283
Chapter 8
Pumps
© Springer Science+Business Media Dordrecht 2015
E. Dick, Fundamentals of Turbomachines, Fluid Mechanics and Its Applications 109,
DOI 10.1007/978-94-017-9627-9_8
Abstract The fundamentals of pump operation have been fully treated in the foregoing chapters. Three particular aspects are discussed in the present chapter. The
first is evaporation of the fluid in liquid state when the pressure inside the pump
becomes lower than the vapour pressure. Cavities with vapour then emerge and the
phenomenon is generally described as cavitation. The second concerns starting up.
Priming is necessary. This is filling the pump and the suction pipe with the liquid
to be pumped. The third topic is the intersection of the pump characteristic with the
load characteristic, which mostly has a large static part. The intersection does not
necessarily result in a stable operating point. Cavitation, priming and stability are
discussed in the present chapter. Further topics are some aspects concerning shaping
of the components, construction and special applications.
8.1 Cavitation
8.1.1 Cavitation Phenomenon and Cavitation
Consequences
Cavitation occurs when a liquid evaporates locally with generation of a vapour
cavity due to static pressure decrease under vapour pressure. The vapour pressure
of a liquid strongly depends on temperature and may be very low at atmospheric
temperature. At 15 °C, the vapour pressure of water is about 2 kPa. It is nevertheless
possible that such a low pressure is attained at the entrance of a pump. Intervening
phenomena are decrease of static pressure due to suction height, pressure drop by
losses and pressure drop by dynamic effects.
Cavitation mainly has two consequences. First, there is erosion. Cavitation-generated vapour bubbles are conveyed by the flow and condense at places where the
static pressure again exceeds the vapour pressure. This occurs in a fast collapse,
termed implosion. The colliding edges of a cavity generate strong liquid jets impacting on material walls. The material surface crumbles locally and pits are formed that
gradually may grow into perforations. The phenomenon is termed pitting. Fatigueresisting materials resist cavitation erosion the best. When cavitation is unavoidable, it is advisable to manufacture the pump from a stainless steel type, e.g. 18/8
Chapter 8
Pumps
© Springer Science+Business Media Dordrecht 2015
E. Dick, Fundamentals of Turbomachines, Fluid Mechanics and Its Applications 109,
DOI 10.1007/978-94-017-9627-9_8
Abstract The fundamentals of pump operation have been fully treated in the foregoing chapters. Three particular aspects are discussed in the present chapter. The
first is evaporation of the fluid in liquid state when the pressure inside the pump
becomes lower than the vapour pressure. Cavities with vapour then emerge and the
phenomenon is generally described as cavitation. The second concerns starting up.
Priming is necessary. This is filling the pump and the suction pipe with the liquid
to be pumped. The third topic is the intersection of the pump characteristic with the
load characteristic, which mostly has a large static part. The intersection does not
necessarily result in a stable operating point. Cavitation, priming and stability are
discussed in the present chapter. Further topics are some aspects concerning shaping
of the components, construction and special applications.
8.1 Cavitation
8.1.1 Cavitation Phenomenon and Cavitation
Consequences
Cavitation occurs when a liquid evaporates locally with generation of a vapour
cavity due to static pressure decrease under vapour pressure. The vapour pressure
of a liquid strongly depends on temperature and may be very low at atmospheric
temperature. At 15 °C, the vapour pressure of water is about 2 kPa. It is nevertheless
possible that such a low pressure is attained at the entrance of a pump. Intervening
phenomena are decrease of static pressure due to suction height, pressure drop by
losses and pressure drop by dynamic effects.
Cavitation mainly has two consequences. First, there is erosion. Cavitation-generated vapour bubbles are conveyed by the flow and condense at places where the
static pressure again exceeds the vapour pressure. This occurs in a fast collapse,
termed implosion. The colliding edges of a cavity generate strong liquid jets impacting on material walls. The material surface crumbles locally and pits are formed that
gradually may grow into perforations. The phenomenon is termed pitting. Fatigueresisting materials resist cavitation erosion the best. When cavitation is unavoidable, it is advisable to manufacture the pump from a stainless steel type, e.g. 18/8