Water in a cryogenic vessel
Water in a cryogenic vessel
(OP)
Hello, I'm a long time reader, first time poster. Thanks everyone for taking time to expand the knowledge of the human race.
I have a problem with a small scale LNG dispensing system. One part of the process allows small amounts of moisture (~1cc per event, 18 times per day) to be injected back into the system and we are now faced with moisture occasionally blocking a 10 micron filter screen in the pump discharge line. I see more blockages after we run the storage vessel level low and then fill the vessel close to capacity (20K Gal).
Does anybody know how water behaves while in a cryogen under pressure (90psi)? Will it sink, float or stay mixed into the LNG? I tested spraying water onto LN2 in an open vessel and it seemed to form chains but because the liquid was boiling it got very churned up. It was hard to see if it sank or floated.
I am considering circulating the whole volume of the vessel through the 10 micron strainer but this will take a long time. I can recirculate to either the top of bottom of the vessel. Any idea which would be better? Should I use a finer strainer?
Another option I considered was to empty the vessel and either flush it with more LNG or heat the inner vessel with warm N2 until the dew point is acceptable. The flush option is obviously preferable to save chilling the vessel again but I am worried that moisture may stick to the walls of the vessel and this won't solve the problem.
I appreciate any assistance you can give.
I have a problem with a small scale LNG dispensing system. One part of the process allows small amounts of moisture (~1cc per event, 18 times per day) to be injected back into the system and we are now faced with moisture occasionally blocking a 10 micron filter screen in the pump discharge line. I see more blockages after we run the storage vessel level low and then fill the vessel close to capacity (20K Gal).
Does anybody know how water behaves while in a cryogen under pressure (90psi)? Will it sink, float or stay mixed into the LNG? I tested spraying water onto LN2 in an open vessel and it seemed to form chains but because the liquid was boiling it got very churned up. It was hard to see if it sank or floated.
I am considering circulating the whole volume of the vessel through the 10 micron strainer but this will take a long time. I can recirculate to either the top of bottom of the vessel. Any idea which would be better? Should I use a finer strainer?
Another option I considered was to empty the vessel and either flush it with more LNG or heat the inner vessel with warm N2 until the dew point is acceptable. The flush option is obviously preferable to save chilling the vessel again but I am worried that moisture may stick to the walls of the vessel and this won't solve the problem.
I appreciate any assistance you can give.





RE: Water in a cryogenic vessel
Regarding how it behaves, I've seen solids block fairly large diameter pipes (maybe up to 1") but I would think the solids are generally going to take the same form as they get frozen in. That is, if water (or liquid air) lies in a small bowl, it'll freeze that way. If water gets in as a droplet, it'll freeze as a droplet. If it gets in as vapor or a fine mist, it'll freeze more like snow. In general, I wouldn't think the frozen particles would have any tendency to freeze together after they're already frozen. Kinda like very cold, dry snow doesn't make a good snow ball.
RE: Water in a cryogenic vessel
I agree that as the moisture starts as frost it probably stays as frost in the system. I just don't know how it will sit in the liquid when the vessel is idle. Will it sink or float? I'm assuming it must float because the blockages mainly occur after we've filled the vessel. If the water contaminant sunk the pump suction would pick it up constantly, the suction nozzle is at the very bottom of the vessel. The filling must churn up the moisture and keep it in suspension until the the liquid stops boiling in the vessel.
Do you have an opinion of the methods I've listed on how to remove the moisture for the vessel?
RE: Water in a cryogenic vessel
Regarding whether water will sink or float, the density LNG is much lower than that of ice, so the ice will sink. The only way I see of it staying in suspension is if the ice particles are relatively small and they can be 'churned up' as you say.
If you absolutely had to remove the water from the tank, the only way I know of is to empty it, warm to atmosopheric temperature, then heat it by circulating hot, dry nitrogen through.
RE: Water in a cryogenic vessel
I think I will need to look at devising a water trap that will allow the moisture to drop away from the filters when warm.
Thanks again for your input this problem has stumped everyone I know.
RE: Water in a cryogenic vessel
RE: Water in a cryogenic vessel
and any idea how I can get it out of the solution?
RE: Water in a cryogenic vessel
After all you do want to catch this stuff.
I would think about two stages of filtering. You could then trap more without restricting flow as much.
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Plymouth Tube
RE: Water in a cryogenic vessel
What do you know about filter media for cryogenic fluid? I'm thinking of recirculating all the fluid in the vessel through a high flow/capacity filter. I'll have to get a filter system fabricated and knowledge of cryo fluids is thin on the ground in my area. It would be nice to have some sort of coalescing filter to absorb the moisture but I'm not sure where to start.
RE: Water in a cryogenic vessel
If all the liquid is circulated through a piping system that filters out the water/ice you can expect to get a lot of heat added to the tank as well. That will cause the pressure to rise and product will most likely end up being vented. So it might be an option for a tank that's heavily used (so tank pressure can be kept down), but if you're on the edge of venting product now, it'll be much worse when you add all that heat to it. You're essentially creating a PB (pressure build) loop.
I don't know enough about coalescing filters to know if you could use one to separate solids but I'm thinking the filter is essentially going to be blocking solids, and those solids aren't likely to 'soak' the filter element. As soon as flow stops, I'd think the solids would simply drop to the bottom of the filter cartridge if they were big enough not to get stuck in the wire. Once they drop off the filter, they would be in the bottom of the filter bowl. At that point, once it warms up and turns to liquid, a port on the bottom of the filter bowl could be opened to a vent stack and the liquids blown out.
Imagine a T style filter with the bowl down as it normally would and the flow going from the OD of the filter element to the ID and then out. The solids are going to drop to the bottom of the bowl once flow stops. Now if there's a gas flow from the filter outlet which goes from the ID of the filter to the OD and then out the bottom of the bowl, any water trapped as liquid inside the filter element will be blown out as well.
You could test this out by soaking a filter element in water then blowing it out with compressed air, the same way you'd do with natural gas.
RE: Water in a cryogenic vessel
We use Norman filters in our centrifugal pumps so I might inquire about a 2 micron filter element through them. The tee filter shouldn't be a problem and I think we could rig up a reverse N2 purge to run when the system is idle. The 10 micron filter in use currently does stay soaked when warm so I will need this to force the water out or we will have to constantly replace the filter with a dry unit.
Does anyone now if ambient dry N2 will actually carry moisture or it does need to be warmed to be effective?
RE: Water in a cryogenic vessel
The dry nitrogen will work, but how about hot nitrogen? Insulate the filter bowl and run the nitrogen through a heating unit to bring it up to about 250 F. Flow rate doesn't need to be very high, but warming it would help considerably and it isn't very expensive. You might even just wrap the nitrogen supply line with a heating tape, then insulate that.
RE: Water in a cryogenic vessel
Deriming an LNG plant(defrosting or deicing). Deriming (defrosting or deicing) means the removal by heat and evaporation, sublimation or solution of accumulated matter, such as water and CO2 from the low temperature process equipment.
RE: Water in a cryogenic vessel