training - flowserve apm pump cw pump part4

7/26/2019 Training - Flowserve Apm Pump Cw Pump Part4

http://slidepdf.com/reader/full/training-flowserve-apm-pump-cw-pump-part4 1/11

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

78

Rev-0 CIRCULATING WATER PUMP - O & M TRAINING 39 of 97




Cavitation

• Vaporization and the collapse of that vapor within a pump

• Reduces pump performance.

• Sustained Cavitation can cause damage to a pump.


Net Positive Suction Head (NPSH)


•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.


…… Vane “Top Edge”

Cavitation Bubbles

…… Impeller Outter Diameter





Net Positive Suction Head Available

Net Positive

Suction Head

Available

Static suction head ft Friction loss, suction side, ft

Vapor pressure, absolute, ft


Surface pressure, ft Atmospheric pressure, ft






NPSHa on a System in Operation

Suction gauge

pressure, ft

Elevation of gauge

above pump

centerline, ft

Vapor pressure,

absolute, ft


Suction velocity

head, ft

Atmospheric

pressure, ft

Net Positive Suction Head (NPSH)


NPSHR is the head required by the

pump to cover the losses in the

pump suction - that is shown on thepump characteristic curve.






Net Posi tive Suct ion Head Required

•Varies with pump design, size, and operating conditions.

•Requires testing for accurate determination.



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.






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.


• The difference between NPSH A and NPSHR must be sufficiently


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


88





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


3.When the liquid has a high vapor pressure

89


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)






Methods to Reduce NPSHr

•Reduce pump speed

•Use pump with double suction first stage impeller

•Use a vertical turbine type pump


Recirculation Cavitation







Recirculation Cavitation

Damage is influenced by:

•Impeller inlet tip speed

•Degree to which recirculation is taking place

•NPSH margin


•Characteristics of the pumped liquid

•Pump energy level

•Materials of construction


Real World Examples






Real World Examples


Q&A



training - flowserve apm pump cw pump part4

Documents