# How to size an orifice on a vent line

• steves1080
In summary, Mike would like to install a vent stone on a 3/4" vent line that releases stored gasous nitrogen at a high pressure. Since the vent stone is only rated for 300 psig, he plans to add an orifice upstream to limit the exit flow rate. He is seeking useful formulas for this application, including the equation for mass flow rate with choked flow and the equation for flow coefficient. He also considered using the equation for dynamic pressure but is struggling to apply these equations correctly. The post has been answered on a different thread and can be deleted.

#### steves1080

I would like to install a vent stone on a 3/4" vent line. The line vents upwards of 1500 psig of stored gasous nitrogen in under 1 second. The vent stones are only rated for 300 psig, so I'd like to add an orifice just upstream to limit the exit flow rate to a point where it can handle the pressures from venting the GN2. What are useful formulas for this application? I was thinking perhaps the equation for mass flow rate with choked flow (http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html), and also the equation for flow coefficient (http://www.fnwvalve.com/FNWValve/assets/images/PDFs/FNW/tech_AboutCv.pdf). Then I started thinking about the equation for dynamic pressure (http://www.grc.nasa.gov/WWW/k-12/airplane/dynpress.html), but I am struggling with applying these equations to get the information I need. Either I am not using the right approach, or I am not using these correctly.

Thanks for any insight!

-Mike

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

## 1. What is the purpose of sizing an orifice on a vent line?

The purpose of sizing an orifice on a vent line is to control the flow of gases or liquids through the vent line. This is important in order to maintain safe operating conditions and prevent overpressure or vacuum situations that could damage equipment or cause safety hazards.

## 2. How is the size of an orifice on a vent line determined?

The size of an orifice on a vent line is determined by calculating the required flow rate and choosing an orifice size that will allow for that flow rate at the desired pressure drop. This calculation takes into account factors such as the fluid properties, system pressure and temperature, and desired flow rate.

## 3. What factors should be considered when selecting the appropriate orifice size?

When selecting the appropriate orifice size, factors such as the type of fluid being vented, the required flow rate, the system pressure and temperature, and the desired pressure drop should all be taken into consideration. Additionally, the size and length of the vent line, as well as any potential changes in elevation, should also be considered.

## 4. What are the consequences of an improperly sized orifice on a vent line?

An improperly sized orifice on a vent line can lead to inefficient operation, overpressure or vacuum situations, and potential safety hazards. It can also result in decreased equipment lifespan and increased maintenance costs.

## 5. Are there any industry standards or guidelines for sizing orifices on vent lines?

Yes, there are industry standards and guidelines for sizing orifices on vent lines, such as those provided by the American Petroleum Institute (API) and the American Society of Mechanical Engineers (ASME). These standards provide recommended practices and calculations for properly sizing orifices on vent lines in various industries and applications.