Bottled nitrogen cooling due to JT effect 3

[PetroBob]

We are letting nitrogen down from high pressure cylinders at 4250 psig / 29,300 kPag, to just above atmospheric. The nitrogen will be used to purge piping and equipment. I understand that the nitrogen will get very cold due to JT cooling.

HYSYS gives very alarming temperatures; eg if bottles are very cold on midwinter day at -22degF / -30degC, then HYSYS predicts the temp of gas once pressure is reduced to atmospheric is -133degF / -92degC. If nitrogen bottles are +50F/+10degC then HYSYS gives -34F/-37C outlet, for which low temp carbon steel is adequate.

I understand that we can take some credit for heat retaining in the metal of cylinders and piping (per previous Eng-tips thread

http://www.eng-tips.com/viewthread.cfm?qid=78680).

However I believe we may still need some heating system so as not to have a metal embrittlement issue.

What solutions are most commonly used to prevent excessive cooling of pressurized gas bottles and downstream piping and equipment? Eg heating jacket on bottle, heat tracing of discharge piping, extra low temp piping?

 

[zdas04]

What kind of volumes are you using for these purges? If it is small volumes and/or short duration, then residual heat will keep from excessively low temps.

For big volumes (e.g., nitrogen frac, pneumatic testing with nitrogen, etc.) then we typically start with liquid nitrogen and heat it up to required pressure. Big volumes with ganged gas bottles are just too high a risk.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

 

[PetroBob]

Thanks David. It's fairly small flow rate - purging natural gas out of 100 m3 of equipment + piping, in say 2 to 3 hours. It sounds as though residual heat will mitigate the problem provided bottles are room temp. Should we be concerned if gas bottles are located outdoors in winter, where ambient minimum temp is -30 to -35degC? Ie do we need gas bottle heating sytem and/or heat tracing to warm the gas to room temp upstream of the regulator?

And another question - we are having trouble sourcing a backup (ie secondary) regulator for such high pressures. Is this likely to require several regulators in series, or a control valve to let the pressure down? The client doesn't want to reply on nitrogen bottle regulator and PSV, and wants a secondary regulator/PSV system downstream of the gas bottle regulator as backup.

 

[waqasmanzoor]

PetroBob

Yes dear. As you are reducing pressure from fairly high value, so it would be good to use 2 or more open loop pressure regulators in series.

What is the size of nitrogen pipe?

I think, you should provide heat tracing to the nitrogen gas cylinders, so that JT effect wouldn't "brittle down" your Carbon Steel piping.

I have seen steel breaking like a biscuit at low temperatures. It was awful to observe.

Regards

 

[itdepends]

Use the HYSYS data to determine the temperature of the nitrogen leaving the bottles and then used the mass of steel in your pipe, it's temperature and the nitrogen/steel heat capacity to determine what the final temperature will be. I wouldn't expect the nitrogen to cool the steel down a lot unless you're purging many, many pipe volumes.

The coldest section will be the small diameter tubing you use to connect the cylinders to your main gas line with.


As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[xeng]

A) POLITELY INFORM THE BOTTLES ABOUT THE VALUES CALCULATED BY YOUR SIMULATOR

B) MAKE SURE THE BOTTLES INTEND TO WORK (AS USUALLY DO IN MOST PLANTS) EVEN AGAINST THE OPINION OF AN EXPENSIVE SOFTWARE...

C) IN CASE OF DOUBTS ASK AN EXPERT HERE AND GET KNOWLEDGE ABOUT BASIC THERMODYNAMICS AND HEAT TRANSFER MECHANISM...

 

[gaswell]

I must agree with xeng,
you do not need a simulator to calculate dT for nitrogen
but basic knowledge of thermodynamics and a table of enthalpies or a mollier chart (available in most textbooks).
In addition you must take in account heat transfer mechanisms,
may be you do not have a sufficient knowledge of thernodynamics
but in case of doubts better to ask an expert to do the work
instead to run a simulator without being able to interpret the results.

 

[LittleInch]

A few things about this post you might like to consider.

1) Don't forget that any system under about 5C inside a plant will produce a lot of condensation and under 0C will rapidly start to grow ice, which for small bore pipes can get to dangerous levels quite quickly.

2) If your gas used for purging is very cold this can result in it sinking below the gas it is trying to purge, especially if the purging actual flow rate is quite small, which is what it sounds like.

3) Predicted temperatures from simulations rarely match reality, but do show you have potential for some pretty low temperatures.

4) Some sort of heater would seem like a good idea, maybe in between your first stage regulator and the final one.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gaswell]

Cylinders are utilized daily in thousands of industries,

http://www.lindepremiumproducts.com/internet.lg.lsg.usa/en/images/psa_gaseous_nitrogen892_21857.pdf

one can ask directly the supplier,
also it seems strange (to me) the idea to run a simulator to calculate JT without keeping in account total mass & heat balance in the system as correctly noted by zdas4.

 

[PetroBob]

Waqasmanzoor, itdepends and LittleInch, many thanks for the useful replies. Xeng & gaswell, I agree HYSYS alone is not sufficient for this thermodynamic calculation; however it was a useful tool to provide a quick preliminary JT check before calculating heat transferred from the metal. The reason I posted on eng-tips was to gather information regarding typical engineering solutions used to mitigate low temperature effects.

 

[MortenA]

@Gaswell: Out of curiosity - could you give me a reference for a N2 HX diagram?

Best regards, Morten

 

[MortenA]

Found something (

http://www.nist.gov/data/PDFfiles/jpcrd39.pdf)

its a table but that works too. And it confirms the HYSYS calculations (i would be surpriced if they didnt i would think that HYSYS uses a table for this type of components). Start out by -30ºC and you will end up around -90ºC.

So how does the millions of bottles around the world work? Well you CANT discount the heat absorption - and also sometimes they dont. If you need large quatities you will need a system that adds heat (like using liquid N2 and having an evaporator).

Best regards, Morten

 

[gaswell]

Morten,
you may google for T-S or equivalent diagrams of nitrogen,
there is even a specific page in wikipedia with
a step by step example of how to evaluate JT effect
with a T-S diagram

http://en.wikipedia.org/wiki/Joule–Thomson_effect

if you wish to simulate the process (solving a
H-P flash operation) with the help of a EOS such
as Peng Robinson or Soave Redlick Kwong or Lee Kesler
then results will be quite equivalent,
to compare the different models I solved the problem
with Prode Properties
(copy available at

http://www.prode.com)

results for
Tin -30C Pin 4250 Psig Pout 0 Psig
calculated Tout with different models

Peng Robinson -91.95 C
Soave Redlich Kwong -85.47 C
Lee Kesler (plocker) -88.377 C

as you see the different models predict similar results,
the most accurate being Lee Kesler
which shows an estimated error of less than 1 K
I have utilized Prode because one can download the
sofware but I expect to obtain similar results
from any simulator such as PROII, Promax, Chemcad, Winsim...
I have not included here the results from extended
EOS models available in Prode Properties cause those
are specific for that software...

Anyway my point in this thread is that the exercise
is unfruitful for evaluating the
temperature of cylinder cause the contribute
of heat transfer is not considered...

you can show how a simulator works
but you do not need a simulator,
you can get similar results from tables or charts...

 

[sshep]

Hey PetroBob,

Google "gas cylinder heater" and check out the pictures of these electric blanket style heaters which go around the cylinder.

I think this is probably the answer for you. The number of stages of letdown will have no theoretical effect. While heating the line (and regulator) is possible, the blanket style heater on the cylinder is an off the shelf solution.

Recently we did this on bottled start-up seal purge for a propane compressor (struggled with low supply pressure during cold), problem solved.

Best wishes,
sshep

 

[MortenA]

@gaswell,

It would suit you to read my post before you start assuming a lot of things - you wrote that a mollier diagram (HX or TS) for N2 is easily foudn - i just couldt find one easy (air is easy but i just couldt find one for N2 and just asked for my future reference. Then i found a table and reached the same temperature for gas as HYSYS found. And in my post i agreed that for such a sysmet you cant ignor heat tranfer - but you cant just assume that it will save your arse every time either imo.

So again: For N2 - can you give me a reference (apart from the NIST table that i found myself? I would check against a table or chart myself for a species like N2 because e.g. SRK is nor developed with N2 in mind. A table is more precise (and a chart is easier to use). The print you included is just something from a piece of software - with to reference to source.

I actually sit in a technical committee i GPA that sponsors research in thermodynamics so i do have some fundamental understanding of the subject.

Bets regards, Morten

 

[MortenA]

bUmmer, just notised the the wikipedia refernce actually include a chart for N2 specificaaly (from NIST refprop). Sorry about that little rant. Maybe i should even try to convice my company to by refprop.

 

[MortenA]

OK, learned a little mroe (happens now and then) - refprop is actually also "just" calculations". Silly me i thought it was based on experimental values.

 

[MortenA]

And it does give a differen answer - or maybe its me?

 

[MortenA]

missed the attachment - try again

 

[MortenA]

OK, a simple "collumn error" when i read the table the first til - actually the table says around -43 deg C (i stated with 300 barg and i just chose the nearest no.

SO: HYSYS, etc is actually off (if im right the 2. time. Would somebody (for my peace of mind) check the referenced table from NIST and see if they can duplicate?