Discharge Blower Reverse Flow

[TiCl4]

In discussion with my colleagues regarding the possibility of installing a dilution air blower on a vent stack, the following question arose:

Let's say a dilution blower is provided that has a deadhead pressure of, say, 3 psig, and the vent stack is in the middle of relieving an emergency event. The relief event is such that the stack pressure is 10 psig at the inlet due to the relieving gas flow (close to where the dilution air blower would discharge into the stack).

My colleagues seemed to think that by providing a "Y" connection would somehow prevent backflow through the blower. I contended that unless the stack were provided with some kind of venturi or other design to accelerate the gas and drop local pressure below 3 psig, the blower would experience back-flow through it.

My overall contention and understanding of a blower is that if discharge pressure is higher than the deadhead pressure, it will experience backflow.

 

[georgeverghese]

Would agree with your predictions on this.

 

[3DDave]

It depends; what amount of momentum will the gas have as it passes the location of the dilution blower. For it to take a different path it needs to change direction; what will cause that direction change? Now, if the tube was capped so there was no flow it would be obvious that the 10 psi source would vent into the 3 psi area, but once it's moving it really depends on the geometry.

An interesting example I came across recently is the Tesla valve which is essentially a solid state diode to gas flow.

 

[TiCl4]

3Ddave: The 10 psig pressure would be the driving force. The vent is already providing a significant resistance to flow.

 

[3DDave]

So you've calculated the velocity? I missed where you did that step.

 

[TiCl4]

3DDave - I believe that the velocity is immaterial if the gauge pressure that is read at the point of the dilution air entry is 10 psig. The only way velocity would matter is if the air stream at 10 psig upstream of the dilution air entry were accelerated via a venturi, which could drop the gauge pressure reading to sub-3 psig at the entry point of the dilution air. This stack is intended to be a straight pipe, so I’m not seeing how velocity plays into this.

You seem to be saying that even if I had this Y connection, but it discharged to atmosphere (no blower), there would be no flow through it because the momentum of the relieving stream would prevent the gas from “turning around”. I find that to be an unbelievable assertion. Adding a blower to that pipe only changes the static pressure the gas flow is fighting against. You are essentially asserting, if I understand you correctly, that gases will not always flow from high pressure to low pressure. If that is a fundamental error on my part, please explain why.

We are currently designing this stack to handle a range of relief conditions, and dilution air is one means of achieving sufficient velocity for dispersion for some of the smaller relief cases. This question arose during that discussion regarding what kind/pressure the blower would be needed.

 

[danschwind]

I agree that there would be backflow. I don't see why a Y shape would avoid that.

Daniel
Rio de Janeiro - Brazil

 

[LittleInch]

If you were talking about 3 psig deadhead and say 3.2 psig at the Y, your colleagues might have a point as it would act to a certain extent like a venturi.

But not 10 psig. No way.

without an NRV suitably rated you will get backflow.

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