Cavitation and Pump NPSHR

Cavitation and Pump NPSHR
Gordon Stables
Engineering Manager CLYDEUNION Pumps Canada
November 2009
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Cavitation and Pump NPSHR

What is Cavitation ?

By cavitation we understand the formation of local vapour bubbles inside a
liquid.

In contrast to boiling, which may be caused either by the input of heat or a
reduction of pressure, cavitiation is a local vaporization of the liquid
induced by hydrodynamic pressure reduction.
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Cavitation and Pump NPSHR

How does cavitation manifest itself

The consequences of cavitation in centrifugal pumps, damage to solid
boundary surfaces, noise generated over a wide frequency spectrum
vibration and loss of capacity / total differential head.

Cavitation will destroy all types of materials.

In centrifugal pumps, cavitation is a determining factor as it sets the lower
limit for the size of the pump and the upper limit for speed.

Cavitation damage takes place when the vapour bubble reaches a zone of
higher pressure where it collapses with very high implosion pressure.
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Cavitation and Pump NPSHR
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Cavitation and Pump NPSHR

Bubble collapse
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Cavitation and Pump NPSHR
Cavitation
Damage caused
by suction
recirculation.
Damage starts on
the vane
pressure surface
and bores
through to the
suction surface.
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Cavitation and Pump NPSHR

Cavitation associated with other
devices.

Initial cavitiation studies were
associated with ships propellers
and hydraulic turbines.
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Cavitation and Pump NPSHR

Cavitation in Valves.

Valves can suffer from cavitation damage if the pressure drop across
the valve is significant and the pressure loss is associated with
sudden expansions.

Valve manufacturers have designed valves that control the pressure
breakdown using torturous paths and limited magnitude losses.
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Cavitation and Pump NPSHR
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Cavitation and Pump NPSHR
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Cavitation and Pump NPSHR
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Cavitation and Pump NPSHR

Why does a pump cavitate ?

Insufficient NPSHA

Net

Positive

Suction

Head

Available
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Cavitation and Pump NPSHR

Net

Positive

Suction

Head

How do we determine the NPSHR of a pump ?

By physical test usually on water
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Cavitation and Pump NPSHR

NPSHA calculation

NPSHA is the value of the suction energy available at the reference
level, usually the centre line of the suction branch.

NPSHR = NPSHA for a pump on test

NPSHA =

Barometric pressure

+ suction line velocity head

+ static head correction to the suction gauge

– the vapour pressure of the test fluid.

NPSH is expressed in ft of fluid
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Cavitation and Pump NPSHR

This is a typical head drop curve with suction pressure
on the x axis and developed head on the Y axis.

The suction pressure is reduced until the 3% head drop
figure is obtained.
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Cavitation and Pump NPSHR

Typical multipoint NPSHR curve
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Cavitation and Pump NPSHR
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Cavitation and Pump NPSHR

Why do we want to know what the NPSHR is for a number of
flows?

Obviously to determine the requirements of the pump however
once we have determined the requirements, we can also calculate
the Suction Specific Speed.

Suction Specific Speed = (speed X flow^.5) / NPSH ^.75
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Cavitation and Pump NPSHR

Why is suction specific speed important ?

Maximum specific speed is usually mandated in customers
specifications

Suction specific speed is to be calculated in accordance with API
610 appendix A; it is the suction specific speed based on NPSHR at
full diameter BEP (Best Efficiency Point) flow.
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Cavitation and Pump NPSHR
 Why is suction Specific Speed important ?
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Cavitation and Pump NPSHR

In the oil crisis of the mid 70’s
users were forced to run
equipment back on the curve.

Pump failures increased
dramatically, particularly seals
and bearings.

This focused the industry’s
attention as plant reliability was
now an even more important
issue.
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Cavitation and Pump NPSHR

Users had taken the term “minimum
flow” to mean MCSF

Manufacturers understanding of
“minimum flow was Minimum thermal
flow.
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
An Exxon Engineer ( Ed Hallam) noticed that not all pumps
experienced the same accelerated failure rates.

He noted that pumps with suction specific speeds (Nss) of more
than 11,000 (usgpm units) were more prone to failure.

The industry picked up on this and 11,000 very quickly became the
maximum allowable Suction specific speed irrespective of pump
type, speed, energy level and density of fluid being pumped.

Today 11,000 prevails as the maximum suction specific speed
allowed in the majority of specifications.
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Cavitation and Pump NPSHR

Why is suction Specific
Speed important ?
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Cavitation and Pump NPSHR

Other things were changing in the Oil industry, namely much larger
equipment was being built to service the massive oil fields in Saudi
Arabia and aggressive development was taking place in the
offshore industry, particularly the North Sea.

In parallel, better instrumentation was becoming available and
more research was being carried on centrifugal pumps.
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Cavitation and Pump NPSHR
 Cavitation Inception.

Work was being carried in the nuclear industry relative to fast
breeder reactors. These reactors used liquid sodium as the heat
transfer medium. The only way to understand what was going on
in the reactor was to monitor high frequency noise level to
determine if any boiling was taking place.

Pump cavitation will send out similar signals.
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
Cavitation Inception.

Research work showed that
instead of the understood
theory that pump cavitation
inception took place at 0 head
drop, cavitation inception
takes place at multiples of 3%
head breakdown.
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
I was lucky to be involved
in the design and
manufacture of a liquid
sodium pump and
Cavitiation Inception
studies were carried out
using head drop, visual
and high frequency noise.

Actual shop tests single
flow inception plot
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Cavitation and Pump NPSHR

Inception vs. Flow Sodium
Pump.
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Cavitation Inception
Zero Cavitation Zone
NPSH
3% Head
Drop Curve
Pump will achieve
Performance but
Will cavitate
Suction Face
Pressure Face
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.
Cavitation and Pump NPSHR

Based on field problems and the then current state of the art, what
did this mean to the pump industry ?

As discussed earlier much larger , more powerful and higher speed
equipment was being installed in the field.

Some of these pumps failed after a number of days due to
cavitation damage, or could not be operated due to surging and
high vibration.
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Cavitation and Pump NPSHR

Vlaming who worked for Saudi Aramco, studied the problem.
Working with various pump manufacturers he came up with a
concept of 40,000 hrs life.

The Impeller inlet would need to be designed to a certain set of
criteria and the appropriate level of NPSH available to guarantee
40,000 hr life.
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Cavitiation and Pump NPSHR
ω Rot
diameter
OUTLET
ation
U m/s
Flow
ω
C = Fluid Velocity m/s
U = Impeller Eye Velocity m/s
Low Pressure areas at High flow
Pressure face
a
U m/s
Tan a = C/U = Flow Coefficient.
“a” is the blade angle.
Suction face
Low Pressure areas at Low flow
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Cavitation and Pump NPSHR

Although cavitation inception could be determined by noise studies
it was understood that pumps could operate with some level of
cavitation and still meet the requirement of 40,000hrs life.

In order to determine what level of cavitation was acceptable,
visual studies were carried out and a standard based on cavitation
bubble length was adopted in the industry. Most major
manufacturers of high energy pumps developed first stage impeller
visual rigs.

Over time, design standards were developed where a level of
confidence was obtained whereby visual studies were no longer
required. With the advent of computational fluid dynamics the
NPSH required for bubble length can now be demonstrated
mathematically.
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NPSHA
NPSH
Cavitation and Pump NPSHR
Acceptable cavitation
To achieve 40,000 hrs
Zero Cavitation
4mm Bubble
Length
40,000 hrs
3%
Ø (Q)
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Cavitation and Pump NPSHR

CFD Analysis

Computational Flow Analysis can now predict cavitation from incipient to
breakdown.
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
CFD Analysis

Computational Flow Analysis can
now predict cavitation from
incipient to breakdown.
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Cavitation and Pump NPSHR

Where do we go from here ??

Some organizations are questioning the absolute nature of the
11,000 Nss boundary.

Is it necessary for lower energy pumps, was the work of Hallam to
subjective and based on pumps with small shafts and single volute
pumps where high radial loads prevailed back on the curve.

Should Fraser's recirculation work be revisited based on CFD
capabilities.
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Cavitation and Pump NPSHR

Recommended Reading From the Proceedings of the 25th
International Pump Users Symposium 2009

Pump Cavitation Various NPSHR Criteria, NPSHA Margins and
Impeller Life Expectancy.

A review of Nss Limitations New Opportunities.
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