Basics of Cavitation in Pumps

7/31/2019 Basics of Cavitation in Pumps

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CAVITATION,NPSHR,

& NPSHA

Pump BasicsPump Basics


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Cavitation?Volute Wall

Minimum NPSHA must be > NPSHR+ a safety factor

Higher thanVaporizationThreshold, butlower thanNPSHA

Higher pressure, butstill moving fast =

Kinetic energy Moving slowly,now highestpressure(Potential)

Sharp speedincrease due to

centrifugal force.Slight pressureincrease. Bubblesstart to implode,releasingdamaging energy

Vaporizationstarts to occurhere


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CAVITATION

Occurs in the impeller andchamber. Sounds like sand goingthrough the pump.

Cause - Liquid vaporizes as itenters the eyelet of the impeller if

the inlet pressure is too low.These vapor bubbles collapse(implode) under increasedpressure during their exit from theimpeller.


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CAVITATION Damage Caused - The implosion

releases energy which erodes the

impeller and bearings, leading toreduced efficiency and possiblepump failure.

How to Avoid Cavitation - Ensurethat the NPSHA is greater thanthe NPSHR! What does thismean?


5/23

NPSH NPSH - Stands for Net Positive Suction Head.

It is the measure of the amount of energy at the

pump suction available to exert pressure on thefluid (head is energy, measured in feet). Net = Result of adding some positives and

some negatives. P ositive = + (always positive)

S uction = The inlet of the pump Head = Energy in feet, absolute (Includes

atmospheric pressure)+


6/23

NPSHR

NPSHR - Stands for Net Positive Suction Head Required.

Cavitation occurs when the Available Suction Head is less thanRequired for a specific pump. When Suction Pressure is toolow, fluid vaporizes. The created bubbles collapse as theyencounter increased pressure while exiting the impeller,resulting in Cavitation


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NPSHR

NPSHR Is Unique - The Net Positive Suction Head Requiredis unique for each pump design. It is a function of thegeometric design and RPM.

NPSHR is a Given! - Manufacturers provide graphs whichillustrate the NPSHR for each pump model at the full

spectrum of GPM range. As GPM increases, NPSHRincreases.


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NPSHR Curve


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NPSH A - Net Positive Suction Head Available is unique for each specific

application and operating condition. NPSHA - Consists of 5 variables.

1. Gas Pressure - Atmospheric or closed system - (Positive, P a)2. Distance of the fluid source above (positive) or below (negative) the pumpsuction inlet (H e)3. Suction plumbing friction loss (Negative, H

f)

4. Vapor Pressure (Liquid Temperature Sensitive) - (Negative, P v)5. Specific Gravity (S.G.)

Positives prevent cavitation Negatives promote cavitation

NPSHA


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1. Gas Pressure (Air, Vapor, etc.) acting on the fluid surface.Its absolute pressure is always positive (although gauge

pressure may be negative). a. For an Open System, determine the atmosphericpressure based on the elevation of the site aboveseal level (e.g. 14.7psi at sea level multiplied by 2.31to convert psi to feet of head = 33.4 feet). Consultcharts which supply this data.

b. For a Closed System, gauge pressure, plusatmospheric pressure based on the elevation abovesea level. Multiply by 2.31 to convert psi to feet ofhead.

E l e v a

t i o n

P r e s s u r e=

NPSHA


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2. Distance of the liquid sourceabove or below the pump inlet,measured in feet: can be positiveOR negative. Positive suctionhead occurs when the top of thewater source is above the suctionInlet of the pump. Negativeelevation refers to suction lift (likefrom a pond, tank lower than thepump, etc.)

+

-

NPSHA


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3. Suction plumbing friction loss - It isalways Negative. It is the friction loss forall components prior to the pump inlet.As flow increases linearly, friction

increases exponentially. Friction loss iscalculated from friction charts for pipe,fittings, valves, and other special fluidcarrying devices. The GrundfosEngineering Catalog and thePlumbing/Heating, Sump, Sewage &

Effluent Handbook include thisinformation. Friction

F l o w L

i n e a r

= F r i c t

i o n

L o s s

E x p o n e n

t i a l

Friction loss inpipes


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4. Vapor Pressure, at any given temperature isalways Negative. Vapor Pressure reduces thepositive effect of NPSH variable number 1, GasPressure, by lowering the gas density astemperature rises. Vapor Pressure sounds likeit should be positive but it is the result ofvaporization due to higher temperature.

Remember it as Vaporizing (Negating) theAtmospheric Pressure effect.

H o t

C ol d

Affect of

Temperature On Inlet Pressure

NPSHA


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NPSHA

5. Specific Gravity - If the specific gravity is < or > 1.0, the atmosphericpressure - vapor pressure must be adjusted.

2.31 * (P a P v)Specific Gravity


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NPSHA

NPSHA = 2.31 * (P a P v) +/- H e H fSpecific Gravity


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A pump is required to produce 45 gpm at a TDH of 324 feet. Thepump is fed by a reservoir which is at an elevation of 20 feet abovethe inlet of the pump. The system is located at an elevation of2,000 feet above sea level. The water is 50 degrees F. The frictionloss in the plumbing leading from the tank to the pump inlet iscalculated at 3 feet of head.

NPSHR, NPSHA, & CAVITATIONAn Example


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0.00 20.00 40.00 60.00

GPM

0.00

100.00

200.00

300.00

400.00

500.00

F e e

t o

f H e a

d

CR8-80

Desired 45 GPM@ 324 Ft.

Actual 46 GPM@ 336 Ft.

Check It With WinCAPS!


18/23

NPSHR Curve


19/23

Atmospheric Pressure Adjusted for Elevation


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Properties of Water


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1. Pump Size - 45 GPM at 324 feet of TDH. Select CR8-80.

2. NPSHR - At 45 GPM, the CR8 submittal data NPSH(R) Curveindicates a minimum of 3.5 feet of NPSH is required to avoidcavitation.

3. NPSHA 1. Gas Pressure (Open air or closed?) - In this case, an open system.

The elevation chart indicates an atmospheric pressure of + 13.7 psi at anelevation of 2,000 feet.

2. Elevation from the pump inlet to the top of the feed water. Given = +20 feet (since it was above the inlet).

CAVITATION, NPSHR, & NPSHAAn Example - The Answer


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3. Friction loss - Given in this case, 3 feet of head. If not given, calculatelosses in the pipes, fittings, and valves from the feed source to the pumpinlet flange. Include losses in the tank discharge port. Friction charts areavailable to calculate losses in pipes and fittings. Manufacturers cansupply friction loss values for special components.

4. Vapor Pressure - Reading from the Properties of Water chart, theVapor Pressure loss at 50 degrees F is 0.18 psia

5. Specific Gravity = 1.001 at 50 degrees F Total NPSHA = 2.31* (13.7psia - 0.18psia)/1.001 + 20 ft. - 3 ft. = +48.2 ft.

NPSHR = 3.5 ft. NPSHA > NPSHR = A good application Approximately how long could the feed pipe be extended if the lost per 100 feet

of the 2 pipe was 3.9 feet? What could you do to increase the distance andnot cause cavitation?

CAVITATION, NPSHR, & NPSHAAn Example - The Answer


23/23

NPSHA > NPSHR = OK

NPSHA < NPSHR = CAVITATION

CAVITATION, NPSHR, & NPSHASummary

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Basics of Cavitation in Pumps