Damage caused by cavitation is visible on the suction side of an impeller blade. Source: Cooper and Antunes via Schiavello and Visser

An important consideration in the design of centrifugal pumps is a phenomenon known as cavitation. This occurs when the pressure of liquid entering the pump is insufficient to maintain appropriate fluid and flow conditions, resulting in performance degradation and damage to the pump which can cause early pump failure.

What is cavitation?

Cavitation happens when pumped liquid entering the low-pressure region of the pump near the impeller vane entrances falls below the vapor pressure of the liquid. In this circumstance, the liquid will begin to boil, undergoing a phase change to gas within the pump and forming vapor bubbles.

Cavitation occurs when pressure at the pump inlet falls below the vapor pressure of the pumped liquid, generating vapor bubbles that later collapse under higher pressure and damage pump surfaces. Source: Flowserve

When the bubbles enter a region of high pressure within the pump, they collapse and generate shockwaves that damage pump surfaces such as the impeller and casing. Tiny chunks of material are essentially blasted off, causing a steady erosion of pump components that degrades performance and eventually leads to failure of the pump. The process is characterized by a distinct sound emanating from the pump, like gravel running through it, along with increased vibration.

To prevent cavitation from occurring, the absolute pressure of the liquid through each section of the pump must be higher than the vapor pressure of the liquid. It should be noted that vapor pressure depends on both temperature and pressure. If the liquid is hot, the pressure at which it will start to boil is higher than if the liquid is cold. Vapor pressure is also specific to the fluid being pumped. For example, the vapor pressure of butane at 25° C (244 kPa) is much higher than that of water at 25° C (3.171 kPa). Liquid butane at 25° C will boil at all pressures below 244 kPa, including normal atmospheric pressure (101.325 kPa).

Cavitation occurs as high pressure causes vapor to transition back to liquid. During the resulting implosion of gas bubbles, microjets of high-pressure liquid are generated that blast pump surfaces, causing damage. Source: Flow Control Equipment

Net positive suction head

To avoid cavitation, designers of pump systems consider two key criteria: net positive suction head required (NPSH_R) and net positive suction head available (NPSH_A). Sufficient margin between NPSH_A and NPSH_R is the design point that designers seek to prevent cavitation issues.

NPSH_R is specified by the pump manufacturer and represents the pressure energy required to prevent cavitation. NPSH_A is the pressure energy available to meet this requirement. NPSH_A has several components and can be calculated according to the following formula.

NPSH_A = h_a ± h_z – h_f + h_v – h_vp

Where,

• h_a is atmospheric or absolute pressure head, representing the atmospheric pressure acting on the surface of liquid contained in an open or vented tank, or the absolute pressure acting on the surface of liquid in a closed tank.
• h_z is the elevation head, representing the potential energy due to the weight of the liquid caused by gravity as a result of the vertical distance between the surface of the liquid in the suction tank and the pump’s centerline.
• h_f is the friction head, representing losses due to friction in the pipes and fittings leading to the pump’s suction inlet.
• h_v is the velocity head, representing the motion or kinetic energy of the liquid based on its velocity in the suction piping.
• h_vp is the vapor pressure head, representing the vapor pressure of the liquid at the operating temperature.

Each of these parameters is expressed in units of length, or meters, consistent with pressure head, which represents the height of a column of liquid that produces a specific pressure at the column’s base. Pressure head is equivalent to a fluid’s pressure divided by the product of its density, multiplied by the acceleration due to gravity.

How much NSPH margin is enough?

Traditionally, NPSH_R was calculated based on the point at which there is a 3% drop in total pump head, referred to as NPSH3. This measure may be insufficient in some applications to prevent all damage from cavitation.

Cavitation inception and damage begin well before performance deterioration results in a three percent drop in pump head, which is the traditional, and in some cases insufficient, design point for NPSH margin.

That is because the onset of cavitation occurs before NPSH3, at a point known as the incipient net positive suction head (NPSH_i). By the time pump performance has deteriorated and pressure head has fallen 3%, some liquid has already changed to vapor and cavitation is already proceeding.

To guarantee no cavitation, NPSH_A should be equal to or above NPSH_i for the entire operating regime of the pump. This would require a generous safety margin, with NPSH_A much higher than NPSH3.

Designing with higher margins between NPSH_R and NPSH_A, however, can increase the cost of both the pump and the system.

In most real applications, complete elimination of cavitation may not be necessary. Indeed, from a cost-benefit perspective, some cavitation may be acceptable. Minor cavitation may not significantly reduce pump performance and may generate rates of impeller erosion that are low enough to result in satisfactory pump lifetime and reasonable maintenance costs.

Choosing an acceptable NPSH margin involves balancing system cost, pump performance and service life. This is not an easy task since many factors contribute to or inhibit damage due to cavitation.

Bruno Schiavello, director of fluid dynamics at Flowserve Corp., and Frank C. Visser, principal design and fluid dynamics engineer at Flowserve, identify and rank several key parameters in order of the importance of their effect on cavitation damage and recommend that all should be considered in a quantitative model for sufficient NPSH margin. These factors include:

• Peripheral velocity at the impeller eye (U_eye) (8 to 10)Cavitation pitting damage on pump casing. Source: Holland Applied Technologies
• Pump design, primarily the impeller (Q_sl/Q_bep-des, Q_rs/Q_bep-des, special design features) and secondarily the suction chamber (5 to 7)
• Operating capacity ratio (as fraction of Q_sl, preferably, or Q_bep at maximum impeller diameter for less critical duties) (4 to 7)
• Ratio NPSH_i/NPSH_R (4 to 6)
• Liquid density (SG) (4 to 5)
• Impeller material resistance to cavitation erosion: commercially available alloys (2 to 4), special patented alloys (5 to 7)
• Corrosion (various types: chemical, galvanic, etc.) (2 to 4)
• Fluid temperature (1 to 4)
• Air content (noncondensable gas content, more generally) (1 to 4)
• Vapor density (1 to 3)
• Thermodynamic fluid properties (specific heat, vaporization latent heat) (1 to 3)
• Suction specific speed (1)

Designers should take all of these factors into account, in addition to examining opportunities to maximize NSPH_A, to arrive at the optimal balance between the cost, performance and service life of the pump.

For guidance in determining the correct margin between NPSH_A and NPSH_R for rotodynamic pumps, designers can refer to ANSI/HI 9.6.1.

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Resources

Schiavello, Bruno; Visser, Frank C. (2009). Pump Cavitation: Various NPSHR Criteria, NPSHA Margins, Impeller Life Expectancy. Texas A&M University. Turbomachinery Laboratories.