When investigating a pump failure, the source of the problem is often not the pump itself, but the suction line. Even a properly selected pump will experience cavitation in an improperly installed suction line; the impeller will melt, and the bearings and packing will wear out prematurely.
In this guide, we cover the physics of cavitation, NPSH calculations, and practical ways to improve the suction line, using numerical examples.
What is cavitation?
Every liquid has a vapor pressure at every temperature. If the liquid’s pressure drops below this value, the liquid boils—even if its temperature does not rise. Water boils at 20 °C when the pressure drops to approximately 23 mbar.
In a pump suction line, pressure reaches its lowest point at the impeller inlet. If the pressure here drops below the vapor pressure, vapor bubbles form. When the bubbles are carried into the region of the impeller where the pressure rises, they collapse suddenly (implosion). The micro-jets formed at the collapse point generate very high local pressures and dislodge particles from the metal surface.
Signs of cavitation:
- A noise resembling gravel or marbles being transported through the pump—the most typical sign.
- Increased vibration, fluctuations in flow rate and pressure.
- Spongy, porous wear on the impeller inlet edges.
- Recurring seal and bearing failures (an indirect result of vibration).
NPSHa and NPSHr
- NPSHr (required): The net positive suction head required at the suction inlet for the pump to operate without cavitation. This is a pump characteristic, read from the manufacturer’s curve, and increases with flow rate.
- NPSHa (available): The net positive suction head that your system can provide to the pump. This is a system characteristic; you calculate it.
Basic rule: NPSHa must be greater than NPSHr—and the difference should include a safety margin.
How is NPSHa calculated?
In meters of liquid column:
NPSHa = (Pa − Pv) ÷ (ρ × g) ± hs − hf
- Pa — absolute pressure at the surface of the suction tank (Pa). Atmospheric pressure in an open tank.
- Pv — vapor pressure of the liquid at operating temperature (Pa).
- ρ — liquid density (kg/m³), g — 9.81 m/s².
- hs — static head difference (m). If the liquid level is higher than the pump, + (submerged suction); if lower, − (suction lift).
- hf — friction losses in the suction line (m): pipe, elbow, valve, strainer.
Example 1 — Water at 20 °C, 2 m suction lift
Open tank at sea level; pump 2 m above the liquid level; suction line losses 0.8 m.
- Pa = 101,325 Pa · Pv(20 °C) = 2,340 Pa · ρ = 998 kg/m³
- (101,325 − 2,340) ÷ (998 × 9.81) = 98,985 ÷ 9,790 = 10.11 m
- NPSHa = 10.11 − 2.00 (lift) − 0.80 (loss) = 7.31 m
If the pump’s NPSHr value at this flow rate is, for example, 3.5 m, the margin is sufficient.
Example 2 — Same system, but fluid at 80 °C
The only variable is temperature:
- Pv(80 °C) = 47,390 Pa · ρ = 972 kg/m³
- (101.325 − 47.390) ÷ (972 × 9.81) = 53.935 ÷ 9.533 = 5.66 m
- NPSHa = 5.66 − 2.00 − 0.80 = 2.86 m
Same system, same pump—but the NPSHa dropped from 7.31 m to 2.86 m. The pump, with an NPSHr of 3.5 m, now enters cavitation. This is why suction lift is not used with hot liquids, and why the tank is positioned higher than the pump.
| Water temperature | Vapor pressure | Net head provided by the atmosphere (sea level) |
|---|---|---|
| 20 °C | 2,340 Pa | 10.11 m |
| 40 °C | 7,380 Pa | 9.65 m |
| 60 °C | 19,940 Pa | 8.44 m |
| 80 °C | 47,390 Pa | 5.66 m |
| 100 °C | 101,325 Pa | 0 m — forced suction required |
Altitude is also a factor
Atmospheric pressure decreases with altitude; in Turkey, this creates a significant difference that cannot be ignored. For 20 °C water and an open tank:
| Elevation | Atmospheric pressure | Net head (20 °C water) |
|---|---|---|
| 0 m (Istanbul, Izmir) | ≈ 1013 mbar | 10.11 m |
| 900 m (Ankara) | ≈ 910 mbar | 9.05 m |
| 1,900 m (Erzurum) | ≈ 805 mbar | 7.98 m |
A plumbing system that operates without issues at sea level will lose nearly 2 meters of NPSHa when applied to a high-altitude site. This adjustment must be made when replicating the design.
How much margin should be left?
NPSHr is generally defined in manufacturer tests as the point where the head drops by 3%. In other words, when NPSHa = NPSHr, the pump is not “cavitation-free”; this is the point where measurable performance loss begins. Therefore, a margin is essential.
The Hydraulic Institute’s ANSI/HI 9.6.1 guideline specifies the margin as the ratio of NPSHa to NPSHr and adjusts it based on the application:
- Low-energy water and HVAC services: 1.1 – 1.3
- High-suction-head, critical, or boiler feed services: 1.5 – 2.0 and above
General rule of thumb: a minimum absolute margin of 1.0 m or a ratio of 1.1—whichever is greater. For critical services, the higher ratios recommended by the standard should be applied.
Perform the calculation based on the most unfavorable scenario: when the tank is at its lowest level, the liquid is at its hottest, the filter is at its dirtiest, and the pump is operating at its highest flow rate.
How to increase NPSHa?
Each term in the formula represents a point of intervention:
| Method | Effect | Note |
|---|---|---|
| Raise the liquid level / lower the pump | Most effective — hs increases directly | A submerged suction is the ideal configuration |
| Increase the suction pipe diameter | hf decreases significantly | Select a suction pipe larger than the discharge pipe |
| Shortening the intake line and reducing the number of elbows | hf decreases | Move the pump closer to the tank |
| Clean the strainer/filter | hf drops | A clogged strainer is the most commonly overlooked cause |
| Cool the liquid | Pv drops — this has a significant effect | Look at the difference in Example 2 |
| Pressurizing the tank | Pa increases | Applicable in closed systems |
| Pump with lower NPSHr / low speed | NPSHr decreases | Reducing speed significantly reduces NPSHr |
Situation with other pump types
- Positive-displacement pumps (gear, lobe, mono): Their NPSHr values are generally low, but rise rapidly as viscosity increases. For viscous fluids, it is essential to increase the suction pipe diameter and reduce the speed—see our guide on viscous fluid transfer for details.
- Air-operated diaphragm (AODD) pumps: These are self-priming and can handle dry suction; however, suction losses reduce flow rate, and cavitation is possible. See the AODD selection guide.
- Magnetic-coupled pumps: Cavitation is particularly dangerous here; the resulting vapor cuts off lubrication to the silicon carbide bearings, which are lubricated by the product. See the magnetic-coupled pump guide.
- Liquid-ring vacuum pumps: Cavitation occurs in the ring when the service water approaches the vapor pressure. See the vacuum pump selection guide.
Suction Line Checklist
- Have the vapor pressure and density of the liquid at its highest operating temperature been determined?
- Has the site elevation been accounted for in the atmospheric pressure calculation?
- Was the calculation based on the tank’s lowest level?
- Is the suction pipe diameter larger than the discharge pipe’s; is the line short and free of elbows?
- Are there any elevations in the suction line that could create air pockets? (The pipe must rise continuously toward the pump.)
- Is there a throttling valve on the suction side? Flow must never be restricted on the suction side.
- Does the difference between NPSHa and NPSHr provide a margin appropriate for the application class?
If you share your suction conditions (fluid, temperature, elevation, tank configuration, line length), we can evaluate the appropriate pump along with the NPSH margin—please contact us.