Pump
Pump Basics
Basics
CAVITATION,
NPSHR,
& NPSHA
Cavitation?
Higher pressure, but
still moving fast =
Kinetic energy
Sharp speed
increase due to
centrifugal force.
Slight pressure
increase. Bubbles
start to implode,
releasing
damaging energy
Vaporization
starts to occur
here
Volute Wall
Moving slowly,
now highest
pressure
(Potential)
Higher than
Vaporization
Threshold, but
lower than
NPSHA
Minimum NPSHA must be > NPSHR
+ a safety factor
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 Required
is 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, NPSHR
increases.
NPSHR Curve
NPSHA
•
•
•
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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
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.
=
Pressure
•
Elevation
NPSHA
NPSHA
•
2. Distance of the liquid source
above or below the pump inlet,
measured in feet: can be positive
OR 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.)
+
-
Friction
=
Friction Loss
Exponential
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.
Flow Linear
•
Friction loss in
pipes
NPSHA
4. Vapor Pressure, at any given temperature is
Hot
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.
Cold
•
Affect of
Temperature
On Inlet Pressure
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
NPSHR, NPSHA, & CAVITATION
An Example
•
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 of
2,000 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.
CR8-80
500.00
Actual 46 GPM
@ 336 Ft.
Feet of Head
400.00
300.00
Check It
With WinCAPS!
Desired 45 GPM
@ 324 Ft.
200.00
100.00
0.00
0.00
20.00
40.00
GPM
60.00
NPSHR Curve
Atmospheric Pressure Adjusted for Elevation
Properties of Water
CAVITATION, NPSHR, & NPSHA
An Example - The Answer
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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) 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, & NPSHA
An 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.
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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, & NPSHA
Summary
• NPSHA > NPSHR = OK
• NPSHA < NPSHR = CAVITATION