### Reducing pressures with Orifice Plates

Posted:

**Fri Apr 18, 2014 10:20 pm**Hi,

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.

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.