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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
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Slurry pump transients and faults can arise from a number of causes
3
• Start‐up
• Shutdown
• Cavitation
• VFD operation
• Multiple pumps in series
• Multiple pumps in parallel
Slurry Pump Transient Operation and Troubleshooting
• 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.
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Startup on Part Full LineHard start against an empty line ‐ Potential for onset of cavitation
4
Pump Head
FLOW >>>
HEA
D >>>
System Curve during normal operation
NPSHA
NPSHR
Onset of Cavitation
Slurry Pump Transient Operation and Troubleshooting
System Curve during startup w/ line part full
The KEY:The pump always operates at the intersection of the pump and system head curves.
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Startup on Part Full LineHard start against an empty line ‐ Potential for power overload
5
Pump Head
FLOW >>>
HEA
D >>>
System Curve during normal operation
Slurry Pump Transient Operation and Troubleshooting
System Curve during startup w/ line part full
Pump Power
Increased power required
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Startup against closed or partially closed valveCavitation and overload avoided
6
FLOW >>>
HEA
D >>>
System Curve during normal operation
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
Slurry Pump Transient Operation and Troubleshooting
Pump Head
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Startup against closed or partially closed valve
7
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.
Slurry Pump Transient Operation and Troubleshooting
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.
Calgary Pump Symposium 2013
With proper control, NPSHR and power remain below normal levels as system fills
Calgary Pump Symposium 2015
Startup with a VFDCavitation and power overload can be avoided
8
FLOW >>>
HEA
D >>>
System Curve during normal operation
Pump Power
Slurry Pump Transient Operation and Troubleshooting
Pump Head
System Curve during startup w/ line part full
Pump Head @ reduced speed
NPSHR
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Startup on Part Full Line
9
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).
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Shutdown: In general, reverse startup protocolExample: Shut valve method
10
Pump Curve
FLOW >>>
HEA
D >>>
System Curve during normal operation
System Curve with closed or partially closed valve
Pump Power
NPSHR
Close valve slowly, then shut pump down
Slurry Pump Transient Operation and Troubleshooting
Don’t forget the slurry solids!
Flushing usually required before
shutdown.
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Pump start up and shutdownfor multiple pump systems
11
• 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.
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
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 >>>
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Pumping in ParallelConsider the case of three pumps operating against a common system
13
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 >>>
Slurry Pump Transient Operation and Troubleshooting
12
3
1 = new wet end2 = worn3 = badly worn
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Long Distance Pipelines
14
• 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.
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Long Distance Pipelines
15
Pumps clustered at beginning of line
Pumps spread out along line
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Pumping in SeriesConsider the case of two pumps in series operating at different speeds
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System Curve
NPSHR
12
3
FLOW >>>
Slurry Pump Transient Operation and Troubleshooting
Efficiency
1 32
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.
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
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)
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
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).
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Water Hammer
19
Ref: https://www.youtube.com/watch?v=f9LY0‐WP9Go
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Water Hammer
20
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
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Water Hammer
21
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.
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Blocked Pump Explosion
22
Typical time from blockage to explosion in slurry transport service: 15 to 30 minutes!
PIECE OF CASING FOUND 100 METERS AWAY
Slurry Pump Transient Operation and Troubleshooting
Warning signs:• No flow• Reduced power draw• Increased pump temp
(well over 100C)
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Blocked Pump Explosion
23
Ref: Brian P. O’Connor http://www.amre.org.za/downloads/seminars/16March2006/Pump%20explosions%20Amplats.pdf
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Blocked Pump Explosion
24
Ref: Brian P. O’Connorhttp://www.amre.org.za/downloads/seminars/16March2006/Pump%20explosions%20Amplats.pdf
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Pumping Through a Pump
25
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.
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Unplanned shutdown & Flow Reversal
26
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.)
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
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
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
CavitationVapor blockage at the pump inlet leads to head loss, noise, vibration and damage
28Slurry Pump Transient Operation
and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
NPSH
29
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
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
NPSHA: Typical calculation for pumpsNPSH “Available”
30
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.
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
NPSHA: Typical calculation for pumpsNPSH “Available”
31Slurry Pump Transient Operation
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.
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
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.
Slurry Pump Transient Operation and Troubleshooting
Calgary Pump Symposium 2013Calgary Pump Symposium 2015
Operational Stability in Pump‐System Interactions
33Slurry Pump Transient Operation
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.
Slurry Pump Transient Operation and Troubleshooting
Operational Stability in Pump‐System Interactions
Calgary Pump Symposium 2013
Slurry Pump Transient Operation and Troubleshooting
Questions ?
Calgary Pump Symposium 2015 35