CAVITATION,NPSHR,

& NPSHA

Pump BasicsPump Basics

Cavitation?Volute Wall

Minimum NPSHA must be > NPSHR+ a safety factor

Higher than Vaporization Threshold, but lower than NPSHA

Higher pressure, but still moving fast = Kinetic energy

Moving slowly, now highest pressure (Potential)

Sharp speed increase due to centrifugal force. Slight pressure increase. Bubbles start to implode, releasing damaging energy

Vaporization starts to occur here

CAVITATION

• Occurs in the impeller and chamber. Sounds like sand going through the pump.

• Cause - Liquid vaporizes as it enters the eyelet of the impeller if the inlet pressure is too low. These vapor bubbles collapse (implode) under increased pressure during their exit from the impeller.

CAVITATION• Damage Caused - The implosion

releases energy which erodes the impeller and bearings, leading to reduced efficiency and possible pump failure.

• How to Avoid Cavitation - Ensure that the NPSHA is greater than the NPSHR! What does this mean?

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 the fluid (head is energy, measured in feet).

• Net = Result of adding some positives and some negatives.

• Positive = + (always positive)• Suction = The inlet of the pump• Head = Energy in feet, absolute (Includes

atmospheric pressure)+

NPSHR

• NPSHR - Stands for Net Positive Suction Head Required.• Cavitation occurs when the Available Suction Head is less than

Required for a specific pump. When Suction Pressure is too low, fluid vaporizes. The created bubbles collapse as they encounter increased pressure while exiting the impeller, resulting in Cavitation

NPSHR

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

• NPSHR is a Given! - Manufacturers provide graphs which illustrate the NPSHR for each pump model at the full spectrum of GPM range. As GPM increases, NPSHRincreases.

NPSHR Curve

• NPSHA - 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, Pa) 2. Distance of the fluid source above (positive) or below (negative) the pump suction inlet (He)3. Suction plumbing friction loss (Negative, Hf)4. Vapor Pressure (Liquid Temperature Sensitive) - (Negative, Pv)5. Specific Gravity (S.G.)

• Positives prevent cavitation• Negatives promote cavitation

NPSHA

• 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 atmospheric pressure based on the elevation of the site above seal level (e.g. 14.7psi at sea level multiplied by 2.31 to convert psi to feet of head = 33.4 feet). Consult charts which supply this data.

– b. For a Closed System, gauge pressure, plus atmospheric pressure based on the elevation above sea level. Multiply by 2.31 to convert psi to feet of head.

Elev

atio

n

Pres

sure=

NPSHA

• 2. Distance of the liquid source above or below the pump inlet, measured in feet: can be positiveOR negative. Positive suction head occurs when the top of the water source is above the suction Inlet of the pump. Negative elevation refers to suction lift (like from a pond, tank lower than the pump, etc.)

+

-

NPSHA

• 3. Suction plumbing friction loss - It is always Negative. It is the friction loss for all components prior to the pump inlet. As flow increases linearly, friction increases exponentially. Friction loss is calculated from friction charts for pipe, fittings, valves, and other special fluid carrying devices. The Grundfos Engineering Catalog and the Plumbing/Heating, Sump, Sewage & Effluent Handbook include this information.

Friction

Flow

Lin

ear

=

Fric

tion

Loss

Expo

nent

ial

Friction loss in pipes

• 4. Vapor Pressure, at any given temperature is always Negative. Vapor Pressure reduces the positive effect of NPSH variable number 1, Gas Pressure, by lowering the gas density as temperature rises. “Vapor Pressure” sounds like it should be “positive” but it is the result of “vaporization” due to higher temperature. Remember it as “Vaporizing” (Negating) the Atmospheric Pressure effect.

Hot

Cold

Affect of Temperature

On Inlet Pressure

NPSHA

NPSHA

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

2.31 * (Pa – Pv)Specific Gravity

NPSHA

NPSHA = 2.31 * (Pa – Pv) +/- He – Hf Specific Gravity

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

NPSHR, NPSHA, & CAVITATIONAn Example

0.00 20.00 40.00 60.00GPM

0.00

100.00

200.00

300.00

400.00

500.00Fe

et o

f Hea

dCR8-80

Desired 45 GPM @ 324 Ft.

Actual 46 GPM@ 336 Ft.

Check It With WinCAPS!

NPSHR Curve

Atmospheric Pressure Adjusted for Elevation

Properties of Water

• 1. Pump Size - 45 GPM at 324 feet of TDH. Select CR8-80.• 2. NPSHR - At 45 GPM, the CR8 submittal data NPSH(R) Curve

indicates a minimum of 3.5 feet of NPSH is required to avoid cavitation.

• 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 an elevation 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

– 3. Friction loss - Given in this case, 3 feet of head. If not given, calculate losses in the pipes, fittings, and valves from the feed source to the pump inlet flange. Include losses in the tank discharge port. Friction charts are available to calculate losses in pipes and fittings. Manufacturers can supply friction loss values for special components.

– 4. Vapor Pressure - Reading from the “Properties of Water” chart, the Vapor 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 and not cause cavitation?

CAVITATION, NPSHR, & NPSHAAn Example - The Answer

• NPSHA > NPSHR = OK

• NPSHA < NPSHR = CAVITATION

CAVITATION, NPSHR, & NPSHASummary