Evacuation of gas in a storage tank

[m2e]

I'm working on a project where some tanks needs to be vented, and need some advice. Not sure this is the best forum to post, or the HVAC/R forum. Please advice if it's incorrect.

The tank is at atmospheric pressure, and we need to fill air into the tank through a 2" inlet pipe and release air through a 3" outlet pipe. The purpose is to flush out the gases from the tank and dilute it with air. The inlet and outlet pipes are only a few inches in length and they will be transitioned to bigger pipes.

I'd like to determine how much air we can put through the tank, with the 2" section being the bottleneck. What is the maximum velocity or flow rate for a short 2" section with a few elbows?

 

[JStephen]

That entirely depends on what you're using to blow air through there. Is this a fan, an air compressor?

If the tanks are very large and/or have low pressure ratings, then outlet flow might be the limiting factor.

 

[m2e]

We can use fan, blower, or compressor, whatever works best. The tank is rated to 15 PSI. We can also use an exhaust fan to suck air from the 3" outlet if necessary.

 

[chicopee]

I guess it depends on the tank dimensions and shape, the relative positions of the 2" and 3" connections to each other, and the type of gas that you may want to vent. If manholes and handholes are available those should be primarily used.

 

[darthsoilsguy2]

chicopee also hit on this...just to be on the safe side...I'm going to assume the tank gases are potentially hazardous and also assume that the purpose of doing this is that someone wants to enter the tank.

apart from needing to follow Confined Space Permitting procedures with trained personnel.... the ventilation of the tank should be done to prevent a 'dead-air' space where a pocket of hazardous gases might remain despite your efforts. if the ports are too close together, you can create a short-circuit in your ventilation. you didn't provide enough info to us to show that would be understood. The fresh air needs to be as close to having a plug-flow relationship with the tank to evacuate at exhaust ports as possible. Sometimes tanks are placed into the market that are very hard to conventionally ventilate for confined space entry and people get creative with ventilation schemes using what is available. This is particularly true for Aboveground Storage Tanks that were designed to be Underground Storage Tanks. i had a job where SCBA was prohibited by the facility, there was no way to remove the dead zone pocket or check that zone of the tank with a sniffer, and they ended up installing a new manway by water-jet cutting the penetration so the tank could be made safe for entry.

pardon me if i'm way off-base here. you didn't state your intent for doing this.