Cavitation and PUmp NPSHr[1]

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Cavitation and Pump NPSHR

Gordon Stables

Engineering Manager CLYDEUNION Pumps Canada

November 2009

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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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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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Bubble collapse


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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 associated with other devices.

Initial cavitiation studies were associated with ships propellers and hydraulic turbines.


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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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Why does a pump cavitate ?

Insufficient NPSHA

Net

Positive

Suction

Head

Available


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Net

Positive

Suction

Head

How do we determine the NPSHR of a pump ?

By physical test usually on water


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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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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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Typical multipoint NPSHR curve


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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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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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Why is suction Specific Speed important ?


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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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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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Why is suction Specific Speed important ?


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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 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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Inception vs. Flow Sodium Pump.


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Zero Cavitation Zone

3% Head Drop Curve

Suction Face Pressure Face

NP

SH

.

Cavitation Inception

Pump will achievePerformance but Will cavitate

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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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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

OU

TLE

Tω

ω Rotation

diam

eter

Pressure face

Suction face

Low Pressure areas at Low flow

Flow

aU m/s

Tan a = C/U = Flow Coefficient.“a”

is the blade angle.

U m/s

C = Fluid Velocity m/sU = Impeller Eye Velocity m/s

Low Pressure areas at High flow

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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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Zero Cavitation4mm BubbleLength 40,000 hrs

3%

Acceptable cavitationTo achieve 40,000 hrs

NP

SH

A

Ø (Q)

NP

SH

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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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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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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.


Documents

Cavitation and PUmp NPSHr[1]