Negative Venting Pressure

[Nanuuq]

Hello,

I have been tasked with finding at what ventilation level will a tank collapse. Workers will be present inside the tank which is why the high ventilation level is necessary.

The tank will be emptied of all product and the only fluid flow which creates a vacuum will be air. Fans will be placed in some manholes, blowing air out, while other manholes and nozzles will be left open. The concern is that the fans may create a negative pressure within the tank thus imploding said tank.

Essentially, what I need to find is the volumetric flow rate of air out that will create a pressure differential which implodes the tank. I imagine it would be fairly high since air can enter the tank relatively easy through the remaining vents and manholes and as long as the flow rates in and out are very similar there won't be a noticeable differential pressure. Any help is appreciated.

 

[bimr]

It is easier for a fan to blow air than to suck air. Once the negative pressure increases the fan will begin to slip.

The static pressure on the common fan is only about 1 inches of water gauge, generally not enough to collapse a tank.

http://www.engineeringtoolbox.com/fan-types-sizes-d_910.html

Typical scenarios that would cause collapse of a tank include:

1. Condensing steam in a closed tank
2. Pumping a non-compressible fluid out of the tank out of a sealed tank
3. Cooling the tank

Verify the tank design parameters and evaluate against the maximum static pressure that is possible with the fan that you are using.

 

[LittleInch]

Air, like any fluid, will only move wen there is a pressure difference to cause that to occur.

As bimr says, "sucking" air out of a vessel by means of a standard Fan assembly is a very poor use of energy and not normally undertaken, in part because of potential concerns about collapse, but mainly because it doesn't work very well.

only when you have data on what actual negative pressure the fan can produce at different flow rates from zero to max can you start to see how much air will enter the tank due to that pressure difference from your many entrances. It might take a few iterations to get the inflow to match the outflow, your common issue being the internal pressure.

Blowing air in also causes a potential issue with overpressure which is sometimes relieved by use of extraction fans to increase flow through what are usually relatively small manholes or vents.

What type of fan / blower are oyu thinking of using? Can you post any details?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nanuuq]

Thanks for the responses,

bimr: I didn't and still don't have much information which is why I didn't include much in my initial post. Just found out that there are going to be 2 EA 20" electric fans with an approximate flow rate of 4000 cfm per fan placed in two manholes. The tank has 3 EA 2" nozzles and a third manhole that will be open. I'll check the static pressures out for a worst case scenario. Thank you. I agree it is easier for fans to "push" but in the past somebody got a headache and blamed it on the fans so we can no longer blow air in...

LittleInch: I'm afraid I have very little information on the fans other than 20" electric fan blowing 4000 cfm. I'll be searching for more info on the static pressure for these.

I don't believe that these two fans will implode the tank but apparently in the past they have when other manholes were blocked (some sort of impact loading? I am unsure. Have never worked with fans before). We don't plan on blocking the other manhole or nozzles while the fans are operating.


Thanks again.

 

[LittleInch]

That's the problem, you might not plan to block the other manholes, but sh1t happens. Maybe a plastic bag, maybe just a large operator squeezing into the tank, you name it, it's happened. Most tanks are able to handle extra pressure better than negative and it's easier to have pressure relief than vacuum relief.

Maybe split it and have one fan blowing in and the other blowing out?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.