Cavitation in Centrifugal Pumps

 
How does a centrifugal pump work
Principles of Centrifugal Pumps-minIn the volute of the pump fluid enters the pump axially through the eye of the impeller (low pressure area) which rotates at high speed. As the impeller and blades rotate, they transfer momentum to incoming fluid. The fluid accelerates radially outward from the pump chasing and a vacuum is created at the impellers eye that continuously draws more fluid into the pump. As the fluid’s velocity increases its kinetic energy increases. Fluid of high kinetic energy is forced out of the impeller area and enters the volute. In the volute the fluid flows through a continuously increasing cross-sectional area, where the kinetic energy is converted into fluid pressure (according the Bernoulli’s principle).

The impeller blades are usually backward-curved, but there are also radial and forward-curved blade designs. The output pressure slightly changes according to the design used. The blades may be open or closed. Also the diffuser may be fitted with fixed vanes to help guide the flow toward the exit. The energy transferred to the liquid corresponds to the velocity at the edge of the impeller. The faster the impeller revolves or the bigger the impeller is, the higher will the velocity head be.

See also: Euler’s Turbomachine Equation – Pump Theory

Major Failure Modes of Centrifugal Pumps
Since centrifugal pumps are one of the world’s most widely used type of pump, their operational parameters and also their vulnerabilities are well known. This article reviews the major failure modes that are found in centrifugal pumps. In general, pump failures result in operational changes that reduce efficiency or may result in a breakdown of the pump. Reliability of hydraulic systems and also centrifugal pumps is of the highest importance in nuclear engineering.

The failure modes of centrifugal pumps can be grouped into three categories:

Hydraulic Failure Modes

  • Cavitation. Cavitation is, in many cases, an undesirable occurrence. In centrifugal pumps, cavitation causes, damage to components (erosion of the material), vibrations, noise and a loss of efficiency.
  • Pressure Pulsation.  Pressure pulsations are fluctuations in the basic pressure. For high-head pumps, suction and discharge pressure pulsations may cause instability of pump controls, vibration of suction and discharge piping, and high levels of pump noise.
  • Pump Recirculation. A pump operating at lower capacity than design limits can suffer from recirculation which occurs internally in the pumps. Pump recirculation can cause surging and cavitation even when the available NPSHa exceeds the supplier’s NPSHr by considerable margin.
  • Radial and Axial Thrust. High radial thrust that results in excessive shaft deflections may lead to persistent packing or mechanical seal problems, and possibly, shaft failure. Axial thrust is imposed along the shaft axis. High axial thrust may impose an excessive load on the bearing.

Mechanical Failure Modes

  • Shaft Seizure or Break
  • Bearing Failure
  • Seal Failure
  • Vibrations
  • Fatigue

Other Failure Modes

  • Erosion
  • Corrosion

Cavitation in Centrifugal Pumps

cavitation - damaged impeller-minMajor places where cavitation occurs are in pumps, on impellers or propellers. In centrifugal pumps, cavitation results from a reduction in suction pressure, an increase in suction temperature, or an increase in the flow rate above that for which the pump has been designed.

There are two basic types of pump cavitation:

Suction Cavitation

suction cavitation - impeller-minSuction cavitation, or also classic cavitation, occurs when a pump is under low pressure or high vacuum conditions. When the liquid being pumped enters the eye of a centrifugal pump, the pressure is significantly reduced. In some cases the pressure drop is great enough to cause the liquid to flash to steam when the local pressure falls below the saturation pressure for the fluid that is being pumped. Bubbles or cavities will form at the eye of the impeller and subsequently the formed vapour bubbles move into regions of higher pressure as they travel towards the pump discharge. In the higher pressure region the vapor bubbles collapse suddenly on the outer portions of the impeller. This can cause significant damage to all moving parts of a centrifugal pump.

Typical causes of suction cavitation:

  • Pump is running too far right on the pump curve
  • Poor suction conditions (NPSH requirements)
  • Blockage in the pipe on suction side
  • Inappropriate piping design
  • Clogged filters or strainers

To prevent this type of cavitation, the Net Positive Suction Head available (NPSHa) in the system must be higher than the required NPSH of the pump. This problem is typical for suction cavitation and therefore this type of cavitation is also called inadequate NPSHa cavitation.

Net Positive Suction Head

Besides the change of the pump, problems with suction cavitation can be also solved by:

  1. Lowering the temperature
  2. Reduction of motor RPM if possible
  3. Increase of the diameter of the eye of the impeller
  4. Use of an impeller inducer.
  5. Use of two parallel pumps with lower capacity.
  6. Use of a booster pump to feed the principal pump.

Special case of cavitation occurs at the suction side as a result of inappropriate piping in suction line. Use of restrictions, sharp elbows and other hydraulic equipment can turbulize the flow this can contribute to cavitation formation.

 
Net Positive Suction Head

Net Positive Suction HeadNet Positive Suction Head
or NPSH for pumps can be defined as the difference between the suction pressure and the saturation pressure of the fluid, expressed in terms of height of liquid column. NPSH is used to measure how close a fluid is to saturated conditions. Lowering the pressure at the suction side can induce cavitation. If cavitation occurs, the violent collapse of the cavitation bubble creates a shock wave that can carve material from internal pump components (usually the leading edge of the impeller) and creates noise often described as “pumping gravel”. Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.

Net Positive Suction Head - definition

In general, there are two suction heads defined in hydraulics:

  • NPSH Available (NPSHa): The absolute pressure at the suction port of the pump. NPSHa is a function of water temperature. As the inlet temperature increases NPSHa decreases, because the saturation pressure decreases.
  • NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating. NPSHa is not a function of water temperature.

NPSHA is a function of your system and must be calculated, whereas NPSHR is a function of the pump and must be provided by the pump manufacturer. During operation the available NPSH must be maintained at a level greater than the NPSH required by the pump manufacturer. It has been found that cavitation rates increase rapidly with the increase in the volume flow rate. This can be seen from the picture, as volume flow rate increases, NPSH required increases, but the available NPSH decreases.

How to increase NPSH available?

To avoid suction cavitation, NPSH available must be increased as much as possible. The only way to increase NPSH available is to increase the pressure at the pump inlet:

  • Lower the pump level
  • Raise the reservoir level
  • Reduction of motor RPM if possible
  • Reduce minor losses upstream of the pump
  • Reduce major losses upstream of the pump
    • Shorten the length of pipe
    • Use a smoother pipe
  • Increase of the diameter of the eye of the impeller
  • Use of a booster pump to feed the principal pump.

Discharge Cavitation

discharge cavitation - pump-minDischarge cavitation occurs when the pump discharge pressure is extremely high or when the discharge flow is restricted and cannot leave the pump (e.g. caused by closed outlet valve). An extremely high discharge pressure results in the majority of the pumped fluid circulating inside the pump.

This type of cavitation originates from two sources. First, this internal circulation (from high-pressure zones into low-pressure zones) is forced through the clearance between the impeller and the pump housing at high velocity resulting in the formation of a low pressure region (as a result of Bernoulli’s principle) in which cavitation can occur. Second, the liquid is circulating inside the volute of the pump and it rapidly overheats.

In both cases, cavitation have similar consequences. The implosion of bubbles trigger intense shockwaves, causing premature wear of the impeller tips and pump housing. In extreme case, discharge cavitation can cause the impeller shaft to break.

Typical causes of discharge cavitation:

  • Pump is running too far left on the pump curve
  • Blockage in the pipe on discharge side
  • Clogged filters or strainers
  • Inappropriate piping design

Cavitation Number

The Cavitation Number (Ca) or Cavitation Parameter is a dimensionless number used in flow calculations. It is conventional to characterize how close the pressure in the liquid flow is to the vapor pressure (and therefore the potential for cavitation) by means of the cavitation number.

The Cavitation Number can be expressed as:

cavitation number - equation

where

CA  = Cavitation Number

p  = local pressure (Pa)

pv = vapor pressure of the fluid (Pa)

ρ  = density of the fluid (kg/m3)

v  = velocity of fluid (m/s)

Cavitation Damage

Cavitation is, in many cases, an undesirable occurrence. In centrifugal pumps, cavitation causes damage to components (erosion of the material), vibrations, noise and a loss of efficiency.

Source: Wikipedia, CC BY 2.5, https://commons.wikimedia.org/wiki/File:Turbine_Francis_Worn.JPG
Source: Wikipedia, CC BY 2.5,
https://commons.wikimedia.org/wiki/File:Turbine_Francis_Worn.JPG

Perhaps the most important engineering problem caused by cavitation is the material damage that cavitation bubbles can cause when they collapse in the vicinity of a solid surface.  Cavitation bubbles collapse is a violent process that generates highly localized shock waves and microjets. They force energetic liquid into very small volumes, thereby creating spots of high temperature and these intense disturbances generate highly localized and transient surface stresses to a solid surface. Signs of erosion will appear as pitting due to the waterhammering action of the collapsing vapour bubbles. It has been found that cavitation damage rates increase rapidly with the increase in the volume flow rate.

Softer materials can be damaged even by short-term occurrance of cavitation.  Individual pits can be observed after a single bubble collapse. Therefore harder materials are used for centrifugal pumps. But with the harder materials used in most applications, the cyclic stress due to repeated collapses can cause local surface fatigue failure. Thus cavitation damage to metals usually has the appearance of fatigue failure.

cavitation - bubble collapse-minWhen the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump’s lifetime.

Cavitation is usually accompanied also by:

  • Noise. Typical noise is caused by collapsing cavities. The level of the noise that results from cavitation is a measure of the severity of the cavitation.
  • Vibration.  Pump vibrations due to cavitation are characteristically low frequency vibrations, usually found in the 0 to 10 Hz range.
  • Reduction in pump efficiency. A decrease in efficiency of the pump is a more reliable sign of cavitation occurring.
 
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See above:

Centrifugal Pumps