Calculate Air Flow or Vacuum Pressure through Orifices

Thin plates fitted inside the pipes of your production line systems, orifices help in measuring and controlling the flow of any liquid or gaseous fluids that pass through the pipes. Each orifice plate features a hole in the center that helps increase the velocity and decrease the pressure of any type of fluid. Any production line that features control valves, anti-cavitation valves, pressure and vacuum measuring devices, or accelerated fermentation or oxidation, typically requires a basic understanding and experience of applying orifices.

At Trident Compressed Air LTD., we specialize in industrial applications that use compressed gas and vacuum, and have a deep understanding of orifices, orifice pressure, and air flow. Read on to learn the basic types of orifices that may be applicable to your business, along with the calculation for air and vacuum flow through the orifices.

Understanding the Types of Orifices

  • Bevelled Square Edge Bore: Bevelled to the current industry standards, the bore and these orifices are the most common method of limiting the plate edge’s thickness.
  • Quadrant Edge Bore: Featuring a rounded inlet edge that is counterbored to the desired plate edge thickness, these orifices are especially handy for viscous fluids, such as syrups, heavy crudes, or slurries with R Numbers below 100,000.
  • Eccentrically Bore: They come with off-centre orifices, with the bore inscribed in a circle that is 98% of the pipe diameter. This allows solids or slurries to pass through making it an ideal flow measurement device in industries, such as steel, paper, heavy and light chemical industries, and petrochemicals.
  • Segmental Bore: These orifices are suitable for measuring flow when solids are entrained in a liquid or gas flow stream. They find application in paper, steel, or chemical industries, as well as water conditioning and sewage treatment plants.

While the shape and manufacturing tolerances of the plates used in ‘measurement’ applications follow international standards, such as ISO, AGA, or ASME, those used as ‘restriction’ devices do not have standardized definitions. Orifices are versatile and inexpensive, making them a better alternative to other available flow or control devices.

Measuring Orifice Pressure and Air Flow

Depending on their type and position of installation, orifices can increase line pressure, decrease flow, or increase the fluid velocity through the line. The air flow through the orifices depends on:

  • Orifice diameter
  • Air pressure
  • Coefficient of the orifice shape (For example, an orifice with a well-rounded entrance boasts a .98 coefficient. It will typically allow almost double the air flow as compared to an orifice with a sharp entrance, which has a coefficient of .53 to .61).

While there are many different formulas for calculating the air and vacuum flow, here are some things to remember:

  • For standard or normal pressure, air flow is measured in Standard Cubic Feet per Minute (SCFM), or in Linear Feet per Minute (LFM). In vacuum, the air or gas expands, hence, it is common to use actual cubic feet per minute (ACFM).
  • When flowing fluid converges at the orifice, the set temperature and pressure determine the fluid velocity upstream and downstream of the orifice.
  • The term ‘psia’ describes the pounds per square inch absolute. When the ratio of downstream psia to upstream psia is above .5, the flow is subsonic or subcritical. When this ratio touches 0.5, the orifice velocity reaches sonic speed and flow becomes critical. It will remain constant for any ratio below .5.
  • For air pressure up to 15 psig, calculate air flow through the standard adiabatic formula. For pressure above 15 psig, you may want to consider the formula proposed by S.A. Moss:

The equation calculates the weight of air in ‘lbs per second’. Divide the result by 0.07494 lbs / cu ft. (the density of dry air at 70°F and 14.7 lbs / sq. in. psia) and then multiply by 60 seconds to get air flow in ‘SCFM’.

Dependable Engineering Services from Trident Compressed Air LTD

While the piping material, wall thicknesses, roughness of the inside walls, and solid buildup inside the piping can cause variances in the calculation, the basic measurement of orifice pressure or air flow remains the same. For more insights into measuring air and vacuum flow through orifices, count on the technical experts at Trident Compressed Air. We provide a comprehensive range of engineering services for designing turnkey systems, compressors, vacuum stations, piping grids, and more. Our qualified and experienced team can customize the solutions to cater to your production lines and business.

Please contact our office at (519) 737-9905 or fill out our online form to learn more about orifice types and orifice pressure. Our service department is on call 24 hours a day and we are eager to help answer any questions or concerns you may have about compressed gas and vacuum systems.