npsh and pump cavitation

Understanding NPSHand Cavitation

Brian GongolDJ Gongol & Associates

March 5, 2015

Do you have any of these symptoms?

Symptoms

growling pumps

Symptoms

worn-out impellers

Symptoms

broken shafts

Symptoms

failed seals

Symptoms

All leading to...a disappointing

change in performance?

You may have low NPSH.

Fortunately, there's hope.

NPSH

Net Positive Suction Head

A centrifugal perspective

Applies to positive-displacement pumps Applies to axial-flow and mixed-flow pumps Also applies to valves

Start with atmospheric pressure

Centrifugal pumps don't suck

...but this parking lot does

Our heavyweight atmosphere:33.9' at sea level

Centrifugal pumps don't suck

They create a low-pressure condition

allowing atmospheric pressure to push

before discharging to higher pressure

More atmosphere, more pressure

In Iowa

In Iowa

Lowest elevation, highest pressure: Lee County at 480' above MSL 33.40' atmospheric pressure

In Iowa

Highest elevation, lowest pressure: Osceola County at 1,670' above MSL 31.96' atmospheric pressure

A difference of 1190', or about1.5' in atmospheric pressurefrom northwest to southeast

1354' to Willis Tower Skydeck

Next: Take deductions

Not the taxable kind

Deduction #1

Vapor pressure

Deduction #1

How hard does the atmospherehave to push backto keep the liquid

from boiling?

Deduction #1

About 1' for waterat room temperature

Deduction #1

Deduction #1

The vapor pressureof water at sea level

at 212°F is about 33.9'

Gasoline, by comparison

Summer-blend gasoline vapor pressure At sea level, 100°F 7.9 psi (18.2')

Deduction #2

Safety factor

Deduction #2

In the civilized world,2' of safety factor

is enough

Deduction #2

2' of safety factor[divided by]

31.96' atmospheric pressure(lowest in Iowa)

[equals]6.25%

Deduction #2

Iowa code requires 6'

[Iowa Wastewater Design Standards,Chapter 13]

Deduction #2

Lowest pressure ever recorded at sea level:29.11'

Or...4.79' below standard pressure.

In the deepest hurricane ever measured.

Deduction #2

Standard pressure at Denver, Colorado:28.0'

Or...5.9' below standard pressure.

Rocky Mountain High

Deduction #2

If your city's elevation rises by one mile,you have bigger problems.

A 6' safety factor is patently unreasonable.

Deduction #2

Highest pressure recorded at Des Moines: 31.06"

Lowest pressure recorded at Des Moines: 28.70"

All-time pressure difference: 2.36", or 2.67 feet of water head

Deduction #3

Total Dynamic Suction Lift

Deduction #3

TDSL =

Actual elevation difference+

Friction losses in suction pipe

Deduction #3

Deduction #3

Suction friction losses also come fromformed suction inlets, strainers,

elbows, and stands

NPSH (available)

Atmospheric pressure minus vapor pressure minus safety factor minus total dynamic suction lift

Give the system what it needs

NPSH (available)must exceed

NPSH (required)for the pump

NPSH (required)

All centrifugal pumpshave a value forrequired NPSH

NPSH (required)

self-priming other suction lift (e.g., with foot valves) flooded suction (e.g., dry-pit submersibles) in-line submersible

NPSHr - axial-flow

NPSHr - end-suction

NPSHr - horizontal split-case

NPSHr - self-priming

NPSHr - submersible

NPSHr - vertical-turbine

NPSH (required)

Just because a pumpdoesn't operate on a suction lift

doesn't meanit isn't affected by NPSH

NPSH (required)

Low NPSH

Many pump system problemsare caused by

inadequate accountingfor NPSH

Low NPSH

Problems are often hidden by other factors

Noise is a great example

Hungry like the pump

Higher flow ratesusually meanhigher NPSHr

(for common Iowa applications)

Hungry like the pump

Relationship of NPSHr to flowdepends upon specific speed

(centrifugal, axial, propeller, radial...)

Balance your NPSH budget

Credits Atmospheric pressure

minus vapor pressure minus safety factor minus TDSL

Debits NPSH required by

pump

Practical (app) makes perfect

Pumps make noise

Practical (app) makes perfect

A quiet pump is generally a happy pump

Hear me out

Mechanical noises

Hydraulic noises

Mechanical noises

Debris Seals, shafts, and moving parts Anywhere lubrication is required

Hydraulic noises

If a sound isn't mechanical,it's likely a problem of hydraulics

Hydraulic noises

Hydraulic problems are usuallysystem problems,

not necessarilypump problems

Hydraulic problems

Was the system designed right? If not, what needs to be fixed? If so, what changed?

Hydraulic noises

Suction cavitation Discharge cavitation Air entrainment (often vortexing) Other (surges, water hammer, excess velocity)

A bubbling problem

Impeller diagnosis

Suction-side cavitation

Discharge-side cavitation

Abrasion

Suction-side cavitation

Let's go to the video

Discharge-side cavitation

Let's go to the video

Air entrainment

Let's go to the video

Remember

Every system isperfectly designedto create the results

it produces

Thank you

Brian Gongolbrian@djgongol.com

515-223-4144

Credits Gasoline vapor pressure:

http://www.chevron.com/documents/pdf/MotorGasTechReview.pdf Iowa topographical map:

http://ortho.gis.iastate.edu/client.cgi?zoom=900&x0=469163&y0=4653689&layer=ned_10m&action=pan&pwidth=600&pheight=450

Satellite photo of Earth: NASA (public domain) http://earthobservatory.nasa.gov/IOTD/view.php?id=78617&eocn=related_to&eoci=related_image

Hurricane photo: NOAA (public domain)

Pump performance curves are the work and property of their respective manufacturers (Gorman-Rupp, Patterson, and Wilo)

All other photos are original work by Brian Gongol