Calculate pressure after an orifice
Calculate pressure after an orifice
(OP)
Hi!
I have a system of piping going from a pressurized 80L cylinder with 300bar with a gas. In the system i have two orifices. First of I want to calculate the pressure and valocity of the gas after the first orifice, so I can calculate the flow of gas after the second orifice. I am not sure what equations i can use to find the pressure after the orifice.
Can anyone point me in the right direction, so i can dimention the needed sizes of my orifices?
Thanks!
I have a system of piping going from a pressurized 80L cylinder with 300bar with a gas. In the system i have two orifices. First of I want to calculate the pressure and valocity of the gas after the first orifice, so I can calculate the flow of gas after the second orifice. I am not sure what equations i can use to find the pressure after the orifice.
Can anyone point me in the right direction, so i can dimention the needed sizes of my orifices?
Thanks!





RE: Calculate pressure after an orifice
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Calculate pressure after an orifice
Remember mass flow rate will be the same through both.
Don't start with too many unknowns
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Calculate pressure after an orifice
"Nobody expects the Spanish Inquisition!"
RE: Calculate pressure after an orifice
I little clarification
@dave
Lets consider the first orifice. i have 300bar going in, I am assuming, that the flow will be choked giving me
P.crit=0.528*P.0 (for air)
Can i then say, that my pressure after the orifice will be:
0.528*300bar=158.4bar
In my optic the backpressure should depend on the size of the orifice.
I know that the velosity of the gas will be M=1, but i cant find a equation that calculate the backpressure from the velosity.
And can anyone confirm, that my flowrate will be the same through both orifices, which make sence to me?
Thanks a lot!
RE: Calculate pressure after an orifice
RE: Calculate pressure after an orifice
In a steady state run, the only thing you have as a constant is mass flow, not volumetric.
The pressure on one side of the first orifice is 300bar. The pressure on the far side of the second orifice is??? - you don't say.
The pressure in between the two orifices can be anything you choose. What it is will vary depending on the relative size of orifice,
As the pressure decreases, the density decreases and hence volume increases.
You just need to choose two orifice sizes and then keep adjusting the pressure until the mass flow rates get close.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Calculate pressure after an orifice
the upstream pressure to the first orifice is 300bar and the downstream of the second orifice is to atmosphere so 1 bar.
Can i assume, that the flow over both orifices will be choked?
and can you point me in the direction for the right equation for calculating the pressure between the two orifices? I have trouble calculating it for choked flow?
thanks a lot!
RE: Calculate pressure after an orifice
however, as I said above, depending on the size of your first orifice it might be choked, it might not.
The aim for two orifice systems would normally be to try and split the pressure drop between the two to stop one being very small and one quite big.
Note that that is a large pressure drop and the erosion of a standard plate orifice will be significant.
Simply search for "restriction orifice" on this site or in google ( or your search engine of choice" and you will find many equations and on line calculators. There are two versions of the calcs, one for choked flow and one for not choked.
See this as a typical
http://www.engineeringtoolbox.com/flow-coefficient...
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Calculate pressure after an orifice
sorry for all the questions!
RE: Calculate pressure after an orifice
"Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment while upstream pressure is fixed." https://en.wikipedia.org/wiki/Choked_flow
This might also help: https://en.wikipedia.org/wiki/Orifice_plate
Orifices aren't an analog to electrical resistors. They don't just add together.
You'll have to work backwards and look at mass flow rate, knowing that the same mass is flowing though both orifices and then determine whether the pressure difference at each orifice is sufficient to produce choked flow. It's non-linear so there isn't a simple equation to solve for the conditions. Were it up to me I'd put the equations into a spreadsheet and make a plot of the mass flow rate vs pressure drops until it met the other conditions.
RE: Calculate pressure after an orifice
"In choke flow the downstream pressure is the downstream pressure. Choked flow through an ideal orifice doesn't/can't affect downstream pressure."
Do you mean, that when the pressure is choked, the downstream pressure will be a constant not dependant on the upstream pressure as long as there is choked conditions?
I have read those wiki pages. my problem is, that i dont know which equation i need to calculate the pressure drop?
Thanks !