Detrimental effects of cavitation on industrial equipments

DETRIMENTAL EFFECTS OF CAVITATION ON INDUSTRIAL EQUIPMENTS

Bruno Gunther
Turbomecánica Ltda.
Concepción
Chile


ABSTRACT

The different aspects of cavitation considering its sources and consequences on the operation of industrial equipment are analyzed in this paper.

Topics :

1. Types of cavitation: hydrodynamic and vibratory.
2. Cavitation as a tribological problem.
3.Industrial equipment subjected to cavitation damage
4. Cavitation monitoring.
5. Detrimental effects of cavitation on industrial equipment:

Cavitation is defined as the formation of bubbles in a liquid when pressure drops to its vapor pressure without incorporating heat.
Cavitation erosion is the damage produced when the vapor bubbles implode when pressure rises locally above the vaporpressure.





1. Types of cavitation :

Hydrodynamic ( Figure 1 )

Vibratory


This type occurs when a solid surface in contact with a liquid moves back and forth at a rate that generates pressures equivalent to the vapor pressure of the fluid when the surface moves away from it. (Figure 2) When the movement is towards the fluid, the bubbles collapse causing erosion. This movement canbe due to wall deformation ( Diesel engine cylinder liners ) or due to relative movement of machine elements ( compressor crankshaft. )
It can be also caused by pressure waves traveling through the liquid causing cavitation on any solid that is submerged in the liquid: (ultrasonic cleaning ) (Figure 3a & 3b)



2. Cavitation as a tribological problem.

“ Metal eating cavitation” canremove hundreds of kilograms of stainless steel per year of a 250 MW hydroturbine runner.
How does cavitation wear out materials?
From a tribological standpoint of view cavitation erosion is a complex process.

There are two different phenomena when vapor cavities collapse:

Pressure Shock Waves of several hundreds of atmospheres hit the surface causing fatigue cracks
High Speed Microjetsremove protective oxide layers and expose subjacent material to corrosion. They also removes particles that break loose from cracks generated by Pressure Shock Waves.
Electrochemical corrosion reactions are accelerated by the mechanical energy input by the microjets and promote the formation of corrosion microcells.

This is why not only mechanical properties are important, but also corrosionresistance characteristics play an important role.
The most commonly used construction material for impellers, runners and parts subjected to cavitation erosion is ASTM A 487 Grade CA6NM steel or “13/4 ” as it is known in industry ( 13% chromium and 4% nickel ).
It is a hard martensite, tempered, corrosion resistant stainless steel of high toughness and endurance limit.
Also Stelliteexhibits high resistance to cavitation erosion.
Elastomeric materials that can absorb and dissipate the impact energy of microjets are also a solution.

The main factors that influence cavitation erosion are:

Material properties:
• tensile strength and surface hardness
• endurance limit ( fatigue resistance)
• corrosion resistance
• toughness
• surface finish ( roughness)

Fluidproperties:

• Composition
• Dissolved and trapped gases.
• Particle content
• Temperature.
Analyzing the influence of temperature it is possible to distinguish three zones ( Figure 4):

“A” no cavitation because vapor pressure is too low.
“B” maximum cavitation erosion.
“C” Hot cavitation implosions of vapor bubbles are too weak to cause erosion.




3. Industrial equipment subjectedto cavitation damage.

Hydrodynamic type:
Centrifugal pumps : impellers, casing, cut water and diffusers
Hydraulic turbines: runner, needle valves and diffuser
Throttle valves:
Orifice plates, venturis, bents.
Ship propellers

Cavitation erosion location:

Centrifugal pump impeller and cut-water




Cavitation erosion in centrifugal pumps can occur in different places...