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Critical Flow Restriction Orifice Plates

Critical Flow Restriction Orifice Plates

Critical Flow Restriction Orifice Plates

I'm looking for advice, guidance or opinons on critical flow RO plate thickness's.
Our vendor uses FLowel software to calculate RO plate bores & thickness's and came up with a couple of plate thickness's of 0.75" thk/0.663" bore for a 2" line and 1.75" thk/1.69" bore for a 4" line.
I thought the plate thickness's were a bit large but, our vendor quoted RW Miller's book for choked flow relating to increased thickness's and ratio's of 1 to 6 for the plate thickness to bore diameter.


RE: Critical Flow Restriction Orifice Plates

His 24" diameter pipe meters must be able to stop antitank weapons.  How much pressure you got?  10,000 psi?

Sounds pretty thick to me.  Check page 4 here, 12" and still 1/4" thick.


"The problem isn't finding the solution, its trying to get to the real question." BigInch

RE: Critical Flow Restriction Orifice Plates

Let's not confuse two very dissimilar concepts here.  For gas measurement, plate thickness is proscriptively defined by AGA 3 / API 14.3 and may not be deviated from (that is what you would see at Daniels fine web site).

One characteristic that is buried in AGA 3 Part 3 (I think, it has been a while) is a concept of permanent pressure drop.  That idea tries to quantify how much of the dP across an orifice plate in gas measurement is transient and how much is permanent.  It turns out that the amount that is transient and is recovered is a function of beta ratio and is reasonably easy to quantify.  As I recall, low Beta ratios have a very high portion of the dP that is transient and largely recoverable with a thin plate.

On the other hand, the title of this thread is "Critical Flow Restriction Orifice Plates".  This is a whole other kettle of cod.  For restriction orifice plates you do not want any transient rebound, so the flow path has to be longer (i.e., the plate has to be thicker).  I've seen a lot of folks try to quantify exactly how long it needs to be to completely dampen the gross-level transients, but the results conflict a lot.  I don't remember Miller talking about restriction orifices (and I'm in Calgary today so I don't have access to my copy of his excellent book), so I can't comment on your vendor's quotation.  I know that the book has a section on critical-flow nozzles, but I don't remember it off the top of my head.

Those thicknesses seem kind of short to me, but that is just a gut feel.  You don't say how much dP you are expecting.  If I were you, I'd make sure that whatever thickness I ended up with would have acceptable bowing with pipeline MAWP upstream and atmospheric pressure downstream with the hole plugged.  I ran the 2-inch and got a 0.000035 inch deflection at 600 psid.  My criteria for pipelines is 0.1 inch so it would be ok from a deflection viewpoint.


RE: Critical Flow Restriction Orifice Plates

At those 0.75 + thicknesses, there's got to be more of a problem with pipe resonance than the plate.  I think its a case of garbage in = garbage out.

David, seems like a deflection limit should be related to pipe id.

"The problem isn't finding the solution, its trying to get to the real question." BigInch

RE: Critical Flow Restriction Orifice Plates

I do this calc for insert or "Skillet" blinds for static tests.  I started doing it after the first time I specified "insert a skillet blind at _____ location" without providing any design details.  The contractor used 1/8" mild steel in a 16-inch line for a 900 psig test.  We created a really cool wok that had extended so far that no matter what we did we couldn't get the flanges spread far enough to remove it.  We ended up cutting the pipe to get it out (other side of the valve was pressurized and we couldn't get authorization to take the whole system down).  Ever since then I've used the 0.1 inch deflection criteria to make sure that I could remove the blind.  I've probably made some skillets that were seriously thicker than was actually needed, but they've all come out when the test was done.

I'm not familiar with that program so I really can't comment on its output.  I've never seen the 1-6D criteria before, I'm thinking that criteria is trying to optimize the permanant pressure drop (i.e., a bigger hole requires a thicker plate because bigger beta ratios have more transients?).  Absent a better understanding of the design criteria I guess you are best off going with your vendor.  


RE: Critical Flow Restriction Orifice Plates

as the calc sheet states, the specified plate thickness is required to achieve the desired pressure drop. Thin plates do not have the same flow characteristics and lead to some pressure recovery.

deformation of plates is another matter, hard to imagine a paddle blind of 1/8" thickness being used for a hyro test. That's why you need to have an engineer on site checking stuff like that. The plate might have ruptured.


RE: Critical Flow Restriction Orifice Plates

It was my first pipeline job, and that mistake was certainly not the worst one I made on that job.  As one of the guys says in their signature line "If we learn from our mistakes, I'm getting a great education".

These days I specify plate material and thickness, and I do the deflection calcs.  On my first job I thought pipelining was easier than it turned out to be.


RE: Critical Flow Restriction Orifice Plates

Indeed, all engineering is that way, and a continual learning for all of us, though it is amazing what the pressures of schedule and cost bring to bear on any project.   

RE: Critical Flow Restriction Orifice Plates

Weegie, while plate thickness does change the pressure recovery, it is not important here.

The plate is so thick because you want them to guarantee choke flow. As thin plate orifices never choke, they have no choice but to make the plate thicker (roughly t/d=1).

You have two choices:
- Accept something close to choked flow, and use a thin plate.
- Insist on true choked flow, and use a thick plate.


RE: Critical Flow Restriction Orifice Plates

In my mind there are 3 reasons for using a thick RO:

a) As a flat plate end piece, if the orifice becomes plugged, the thickness of teh plate must be sufficient to prevent failure of the endplate into the downstream lower pressure system

b)The only flow correlation the designer had is based on a minimum t/d ratio, and they do not want to be responsible for using another correlation.

c) to compensate for long term erosion by wet steam or particulates.  

RE: Critical Flow Restriction Orifice Plates

"As thin plate orifices never choke, "


Could you please explain?  If the pressure ratio is sufficient, any orifice plate should be able to achieve choked (compressible) or critical (incompressible/flashing vapor) flow conditions.  Or so I thought I'd been taught.

RE: Critical Flow Restriction Orifice Plates

btrueblood, thin plate orifices do not choke in vapour flow (Cunningham 1951), as the area of the vena contracta simply increases at high pressure drops.

It is unclear (to me anyway) whether a thin plate orifice can choke in two phase flow. What is clear is that the residence time is too short for vaporization to occur (Lees).

Yes, you are correct. Our university professors did not tell us the whole truth.

That all being said, the error in assuming a thin plate chokes is usually not that large.


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