Pump Cavitation Risk Assessment: Calculation, Prevention, and Free Online Calculator

Section summary
1. Definition : what is pump cavitation ?
2. How to fix pump cavitation : calculation
3. Consequences : what are the effects of pump cavitation
4. Calculator : pump at risk of cavitation ?

1. What is pump cavitation ?

The phenomena of pump cavitation is due to the vaporisation of the liquid pumped at the entrance of the pump impeller. Bubbles formed then collapse within the impeller when the pressure goes up again. The quick collapse of the bubbles translates in high forces that can damage the pump impeller.

Cavitation is very harmful for pumps.

2. How to fix pump cavitation : calculation

Cavitation will happen if the pressure at impeller entry is below the saturation pressure of the liquid sucked. Static pressure decreases within the pump suction due to the acceleration of the fluid at the suction and pressure can go below the saturation pressure of the liquid.

Decrease of pressure at pump suction and risk of cavitation

Figure 1 : Pump cavitation - decrease of pressure at pump suction

Cavitation condition : P_suction - NPSH < P_sat

Non Cavitation condition : P_suction - NPSH > P_sat + margin

The NPSH is given by the pump manufacturer.

In practice, a margin must be included in the design to account for process variations (temperature...), it is to be determined by the engineer according to what is acceptable for the case studied (consider different liquid pressure, for example, which can have as effect to change the vapor pressure of the liquid and thus the pressure at which cavitation can appear), according to the calculation codes applicable, or experience - some supplier, if no other value is specified, will consider 0.5 m of margin.

The suction pressure is calculated by summing the pressure in the vessel in which the liquid is pumped with the static pressure of the liquid in the vessel minus the pressure drop due to the suction line. It is therefore necessary to ensure that there is sufficient pressure at the pump suction in order to avoid cavitation.
P_suction = P₁ + h₁*spgr/10.2 - ΔP_f

Note : conversion from bar to m of liquid : ΔP = H*spgr/10.2
- ΔP in bar
- H in m
- spgr in kg/m3

Pump circuit

Figure 2 : Pump circuit

3. Consequences : what are the effects of pump cavitation

Cavitation in pumps has the following effects

Possible reduction in pump capacity due to the vaporization, this drop in performance can be translated by drop of head at same flowrate or a change in the efficiency of the pump
Noise
Vibrations
Wear of impeller and pump housing due to the collapse of the bubbles and the pressure associated. It can decrease pump performance and lead to mechanical failure of the pump. The elements impacted by cavitation take a very specific "pitted" aspect.

4. Calculator : pump at risk of cavitation ?

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.

System Parameters

Fluid Type (for vapor pressure reference)

Vessel Pressure (P1) bar

Static Head (h1) m

Specific Gravity (spgr) dimensionless

Suction Line Pressure Drop (ΔPf) bar

Liquid Saturation Pressure (Psat) bar

Pump NPSH Required (NPSHr) m

Safety Margin m

Results

-

Suction Pressure (Psuction) - bar

-

NPSH Available (NPSHa) - m

-

NPSH Margin (NPSHa - NPSHr) - m

Enter parameters and click calculate

Formulas Used:
Psuction = P1 + (h1 × spgr / 10.2) - ΔPf
NPSHa = (Psuction - Psat) × 10.2 / spgr

Cavitation Assessment:
• Safe: NPSHa > NPSHr + margin
• Warning: NPSHr < NPSHa ≤ NPSHr + margin
• Danger: NPSHa ≤ NPSHr

FAQ: Pump Cavitation

1. What is pump cavitation?

Pump cavitation occurs when the liquid at the pump impeller entrance vaporizes due to low pressure, forming bubbles. These bubbles collapse within the impeller, causing high-pressure shocks that can damage the pump.

2. What causes pump cavitation?

Cavitation is caused when the suction pressure (\( P_{\text{suction}} \)) minus the NPSH (Net Positive Suction Head) drops below the liquid's saturation pressure (\( P_{\text{sat}} \)).

3. How can cavitation be prevented?

Ensure \( P_{\text{suction}} - \text{NPSH} > P_{\text{sat}} + \text{margin} \). Increase suction pressure, reduce suction line losses, or use a pump with lower NPSH requirements.

4. What is NPSH, and why is it important?

NPSH (Net Positive Suction Head) is the minimum pressure required at the pump inlet to prevent cavitation. It is provided by the pump manufacturer and is critical for design.

5. How is suction pressure calculated?

Suction pressure (\( P_{\text{suction}} \)) is calculated as: \[ P_{\text{suction}} = P_1 + \frac{h_1 \times \text{spgr}}{10.2} - \Delta P_f \] Where: - \( P_1 \) = Vessel pressure (bar) - \( h_1 \) = Static head (m) - \( \text{spgr} \) = Specific gravity - \( \Delta P_f \) = Suction line pressure drop (bar).

6. What are the consequences of cavitation?

Cavitation causes reduced pump capacity, noise, vibrations, and wear on the impeller and housing, leading to decreased performance and potential mechanical failure.

7. How is NPSH Available (NPSHa) calculated?

NPSH Available (\( \text{NPSHa} \)) is calculated as: \[ \text{NPSHa} = \left(P_{\text{suction}} - P_{\text{sat}}\right) \times \frac{10.2}{\text{spgr}} \]

8. What is a safe NPSH margin?

A safe NPSH margin is when \( \text{NPSHa} > \text{NPSHr} + \text{margin} \). A common margin is 0.5 meters, but it varies based on application and codes.

9. Are there tools to assess cavitation risk?

Yes, our website offers a free online calculator to determine if your pump is at risk of cavitation based on system parameters.

10. What precautions should be taken when using the calculator?

The calculator is for illustrative purposes only. For detailed design, consult a reputable engineer or designer.

Source

Source [Sihi] Basic Principles for the Design of Centrifugal Pump Installations, 2003

Pump cavitation