Flow capacity is one of two common ways to compare and size valves. Port thread size is another option, but isn’t as relevant since two valves with the same thread won’t necessarily have the same flow capacity. Generally, manufacturers will review the opening through associated fittings of that size and design the valve’s flow capacity accordingly. Learn how calculating flow capacity influences the valve selection process.
ifferent Methods of Expressing Valve Flow
Flow rates are calculated in different ways, and some methods are more precise than others.
You can express flow through a valve as a coefficient (Cv or Kv). Other options include liters per minute (LPM), cubic feet per minute (CFM), gallons per minute (GPM) and estimated sharp edge orifice dimension (ESEOD).
Direct Measurement vs. Calculated Values
There are two basic methods of determining valve flow: Calculated and direct measurement. Direct involves physically measuring the flow through the body of the valve, while calculated involves formulas and parameters to establish approximations.
Both options are useful for determining valve sizing. Due to potential human error, calculated measurements like Cv and Kv should be used as a guide for general industrial applications and product comparison. ESEOD is another calculated value that’s typically applied to pneumatic spool-type and poppet-type pneumatic valves. If your system requires exact flow values, then it must be sized with direct measurements and simulated upstream and downstream application conditions.
Benefits of direct measurement:
· High accuracy
· Use of flow meters, pressure gauges and calibrated systems
· Suitable for field testing
· Real-time data collection
Benefits of calculated measurements:
· Effective for designing, sizing and selecting valves
· Media and operational flexibility
· Cost-effective, most manufacturers publish this specification
· Good for comparing multiple valves
Measured Flow Values
A measured flow value is based on the actual rate that’s recorded through a valve. LPM, CFM and GPM are considered measured values since they’re generated by a stable upstream pressure and volume and by flow meters downstream from the valve body. They’re stated as “flow at a given pressure.”
Flow curves of a valve that are measured at various pressures aren’t linear, so dividing a valve with a 100-psi rating in half won’t provide an accurate estimation for a model at 50 psi. However, it’s relatively close and can be used as a good estimation. Each measurement is specific to an individual test and is influenced by the system layout and media temperature and pressure.
