Training - Flowserve Apm Pump Cw Pump Part4
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7/26/2019 Training - Flowserve Apm Pump Cw Pump Part4
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Q&A
Flowserve Corporation Property and Confidential 77
Centrifugal Pump Basics
Cavitation
•If the pressure energy falls below the vapor pressure of a liquid, then
vapor is formed.
•Increase in volume from liquid to vapor depends on the liquid,
temperature, and vapor pressure.
1 ft³ of water, evaporated at 212°F and 14.696 psia, becomes
Flowserve Corporation Property and Confidential
1606 ft³ of dry saturated steam, (vapor).*
•Vapor bubbles on their way through the pump re-condense or collapse
when they reach regions of higher pressure
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Centrifugal Pump Basics
Cavitation
• Vaporization and the collapse of that vapor within a pump
• Reduces pump performance.
• Sustained Cavitation can cause damage to a pump.
Flowserve Corporation Property and Confidential 79
Net Positive Suction Head (NPSH)
Centrifugal Pump Basics
•NPSH A, is the head available in the system, taking into account friction
loss in suction piping, valves, fittings etc. NPSHa should always be
given in absolute values.
•In all cases the NPSHA must exceed the NPSHR in order to push the
liquid into the pump.
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…… Vane “Top Edge”
Cavitation Bubbles
…… Impeller Outter Diameter
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Centrifugal Pump Basics
Net Positive Suction Head Available
Net Positive
Suction Head
Available
Static suction head ft Friction loss, suction side, ft
Vapor pressure, absolute, ft
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Surface pressure, ft Atmospheric pressure, ft
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Centrifugal Pump Basics
NPSHa on a System in Operation
Suction gauge
pressure, ft
Elevation of gauge
above pump
centerline, ft
Vapor pressure,
absolute, ft
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Suction velocity
head, ft
Atmospheric
pressure, ft
Net Positive Suction Head (NPSH)
Centrifugal Pump Basics
NPSHR is the head required by the
pump to cover the losses in the
pump suction - that is shown on thepump characteristic curve.
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Centrifugal Pump Basics
Net Posi tive Suct ion Head Required
•Varies with pump design, size, and operating conditions.
•Requires testing for accurate determination.
Flowserve Corporation Property and Confidential 85
Centrifugal Pump Basics
Classical Cavitation
•Classical cavitation occurs when the absolute pressure of a moving
liquid is reduced to, or even below the vapor pressure of the liquid in
the impeller eye.
•Bubbles are formed in the impeller eye.
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Centrifugal Pump Basics
Classical Cavitation
Somewhere along the low pressure side of the impeller vane, the
pressure may once again exceed the vapor pressure and cause the
bubbles to collapse.
Flowserve Corporation Property and Confidential 87
• The difference between NPSH A and NPSHR must be sufficiently
Centrifugal Pump Basics
large to reduce the formation of cavitation.
• There should be a reasonable margin between NPSH A and NPSHR
General guidelines are that NPSH margin should be 5 ft. or20% whichever is greater
HI has come up with a more specific guideline chart which is
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Centrifugal Pump Basics
Factors affecting NPSH A of a Piping System
NPSH available should always be determined during the design of a
pumping system.
1.When the pump is installed an appreciable height above the liquid level
2.When the pump takes suction from a tank under vacuum
Flowserve Corporation Property and Confidential
3.When the liquid has a high vapor pressure
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Centrifugal Pump Basics
Factors affecting NPSH A of a Piping System
NPSH available should always be determined during the design of a
pumping system.
4.When the suction line is unusually long
5.When the pumping system is at an altitude considerably above sea
level (lower atmospheric pressure)
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Centrifugal Pump Basics
Methods to Reduce NPSHr
•Reduce pump speed
•Use pump with double suction first stage impeller
•Use a vertical turbine type pump
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Recirculation Cavitation
Centrifugal Pump Basics
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Centrifugal Pump Basics
Recirculation Cavitation
Damage is influenced by:
•Impeller inlet tip speed
•Degree to which recirculation is taking place
•NPSH margin
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•Characteristics of the pumped liquid
•Pump energy level
•Materials of construction
Centrifugal Pump Basics
Real World Examples
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Centrifugal Pump Basics
Real World Examples
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Q&A
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