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Feedwater Systems Reliability Users Group (Jan 2012)
San Antonio, TX
SGFP Minimum Flow Operations –
Impact on Reliability
Presenter:
Art Washburn, P.E.
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• ICE BREAKER
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TOPICS
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Two Rules to Get You Home
Minimum Flow Protection – WHAT and WHY?
Minimum Flow Operation - Problems
Minimum and Maximum Flow Rates
Minimum Flow Energy Levels
Calculate Recommended Minimum Flow Rate
Guide for Minimum Flow Operation of Pumps
Recirculation - Large vs. Small Eye Impeller Design
Predicted Performance Curve for Replacement Feedwater Pump
Recirculation – Affect on Part Flow Operation and Corrected by
Computational Fluid Dynamics (CFD) Analysis
Common Discussions Between User and Pump Supplier
Using Minimum Flow Protection for Discharge Piping Overpressure
Protection
Minimum Flow Protection – Why ?
Source References
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Two Rules:
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Two Rules:
Rule #1 – Make Conservative Decisions
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Two Rules:
Rule #1 – Make Conservative Decisions
Rule #2 – Maintain Design Control
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Two Rules:
Rule #1 – Make Conservative Decisions
Rule #2 – Maintain Design Control
Plant Priorities:
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Two Rules:
Rule #1 – Make Conservative Decisions
Rule #2 – Maintain Design Control
Plant Priorities:
1. Nuclear Safety
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Two Rules:
Rule #1 – Make Conservative Decisions
Rule #2 – Maintain Design Control
Plant Priorities:
1. Nuclear Safety
2. Legal Requirements
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Two Rules:
Rule #1 – Make Conservative Decisions
Rule #2 – Maintain Design Control
Plant Priorities:
1. Nuclear Safety
2. Legal Requirements
3. Efficiency
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Minimum Flow Protection – WHAT and WHY?
- Assure Shutoff Head (zero flow) Operation Does Not Occur
to Protect Pump from Catastrophic Damage
- Extreme Minimum Flow (Unacceptable Temperature Rise)
- Excessive Vibration (Damaging to Equipment)
- Minimum Flow Path Recirculation Back to Pump Suction to
a Large Volume Tank (Deaerator, Condenser)
- Minimum Flow Controlled by Valve, Orifice Assembly
- Main System Pump Suction Flow Velocity (8-10 FPS)
- Main System Pump Discharge Flow Velocity (18-20 FPS)
- Minimum Flow Piping Velocity (25-30 FPS)
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Minimum Flow Operation – Problems
- Pump Oversized for Application
- Inlet Eye Diameter (too large, not optimum)
- Head per Stage excessively high (OEM Pumps
from 1970s)
- Operating Strategy Changes since original Plant
Specification
- Minimum Flow Rate (thermal protection vs.
continuous minimum flow rate)
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Minimum and Maximum Flow Rates
Recommended flow rate ranges for minimum and maximum,
continuous and short-term, operation. From Gülich,
Centrifugal Pumps.
Specific Speed, ns
ns = (n x Q1/2)/(H3/4)
nq = ns x 0.01936
Suction Specific Speed, nss
nss = (n x Q1/2)/(NPSHR3/4)
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Minimum and Maximum Flow Rates
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Minimum and Maximum Flow Rates
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Minimum and Maximum Flow Rates
Ref. ISO/CD 10816-7:2004(E)
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Minimum Flow Energy Levels
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The following criteria has an effect on determining minimum
flow rates:
Specific Speed (Ns)
Suction Specific Speed (Nss)
Energy Level
Vibration / Pulsation Requirements
Service
Hours of Operation
Pumpage
Temperature Rise – Vapour Pressure
Ratio NPSHA to NPSHR
Impeller Material
Impeller Design
Lip Clearance
Normal Duty Energy Level Pumps
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Minimum Flow Energy Levels
• "Normal Duty" Energy Level for Pumps
• "Heavy Duty"
• "High Energy" (API-610 - greater than 650 FT/STAGE and
greater than 350 HP/STAGE)
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Calculate Recommended Minimum Flow Rate
SGFP – Sulzer 16 x 18 x 17 CD 1 Stage, 5200 RPM
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Figure 1 - Guide for Minimum Flow Operation of Pumps
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Application Notes for Figure 1
1) The minimum flow values given are for continuous operation. Continuous
operation is considered as more than 2-hours in any 24 hours of
operation.
2) For intermittent operation, reduce the minimum flow values given by 10
percentage points.
3) The minimum flow values given are for suction specific speeds (Nss) less
than 10,000. Above 10,000 Nss, then increase the values given by 2
percentage points for each 1,000 Nss greater than 10,000 Nss.
4) The minimum flow values are percentages of flow at Best Efficiency Point
(B.E.P.) flow taken as 100%.
5) Specific Speed (Ns), Suction Specific Speed (Nss) and head per stage are
determined at best efficiency point for the actual impeller trim
performance.
6) For multistage pumps, use the Specific Speed (Ns), Suction Specific
Speed (Nss), and the head per stage of the first stage impeller.
7) Specific Speed (Ns) and Suction Specific Speed (Nss) for double suction
impellers are based on 50% of total flow.
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Calculate Recommended Minimum Flow Rate
SGFP – Sulzer 16 x 18 x 17 CD 1 Stage
n = 5,200 RPM
Q = 20,000 GPM @ BEP
H = 2,300 FT/STAGE @ BEP
NPSHR = 190 FT @ BEP
ns = (n x Q1/2)/(H3/4)
= (5200 x 20,0001/2)/(23003/4) = 2200
nss = (n x Q1/2)/(NPSHR3/4)
= (5200 x 10,0001/2)/(1903/4) = 10,000
Min Flow = (0.60)(20,000) = 12,000 GPM
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Recirculation – Large Vs Small Eye Impeller Design
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Suction and Discharge
Recirculation occurs at off
peak (low flow) conditions.
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The turbulent mixing action
due to suction or
discharge re-circulation
leads to surging, pulsations,
vibration, and noise
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Recirculation – Large Vs Small Eye Impeller Design
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Recirculation & Cavitation
Damage
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Large Eye = High Nss
(suction specific speed)
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Predicted Performance Curve for
Replacement Feedwater Pump
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Recirculation – Large Vs Small Eye Impeller Design
Large eye NPSH performance test, notice
the unstable high vibration at low flows.
NSS = 17,000
Eye Diameter = 7.38” (Large Eye)
6 x 10 x 14 CD
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Recirculation – Large Vs Small Eye Impeller Design
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Vibration Level Veff (10-10-1000 Hz) / mm.s-1 (RMS)
Recirculation – Large Vs Small Eye Impeller Design; Pump Vibration Levels
‘Large eye’
4 vanes
96.4%
Max.Dia.
‘Small eye’
5 vanes
96.4%
Max.Dia.
Slide shows the difference between the two
impeller designs in vibration levels.
Flow Rate / gal (UK)/Min
Permissible
Vibration
Limit
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Recirculation – Large Vs Small Eye Impeller Design
Stall ring design is a temporary fix, to have the same influence as
the small eye design on the pump suction performance, note the
B.E.P will not be repositioned with this fix
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Recirculation – Large Vs Small Eye Impeller Design
Equation for Optimizing Eye Diameter:
Eye Diameter (I.D.) =
[(51.27)(Q/N)^0.666 + (Dia Shaft)^2]^0.5
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Recirculation – Affect on Part Flow Operation and
Corrected by Computational Fluid Dynamics (CFD)
Analysis
Closed Impellers typically are lift set
based on total mechanical endplay,
and thermal growth. The location of
the impeller to volute cutwater on
discharge and suction can affect
suction and discharge recirculation.
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Recirculation – Affect on Part Flow Operation and
Corrected by Computational Fluid Dynamics (CFD)
Analysis
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Recirculation – Affect on Part Flow Operation and
Corrected by Computational Fluid Dynamics (CFD)
Analysis
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Recirculation – Affect on Part Flow Operation and
Corrected by Computational Fluid Dynamics (CFD)
Analysis
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Common Discussions between Utility and
Pump Supplier for Replacement Pump or
Repaired Pump:
• Continuous Minimum Flow Required by
Specification vs. Existing Limitations of Minimum
Flow Piping Size
• Vibration Limits Required by Specification (recent
limits are more stringent)
• Demands to Guarantee Mechanical Performance
(Vibration) on Test Stand and in Field
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Using Minimum Flow Protection for Discharge
Piping Overpressure Protection
Common on
Heater Drain
Pumps
Be Aware When
Rerating as Part
of EPU
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Minimum Flow Protection – Why ?
- Shutoff Head Operation
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SOURCE REFERENCES:
• Centrifugal Pumps: Design & Application, Lobanoff and Ross
(practical application)
• Centrifugal and Axial Flow Pumps, Stepanoff
(theory)
• Centrifugal Pumps, Gülich
• Vertical Turbine, Mixed Flow & Propeller Pumps, Dicmus
• Centrifugal Pump Handbook, Sulzer
• Pump Handbook, Karassik
• Centrifugal Pump Clinic, Karassik
• Centrifugal Pumps Selection & Operation, Karassik
• Centrifugal Pumps, Karassik
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• ICE BREAKER
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THE (HAPPY) END
QUESTIONS ?