Restriction Orifices in Choked Flow?

[byrdo]

Restriction Orifices in Choked Flow?

I am working on a restriction orifice in a gas service with the downstream pressure is less than Pchoked. The original design has a Po=285psia and requires a 200psi permanent DP. For this system Pchoked ~155 psia.

This is where things get a little confusing. I know the maximum flow through the orifice is at DP=Po-Pchoked or 130psi, so how do I obtain the needed 200psi DP. My gut feel is that the downstream pressure will be whatever the backpressure associated with the choked mass flow rate through downstream piping, i.e.; if I was discharging into the atmosphere the backpressure would be zero and the DP across the orifice would be ~270 psi.

Is this how a sonic system acts? Any comments are appreciated.

Matt

 

[TD2K]

The 200 psig needs to be 'set' by your downstream piping and the hydraulics in that system.

Let's say you had a control valve downstream of your RO controlling the pressure. You could set that control valve at any pressure less than the pressure that gives you the 'choked' flow across your RO with no change in flow through the RO. The control valve has set the dP for your RO, not the RO itself.

The RO is sized for a given flow and pressure drop (or part of the total pressure drop when you have choked flow) but it doesn't really 'create' that dP in the system.

I know I'm not explaining this well. Basically, wherever the flow that goes through the RO goes to, that downstream system and whatever controls is on it sets your downstream pressure, not really the RO itself (a sketch might help you here). Let's say you had a control valve right downstream of the RO. YOu could change the downstream pressure on the RO to any pressure less than the dP corresponding to your choked flow without affecting the flow through the orifice. If you put in a bigger orifice, you still wouldn't change the pressure downstream of the RO as long as the control valve had sufficient capacity to handle the additional flow through the RO.

You need to look at the system hydraulics when thinking about the DP the RO will take. It's similar to a control valve in that the control valve doesn't 'take' a pressure drop, it has a pressure drop for it established by the system it is installed in.

 

[thermicatech]

Byrdo,
I am going to make it simple, just replying to your question.
Yes, you are right with your description as long as the downstream backpressure developed by your set flow is less than the critical pressure.
Under choked flow conditions the orifice area is directly proportional to the flow, upstream pressure, MW, cp/cv and it is independente of the back pressure.

 

[Hookem]

Does anyone have any excel spreadsheets on this topic?

 

[ARCH01]

To the original question:

Yes, you can have a downstream pressure below the critical value. This pressure will be governed by the choked flowrate (or maximum flowrate at dP critical) and downstream system characteristics.

If you know upstream pressure and orifice size, you can determine maximum flowrate at critical conditions. Use this flow, pipe size, pipe length and final system pressure (i.e., atmospheric) to determine downstream pressure. Keep in mind that the final pipe discharge to atmosphere might also act as a choke; depending on the flowrates and pipe size.