Flow Control When Testing Dry-Powder Inhalers

Area of Science:

• Pharmaceutical Sciences
• Mechanical Engineering
• Fluid Dynamics

Background:

• International compendia mandate a pressure ratio ≤0.5 for dry powder inhaler (DPI) testing.
• Current guidance lacks requirements for flow control equipment to adhere to thermodynamic theory.
• There is a need to define and test acceptable flow control valve performance for DPI testing.

Purpose of the Study:

• To present flow control data for two common valves used in inhaler testing.
• To describe the necessity for qualification specifications for flow control equipment.

Main Methods:

• A laboratory apparatus was developed to determine mass flow rate through two commercial flow control valves.
• The test method involved lowering the pressure ratio across the valve from ~0.7 to 0.3.
• This range covers the standard test requirement (pressure ratio ≤0.5) and maps compressible flow transition.

Main Results:

• One common industrial valve demonstrated nearly constant flow when the pressure ratio was <0.5.
• A second widely used valve showed a slight, continued increase in mass flow rate as the pressure ratio decreased below 0.5.

Conclusions:

• Inhaler testing laboratories should verify if their equipment achieves constant flow under a pressure ratio of 0.5 or less.
• Future quantitative specifications for qualifying flow control equipment must address both transient and steady-state flow conditions in DPI testing.