New Facility Purge -- (Purge Oxygen with Fuel Gas)

[sowetoddid]

Specific Question: Is it safe to purge a new production facility of air using fuel gas?

Background Information
1. The facility is for oil production in a corrosive environment. Pipe is internally coated or stainless steel to address CO2 in the production stream.

2. CO2 is available for purge gas but would require a heater due to the high pressure and resulting freeze effect from dropping the pressure.

3. Nitrogen is an option for purge gas that comes at 4x the cost of fuel gas (accounting for the trucking and other fees)

4. Fuel gas is readily available and appears to be the cheapest solution.

5. Received some feedback/opinions on the operations recommendation to purge three times with fuel gas (pressure up to 100 psi and bleed down). I need help to validate these...

5A) Partial pressure of air would be too low to create an explosive mixture

5B) There is no ignition source. A static charge is not likely with fuel gas because of a lack of sufficient friction. Static charge would be "expected from an ethylene mixture that decomposes more in transport"??

5C) Pressure up and vent is better than a clearing purge due to the multiple branches and only two main vent stacks.

I would appreciate any experience to fill in the gaps? I have heard from a number of people that "it is common practice and not an issue". I personally need to improve my comfort factor with it.

 

[owg]

For a multiple branch system I would evacuate and break the vacuum with an inert gas. You should develop the details of how to do this, e.g. maybe break twice with inert gas then finally with fuel gas. Check the system is good for vacuum. Also I do not buy your 5A statement.

HAZOP at

http://www.curryhydrocarbons.ca

 

[JLSeagull]

I wonder, could air and hydrocarbon possibly ignite and not just explode? Perhaps something like commissioning and checkout folks know a bit about nitrogen, CO2 or other compounds to be inert in lieu of to be explosive, perhaps.

 

[danw2]

I'm not sure what fuel gas is, but there was an explosion at a new construction power plant here in the US recently when new piping was purged by a gas blow of methane (called natural gas in the US).

The Chemical Safety Board video

http://www.csb.gov/videoroom/detail.aspx?vid=49&F=0&CID=1&pg=1&F_All=y

covers its investigation of the explosion. The first 8 minutes involve a heater maintenance case. The 2nd case in Middletown, Connecticut covers the power plant explosion resulting from the gas blow starts at 8:00 minutes into the video.

The board's conclusion is that gas blows are a "common but dangerous practice".

I suspect that the economic factors cited will determine which method is used. I just don't want to be nearby or on-site the day when a gas blow purge occurs.

 

[owg]

Good post danw2. In answer to your comment on fuel gas, it varies with facility. Oil refineries for example can have a fuel gas utility header with the gas made up of by-product gases made in the various units. The fuel gas could contain methane, ethane, hydrogen, a bit of propane, maybe some light olefins. Pressure controlled perhaps by a natural gas connection. So that fuel gas, on occasion, could be all natural gas.

HAZOP at

http://www.curryhydrocarbons.ca

 

[dcasto]

The facility that blew up in the US didn't blow up because of purging. The purging/commisioning standard called for blowing natural gas out of the line at very high velocity. A piece of plywood painted white was placed in from of the line and a full port valve was slammed open. The board was inspected for signs of weld slag and junk. If there was none, then the line was deemed clean. There are cases where doing this operation caused high velocity particle to strike metal which caused a spark.

Now we have a new document nfpa 56 which expects people to look for aircraft in the area before purging, OVERKILLLLL.

Using nat gas is best, but we sometimes use a bottle of N2 between the air and nat gas, cheap insurance.