PSI vs. SCFM

When sizing and operating Air-Operated Double Diaphragm (AODD) pumps, two air supply metrics dictate performance: PSI (pounds per square inch) and SCFM (Standard Cubic Feet per Minute).

Because these two terms are frequently confused in the field, operators often struggle with stalling or reduced flow rates. In short: PSI represents force, while SCFM represents volume.

Here is a breakdown of how PSI and SCFM affect AODD pumps and how to properly manage both metrics in your fluid handling system.

What is the Difference Between PSI and SCFM?

• PSI (Pounds per Square Inch): Measures the pressure or force of the compressed air supplied to the pump. Higher PSI provides more force to move fluid against resistance.

• SCFM (Standard Cubic Feet per Minute): Measures the volume of air moving through the system over time. AODD pumps require a specific SCFM rate to operate efficiently at a given capacity.

How PSI Affects AODD Pump Discharge Pressure

AODD pumps transfer fluid across diverse industrial setups, each presenting varying levels of discharge pressure (backpressure). Common sources of discharge pressure include:

• Throttled or partially closed valves

• Vertical discharge lines (pumping fluid to higher elevations)

• Pressurized discharge vessels

• Restricted or smaller-diameter discharge piping

• Line blockages or high-viscosity fluids

The 1:1 Pressure Ratio

Standard AODD pumps operate on roughly a 1:1 ratio between input air pressure (PSI) and output discharge pressure.

Key Rule: If system pressure exceeds the supplied air pressure, the pump will stall until system pressure drops or supply PSI increases.

How SCFM Controls Pump Flow Rate (GPM)

While PSI gives the pump the force to overcome pressure, SCFM determines how fast the pump can run and how much fluid it moves (measured in Gallons Per Minute, or GPM).

What Happens When SCFM Is Insufficient?

Consider a scenario where an AODD pump requires 60 SCFM at its operating point, but your air compressor only delivers 40 SCFM:

1. Initial Operation: If the compressor's air receiver tank is fully charged, the pump may run normally at first.

2. Volume Drop: Once the compressor kicks on, it supplies air at only 40 SCFM. Because air volume is restricted, the pump slows down, reducing overall fluid output (GPM).

3. Insufficient SCFM: The air compressor will not be able to keep up with the demand the pump is drawing. This we result in a restricted GPM output of the pump for the life of the system. 
   
    

How to Regulate PSI and SCFM for Optimal Longevity

Proper air regulation protects AODD pump components from premature wear and catastrophic failure. Two essential accessories belong on every air intake line:

1. Filter Regulators (FR)

Installing a quality filter regulator—such as the Yamada FR-5A—serves two purposes: it filters contaminants out of the supply air and caps the maximum PSI delivered to the pump. Keeping pressure within specified manufacturer limits prevents diaphragm rupture and internal casing damage.

2. Air Intake Ball Valves

Placing a standard ball valve directly before the air motor inlet acts as a manual speed control. By throttling airflow (SCFM) into the pump, you can run the pump at its optimal stroke rate, significantly increasing diaphragm and seal longevity.

Key Takeaways

• PSI = Power (Force): Overcomes system backpressure to keep the pump from stalling.

• SCFM = Volume (Speed): Controls fluid displacement rate (GPM) across the performance curve.

• Air Management: Using filter regulators and intake control valves ensures reliable pump operation and extends service life.