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Reducing pressures with Orifice Plates

Posted: Fri Apr 18, 2014 10:20 pm
by nargo

We are installing two orifices, one on a CO2 line, one on argon line, but not for flow measurement, but rather to limit the flow in case the lines gets broken (thus generating a large pressure drop), but for the normal operating conditions, our flow is very small so the pressure drop thru the orifices will be also very small, not more than 0.5 psi. I used this calculator, and found an excellent match with Perry's formulas, down to the minimum beta ratio (d orif / d pipe) that is included there, which is 0.2. This calculator doesn't have that limitation, so I used for a beta ratio of 0.129 in our application. So, I have three questions:

1) What would be a good number for the surface roughness of SS tubing, 0.5" OD, manufactured by Swagelok (this is what we have). I have found two bad points in this regard: these roughness values vary a lot among internet sources, and the calculations vary a lot depending on it.

2) Does anybody have experience with orifices working to reduce pressure, instead of working to create a pressure drop to be measured? In other words, are these calculations reliable, for a case like ours, where we don't have pressure measuring taps, so we don't have all the coefficients that are related to them?

3) What is the use of the gas constant R (not the universal but the particular), and the k (spec. heat ratio), that are inputs in the orifice plate calculator, but are not included in the equations shown in the
"Theory" page?

Thanks in advance for any feedback or opinion.

Re: Reducing pressures with Orifice Plates

Posted: Fri Apr 18, 2014 11:03 pm
by admin
Gas constant for CO2 is R=189 J/kgK and k=kappa=Cp/Cv=1.3 for CO2. These values are needed for calculation. You have table at:
I suggest you to use check button with "use ISO constraint" just to be sure if calculation is within its purpose or not.
Calculation is based on ISO 5167 and it has many constraints in order to be accurate like pipe lenghts in front and after the orifice, etc. but I am sure that it can give decent results if you are looking to find close aproximation for your problem and not 99% or more accuracy.

Re: Reducing pressures with Orifice Plates

Posted: Mon Apr 21, 2014 4:24 pm
by nargo
Thanks Admin, for the fast reply.

And yes, I have tables of R and k values for gases, but what I was looking for, is to know in which specific formulas of this calculator are those factors being used because, as I wrote in my first post, they are not shown in the formulas of the "Theory" page.

To your other point, I can't check that "use ISO constraints" check button, because then I get an error message telling me that D1 has to be above 2", but it is only 0.402" (1/2" tubing) in our case. And again, we are using the orifices just to restrict flow in case of accidental rupture of lines, so we are not using them to measure flow and we don't have pressure taps before and after them. We don't need 99% accuracy on the calculated results, we are just looking for estimates, as good as we can get them without using an expensive simulation package, about two things: pressure drop with the very low flows that we need (about 0.5 lbm/hr of each gas), and flows with the large pressure drop that would occur if the lines get broken, with the 0.052" orifice we are planning to use.

Thanks again for any feedback.

Re: Reducing pressures with Orifice Plates

Posted: Mon Apr 21, 2014 8:39 pm
by admin
Kappa is for expansion factor ISO 5167 - Y for orifices, you have it in the right middle of this page: ... ttings.php
R is simply used for calculations between rho, T and p in ideal gas state equation, depending on user inputs.

I think that you should use gas pipe discharge flow rate calculator. It is taking into consideration choked flow that you might have in case of pipe line break.

Re: Reducing pressures with Orifice Plates

Posted: Wed Apr 23, 2014 4:19 pm
by nargo
Thanks again admin, for your feedback.

Regarding my original question about the use of k on these particular equations (this k is cp/cv, as you wrote before), you just pointed me in the right direction, because now I see it, in that Y equation you mentioned, but its symbol there is X, not k, that is why I missed it. But, regarding R, I know it is a factor in the state equation PV=mRT, but I am still looking for R in the specific equations for orifices included with the calculator.

Also, I used the other calculator you mentioned for the initial calculation of the flow in the ruptured line without orifice so, when it gave me a very high flow, we decided to put orifices to restrict the flow in the event of such accident. Then, it is the orifice calculator the one we are trying to understand now regarding the use of R in it, and regarding its applicability as restrictors in small lines like ours where we are not measuring flow.

Again, we would appreciate any feedback respect to these points and also to our other questions shown in the original post.

Re: Reducing pressures with Orifice Plates

Posted: Wed Apr 23, 2014 10:12 pm
by admin
Calculator gives option for enter either density or temperature. If you choose to enter density, than calculator using R is calculating T prior to other calculation is done.
I can't tell you about pipe roghness you should use. I think that it is the best way to be on safe side and choose values of pipe roghness that gives higher flows.