FSRUG

Feedwater Systems Reliability Users Group (Jan 2012)

San Antonio, TX

SGFP Minimum Flow Operations –

Impact on Reliability

Presenter:

Art Washburn, P.E.

FSRUG

• ICE BREAKER

FSRUG

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

FSRUG

Two Rules:

FSRUG

Two Rules:

Rule #1 – Make Conservative Decisions

FSRUG

Two Rules:

Rule #1 – Make Conservative Decisions

Rule #2 – Maintain Design Control

FSRUG

Two Rules:

Rule #1 – Make Conservative Decisions

Rule #2 – Maintain Design Control

Plant Priorities:

FSRUG

Two Rules:

Rule #1 – Make Conservative Decisions

Rule #2 – Maintain Design Control

Plant Priorities:

1. Nuclear Safety

FSRUG

Two Rules:

Rule #1 – Make Conservative Decisions

Rule #2 – Maintain Design Control

Plant Priorities:

1. Nuclear Safety

2. Legal Requirements

FSRUG

Two Rules:

Rule #1 – Make Conservative Decisions

Rule #2 – Maintain Design Control

Plant Priorities:

1. Nuclear Safety

2. Legal Requirements

3. Efficiency

Minimum Flow Protection – WHAT and WHY?

FSRUG

- 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)

Minimum Flow Operation – Problems

FSRUG

- 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)

Minimum and Maximum Flow Rates

FSRUG

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)

Minimum and Maximum Flow Rates

FSRUG

Minimum and Maximum Flow Rates

FSRUG

Minimum and Maximum Flow Rates

FSRUG

Ref. ISO/CD 10816-7:2004(E)

FSRUG

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

FSRUG

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)

Calculate Recommended Minimum Flow Rate

FSRUG

SGFP – Sulzer 16 x 18 x 17 CD 1 Stage, 5200 RPM

Figure 1 - Guide for Minimum Flow Operation of Pumps

FSRUG

Application Notes for Figure 1

FSRUG

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.

Calculate Recommended Minimum Flow Rate

FSRUG

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

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

FSRUG

Recirculation – Large Vs Small Eye Impeller Design

• Recirculation & Cavitation Damage

• Large Eye = High Nss (suction specific speed)

FSRUG

FSRUG

Predicted Performance Curve for Replacement Feedwater Pump

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.

FSRUG

Recirculation – Large Vs Small Eye Impeller DesignFSRUG

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.

FSRUG

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

FSRUG

Recirculation – Large Vs Small Eye Impeller Design

FSRUG

Equation for Optimizing Eye Diameter:

Eye Diameter (I.D.) =

[(51.27)(Q/N)^0.666 + (Dia Shaft)^2]^0.5

Recirculation – Affect on Part Flow Operation and Corrected by Computational Fluid Dynamics (CFD) Analysis

FSRUG

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.

Recirculation – Affect on Part Flow Operation and Corrected by Computational Fluid Dynamics (CFD) Analysis

FSRUG

Recirculation – Affect on Part Flow Operation and Corrected by Computational Fluid Dynamics (CFD) Analysis

FSRUG

Recirculation – Affect on Part Flow Operation and Corrected by Computational Fluid Dynamics (CFD) Analysis

FSRUG

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

FSRUG

FSRUG

Using Minimum Flow Protection for Discharge Piping Overpressure Protection

Common on Heater Drain Pumps

Be Aware When Rerating as Part of EPU

FSRUG

Minimum Flow Protection – Why ?

- Shutoff Head Operation

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

FSRUG

FSRUG

• ICE BREAKER

THE (HAPPY) END

QUESTIONS ?

FSRUG