A centrifugal pump that made a rattling noise. The pump was a 50 mm end-suction, transferring water from a tank 3 meters below. The noise started after 6 months. It sounded like marbles rattling inside the pump casing. The vibration increased. After a year, the impeller was pitted. The customer thought the pump was worn. It wasn’t — the pump was cavitating. The net positive suction head available (NPSHa) was less than the NPSH required (NPSHr). This is about pump cavitation and suction system design.
What is NPSH
NPSH is the pressure head at the pump suction, above the vapor pressure of the fluid. If the pressure at the impeller eye drops below the fluid’s vapor pressure, bubbles form. When the bubbles reach the high-pressure region of the volute, they collapse. The collapse creates shock waves that pit the impeller. The noise sounds like marbles. The vibration destroys the bearings.
NPSHr (required) is published by the pump manufacturer. For a 50 mm pump at 500 L/min, NPSHr is about 3 meters. NPSHa (available) is calculated from the system:
NPSHa = P_surface/ρg ± z – h_loss – P_vapor/ρg
Where P_surface is the tank pressure (atmospheric = 10.3 m), z is the elevation from pump centerline to tank fluid level (negative if suction lift), h_loss is the friction loss in the suction pipe, and P_vapor is the vapor pressure of the fluid (0.24 m for water at 20°C).
For a tank 3 m below the pump (suction lift): NPSHa = 10.3 – 3 – h_loss – 0.24. With 3 m suction lift and 2 m of pipe friction (1.5 inch pipe, 500 L/min, 5 m of pipe): NPSHa = 10.3 – 3 – 2 – 0.24 = 5.06 m. That’s above the NPSHr of 3 m. The pump shouldn’t cavitate. But the suction strainer was clogged (5 m of additional friction). NPSHa dropped to 0.06 m. Below NPSHr. Cavitation started.
What was changed
1. Cleaned the suction strainer. The clogged strainer was the main cause. Cleaning it dropped the friction loss from 7 m to 2 m. NPSHa returned to 5 m. The cavitation noise stopped immediately. The impeller was already pitted (replaced during overhaul).
2. Increased the suction pipe size. The suction pipe was 1.5 inch. Upsized to 2 inch. The velocity dropped from 2.5 m/s to 1.4 m/s. The friction loss dropped to 0.5 m. NPSHa increased to 6.5 m. The pump had margin. The upsized pipe costs $50 but prevents cavitation permanently.
3. Raised the tank level. The tank was 3 m below the pump. The customer raised the tank by 1 m (gravity feed). The suction lift dropped to 2 m. NPSHa increased by another 1 m. The pump now operates with 3.5 m margin above NPSHr. No cavitation possible.
The NPSH margin rule
| Service | Minimum NPSHa – NPSHr |
|---|---|
| Cold water, clean | 1 m |
| Hot water (over 60°C) | 2 m |
| Hydrocarbons (low vapor pressure) | 1.5 m |
| Slurry, dirty | 3 m |
A pump with no margin (NPSHa = NPSHr) cavitates intermittently. Even a small clog in the strainer triggers it. The margin should be at least 1 m for clean water, 2 m for hot water. The pump should never operate at the NPSHr curve point — always on the margin.
The NPSH rule: keep NPSHa at least 1 m above NPSHr, check the suction strainer monthly. The rattling pump wasn’t worn — it was cavitating because a clogged strainer dropped NPSHa below NPSHr. Use oversized suction pipe (under 1.5 m/s velocity). Raise the tank level if possible. Cavitation sound like marbles — never ignore it, it pits the impeller.
