Hydronics & Pump Logic | Module 306
NPSHA Field Math Audit
Field Status: Cavitation is the formation and collapse of vapor bubbles within a pump’s eye. If the absolute pressure of the fluid at the pump suction falls below its vapor pressure, the fluid “boils” locally. The subsequent implosion of these bubbles creates microscopic shockwaves that gouge metal from the impeller, causing vibration, noise, and total pump destruction.
NPSHA (Available) must be greater than NPSHR (Required) by at least 3-5 feet of head. If your system NPSHA is too thin, the pump will cavitate the moment the system load increases or the suction strainer starts to plug.
SYS AUDIT: NPSHA Energy Margin
SAFETY MARGIN: —
CAVITATION STATUS: —
—
Pump Suction Dynamics
Increasing system temperature drastically changes the vapor pressure of the fluid. A pump that runs perfectly at 70°F might start cavitating at 120°F because the vapor pressure threshold has shifted, effectively shrinking the NPSHA margin.
| Margin (Ft) | Risk Level | Required Action |
|---|---|---|
| < 0 | Severe | Stop pump. Cavitation is destroying the impeller. |
| 0 – 3 | Unstable | Address suction restrictions (strainers) or reduce fluid temp. |
| 3+ | Secure | Maintain system monitoring. |
Diagnostic Gateway Challenge
A pump is making a sound like “pumping rocks.” You determine NPSHA is 14ft and NPSHR is 13ft. Why is the pump cavitating despite the margin being technically positive?