Air release rate. 1

[Mike692]

I have a question for you guys.
I am a sprinkler fitter and we have certain
Systems that are full of air where freezing is a concern. My question is this, would a higher air pressure release through an open outlet at a faster rate than a lower air pressure?

Eg. Would 80psi Drop at a faster rate than 40 PSI? At least maybe initially?

 

[TD2K]

Yes, you should initially get about 75% more flow.

The flow of gases is different than the flow of liquids. If you had water at 80 psi versus 40 psi, you would flow about 1.4x more water as you likely know.

Air though is compressible and in trying to go from 80 psig to atmospheric pressure or 40 psig to atmospheric pressure, both air streams streams become choked trying to flow through the orifice. However, since the 80 psig gas has a higher density than the 40 psig air, you get a higher initial flow rate with the 80 psig air.

 

[rotw]

I think the higher the pressure, the higher the flow till you have reached sonic conditions through the orifice, at which point the flow does not increase further when pressure is increased.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[Mike692]

Thanks guys.

Thats what I thought.

Is there and mathematical formula or chart that you know of, to see the difference in flow rate between the 2 pressures?

 

[Mike692]

like,
is the 80psi system going to deplete at say 5psi/second where as the 40psi system depletes at 2psi/second?? ----is there any way to find out the rates of depletion of each system?

 

[TD2K]

< at which point the flow does not increase further when pressure is increased. >

No. If you have a fixed pressure and drop the outlet pressure and get choked flow, future reductions in the downstream pressure does not increase the flow.

However, if you start at a higher pressure, when you choke the flow through the orifice will be higher than in the case of the flow of a lower, but still choked, pressure.

< Is there and mathematical formula or chart that you know of, to see the difference in flow rate between the 2 pressures? >

For both 40 psig and 80 psig flow to the atmosphere, you will get choked flow. As a rough guide, a pressure drop of 1/2 the initial absolute pressure (psia which equals psig + atmospheric pressure) will be getting you close to choked flow.

The ratio of the two pressures in psia will give you a reasonable estimate of the two flow rates, assuming the pressures aren't high enough to start to affect parameters like compressibility which won't be the case for these pressures.

(80 + 14.7) / (40 + 14.7) = 1.73 The orifice with 80 psig inlet pressure will flow about 1.73 times the 40 psig case.

< s the 80psi system going to deplete at say 5psi/second where as the 40psi system depletes at 2psi/second?? ----is there any way to find out the rates of depletion of each system? >

Depends on the size of the orifice and the volume of the system.

 

[rotw]

TD2K,
I stand corrected. Thanks for the info.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

 

[LittleInch]

The original question is clear - yes it will deplete faster, but for a sprinkler system this is only of use if the incoming water is at a higher pressure than your air. If it isn't you will still wait until the water pressure equals the air pressure before you get water flow... If this is less than your static air pressure you will end up waiting longer for water to arrive.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Mike692]

Thanks guys,
The reason I ask is because on sprinkler dry Systems you sometimes have an Excelerator. Which is a device that speeds up the tripping of the dry valve. It's based on the rate of air loss in the system. It senses the drop and don't air pressure from on top of the clapper to blow it tripping the valve sooner than it would otherwise. So I was wondering with a half-inch orifice in those pressures given,[pre][/pre] how fast would hire pressure trip the valve as compared to lower pressure.