Transferring propane by feeding vapour into bulk tank?

Propane, Butane, LPG, GPL, C3H8, C4H10
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evranch
Posts: 28
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Transferring propane by feeding vapour into bulk tank?

Post by evranch »

Haven't been here in awhile as all my propane equipment is running great. I have added a few more pieces of equipment to my LPG fleet and now have a liquid withdrawal bulk tank and can fill tanks on farm, unfortunately only by siphoning/venting at this time.

I would like to stop venting propane to atmosphere as it's both bad for the environment and a waste of fuel. I've done some tests and estimated I can waste 0.5-1lb of propane filling a 30lb cylinder by venting if I'm in a hurry.

I noticed that propane will still siphon down from the bulk tank to a cylinder being filled when they are at equal temperatures, which can get the smaller 20-30lb tanks up to 60% pretty quickly without venting, especially at sundown when the bulk tank is warmer. However this is not an option for vehicle mounted tanks that are too high off the ground, or larger cylinders such as 100lbs (which would probably siphon fill if laid on their side, but... nope).

I know that vapour compressors can be used to move liquid by pressurizing the bulk tank with returned vapour. Unfortunately, the POL valve tanks I use for offroad equipment have no vapour return outlets, and I of course don't have the vapour compressor.


So here's what I was thinking, what if I warm up a 20lb tank inside the house or with an electric tank blanket, so that it is at, say, 30C while the bulk tank and empty tanks are at 0C. Then connect the vapour outlet of this tank to the bulk tank's vapour outlet, through a purged hose of course. Could this be sufficient to increase the vapour space pressure of the bulk tank enough to drive liquid flow, or will the vapour supplied simply condense immediately inside the bulk tank?

The tank is also mounted near a hillside so I suppose I could run a hose down the hill to get more siphon head pressure. I'm not a fan of long LPG hoses though, and flex hoses longer than 25' are against gas code here, so even though any yard with grain dryers usually has hundreds of feet of propane flex hose laying around :roll: I still prefer to follow the rules and not be "that guy".

C3H8
Posts: 1083
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: Transferring propane by feeding vapour into bulk tank?

Post by C3H8 »

Years ago it was common for farmers to place the propane bulk tanks on a stand or higher ground to use the siphoning action of the fuel. If the bulk tank is higher then the vehicle tank it will slowly fill the tank providing the pressures are the same. After all the weight alone of the fuel in the bulk tank far exceeds the weight of the fuel in the vehicle tank. Warming a 20 pound will not help much. Too small to be of any real benefit. There are some other choices.

One is to artificially warm the bulk tank or cool the vehicle tank. There are warmers available for bulk tanks to increase the tank pressure in very cold weather. Secondly is to place the bulk tank in an area where it gets lots of sun while parking the vehicle in a shaded area. One thing that might provide information on the best time to transfer fuel is to install pressure gauges in both the vehicle tank and bulk tank to determine the time of day when the largest pressure difference exists. This would likely occur late in the evening or overnight as the vehicle tank will cool much faster then the bulk tank. Mornings usually provide the best opportunity.

Lastly, to facilitate the quickest transfer the bulk tank should be maintained at the highest level reasonable. I realize this is difficult as you don't want to be calling for small amounts of fuel to refill but if your bulk tank is large enough try not to go below 30 or 40% before calling for refills. One thing to keep in mind is that if your bulk tank is being used for fairly large capacity units (furnaces, etc) the tank is constantly being cooled by internal vaporization. Plan your transfers for when the least draw is on the tank.

Another solution is the Krug manual transfer pump. It transfers fuel safely and it provides some great exercise :lol: . Here's the link to the company that sells them. We used these to empty propane tanks for servicing at the dealer garages. That way the fuel wasn't wasted. It's a little slow but it pumps on both the forward and reverse strokes I believe.

http://www.krug-products.com/custom/products

evranch
Posts: 28
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Re: Transferring propane by feeding vapour into bulk tank?

Post by evranch »

Thanks, pressure gauges especially on the bulk tank are a really good idea. I usually fill a bunch of portable tanks on an evening when the temperatures seem appropriate, letting them siphon and then vent out a little to get them up to 80%. Then I have enough tanks available to work for days or weeks before I have to fill again, but with plans this winter to fix up a grain truck with a large frame mounted tank, this method is not going to work. I also want to do a half-ton or other on-road truck which obviously cannot have forklift tanks strapped to it.

Unfortunately it's not really feasible to put the 1000 gallon shop heating tank on a stand :mrgreen: So the most practical option might be then to use a long hose or a permanently buried copper line down the hill to a fill station for vehicles and larger tanks. I have about a 15' drop which should be more than sufficient.

With a permanent liquid fill line, I suppose there is a high risk of bursting when it ends up 100% full and valved off at both ends. What is usually done around here for grain drying, correct or not, is to valve the far end of the flex hose and leave the tank valve open when moving the hose between bins. Any pressure that builds up will then expand into the vapour space in the tank. Then of course the entire hose is vented and rolled up after grain drying season is over.

I don't like the idea of leaving the tank valve open for days when not connected to anything at the far end. Also I don't like the idea of having to regularly vent a long line needlessly to atmosphere. Do you think a small expansion tank teed into the uphill end of the fill line could provide protection for the line and allow me to valve off the ends?

Alternately I have seen the Krug pump and even tried to build my own from a double-acting pneumatic cylinder when I first started getting into propane. It actually worked quite well (and is indeed good exercise), but ultimately I could not find a lubricant for the piston rings that wouldn't dissolve in liquid propane. It would quickly start to squeal and required disassembly and lubrication after every use, so I set it aside. Is there a specific lubricant available for these pumps?

C3H8
Posts: 1083
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: Transferring propane by feeding vapour into bulk tank?

Post by C3H8 »

Let's look at the regulations. Anytime you have a line on liquid propane with shutoff valves on both ends it has to be protected with one of two items. Either the supply valve must have an internal relief allowing propane to flow back into the tank even if the valve is shut off, or the line must have a hydrostatic relief valve installed to protect the line between the shut off valves. A typical hydrostatic valve is usually set to relieve pressure at 375 PSI. An approved propane line is normally approved at a minimum of 3.5 times the hydrostatic pressure. Bottom line is if the proper safety devices are incorporated pressure build up in the line should not be an issue. Lastly the service valve in the tank should incorporate an excess flow valve that would close if a major leak was present. The other choice in addition to the safety devices would be to run a schedule 40 galvanized steel line for the majority of the distance and just have a minimal amount of flexible hose where you want to fill the tanks. Canada has set reasonable and easy to follow recommendations for almost all situations. All we have to do if follow them.

evranch
Posts: 28
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Re: Transferring propane by feeding vapour into bulk tank?

Post by evranch »

the supply valve must have an internal relief allowing propane to flow back into the tank even if the valve is shut off
I'm familiar with this style of valve for liquid expansion in other industries, and this is such an elegant solution I should have realized it was an option. I will check the model number of my liquid withdrawal valve and see if this is a feature it includes. That solves the problem right there if that is the case. Edit: it is a Rego 7550 which does not have an internal relief. It does have a port to install an external relief on the discharge side, though.

I realize any time liquid propane is in a vessel it must have a hydrostatic relief valve and perhaps I phrased that wrong - I was wondering if including an expansion tank in addition to the relief valve would allow operation of the system without risk of ever popping the relief and venting propane. I remember once seeing a similar setup sitting beside a liquid tank at a warehouse, where a 20lb propane tank was teed into a flexible hose, presumably providing both expansion room and the built-in 375PSI relief. I believe it was likely used to fill their forklift cylinders.

The service valve does have a main excess flow valve at ~20GPM and an auxiliary excess flow valve set at 4GPM which is supposed to match the 1/2" line. However I have heard these can fail to operate in any failure smaller than a complete severing of the hose and should not be trusted to prevent liquid loss.

Galvanized line sounds like a great idea, our gasfitters here only ever seem to use copper or black steel so I didn't realize galvanized was even an option. I know SK is quite restrictive about materials and appliance hookups though, compared to BC where I grew up.

C3H8
Posts: 1083
Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: Transferring propane by feeding vapour into bulk tank?

Post by C3H8 »

All great comments. I suggested the galvanized to ensure no rusting, however, just maintaining a proper paint coating on the line may suffice also. I must also state that some of the regulations may have changed on the gasfitters side since I was last involved. The usual steel line was black iron, minimum schedule 40, however again that may have changed. As for the expansion tank, that would certainly protect a line filled with liquid. You would have to consult with a gas fitter on it but I can't think of any reason it would not be permitted as long as the line included a hydrostatic relief valve. Obviously the tank would provide the same protection as an internal relief would and it would prevent a sudden spike in pressures preventing the release of liquid.

You are correct about the excess flow valves. They only shut if a major leak occurs plus at very cold temperatures the valve may not close on a longer line due to lower pressure in the tank resulting in slower flow.

evranch
Posts: 28
Joined: Thu Dec 13, 2018 11:11 am
Location: Saskatchewan, Canada

Re: Transferring propane by feeding vapour into bulk tank?

Post by evranch »

Thanks, had a chat with a gasfitter buddy and they said they use black iron for fear of zinc flakes getting into a regulator or orifice. However I have access to pipeline tape i.e. Denso tape that can protect iron from rusting for decades, so I might use that when I build the permanent setup. Then it can be buried or at least in contact with the ground.

I built a flexible fill line for now using 1/2" grain dryer hose, and set it up so there is a short whip from the bulk tank to a 20lb expansion tank, then the long fill line. I can also use it for drying grain I guess once the iron line is installed. I valve off the 20lb tank while I am filling so that it doesn't fill with liquid, then shut off the ends and open the tank valve for expansion. It works well and just a little liquid ends up trickling from the line into the 20lb - so when it starts to get heavy I can just swap it out and use it on the BBQ or welding torch. I haven't tried to siphon propane down the hill yet since there is a blizzard going on and I wanted small tanks full for heaters and generators more than I wanted to ride a 100lb down the hill :lol:

Since the test worked I think I should change the tee for a cross and add a dedicated relief valve in case the tank valve ends up shut by accident, which would take its built-in relief out of play.

While buying the parts the gas supplier was curious about my plans and really liked the idea - figured that every grain dryer line should be built that way since a lot of propane gets wasted moving them around.

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