Feedwater Systems Reliability Users Group (Jan 2017) Austin, TX Stretching NPSHA vs. NPSHR to the Limit Presenter: Art Washburn P.E.
DISCUSSION POINTS Two Rules to Get You Home and Plant Priorities Customer Challenge To Perform Inspections & Maintenance Online Manage In-Leakage to Accomplish Inspections by Creative Means Sump Pump and System NPSHR vs. NPSHA Source References
Two Rules:
Two Rules: Rule #1 Make Conservative Decisions
Two Rules: Rule #1 Make Conservative Decisions Rule #2 Maintain Design Control
Two Rules: Rule #1 Make Conservative Decisions Rule #2 Maintain Design Control Plant Priorities:
Two Rules: Rule #1 Make Conservative Decisions Rule #2 Maintain Design Control Plant Priorities: 1. Nuclear Safety
Two Rules: Rule #1 Make Conservative Decisions Rule #2 Maintain Design Control Plant Priorities: 1. Nuclear Safety 2. Legal Requirements
Two Rules: Rule #1 Make Conservative Decisions Rule #2 Maintain Design Control Plant Priorities: 1. Nuclear Safety 2. Legal Requirements 3. Efficiency
Customer Challenge to Perform Inspections & Maintenance Online - Inspect Circulating Water Inlet Piping, Safety Related - Cross Tie Valve Between Plants Leaks ~750 GPM - Dual Unit Outage Required to Allow Personnel Access to Inspect Piping, Water Box Valves
Creative Ideas Needed - Site Planning Dual Unit Outage, Estimated Cost Greater Than $10 Million - Maintenance Organization Identifies Creative Idea to Complete Inspections with 750 GPM In-Leakage - Provide Sump Pump: 1000 GPM @ X FT of Head
Circulating Water Inlet Piping, Water Box, Condenser
Inlet Circulating Water Piping Access Point, 30 Manway
Sump Pump Predicted Performance Test Curve
Net Positive Suction Head Available vs. Required Energy acting on the inlet (suction) side of the pump include: 1. Atmospheric Pressure (positive) 2. Lift (negative) 3. Pump Impeller requirements (negative) 4. Friction losses (negative) In order for the pump to move the water, the Net Positive Suction Head Available (NPSHA) must be greater than the Net Positive Suction Head Required (NPSHR). NPSHA > NPSHR (FT absolute) Atmospheric Pressure > Lift + Impeller Requirement + Friction Losses
Net Positive Suction Head Available vs. Required The available suction head (NPSHA) comes from atmospheric pressure acting on the surface of the water. 1 Atmosphere ~ (14.7 PSIA)(2.31 FT/PSI)/(1.0) ~ 33.9 FT abs NPSHA = 33.9 FT abs WATER
Net Positive Suction Head Available vs. Required The amount of Lift is elevation differece between the surface of the water and the centerline of the pump impeller. Lift can be either positive or negative depending on the actual configuration in which the pump is running. Sump surface above the pump lift is positive Sump surface below the pump lift is negative Lift = 20.5 ft WATER
Net Positive Suction Head Available vs. Required The pump impeller requirements are a function of the flow rate of the individual pump. The higher the flow rate, the greater the NPSH requirement. The NPSH curve the the Oconee Sump Pump is shown below. At maximum expected flow (1000 GPM), NPSHR is approximated to be 8.6 FT abs.
Net Positive Suction Head Available vs. Required The friction losses in the pipe were an unknown for this pump. While significant data exists for friction losses in steel pipe, losses in PVC pipe and fittings is not readily available. Also, due to the size of the pipe (6 inch) the flow velocities (12 ft/sec) are much higher than recommended (5 ft/sec). Based on estimates for friction losses, we knew that we would be near the limit of NPSH at the 1,000 gpm flow rate. This is why the actual test was required to determine for certain that the sump pump would perform as required.
Net Positive Suction Head Available vs. Required SUMMARY: The final equation for NPSH is as follows: NPSHa > NPSHr Atmospheric Pressure > Lift + Impeller Requirement + Friction Losses 33.9 ft > 20.5 ft + 8.6 ft + Friction Losses 33.9 ft > 29.1 ft + Friction Losses Therefore Friction Losses must be less than 4.8 ft in order for the pump to perform its intended function. What we learned in the first test is that the friction losses in the suction line achieve 4.8 ft when the flow rate goes above approximately 950 gpm. Therefore, any flow rate below this will have sufficient NPSH for the pump to perform its intended function.
Test Pit: Maximum Attainable Flowrate at Duplicated Suction Condition
Sump Pump: Inadequate NPSHA Performance
Foot Valve Head Loss Curve: Simmons 8 Valve
Suction Piping: Friction Losses - 6 Sch. 80 Pipe: 1000 GPM, 12.5 FPS, Friction Loss (7.2 FT / 100 FT) - 7.2 / 100 = X / 20, X = 1.44 FT - Limits: - Steel Pipe, 8 12 FPS - PVC Pipe, 5 FPS max (Wear, Noise)
Discharge Piping: Friction Losses - 6 Sch. 80 Pipe: 1000 GPM, 12.5 FPS, Friction Loss (7.2 FT / 100 FT) - Unit 3: 7.2 / 100 = X / 500 FT, X = 32.4 FT - Unit 1: 7.2 / 100 = X / 1100 FT, X = 79.2 FT - Limits: - Steel Pipe, 8 12 FPS - PVC Pipe, 5 FPS max (Wear, Noise)
Sump Pump Predicted Performance Test Curve
Sump Pump Skid Assembly
Sump Pump Skid Assembly - Finished
Impeller
Sump Pump Foot Valve Assembly Sized a Vortex Breaker Plate Plate Diameter Still Fit Thru 30 Access Manway Added Drain Valve to Allow Quick Opening from Outside Pit, Emergent Draining of Water (250 lbs), and Removal of Suction Piping System Valve Leather Flapper
Suction Piping
Suction Piping Joints, Fittings, Zero Leakage
Suction Piping Joints, Fittings, Zero Leakage
Test Pit: Suction Piping Mock Up of Field Configuration
Test Pit: Suction Piping Mock Up of Field Configuration
Test Pit: Suction Piping Mock Up of Field Configuration
Test Pit: Maximum Attainable Flowrate at Duplicated Suction Condition
Sump Pump: Inadequate NPSHA Performance
Field Configuration: - Suction Pipe - Fill & Vent Connects - Manway Cover Positioned for Installation in an Emergency - Limited Access Thru 30 Manway
Field Installation: Discharge Piping
Unit 1: Pump Flowrate Achieved
Field Installation: 30 Manway Access
In-Leakage: Dumping Right on Top of Foot Valve
In-Leakage: View from Turbine Building Bottom Floor
In-Leakage: View from Turbine Building Bottom Floor
In-Leakage: Video from Turbine Building Bottom Floor
Oconee Unit 3: Sump Pump System Configuration
Oconee Unit 3 Inspections: Sump Pump Performance Highlights - Allowed for Inspections of Circulation Water Piping - Unwatering Circulating Water Inlet Piping: X Gallons - Discharge Piping Length of Run: 450 FT - Capacity: 750 GPM In-Leakage, 900 GPM Maximum - Discharge Piping 10 FT Sections: Zero Leakage - 6 Discharge Piping Material: PVC - Discharge Returned Water Back to Lake Keowee
Oconee Unit 1: Sump Pump System Configuration
Oconee Unit 1 Inspections: Sump Pump Performance Highlights - Allowed for Inspections of Circulation Water Piping, Water Box Valves, and an Amertap System Modification - Unwatering Circulating Water Inlet Piping: 3,542,000 Gallons - Discharge Piping Length of Run: 1,100 FT - Capacity: 400 GPM In-Leakage, 880 GPM Maximum Pumped - Discharge Piping 10 FT Sections: Zero Leakage - 6 Discharge Piping Material Changed from PVC to Aluminum. - Material Change Relieved Fire Watch Requirements, as the Piping is Routed Thru Operating Units (2 & 3). - Discharge Returned Water Back to Lake Keowee - Chemistry Department did not have to Manage 3.5 Million Gallons of Water Thru Chemical Treatment Ponds
Oconee Unit 1 Inspections: Pump Lost Prime 3 Times - Pump Prime was Lost One Time as a Result of Low Water Level - Water Wave Generating a Vortex - Scaffold Builder Bumped Dump Valve, Opening Valve
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
Gene Cernan, Last Man on the Moon, Dead at 82 R.I.P.
THE (HAPPY) END QUESTIONS?