Interchangeability between SNG and NG

[sizgalil]

Hi guys.

Hope someone can help on the following issue.
Our client wishes to backup the NG with SNG (LPG and Air) that supplied to a mono-fuel burner.
I calculated the Wobbe Index of the NG and of the LPG and determined the correct LPG-Air ratio required to attain an identical Wobbe Index of the SNG to the NG.
The burner currently consumes 100 Sm3/hr NG.
Would it be correct to now convert this volumetric flow to the calorific flow per Sm3 (using the LHV of NG) and then convert the result to the volumetric flow of SNG (using the LHV of the SNG)?

Thanks.

 

[georgeverghese]

Procedure sounds correct, but you may also have to modify the nozzle - orifice on the burner to suit the new spec gravity of the SNG. Turndown on the burner may also be affected.

Though I havent worked with SNG, LPG mixed with air has HSE risks - do you have a FSL and / or PSL on the feed gas line to shut off fuel to prevent flashback? As a backup, would it be necessary to install a flashback arrestor ?

Do you have info on the minimum forward flow velocity across the burner orifice to prevent flashback in case of low flow ?

 

[sizgalil]

Thanks.

The air content in the calculated SNG mixture is way beyond the HEL of the LPG.
So, the mixture is not explosive.
Of course the mixing unit has to be well designed to ensure this.

I in contact with the burner manufacturer to try get answers regarding the orifice modification and flashback.

However, I was under the initial impression that if the wobbe index is identical then, regardless of the specific gravity difference, it would not be necessary to modify orifice. I seem to have been wrong.

 

[georgeverghese]

HEL (or UFL) for propane is at 9.5% vol / vol of air , while that for nC4 is 8.4% v/v.

Also suggest we check if there is change to flame pattern or flame emissivity that may affect heat transfer in the fired heater.