[labnetwork] LN2 vs N2 generation on site

Tue Mar 31 14:00:17 EDT 2015

All,
  To add some information on liquid nitrogen boil-off / loss:

In 2007 the UC Berkeley Microlab did a survey of our nitrogen usage in
order to confirm the accuracy of our metrology before moving to a recharge
model for nitrogen use in labs not directly associated with the Microlab.
>From this analysis we discovered an 8% discrepancy between the amount of
nitrogen we were consuming versus the amount of nitrogen we were being
delivered.

This number is from a 9000 gallon tank, with purely gaseous use, that was
being drawn on at the time at about 1800 L/m.

Danny Pestal

Facilities Manager
Marvell NanoLab
University of California, Berkeley
510-809-8600

On Tue, Mar 31, 2015 at 6:26 AM, John Shott <shott at stanford.edu> wrote:

>  Rick:
>
> In an earlier message, you had commented about the large volume difference
> between what you use and what gets delivered.  To me, something seems
> strange if you are getting 2.6M cuft per month but are using only a bit
> more than half of that.
>
> I believe that large LN2 tanks are noticeably more efficient than that for
> gaseous usage and would expect that the amount that you use to be at least
> 90% of what is delivered.
>
> Here is my understanding ... and I've tried to check these numbers out
> with our LN2 supplier:
>
> A big tank that is just sitting there (that is with little or no usage)
> would lose something like 0.5-1.0% per day due to heat transfer and gas
> that ends up getting vented but not used.  However, for any system that is
> using a significant volume of product per day ... and yours clearly is ...
> I would expect that venting losses would be very close to zero.  When you
> are using a significant volume of product, that would tend to reduce the
> pressure in the tank, the pressure building circuit kicks in ... but that
> doesn't result in any loss because the liquid that is vaporized to build
> pressure goes into your tank.
>
> There is certainly a significant loss anytime that you have a delivery ...
> and, if your vendor is like ours, you pay for all of that nitrogen.  I
> believe that the largest fraction of that consumption is the nitrogen that
> is "depressurized" from the tanker following your fill to reduce the tanker
> pressure from above your tank pressure down to the legal limit for them to
> drive again on the highways which is, I think, 10 PSIG.  The nitrogen
> consumed during your fill that never goes into your tank including pre- and
> post-fill purges and this tanker "blow down" is significant.  Others may
> have a better number than I, but I count on losing at least 20,000 cuft of
> gas per delivery.  On average, we get 8 deliveries per month of 600,000
> cuft per delivery for a total delivered volume of about 4.8M cuft.  So,
> losing 20,000 cuft per delivery is only a 3% loss.  Of course, with a
> smaller tank, the per-fill loss percentage goes up assuming that they are
> still filling with a 53' trailer.
>
> Note: you will have higher losses if you are using liquid withdrawal to
> any degree because there is a fair volume of nitrogen that is "lost" as
> gaseous nitrogen for every liquid gallon that you extract.  If your tank
> pressure is set up for gaseous usage with a tank pressure of 120-150 PSIG,
> then any liquid usage is probably not terribly efficient because of flash
> losses, etc.  ... basically the gas that is vaporized in the process of
> getting things cooled down so that you get good liquid extraction.  I don't
> have much experience in that area so can't really offer a precise number
> for loss during liquid withdrawal but that is likely to be significantly
> lower than the 90% number that I think you should expect for gaseous
> withdrawal.  However, if your system is predominantly used as a source of
> gaseous nitrogen, I would expect that your overall efficiency should be
> closer to 90%.
>
> How well are your flow meters calibrated?  Do you have flow meters on all
> possible usage?  Are your flow meters fully temperature compensated or are
> you measuring gas that is noticeably colder than room temperature?
> Anecdotally, we just sent our two main flow meters out for calibration:
> when they came back, they are now giving readings that indicate that we are
> consuming more nitrogen than our vendor is supplying.  In short, I'm not
> sure that I trust the absolute readings that I get off these big flow
> meters for an absolute reading.  In short, if your tank is used
> predominantly for gaseous nitrogen, I would expect that your overall tank
> efficiency should be closer to 90% and would look carefully at whether I
> was monitoring all usage and whether my usage flow meters were giving me
> believable results.  If your liquid withdrawal and usage are significant
> everything that I have said may be off base because I expect that liquid
> withdrawal is an inherently lower efficiency process.
>
> I trust that my colleagues will correct me if they believe that I've badly
> misstated things here.
>
> Good luck,
>
> John
>
>
> On 3/27/2015 11:08 AM, Morrison, Richard H., Jr. wrote:
>
> Thanks for your comments so far and I looked forward to more discussion.
> BTW after fees and surcharges I pay $1.23 per 100cuft and I use 1.45million
> cuft per month (data from flow meters). The company delivers the equivalent
> of 2.6million cuft so I lose to evaporation and cooling 1million cuft per
> month, does this sound right or do I have a big leak some place?
>
>
>
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