Orifice Plate and Flow Restriction question

[MajesticFerret]

So let's say we have a pressurized volume that needs to blow down to atmospheric pressure.

In between said volume and vent we have blowdown piping that starts out as 2" and eventually expands it's way to 10" before connecting to the vent inlet. Let's say at the inlet connection we have an orifice plate with an orifice that matches the ID of the 2" pipe.

Does said orifice plate provide any flow restriction in this instance? Or would the pipe of matching ID to the orifice provide the same flow restriction?


Thanks to anybody in advance!

 

[BigInch]

you will get a pressure drop at all restrictions to flow, including within the 10" pipe.

 

[davefitz]

based on compressible flow theory, the choke point is based on the location of smallest cross sectional area ( if proper streamlines are considered, based on 2nd law of thermo). If it is a simple restriction orifice , then one should expect add'l "oblique" shock waves formed due to the lack of streamlining at the device; the flow will be restricted at the final restriction orifice based on acoustic velocity. The pressure at the inlet of the oriofice, and the resulting flow, can be calculated iteratively , working backwards from the orifice back to the reservoir. Assume a choked flow, calculate the needed orifice inlet pressure, then calculate the friction pressure drop in the upstream piping.

"In this bright future, you can't forget your past..." Bob Marley

 

[LittleInch]

"Does said orifice plate provide any flow restriction in this instance?" Yes

"Or would the pipe of matching ID to the orifice provide the same flow restriction?" No, unless the length of the pipe was very small.

You need to do a bit of reading on orifice plates and restriction orifices.

A lot depends on whether it is choked flow or not.

Restriction orifices are calculated and looked at differently to measurement orifices. Do a search on this site and there are many posts on the same issue and the odd FAQ.


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[MajesticFerret]

Thanks for the replies.

So I'm assuming the key variable here that differentiates the Orifice Plate vs. the same ID "nozzle" in this case is the coefficient of discharge being much less for the plate?

As far as mass flow is concerned, can chokes "stack?" Like if there’s a choke that limits mass flow to a lesser extent than the downstream orifice, would be mass flow be set by the most restrictive choke or would the lesser choke limit mass flow further?

 

[georgeverghese]

Its a bit more than just comparing the ID of the restricting devices. For high velocity compressible gas flow, there is a significant pressure profile as you go from the final discharge point pressure backwards to the restricting device and then further back to the source reservoir pressure.
The same choking device can give you 2 different flows depending on where you've placed it.

 

[LittleInch]

In a steady state system mass flow is constant. Therefore any device which restricts mass flow is the primary device.

However you can get dynamic situations where, in your example, the pressure in the 10" section might rise over time ( could be 1 second, could be 10 minutes) causing the end restriction to go to critical flow and make the first one non critical as the pressure drop across it falls.

The key to the first point is "steady state". It might take some time or perhaps never to achieve steady state in such a system when pressures and flows are changing all the time.

Vent systems normally work on a high pressure inlet with an RO feeding into a large low pressure vent. Systems with many inputs are quite complex to model to get all the systems blowing down down to the same level after 15 minutes.

Large plants often have zones which blowdown one after another or restricted to an ESD or fire event in one part of the plant to reduce vent / flare sizes.

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