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FSRUG
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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FSRUG
• ICE BREAKER
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TOPICS• 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?
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- 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
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- 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
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Recommended flow rate ranges for minimum and maximum, continuous and short-term, operation. From Gülich, Centrifugal Pumps.
nq = ns x 0.01936
Specific Speed, ns
ns = (n x Q1/2)/(H3/4)
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
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Ref. ISO/CD 10816-7:2004(E)
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Minimum Flow Energy Levels
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
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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
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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
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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 Suction and Discharge
Recirculation occurs at off peak (low flow) conditions. 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
• Recirculation & Cavitation Damage
• 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
Eye Diameter = 7.38” (Large Eye)NSS = 17,000
6 x 10 x 14 CD
Large eye NPSH performance test, notice
the unstable high vibration at low flows.
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Recirculation – Large Vs Small Eye Impeller DesignFSRUG
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Recirculation – Large Vs Small Eye Impeller Design; Pump Vibration Levels
‘Large eye’ 4 vanes96.4%
Max.Dia.
‘Small eye’ 5 vanes96.4%
Max.Dia.
PermissibleVibration
Limit
Vib
rati
on L
evel
Vef
f (1
0-10
-100
0 H
z) /
mm
.s-1 (
RM
S)
Flow Rate / gal (UK)/Min
Slide shows the difference between the twoimpeller designs in vibration levels.
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Recirculation – Large Vs Small Eye Impeller DesignStall 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
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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
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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 ?
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