufacturing...

When you have a tanker delivering a liquiefied gas to site.... when it's unloading do you attach a gas supply to the top of the tanker to maintain a back-pressure???

If so does this tend to be an inert gas? Is it carried on the tanker? Can you use an air compressor (fluid dependant of course).

Also, when your pumping out a tanker what kind of pump do you use? Centrifugal but avoid totally emptying the tanker? Or maybe a scavenging pump to get round it? or would you use PD?

thanks for any help

[Art Montemayor]

JE:

This is a simple question(s) – depending on who you ask.

When you transfer LPG (Liquefied Petroleum Gas) or some of its components - such as Propane or Butane – you do in one of two ways:

1. You connect an equalization vapor line between the source tank vapor space and the target tank’s vapor space; you pump the liquid product from one tank to the other, displacing the vapor in the target tank into the source tank. Since both tanks share common quality of product, there is no harm done in the vapor transfer. The pressure in both tanks is the same before, during, and after the transfer operation.
   
2. You install special spray nozzles inside the vapor space of the target tank and you simply pump the liquid product from the source tank into the target tank – simple, and no other connection is needed between both tanks. The sprays produce a pressure drop which cools the vapor space in the target tank and allow for that tank’s vapor space pressure to remain low and not increase due to the potential compression of the vapor space if there is no equalization vapor line between the two tanks.

I definitely have always preferred and used the first method. I have multiple reasons for this decision and the NPSHa for the transfer pump is primarily one of them. The other is that it is a direct, simple, and consistent procedure that has never failed and totally ensures that the pressure in both tanks will never increase – in fact, it will be “equalized”.

Now to answer your questions:

when it's unloading do you attach a gas supply to the top of the tanker to maintain a back-pressure??
No. As I explained above, it is for equalizing both vapor space pressures.

when your pumping out a tanker what kind of pump do you use?
You can use a positive displacement pump – gear or vane - or a centrifugal pump as well. I’ve used a variety of both.

Centrifugal but avoid totally emptying the tanker?
This makes little sense. I think you mean you want to “totally” empty the tanker truck – which is something that you or anyone else will never do – regardless of pump type. There will always be residual liquid left in the truck, lines, and pump casing. There is no way on God’s Green Earth that you can avoid that from happening. I have assumed you are relating to LPG because you haven't identified the liquefied gas you are handling. If it turns out to be Ammonia or Chlorine, I wouldn't worry about what you leave in the truck. It's better off there than out in the atmosphere of the surroundings.

[JEBradley]

Thanks very much for those answers - I had found myself in the unfortunate position of specifying a piece of kit I don't really know much about.

The chemical is a fire extinguisher - heptafluoropropane.

Im sorry for the lack of clarity on the centrifugal pump question - I was in a bit of a hurry.

I was concerned that if i stuck a straightforward Cent. Pump onto the tanker I could end up dragging a vap/liq mixture through the pump when the tank was almost empty. Hence, would I have to be specifying a scavenger pump?

Thanks again

(And Congrats on your 1000th post!!)

[djack77494]

JE,
I'd suggest you find some manufacturer's literature from a vendor that sells equipment for transferring liquified gases. There are many solutions, and what might be good for LPG or anhydrous ammonia, for example, might not be good for refrigerants.

I have some experience in moving around liquified anhydrous NH3. We actually used compressors to load and unload tankers. The principle is very simple. Connect liquid lines between the two "vessels". Then take vapor from the receiving vessel and run it into a compressor (e.g. Corken). The compressor's discharge goes to the delivering vessel. In this way, a relatively small differential pressure is maintained between the two vessels and that is sufficient to effect the liquid transfer. You can essentially empty the delivery vessel with this approach.

If you want to use a pump, pick one intended for this type of service. I've used Blackmer Pumps, which can deal with low NPSHa and very volatile liquids. There are other choices and other vendors, and I'm sure they would be glad to help.

Good luck,
Doug

[lp2]

I have a followup question on transferring liquid NH3. My NH3 storage tank is partially refrigerated - pressure about 55 - 57 psig at the top and temperature about 40'F. The pressure is controlled with a compressor drawing vapors off the top, returning condensed liquid to the storage tank. The pressure and temperature of our railcar deliveries vary with ambient temperature - pressure between 100-150 psig. We use a side stream off the storage compressor discharge to pressure the transfer of NH3 into the storage tank.

I am installing some new unloading piping and will have a longer run into the storage tank. My question is this. How do I ensure I get a high unloading rate and keep from flashing with my high pressure drop (around 90 psig in summer). The compressor discharge runs about 170 psig. If the railcar is at 150 psig, do I have the delta of 20 psig for liquid pressure drop before flashing? Is there a way to calculate what my unloading rate will be with the new piping?

Thanks for your help.

[Art Montemayor]

lp2:

This is a very practical and important question on Liquid Ammonia handling and unloading that you have presented. However, it is now tied to someone else's thread.

I would like to recommend to you that you copy this same query into a separate and independent thread where all of the Forum members can concentrate and discuss this one subject and anything linked to it. I consider this subject very specific, common, and necessary for all engineers to be at least aware of. It entails a lot of safety and operational issues that will certainly surface once the thread starts to get active.

I also recommend you attach or submit a sketch or drawing of your system in order to allow us all to quickly get to the meat of the operations as you presently have them.

I have already been dealing with a very similar subject involving refrigerated NH₃ storage and ocean tanker loading in the Spanish Student Forum. I have created a workbook that you can down load there and see how I've described the operation. I apologize for the material only being in Spanish, but that's how that thread developed. We can deal with all the specifics with the same detail and attention to your concerns and queries.

I await your response.