[safety] How to make low pressure flow head w/pressure release

After a visit on kingpins website I discovered he has a dewar with handmade low pressure flow head w/pressure release. I want to make this to.

At this moment I need to tilt the dewar sidewides to poor the ln2 in a smaller 1L thermobottle. This is the most dangerous part of working with LN2 imo. Spilling over your hands is easily done.

Can you guy's help me building a system like this in a safe way?
I had an idea after discussing this with unknow_road. The idea is the following.

(beware my paintskills)
http://tweakers.net/ext/f/37e4bd9909...c153b/full.jpg

The heating element will evaporate some LN2 witch turns into gas. The gas will build up a pressure and push LN2 true the tube. The overpressure releasevalve is already in the dewar preventing it from exploding :rolleyes:

My question is, will this work? (I think so)
and how much heat will be needed to create enough pressure? And what kind of heating element can I use (witch will work submerged under LN2)
What materials should I use as pipe? and how can I hold the outside part of the pipe at normal (safe) temps?

I found a nice website with information of a comercial system:
http://www.norhof.com/

The follwing part looks very interesting:

Quote:

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WORKING PRINCIPLE

The pressure above the liquid level inside the dewar is carefully controlled and therefor the flow.

With only 100 mBar of overpressure, the lquid will gentle rise out of the rise pipe and fall into the fill hose.

Because we evaporate some LN2 to build pressure, there is no adding of ice inside the dewar, such as with manual systems which use air from the envrioment.


In the Norhof LN2 cooling systems Liquid Nitrogen is stored in pressure less Dewars. When LN2 is required a small overpressure is generated by a micro processor controlled heater element in the LN2, and liquid flows out of the system like water from a tap, without spilling, noise, vibrations etc.

http://www.norhof.com/900-stuff/flow5.gif

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I hope this can be made safely, and help improving safety!

No heatsource is needed inside the dewar, evaporation will build pressure naturally within the dewar if it's "capped". Controlling the internal pressure to maintain maybe 5-10 lbs. would provide enough force to "push" the liquid out of the dewar. But you must be extremely careful, whatever you do the internal pressure must not be allowed to increase too much or the dewar will burst.

The pressure release valve in the dewar (witch is already there in every dewar) will prevent the dewar from bursting.

The trick is to find out of much heat is needed to build up a pressure (the site I linked at above says only 100mBAR is needed for their model) witch is large enough to push the LN2 true the tube. The tube also must be wide enough not letting LN2 out when no heat is generated.

I had an discusion about this with unknow_road. A variable heating element is needed (heating wire witch can be controlled).

I don't have enough knowlegde to built this on my own in a safe way, that why I made this tread.
I it works, is more safer then pooring LN2 straight out the dewar in an 1L thermobottle (I don't want to spill over my hands and FLOOR!)

The saftey built into the dewar is a burst valve only, not a means to continually control internal pressures. Oxford does make a system that pumps cryogenic liquids without use of internal pressure but as far as I know it does not use a heat source. Ask K|ngp|n if his hisses when it's just sitting, I'd bet it does and that means it uses internal pressure. Using a heat source would only be required in an un-pressurized dewar.

Quote:

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Originally Posted by Extera

At this moment I need to tilt the dewar sidewides to poor the ln2 in a smaller 1L thermobottle. This is the most dangerous part of working with LN2 imo. Spilling over your hands is easily done.

I hope this can be made safely, and help improving safety!

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Use gloves everytime you deal with LN2! Cryo gloves aren't that price compared to how long you want use your hands (80-100 years)... and then 200-350$ is worth the safety equipment!

At the other side, I'm interessted too in such a ln2 pouring system which works with internal pressure, because it's damn hard to control 3 ln2 pots with normal filling method...

No wires here ;)

The Norhoff device looks alot more complicated to me, sensors, wiring, software, heating element and so on.

http://www.norhof.com/monitor/mon_mode3.gif

Quote:

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Originally Posted by G H Z

No wires here ;)

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That's exactly what I want. I don't think its that hard to make?
The norhoff system was only an example. I want the version without the temp sensor :p: Just easy pouring.

I think they don't heat the LN2 up to build up pressure, but use the natural pressure built up in a closed dewar. By adding a pressure release valve the over-pressure can be released. A valve that keeps the pressure in the tank about 100mbar would do I think.

Heating up the LN2 is only needed when you want to controll the amount of flow (?)

I don't think thats dangerous, letting a dewar burst or explode is going to take waaay much pressure.

I will get LN2 tomorrow, I will ask the people from the LN2 company how much a system like this costs, and if it can be mounted on my dewar.

Quote:

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Originally Posted by G H Z

The saftey built into the dewar is a burst valve only, not a means to continually control internal pressures. Oxford does make a system that pumps cryogenic liquids without use of internal pressure but as far as I know it does not use a heat source. Ask K|ngp|n if his hisses when it's just sitting, I'd bet it does and that means it uses internal pressure. Using a heat source would only be required in an un-pressurized dewar.

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it's not a burst valve! it's an overpressure release valve that closes again.

edit: I just realised that not all dewars are alike but the dewars I've seen often let a little gas out of the overpressure valve. If it is a burst valve where does the pressure get's out for safety?

That depends, usually small dewars use a cap that doesn't fit tightly so the pressure just vents past it. Larger dewars on the other hand are typically pressurized and thus are controlled by valve's. The built in release (for lack of a better term;)) on my dewar does not self close, once it pops it's top you have to manually replace it.

My dewar had the same system Ghz. Its a kind of grease in a small hole in de dewar. (don't know how else to explain) The lid on top of the dewar does not close the dewar tight so no pressure can built up.

The LN2 feed assembly seals the top so it can pressurize the dewar, the key is keeping the pressure low via a bleed valve but still maintain enough pressure for LN2 flow.

Quote:

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Originally Posted by Unknown_road

it's not a burst valve! it's an overpressure release valve that closes again.

edit: I just realised that not all dewars are alike but the dewars I've seen often let a little gas out of the overpressure valve. If it is a burst valve where does the pressure get's out for safety?

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That correct All dewars are not alike, building a home device unless you have thermodynamic and engineering know how can be deadly.

unknown what type insulation on the wire will not crack at those temps? what if there is a direct short? how fast will that build heat and pressure? how are you going to regulate pressure?the flow?how will you sense the liquid level? as the lower the level the less heat required for the same pressure? what type safety's would you uses to make it "failsafe"? How will modifications affect original design? etc.etc. I could keep going.

If you modify the Dewar it will no longer be D.O.T. approved which they must be to transport LN2 on a highway in USA.

LN2 can be very dangerous in enclosed containers, dewars are made for specific purposes and modifying one is not suggested.

The simple way is to make a holder (gimble)that the dewar fits in and has a pivot point about half way down on both sides, mounted in a permanent upright frame.so you can tip the dewar with just one hand at nice slow evenly controlled rate. Be sure no matter what the level in the dewar if you let go it returns to the upright position slowly to prevent splashing or stays put with a friction device . you can attach a holder for your secondary container also(I assume a thermos)on a pivot point also ,so as the Dewar is tipped the thermos will always remain under the spout and perpendicular to the floor

if you don't understand let me know and I'll make a drawing.


here just the basic idea a cement mixer but make a gimble and make the pivot point about half way. this should get you thinking,Just make a smooth operating tipping device. lets see what kind of mechanical engineer your are before you try thermodynamic and electrical

motornot required LOL

The simplest solution is often the best!!!!!

Thanks for your help Wdrzal!

I want to do this al safely, I this tread I have learnt making a LN2 withdraw device myself is unsafe. I will look for a commercial solution today when i'm letting my dewar filled. Till then i'll just be very carefull pooring the LN2 in the thermosbottle.

You solution of making a pivot is nice, but I don't have enough room to store one. Pooring LN2 in my thermos like a beertender would be the best solution.

I will inform about the costs of this, and if this is possible on my dewar. I reather spend some money for safety then loosing my hand or worse.

Very useful thread, and answers many quesitons ive posed around here with no response :)

ln² spilling over your fingers is totaly not a problem, i spilled it onto my pants, hands and never had any problems with it.

the norhof device is nice thing:)

Almost everybody does it the "normal" way ;)

Just got back with 16L LN2, a withdraw device costs 620 euro. Thats just to much for me :P

Oldfashion way, here I come!

My withdrawl device was made from spare parts picked up at a few diff ln2 supply places and some different brass fittings and valve. Without the proper parts designed for this application, it would be pretty difficult to make one.

No heat or electicity at all is needed for the nitrogen to flow. It will flow using the pressure it creates from boiling off in the dewar.
I have a 5PSI relief valve on my pump head which keeps the pressure inside the tank from ever reaching over 5-6 PSI. That is more than enough to keep it flowing once pressurized. There is also a 15PSI pressure guage on there to monitor the pessure in the tank. I also have a vent valve on it that I open up to discharge all the pressure before I remove it from the top of the dewar to fill.
YOU MUST have some sort of a pressure release built into the pump head. Nitrogen is constantly boiling off, even in the best of dewars. That pressure needs to be taken care of and released in a closed system, or it could be a disaster. Put about 12oz of nitrogen into a 2litre pop bottle, and seal the cap and watch what happens to the bottle. Stand back far, because it will blow up bigtime!

Parts you will need to make a sound discharge device are:
Dip tube for withdrawl
Discharge spout
Discharge valve
Rubber plug to match the ID of dewar opening
Pressure relief valve (5PSI works good)
Vent valve
Some type of a large wing nut to tighten and seal it once seated on the dewar
Safety leash--This is very important to have, because there have been times when I have been riding in the car with the dewar shotgun, and I hit some rough road bumps (michigan roads lol) and the nitrogen actually popped the pump head off of the dewar and nitrogen gas and some liquid was spraying everywhere in the car :D , so bad I had to pull over and jump out. Without the safety leash attached the pump head to the dewar, there would have been a fountain of nitrogen going off in the car resulting in some serious injury. Not to mention the pump head can become a missile if it's not leashed on and secured tightly.

My dewar sits in my bench room all day long hissing every once in awhile to bleed off built up pressure. My dogs try to attack it when it starts hissing..they totally hate it :)

Building one of these withdrawl devices should be taken very seriously and every safety measure should be used. One can be purchased online and a few different places for around 500.00 usd if you dont think your up to making one.

thank you kingpin, for this info.

500usd is a lot less then the 620euro they asked for it here in holland.
I made up my mind, I'll bench the old-fashioned way and poor the LN2 in de thermosbottle manualy.

The way you told the system works is exactly how tought it would work, but i'm not going to take the risk om building one myself.

I just picked up 16L, and had the dewar on the passenger seat next to me, the 3km to my house looked like 100 :P the lid was totaly frozen. (the frost is now gone)

when I have some money left in the fewture, I think I will buy a withdrawl device from a store in the USA.

Thanks a lot, i'll hope it will al work out safely, first time ln2 benching in a few hours :P

Good luck Extera, I look forward to seeing how the waus-pot goes, I have one also ;)

http://tweakers.net/ext/f/b5890b0247...34120/full.jpg

I made a hole in it for the dt200 sensor :) filled up with artic ceramiqe

when we transported ln² we used the seatbelt of the car to secure the dewar;)

good luck extera with the ln² cooling

go for it extera, i think its gona be ok with the waus-Tube

i ran my E6600 at 5ghz just as stable like a 24/7 desktop computer, working fawless.

when do we get some results from johnny bravo :D


@
kingpin
thanks for the explanation m8, gona look in too it to make a few of them, pouring in the thermalbottle is ok too but sometimes a little time to fill up during benching controlled at -100 ;)
ive ask't same question in a pm too you , but i think you where to busy playing a moviestar i guess .....lol