### Valves

### How to Determine Proper Air Valve Size

**Air at a temperature of 68 °F (20 °C)**

Absolute downstream or secondary pressure must be 53% of absolute inlet or primary pressure or greater. Below 53%, the air velocity may become sonic and the Cv formula does not apply.

**Nomenclature**:

**B** | Pressure Drop Factor | |

**C** | Compression Factor | |

**Cv** | Flow Factor | |

**D** | Cylinder Diameter | (I N) |

**F** | Cylinder Area | (SQ IN) |

**L** | Cylinder Stroke | (IN) |

**p1** | Inlet or Primary Pressure | (PS I G) |

**p2** | Outlet or Secondary Pressure | (PS I G) |

**?p** | Pressure Differential (p1 - p2) | (psiD) |

**q** | Air Flow at Actual Condition | (CFM) |

**Q** | Air Flow of Free Air | (SCFM) |

**t** | Time to complete one cylinder stroke | (SEC) |

**T** | Absolute temperature at operating pressure. Deg R = Deg F + 460 | (°R) |

Valve Sizing for Cylinder Actuation Direct Formula

Most manufacturers’ catalogs provide flow ratings for valves in Cv, based on National Fluid Power Association (NFPA) standard T3.21.3. The following tables and formulas will enable you to quickly size a valve properly. The traditional, often-used approach of using the valve size equivalent to the port in the cylinder can be very costly. Cylinder speed, not port size, should be the determining factor.

The following Cv calculations are based upon simplified formulas which yield results with acceptable accuracy under the following standard condition: