5 replies to this topic

Dear All,

I am a new member to the community, and am faced with a bit of dilemna regarding the blanketing calculations for inbreathing in tanks.

API 2000 tabulates the inbreathing values as volumetric flowrates in Normal cubic metres per hour with AIR as the fluid.

In case where we are to use NITROGEN as the in breathing fluid (which is in general more common nowadays with different facilities), the following are the queries that I have:

1. Will it be correct to use the values specified in the API2000 directly? It seems correct as a direct volumetric displacement occurs in case of pump out or thermal inbreathing, and hence it is independent of the type of gas.
2. I came across a few vendor (Fischer/Emerson) information on the sizing of the PCV for tank inbreathing, where a conversion factor has been used to convert the air flowrate in Nm3/hr to nitrogen flowrate in Nm3/hr. The conversion factor used is inversely proportional to the square root of the molecular weights. {That is sqrt(28.9/28)}. What is the basis of having such a conversion factor?

Hoping to get a solution from the forum.

Thanks

Deb

[proinwv]

Deb, I would not be concerned about the difference in density of air vs nitrogen. The difference is small, and their is much more variation in the accuracy of the calculations, the exact transfer rate, and the actual capacity of the supply valve than the differences in these densities.

If it bothers you, then certainly use the numbers for N2 as that is your gas. Of course, if you were using CO2 or another gas with a significantly different gravity, then you will always need to use those numbers.

I hope that this helps.

As to the ratio of sqrt of MW, this comes from the flow equations for gas in which flow rate is inversely proportional to the sqrt of the s.g.

Paul

Thank you Paul,

Yes I would agree with the view that since the molecular weights and hence the densities are pretty close for air and nitrogen, in all practical purposes, the flowrate will be in the same range. And the assumptions and the Basis of API 2000 are very general and broad and will definitely cater to such small variations. So making the density correction for Nitrogen and Air will definitely make the calculations more perfect, but it will be an insignificant development in the overall scheme of things. But where the densities vary by a good amount(methane rich fuel gas, or CO2 for example) this calculations will be worthwhile to do.

But my confusion with the second point still persists. Definitely the gas flow and the densities are related in the way that you have stated. But for the blanketing scenario, more than the flowrate, our deciding criteria, I feel (which may definitely be wrong), is the volume (and not the flowrate) of the gas that needs to be introduced (or displaced, as the case may be). And hence, if say 100 m3 of fluid is pumped out in one hour, an equivalent volume of air, 94 Nm3 as per API2000, needs to fill in the void to maintain the pressure. So, for Nitrogen (or for that matter any gas), the same volume needs to be made up. It may seem that this basis is too simple and may be stupid as well, but my thoughts cannot find a way around this to see the real picture.

Awaiting your reply on the same.

Deb

Edited by Debopam, 20 January 2010 - 10:41 PM.

[fallah]

And hence, if say 100 m3 of fluid is pumped out in one hour, an equivalent volume of air, 94 Nm3 as per API2000, needs to fill in the void to maintain the pressure. So, for Nitrogen (or for that matter any gas), the same volume needs to be made up.

Bringing both equal system volumes (e.g. Air and N2) in Normal condition leading to a little bit different volumes.For flowrates also we have difference when we convert the condition for both into Normal,while we may had same flowrates in real conditions.

Flowrate in Normal conditions to be calculated in order to be reported to vendor for sizing/selection of the proper valve.

[Qalander (Chem)]

Dear Debopam hello/Good Morning,

Somehow I feel as if you are facing difficulty in

1)Having nitrogen volumes estimations worked out for availability on sustainable(continuous availability)basis!

2)How to maintain Nitrogen's inert atmosphere for the blanketed tank(or system)

In all fairness under operating circumstance this boils down to volumetric flow rate needed to compensate any draw-off/pump-out operations and we require to maintain some reservoir(s) to receive back routed in Nitrogen whilst run-down or pumping-in takes place,Conserv and recirculate.

The system/process can be easily understood if you search through forum or visit'Paul's website'.As this will involve some losses of Nitrogen and replenishment needs and definitely conservation systems in-place.

Additionally the perception of system's extreme rigidity is not very accurate as definitely there occurs certain degree of evaporaton(s) of stored liquid and also the gas(nitrogen can expand volumetrically) thus keeping and considering all these factors and allownce for Air vs Nitrogen density etc. difference(s) you may reach to amicable solution to your query.Hope this proves helping.

Best of luck in way forward.

[Dacs]

But my confusion with the second point still persists. Definitely the gas flow and the densities are related in the way that you have stated. But for the blanketing scenario, more than the flowrate, our deciding criteria, I feel (which may definitely be wrong), is the volume (and not the flowrate) of the gas that needs to be introduced (or displaced, as the case may be). And hence, if say 100 m3 of fluid is pumped out in one hour, an equivalent volume of air, 94 Nm3 as per API2000, needs to fill in the void to maintain the pressure. So, for Nitrogen (or for that matter any gas), the same volume needs to be made up. It may seem that this basis is too simple and may be stupid as well, but my thoughts cannot find a way around this to see the real picture.

Awaiting your reply on the same.

Deb

From my understanding (anyone correct me if I'm wrong), liquids too have vapor pressure. They will release some amount of vapor to the vapor space, hence the apparent disparity of volume displacement. If you have a copy of API 2000, try to read the basis behind the calculation. IIRC they assumed Hexane as the storage medium.

In my experience, I don't bother much on the disparity between air and N2, since air itself is mostly nitrogen.