Gas plant

Hello Jerry,

You don't state what actual type of gas you are making/processing - Advise further if needed.....

The DRY N₂ (Nitrogen) is normally used because it is non-flammable, even when mixed with air, also cheap.

If you have a system of pipes/pumps/storage tanks etc, and these are assembled in the normal way, that entire assembly will initially be full of air.

When the system is flushed/pressurized with Dry Nitrogen (N₂₎ there is normally no problem.

Using CH₄ (Methane) to flush/pressurize the system is not regarded as safe, because in certain Methane/Air ratios, the mixture is highly explosive.

As the flushing/pressurizing gas surges along pipes and into vessels, swirling occurs, and often static charges occur in that swirling gas.

Static discharges may then take place and if an explosive mixture is present = Your proposed CH₄/Air mixture, then it may cause a violent explosion, with consequent damage to personnel and the expensive equipment.

It is always best to use a cheap, dry, non hazardous flushing/pressurizing gas, and Dry Nitrogen has proved to be the best for this type of situation.

I am presuming your "permanent gas" as processed in the plant is to be one which will not combine with Dry Nitrogen at the temperatures/pressures used.

It is important that your Nitrogen source supplies dry gas, or else you will have dewpoint problems inside that equipment.

Advise here further, as needed.....

A couple other points, if you have had the system open, pressurizing with Nitrogen allows you to go check for leaks without the danger of a hydrocarbon gas leaks. If your process is sensitive to water, circulating N2 helps to dry out the system as well.

If you do use hydrocarbon then yes a HAZOP needs to be done.

Nitrogen = nonflammable; Methane= flammable.

Human lives saved by sticking with the documented nitrogen purge process= Priceless.

milo

Methane is a potent greenhouse gas as well. Use and release of such a gas may eventually be regulated. If you have lots of methane lying around or present as a process byproduct, methane can be used to power a gas turbine cogen plant, to the benefit of the environment, and your electric bill. Waste management inc. could tell you more about the process.

The need of nitrogen filling prior to commissioning hydrocarbon vessels (piping & equiment) is to replace oxigen with nitrogen from vessels. This is because hydrocarbion gases, specially methane, when mixed with oxygen and compressed to required pressure of vessels might ignite due to mileculer friction of gases and cause accidents. This is why nitrogen or carbondioxide is purged into piping before commissioning. Yes! It does requre HAZOP.