Nitrogen in LPG tank

[RFZC]

We have taken LPG's sample (vapor) from the top of the tank and we have found a significant value of nitrogen.


1 The presence of nitrogen can affect the conditions of storage (pressure/temperature)?
2 Do we have to purge it totally?
3 If yes, which is the best method?
4 Which is the best method to detect nitrogen?

Thanks for your comment and help

 

[danthemeterman]

How much Nitrogen? Are you saying NH3? We recently had contamination from an oil refinery. all we could discern from NFPA ? was anything under 30PPM was acceptable, although the contamination was causing greenig of the brass connectors where theerre was venting. I hope this helps.

 

[krecian]

Depending on local safety regulations, sometimes N2 is added deliberately to dilute the calorific value of the gas. This is for the gas transported through pipes. - To limit the calorific value within soecified range. Find out your local norms and see if it so.

KRECian

 

[redford]

I'm assuming this is an LPG sphere RFZC:

1) N2 will affect the operating pressure of the sphere as a function of the liquid level (this assumes you have a free N2 "bubble" in the sphere). Caution needs to be exercised in that depending how much N2 is in the sphere, could result in overpressure as the level compresses the N2 increasing the pressure in the sphere. Spheres will have some form of PRV, either to atmosphere or flare system. Atmospheric releases of LPG are not enjoyable. Until you remove the N2 from the sphere you could calculate the maximum operating level to ensure this does not occur (function of sphere temperature, LPG composition).

2) Depending on your specific circumstances, N2 in the sphere may or may not be required. I have seen N2 purposely injected into LPG spheres in very cold climates where insufficient NPSH is available for LPG pumps. Care again has to be exercised as once the sphere contents warm up the pressure in the sphere increases (ie. TVP of contents). So depending on your circumstances will dictate if the N2 has to be totally removed. N2 will, over time, be removed with the LPG exiting the sphere (ie. N2 has some solubility in LPG's) as long as the N2 source has been eliminated.

3)purging can be done 2 ways. Assuming the N2 source has been eliminated N2 will be removed with the LPG over time. If you can't wait that long then venting off the sphere (to flare preferably) is an alternate route. Again caution in undertaking this as you need to watch for frosting off the line and the downstream piping especially if into a flare system. Some locations have "portable" flare which can be utilised for this purpose.

4) detecting N2. Not sure what you exactly mean. GC method is best bet for sphere sample. If you mean "how do we detect if we get N2 into our sphere?". Probably too costly to install an online GC for this specific purpose. You can infer N2 (or other non condensibles) via pressure measurement on the sphere. Installing a PT with a subsequent high pressure alarm should alert you soon enough.