6 replies to this topic

[benabed]

Dear freinds,
After each inspection turnaround we are purging our LPG storage bullets with nitrogen before putting them online. Bullet Capacity is 240m3,p=14.5barg&T°=ambient.
I want to ask you if it is possible to avoid such purging. I have to mention that these bullets are blanketed with natural gas.
Your feedback will be very helpfull to me.
Thanks.

regards;
BENABED

[Dacs]

While I'm not experience with LPG storage, doing away with N2 purging will result in having LPG vaporized (to introduce pressure) in your vessel that will surely result in low temperatures.

Besides, N2 blanketing before LPG introduction ensures that you'd purge any oxygen from your vessel.

[kkala]

1. Natural gas blanketing does not comply with my understanding, since a bullet is a pressurised vessel (normally closed when no LPG is transferred). Liquid LPG is under pressure of its vapors above it. Can you look into this point and clarify? Natural gas might be introduced to increase LPG pressure, thus facilitate transfer out of the bullet (just one case, there may be others).
2. You purge vessels or piping with inert gas (e.g. N2) to remove remaining O2 (as Dacs has pointed out), if these were previously opened to atmospheric air. Only then can you introduce flammable LPG. Evidently this safety procedure cannot be avoided.
3. First remark by Dacs is also noted. Continuously introducing e.g. liquid propane to an LPG vessel, containg only N2 at 0 Barg, will locally result at first moment in -42 oC (normal boiling point of C3H8), gradually increasing. To my knowledge, minimum design temperature of a modern propane vessel is specified -42 oC at 0 Barg, but there is evidence that in the past some vessels had to be pressurized with N2 to higher than 0 Barg before filling, to avoid low temperatures (upgrading seminar at work, 1984).
Note: Higher than -42 oC minimum temperature could be supported, similarly to emergency depressurization temperature in pipes discussed in this forum. But this does not concern the query (besides I am not aware of it).

[benabed]

Dacs & Kkala

Thank you for your replies. concerning the purpose of blanketing with Natural gas this is done in order to minimise LPG LOSSES TO FLARE so we we cannot rely on the LPG vapor pressure to pressurise the bullets,
The point i want to make is that if we pessurise the vessel with natural gas to 14,5 bargs the REMAINING oxygen concentration in the vessel will be very LOW about 1,2% and this will not represent a safety concern in ordrer to put the bullet online, I need a point of view of someones who are familiar with operations like these.

regards
benabed

[kkala]

After the recent explanations, some more questions have been arisen, which Benabel is kindly requested to consider.
-During inspection the bullet is opened to ambient air. Then it is closed and filled with natural gas. But before this, the vessel is assumed to be purged with e.g. nitrogen, to reduce its O2 concentration. Is it so?
-After bullet gets pressurized to 14.5 barg with natural gas, what happens? Nitrogen is introduced to purge natural gas and O2?
-Where the 1.2% O2 comes from? Probably at 0 barg pressure, during filling with natural gas, the bullet contains roughly 1.2*15.5=18.6% O2? (Natural Gas introduction to the vessel, while containing air?).

I believe clarifications on the filling procedure (with corrections on the above with clarifications) can be useful for somebody able to advise. - Best regards.

Edited by kkala, 14 August 2012 - 01:28 PM.

[benabed]

kkala
Actualy each bullet is purged with nitrogen for 03 times before it is filled with natural gas to 14.5 bargs.This nitrogen is supplied by truck as liquid.But if we pressurise the bullet with NG without nitrogen purging the O2 concentration in the bullet will be around 1.2% VOL.SO my goal is to put the bullet online without purging because 1.2% O2 is very low and does not represent a safety concerne. Hope my ideas is clear.

benabed

[kkala]

Picture is rather clear now. Serious drawback of pressurizing the bullet without prior N2 purging would be that O2 concentration during natural gas filling would roughly vary from 21% to estimated 1.2%. Explosive limits for CH4 in air are 5-15% (for C2H6 3-12.4%), which will be created soon after start of filling and cause risk of explosion for some time during filling.
Having felt the careful N2 purging of gas hydrocarbon lines before use, one can conclude that introduction of natural gas into the bullet without previous purging would not be permitted for safety reasons.
Besides I have not heard of liquid nitrogen supply for purging, but my knowledge on the subject is limited. In local refineries liquid nitrogen is evaporated before used for purging.
Others' advice on the topic would be also welcomed.

Note: For explosive limits in air look at http://www.engineeri...mits-d_423.html '> http://www.engineeri...mits-d_423.html .

Edited by kkala, 14 August 2012 - 03:28 PM.