slurry pump transient operation and troubleshooting · pdf fileslurry pump transient operation...

Calgary Pump Symposium 2013

Slurry Pump Transient Operation and Troubleshooting

PRESENTED BY

Robert J. VisintainerV.P. Engineering, Research and Development

GIW Industries Inc, Grovetown, GA USA

Calgary Pump Symposium 2015 1

Calgary Pump Symposium 2013Calgary Pump Symposium 2015

Robert J Visintainer

Robert has worked in the design, testing, sales and manufacture of centrifugal pumps since 1981, with special focus on wear prediction methods, pipeline testing, and all aspects of slurry pump hydraulic and mechanical design.

He is a graduate of the Georgia Institute of Technology with degrees in Mechanical Engineering and Physics.

Presenter

2Slurry Pump Transient Operation

and Troubleshooting


Slurry pump transients and faults can arise from a number of causes

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• Start‐up

• Shutdown

• Cavitation

• VFD operation

• Multiple pumps in series

• Multiple pumps in parallel


• Blocked pumps

• “Pumping through” idle pumps

• Reverse rotation

• Long distance pipelines

• Water Hammer

• Pump‐system interactions

We’ll examine each of these today, focusing on the hydraulic issues.


Startup on Part Full LineHard start against an empty line ‐ Potential for onset of cavitation

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Pump Head

FLOW >>>

HEA

D >>>

System Curve during normal operation

NPSHA

NPSHR

Onset of Cavitation


System Curve during startup w/ line part full

The KEY:The pump always operates at the intersection of the pump and system head curves.


Startup on Part Full LineHard start against an empty line ‐ Potential for power overload

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Pump Head

FLOW >>>

HEA

D >>>




Pump Power

Increased power required


Startup against closed or partially closed valveCavitation and overload avoided

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FLOW >>>

HEA

D >>>


System Curve with closed or partially closed valve

Pump Power

NPSHR and power near shut head are low NPSHR

Open valve slowly as system fills


Pump Head


Startup against closed or partially closed valve

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Dos and Don’ts• Use a slurry gate valve.

• Mount valve near pump discharge to avoid water hammer.

• Start opening within 60 seconds of pump startup to avoid excessive vibration and heat buildup in pump.

• DANGER: A typical slurry pump run with closed valves can heat up and explode within 15 to 30 minutes!

• Startup on water, if possible

• Open slowly, to give system time to fill and to avoid water hammer.

• Open fully, to avoid valve wear and energy waste.


Dos and Don’ts• Use a slurry gate valve.

• Mount valve near pump discharge to avoid water hammer.

• Start opening within 60 seconds of pump startup to avoid excessive vibration and heat buildup in pump.

• DANGER: A typical slurry pump run with closed valves can heat up and explode within 15 to 30 minutes!

• Startup on water, if possible

• Open slowly, to give system time to fill and to avoid water hammer.

• Open fully, to avoid valve wear and energy waste.


With proper control, NPSHR and power remain below normal levels as system fills


Startup with a VFDCavitation and power overload can be avoided

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FLOW >>>

HEA

D >>>


Pump Power


Pump Head


Pump Head @ reduced speed

NPSHR


Startup on Part Full Line

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Must account for NPSHA, NPSHR and pump power required during filling at increased flowrate or...

• … start against a closed or partially closed valve.

• … or use a soft start.

• … or use a VFD.

Start on clear water if possible (reduces power and potential for blockage).



Shutdown: In general, reverse startup protocolExample: Shut valve method

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Pump Curve

FLOW >>>

HEA

D >>>


System Curve with closed or partially closed valve

Pump Power

NPSHR

Close valve slowly, then shut pump down


Don’t forget the slurry solids!

Flushing usually required before

shutdown.


Pump start up and shutdownfor multiple pump systems

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• The closest pump to the feed source should be started first.

• Verify that the NPSH requirements of every pump in the system will be met during the start up and shutdown sequence.

• Verify that no pump in the system will see a pressure in excess of its rated value.

• Shutdown pumps in the reverse order that they were started.



Pumping in ParallelConsider the case of three pumps operating against a common system

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System Curve

HEA

D >>>

1 Pump 2 Pumps 3 Pumps

Head

NPSHR

Power

A B C

A = 3 pumps runningB = 2 pumps runningC = 1 pumps running

WARNING: As pumps are dropped off line to reduce flowrate, the NPSHR and Power requirements for the remaining pumps INCREASE!

FLOW >>>



Pumping in ParallelConsider the case of three pumps operating against a common system

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System Curve

HEA

D >>>

Individual pumps 3 Pumps

Head

NPSHR

Power

Various states of wet end wear can result in variable head curves and unequal sharing of the pumping load. Pumps operating in parallel should be kept in similar states of repair.

FLOW >>>


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3

1 = new wet end2 = worn3 = badly worn


Long Distance Pipelines

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• Long distance pipelines often require multiple pumps operating in series.

• Pumps can be located at the beginning of the line or spaced along the line.

• The pressure along the line (or hydraulic gradient) can vary widely, depending on pump placement and startup sequence.

• Where speed and size are the same, pumps will operate under similar conditions.

• When pumps run at a different speeds, or are of different sizes, more consideration is needed.



Long Distance Pipelines

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Pumps clustered at beginning of line

Pumps spread out along line



Pumping in SeriesConsider the case of two pumps in series operating at different speeds

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System Curve

NPSHR

12

3

FLOW >>>


Efficiency

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Duty 1: HS pump on under‐discharge.LS pump on over‐discharge.Efficiency equal, wear not equal.NPSH of LS pump is better.

Duty 2: HS pump @ BEP.NPSH of LS pump now exceeds that of HS pump.

Duty 3: LS pump contributes no head.


Long Distance PipelinesSeveral operating scenarios

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Pump 1 Pump 2 Pump 3 Pump 4HEA

D (P

RESSURE

) >>>

DISTANCE ALONG PIPELINE >>>

Danger of over‐pressure

A. All pumps at beginning of line ‐ Full, steady flowB. Pumps spaced along line ‐ Full, steady flowC. All pumps at beginning of line ‐ Just after hard start (little or no flow)D. Pumps spaced along line ‐ Just after hard start (little or no flow)



Water Hammer

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Vapor Pocket FormsWater Hammer

Sudden Valve Closure

FLOW

Traveling Shock Wave

Pipeline water hammer is caused by the collapse of a large vapor pocket within the pipeline.

Vapor pockets can be caused by valve closure or anything which creates a sudden restriction in flow. They can also be caused by pump cavitation.

The sudden collapse of the vapor pocket creates a pressure wave that moves in both directions away from the source of the collapse at the speed of sound in the fluid. For water, this is approximately 3300 miles/hour ( 5300 km/hr).



Water Hammer

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Ref: https://www.youtube.com/watch?v=f9LY0‐WP9Go



Water Hammer

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A cavitating pump can also cause a water hammer.

In this case, the closure velocity can be as high as 2x the pipeline velocity.

Larger structures, such as pumps and enclosed tanks may be especially susceptible to water hammer, since the shock wave can spread out in these areas and lead to larger forces.

Blockage

Cavitation andVapor formationWater Hammer

Traveling Shock Wave



Water Hammer

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FLOW

Water Hammer(REDUCED)

Vacuum Breaker Opens

Vapor Pocket Forms(MIXED WITH AIR)

Sudden Valve Closure

Traveling Shock Wave(REDUCED)

Rupture disks may offer no protection against water hammer, due to the speed at which the shock wave propagates.

“Vacuum breakers” or “Stand Pipes”, if properly placed, can reduce the effect of water hammer by allowing a cushion of air to form at the vapor collapse site. However, they are subject to clogging and should not be relied upon, or used as a substitute for proper operating procedures.



Blocked Pump Explosion

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Typical time from blockage to explosion in slurry transport service: 15 to 30 minutes!

PIECE OF CASING FOUND 100 METERS AWAY


Warning signs:• No flow• Reduced power draw• Increased pump temp

(well over 100C)



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Ref: Brian P. O’Connor http://www.amre.org.za/downloads/seminars/16March2006/Pump%20explosions%20Amplats.pdf




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Ref: Brian P. O’Connorhttp://www.amre.org.za/downloads/seminars/16March2006/Pump%20explosions%20Amplats.pdf



Pumping Through a Pump

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An impeller driven in its normal direction of rotation by the fluid (e.g. being “pumped through”) produces negative torque may unscrew!

If pumping through cannot be avoided:

Run pump at full power under normal conditions before allowing it to be “pumped through”.

If this is not possible:

Pre‐tighten impeller to 20% full running torque.



Unplanned shutdown & Flow Reversal

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KEY to CHART:

1. Normal operation at 100% speed.

2. Reverse rotation due to backflow through pump (e.g. due to power failure).

3. Backflow valved‐off or diverted.

TORQUE REVERSAL CAUSES IMPELLER TO UNSCREW!

TORQUE POSITIVE

TORQUE NEGATIVE

1

2

3 To prevent unscrewing:a) Do not interrupt backflow.b) Allow pump to come to rest

without interference.c) If valving is necessary, close

VERY slowly. ( 3 minutes for a system with mostly static head.)



Cavitation

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In most cases, the following conditions must be met:

boiling liquid (usually caused by a local pressure drop) in a moving flow with pressure increase downstream (leading to vapor bubble collapse)

FLOW

PRESSURE DROP

PRESSURE RISE andBUBBLE COLLAPSE



CavitationVapor blockage at the pump inlet leads to head loss, noise, vibration and damage


and Troubleshooting


NPSH

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NPSH = the system head above that required to prevent vaporization.

Its usefulness: when NPSH falls to zero, the liquid boils.

It has the same basic formula and units as the slurry system head:Head = P/g + V2/2g + z

Main difference between system head and NSPH is the datum. (Note: All head terms are defined relative to fixed datums)

Head term System Head Datum NPSH Datum Height (z) Pump centerline Pump centerlineVelocity (V) Zero ZeroPressure (P) Atmospheric Fluid vapor pressure



NPSHA: Typical calculation for pumpsNPSH “Available”

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NPSHA = (PA ‐ PVP)/g + z – HFPA (Pa , psf) = Pressure at the liquid free surface. Usually atmospheric.PVP (Pa , psf) = The vapor pressure of the liquid at the suction. (kg/m3 , slug/ft3) = Fluid density (including solids in case of slurry).g (m/s2 , ft/s2) = Acceleration of gravity.z (m , ft) = Height of the free surface above the suction inlet. HF (m , ft) = Friction and shock losses in the inlet piping.



NPSHA: Typical calculation for pumpsNPSH “Available”


and Troubleshooting

NPSHA = (PA + PS ‐ PVP)/g + V2/2gWhere:PS (Pa , psf) = Static Gauge Pressure at the pump suction.V (m/s , ft/s)= Fluid velocity at the pump suction.


NPSHR and Pump PerformanceNPSH “Required”

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In theory, cavitation at the pump suction inlet will occur when the NPSHA there falls to zero.

In practice, localized cavitation occurs elsewhere in the pump at some suction inlet NPSHA value that is greater than zero.

This is usually the result of areas of reduced pressure caused by turbulence around the leading edges of the impeller vanes, or by other characteristics of the pump inlet geometry.

The value of suction inlet NPSHA which results in actual cavitation somewhere in the pump is normally called the „required“ NPSH (NPSHR) and must usually be determined in the test lab.



Operational Stability in Pump‐System Interactions


and Troubleshooting

Calgary Pump Symposium 2013Calgary Pump Symposium 2015 34

KEY POINTS

1. Flowrate is always driven to the intersection of the pump and system head curves. The stability of this intersection point is a key factor in determining system stability.

2. Consider all transient conditions.

3. Operation on downward sloping system curves with centrifugal pumps is inherently unstable and should be avoided.

4. Newtonian Fluids, Settling Slurries and Non‐settling Slurries may have completely different dynamics where stability is concerned. Be cautious in applying experience from one slurry to another.

5. Properly used, variable speed drives can overcome many stability problems.


Operational Stability in Pump‐System Interactions



Questions ?

Calgary Pump Symposium 2015 35

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