## Calculate the Cd of an orifice plate before using.

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bakerlite
Posts: 3
Joined: Sat Mar 03, 2012 1:18 am

### Calculate the Cd of an orifice plate before using.

Ok I'd like to know how to calculate the approximate coefficient of discharge of an orifice plate flowing air, depending on the type of orifice (in this case straight edged ) and the thickness of the plate. Most sharp edged ones seem to be about .60 to .62 Cd. But I want to use a straight edged so I can reverse the flow and still maintain the Cd.

Example: If I have an orifice plate with a 1 inch straight edged hole in it and a plate thickness of 250 thou then how would It's Cd be compared if I had the same hole but with a material thickness of 125 thou?
Cheers

Posts: 359
Joined: Mon Feb 08, 2010 7:47 pm

### Re: Calculate the Cd of an orifice plate before using.

If you do not have bevel of the orifice than the thickness should be between 0.005D and 0.02D where D is inside pipe diameter. But I do not know your inside pipe diameter so can not calculate exact number. You must take care of the pressure difference created, not to cause plate deformation above tolerance.
Anyway, you can use orifice plate calculator where discharge coefficient is also calculated.
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bakerlite
Posts: 3
Joined: Sat Mar 03, 2012 1:18 am

### Re: Calculate the Cd of an orifice plate before using.

Thank you for that but the calculator does not take into account the thickness of the orifice plate, nor the type of edge that is being used. That's what I'm inquiring about
If:
D1 = 5 inchs
D2 = 1 inch
P1 = 28 inH20
P2= 16 inH20
Then we get a Cd of .59862
But what happens if the Orifice plate is not sharp edged and is 0.250 inch thick? The calculator does not seem to take into account the type or thickness of the orifice. How can I change the calculator settings to sharp edge, square edged or even radius edged?
Cheers

Posts: 359
Joined: Mon Feb 08, 2010 7:47 pm

### Re: Calculate the Cd of an orifice plate before using.

For D1 = 5 inches, orifice thickness should be between 0.025 and 0.1 inch.
For bidirectional flow only sharped edged orifice should be used (ISO 5167-2) - plate should not be beveled and not with radius - maximum radius is 0.0004 x D2 = 0.0004 inch. Angle of orifice and and plate face should be 90 deg +/- 0.3 deg.
If this requirements are met, coefficient of discharge is properly calculated, as given in ISO 5167-2
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bakerlite
Posts: 3
Joined: Sat Mar 03, 2012 1:18 am

### Re: Calculate the Cd of an orifice plate before using.

Ok, thank you. Unfortunately this does not answer my question. It now seems to me that we have a problem when trying to work out the Cd when we go to thicker plates. I'm trying to find out what would happen with changes to plate thickness.. Its seems all these calculations assume that we have a thin plate.
I do not have a thin plate in what I want to test.
I've asked you what the CD would be for that plate if it was 0.250 thick not 0.025 etc. Is there a problem when trying to make this calculation?
If we use the numbers in my previous post and change the thickness of the plate from 0.100 to 0.250 what would the change in Cd be?