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Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/20/2016 11:39 AM

We have a N2 tank with a capacity to hold ~3000kg of liq.N2. We have a pressure regulator @4bar downstream ..the pipelines are such designed that via 1st channel we can direct gaseous (internal line is above liq.N2 level) N2 downstream.

If needed (in winters internal pressure is quiet low) 2nd channel directs liq.N2 through a bunch of external coils (mini-evaporator, for instance), this internal circulation of N2 back & fro from tank<>coils rises its internal pressure & we can thus revert to using 1st channel. I don't have a pic for now :(

Problem is that even in winters nowadays (~5-8celsius) when previously we have to raise internal pressures via 2nd channel; now the tank is suffering abnormally high pressures of +8bar, this pressure in summer (40-45celsius) is plausible but not in winters! The aftermath is that we have to intentionally bleed liq.N2 downstream to reduce such pressures which thereby increases our N2 consumption.

I am aware that tank is double-walled vacuum pulled, like a thermo-flask, however, I was also told recently that company intentionally introduces some kind of filler b/w the walls so as to maintain desired internal pressure (adequate heat transfer=adequate phase change) which should be higher than than the regulator setting of 4bar or otherwise it will act as check valve. So my first question ...does anybody know about such fillers, what are they, any material I can read?

Provided that I have replaced valve (in 2nd channel) in b/w tank & external coils which I guessed might be passing & unknowingly increased internal pressures ..but its checked & OK. My 2nd query what may be the problem with the tank internally? Is there any physical symptom I should check?

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#1

Re: Liquid Nitrogen Tank repeatedly approaching HIGH internal pressure

01/20/2016 11:46 AM

Can you monitor the vacuum pulled pressure> If that has increased the insulation will conduct more heat = which fits your problem.Is there a pump out port and valve where you can pump down to the manufacturers pressure?

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#2

Re: Liquid Nitrogen Tank repeatedly approaching HIGH internal pressure

01/20/2016 12:14 PM

First check the pressure build-up valves to ensure they are not leaking, if you see frost on them all the time it is a good indicator of a leak. Keep in mind that the teflon seals will look good at greater than cryogenic temperatures but will show a significant gap at a crack when cryogenic.

All the tanks I dealt with in the past had vermiculite insulation. There should be a port where you can determine the vacuum level, but keep in mind when it is cold you will get a false low pressure.

Every few years you should purge and vacuum your cryogenic tanks. When the purge heats up the tank you will get a better vacuum draw.

The standard we followed stated that the periodic inspections would include a purge and vacuum (every 5 years I think?) or if the charts showed product loss.

Drew K

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#3

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/20/2016 6:43 PM

The first symptom to check is that, "I was also told recently that company intentionally introduces some kind of filler b/w the walls so as to maintain desired internal pressure".

"You were told" is a very poor statement for someone to make, without any validation.

I was told once that I was a very handsome man. I never got the money back that I loaned her.

Who is in charge here?

Also, what does "b/w the walls mean?

Have you Googled today?

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#7
In reply to #3

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 5:36 AM

They call this 'super insulation' and it consists of multiple layers of aluminized mylar wound around a core with a thin layer of fluffy insulator(usually cab-o-sil but fly ash is cheaper and almost as good. This is 1-2 inches thick. A hard vacuum is drawn on this and it is heat soaked under vacuum until a stable vacuum persists = sealed.

The theory being there is a minimal conductive path through the cabosil and the reflective mylar produces a minimal radiative path. It is a Dewar flask with added superinsulation.

The remanent heat leakage is via conduction through the entry port(s)

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#9
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 9:09 AM

Your criticism is entirely valid however I took it for granted because supplier representative told me! Now I have discussed this in detail and came to know that filler is indeed added in the annular (filler is glass wool based) so to fill the void thereby decreasing time to pull close to perfect vacuum.

As noted in my previous comments the PURPOSE of filler is wrongly described neither in other comments this purpose is clearly defined?

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#10
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 9:10 AM

.. Comment posted twice ..

this website must be mobile optimized &#55357;&#56841;

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#11
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 9:21 AM

Your criticism is entirely valid however I took it for granted because supplier representative told me! Now I have discussed this in detail and came to know that filler is indeed added in the annular (filler is glass wool based) so to fill the void thereby decreasing time to pull close to perfect vacuum.

As noted in my previous comments the PURPOSE of filler is wrongly described neither in other comments this purpose is clearly defined?

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#4

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 12:40 AM

Hard to say more than Drew K already has, that's what I was going to say! It's either a passing valve to the pressure build coil or lost vacuum. You will need to check your vacuum but a good rule of thumb is to feel the dome ends, are they cold or have ice build up, if so you have lost vacuum. The stainless inner tank is nearest and is supported at the dome ends so cold can conduct/convect to the outside.

If so your going to have to drain all the LN2, hot air purge the inner tank and pull a vacuum on the annulus. Speak to your LN2 supplier, they should be able to do this as a service.

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#5

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 1:54 AM

Any changes to the reflectivity of the outer tank - mold, rust, dirt, or any combination of the three?

Any new sources of heat - process exhaust, A/C impinging on tank?

Internal pressure should be what ever you set it at LN2 is -197 deg Celsius - plenty of temperature differential,

Other commenters have made the same suggestions I would have. Only thing I would add is Pressure Build regulator might not be set properly (or freezing in winter).

As for adding fill - don't go there, material settles and insulating quality lost.

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#6

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 4:37 AM

Sounds very much like the vacuum has gone soft, as noted above it should be possible to connect a monitor to check it IF the manufacturer has left a suitable valve in place, don't uncap anything if the tank is rented, call the supplier in. Our old VIE kept going soft, so the owners engineers pumped it down every 6 months or so, their net profit from the extra boil off loss and the cost of installing a new VIE to them and us (down-time and access issues) meant it went on for years like that...

In Europe PFA (pulverised fuel ash) from coal fired power stations is the usual void filler between the inner and outer shells these days.

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#8

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 7:21 AM

Put simply. The N2 is warming up.

Assuming the storage system used to work OK, then it is an increase in ambient temperature, or loss of insulation, or failure of the cooling device, or a combination of all three.

Many causes and remedies have already been given.

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#12

Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/21/2016 9:51 AM

What kind of level indicators do you have on this tank? In the past I have seen several problems with the plumbing on the pressure gauge and level gauge assemblies.

I managed the bulk cryogenic storage of liquid oxygen and nitrogen for aviation and am familiar with the the systems and procedures for storage and dispensing. Let me know if you have any other questions, I would be glad to share what I know.

Drew K

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#13
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/22/2016 5:36 AM

Instrumentation is differential pressure indirect dials both for pressure/level measurements. Insulation is expanded perlite. Tank MOC is SS. Internal wall is 3mm thick while external 20mm. I think after having valuable inputs here ..a Pirani gauge is needed to have a vacuum check? I happened to find that port where it should be measured

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#14
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/22/2016 8:13 AM

On my 2000 gallon tanks I think the port was in the top left of the box. I would remove a cover and connect an electronic device. At the time I was not concerned enough with what it was called to remember now, but they said it measured conductivity or potential or temperature across a gizmo.

Those differential pressure gauges do not like even minor leaks in the tubing. We had to replace all of it on one tank to get it to read consistently. We used soft copper tubing to make the tubing assembly with flared fittings tightened perfectly. I think I read they work with a glass cylinder and don't like being dropped (what gauge would?). When we were done with the overhaul everyone was amazed that the gauges worked correctly for the first time in years, even with the tank vented and no pressure on it.

Drew K

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#15
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/23/2016 12:49 AM

I used to witness periodic pressure tests of tanks, and annular vacuum was one parameter measured to verify that any pressure drop was not due to leakage from internal tank into annular space.

The tank sizes ranged from 300 to 13000-G, and the oldest that I witnessed a test on was built in 1960 (according to the ASME Data Plate), although most of the tanks were built between 1970 and 2000.

Some were new, and some had been refurbished, the work usually consisting of new piping and fittings and a new paint job after sandblasting of the outer vessel to remove rusty and any other surface debris.

Vacuum was measured using a meter that plugged into a fitting having pins that resembled those on the vacuum tubes found in earlier radios, TV's and other electronic devices.

Both this fitting and a valve for attaching a vacuum pump were located on the dished head of the outer vessel, bottom for vertical tanks.

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#16
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Re: Liquid Nitrogen Tank Repeatedly Approaching HIGH Internal Pressure

01/25/2016 8:12 AM

Yes, that sounds familiar. I know I plugged that thing in so many times because we had a faulty reader and took way too long to figure out where the fault was.

Drew K

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