Welcome to the Centrifugal Pump Webinar Audio received through the computer via via WebEx, WebEx, or call the WebEx WebEx toll number (US/Canada) 1-408-600-3600 Participants will be muted during the presentation, but can ask questions in the Chat toolbox Question and Answer session to follow the presentation Presented by Jeff Sines, ESI Training Lead
Objectives •
In this webinar we will: –
Identify the major parts of a centrifugal pump
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Review the theory of operation of centrifugal pumps
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Understand Understand the importance of the pump curve and the key information information found on the curve
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Describe NPSH and its impact on pump cavitation
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Learn the Pump Affinity Rules
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Learn how to calculate pump power and operating costs
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Review the process for for sizing a pump
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Understand how a pump and system interact
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Understand Understand parallel and series pump operation
Pumps Convert Energy •
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Output of the driver (motor, (motor, turbine, or engine) is mechanical energy in the form of a rotating rotating shaft The pump converts this mechanical energy into hydraulic hydraulic energy and adds it to the fluid in the form of pressure and flow –
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Centrifugal pumps use a rotating rotating shaft and impeller Positive displacement pumps use rotation or linear motion to displace fluid
Major Pump Parts
Discharge Casing
Pump Shaft Impeller Bearings
Suction
Pump Seal Volute
Theory of Operation •
A pump converts mechanical energy into hydraulic energy –
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Fluid enters the pump suction and flows into the eye of the rotating impeller As fluid approaches the eye, fluid pressure drops as it feels the centrifugal forces and changes direction from axial flow to radial flow The eye of the impeller is the lowest pressure point in the pump.
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The low pressure at the eye helps pull more liquid into the suction
Theory of Operation –
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The fluid proceeds radially outward from the eye and is accelerated along the impeller vanes by the centrifugal forces created by the rotating impeller The fluid leaves the tips of the vanes at a high velocity •
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•
Fluid enters the volute, a region of increasing cross sectional area, where the fluid slows down and increases pressure Velocity head is converted into pressure head The fluid exits the pump at a higher pressure
Velocity & Pressure Profile
Pump Casing •
The casing contains the diffusing element that converts velocity head to pressure head
Single Volute
Double Volute
Diffuser
Radial Loads
Operating Point Affects Reliability High Temperature Rise
Low Flow Cavitation
Low Bearing and Seal Life
Suction Recirculation
Reduced Impeller Life
Discharge Recirculation Low Bearing and Seal Life Cavitation
Impeller Types Open Impeller
Semi-Open Impeller
Closed Impeller
Impeller Leakage Semi-open Impeller
Closed Impeller
Pump Bearings and Seals
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Pump Performance Curve •
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Pump curve defines the relationship between the flow rate and the total head developed by the pump –
Needed for selecting a pump to fit the system
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Used for monitoring the “health” of a pump
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Good tool for troubleshooting pump operation
If the pump is operating as designed, it must operate at a location on the pump curve based on the resistance of the system it is pumping into The pump curve is the most critical piece of information needed about the pumping system
Typical Pump Curve
Individual Pump Curve
Curves for Different Types of Centrifugal Pumps
Pump Efficiency •
Efficiency is the ratio of the energy added to the liquid to the energy applied to the shaft
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Provided by manufacturer, typically on the pump curve Best efficiency point occurs where there is the least amount of vibration of the pump Efficiency decreases to the right and left of the BEP Seal and bearing maintenance increases the farther the pump is operated from the BEP
Pump Power Calculations •
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•
Water horse power (whp): power added to the fluid (pump output power)
Brake horse power (bhp): power applied to the pump shaft; also the output power of the motor
Electrical horse power (ehp): electrical power applied to the VFD or motor
Power Added to the Fluid •
•
Area formed by a rectangle at the operating point on the pump curve represents the amount of power added to the fluid (whp) BHP takes pump efficiency into account
Calculating Pump Operating Cost
where:
Q = volumetric flow rate (US gpm) H = total head (ft) ρ =
fluid density (lb/ft3)
η p =
pump efficiency
η m = motor efficiency η VSD =
variable speed drive efficiency
hrs = time period for analysis (hrs) $/kWh = energy cost per kilowatt-hour
Example: Pump Power and Operating Cost •
Calculate whp, bhp, ehp, motor power (kW), and operating cost for a fixed speed pump with a 17.3125 ” diameter
impeller pumping 6,000 gpm of water at 60 °F
Given: •
Time = 8,000 hours
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Power cost = $0.08 / kWh
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motor = 93% ρ = 62.4 lb/ft3
From Pump Curve: •
TH = 250 ft
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pump = 87%
Example: Pump Power and Operating Cost
Cavitation •
The formation and subsequent collapse of vapor bubbles as the fluid’s pressure drops below, then
rises above, the vapor pressure –
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Fluid pressure reaches its lowest point at the eye of the impeller If pressure falls below the vapor pressure, vapor bubbles will form As the fluid moves along the impeller vanes, pressure increases above the vapor pressure The vapor bubbles collapse, creating a high velocity jet of water that implodes on the surface of the impeller vane or pump casing
Effects of Cavitation •
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Vapor bubbles take up more space than the same mass of liquid, causing a reduction in the mass flow rate through the pump Collapsing vapor bubbles may be barely audible or very loud like the sound of gravel passing through the pump
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Pump discharge pressure may oscillate
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Increased vibration levels may result in bearing or seal damage
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Long term cavitation will damage the impeller and degrade pump performance, or cause catastrophic failure of the impeller
http://en.wikipedia.org/wiki/File:Cavitation.jpg http://commons.wikimedia.org/wiki/File:Kavitation_at_pump_impeller.jpg
Net Positive Suction Head (NPSH) •
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NPSH: the amount of fluid energy at the pump suction NPSH required: the amount of fluid energy at the eye of the impeller when cavitation is just beginning to occur NPSH available: the amount of fluid energy the system provides at the pump suction
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NPSHa must be greater than NPSHr to prevent cavitation
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NPSHr is determined by the pump manufacturer –
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Pump is run at a constant flow rate in a test system Supply tank pressure is lowered until the pump cavitates, as indicated by a 3% drop in pump total head from the pump curve
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NPSH is calculated
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Test is repeated at various flow rates
Calculating NPSHa =
+ − ×
144
+ + − − ℎ
where: P tank = pressure on the liquid surface of the supply tank (psig) P atm = local atmospheric pressure (psia) P vp = vapor pressure of the liquid entering the pump suction (psia) ρ = fluid density (lb/ft3) Z tank = bottom elevation of the supply tank (ft) Z level = liquid level measured from the bottom of the tank (ft) Z pump = elevation of the centerline of the pump suction (ft) h L = total head loss in the suction pipeline (ft)
A reasonable approximation using pump suction pressure gage: where: P in
≅
+ − ×
144
= pressure gage reading at the pump’s suction (psig)
NPSHa Calculation for Flooded Suction Pvp = 0.256 psia ρ = 62.4 lb/ft 3
=
=
+ − ×
144
+ + − − ℎ
144 14.7 − 0.256 × + 100 + 5 − 95 − 1.7 = 41.6 62.4 3
NPSHa Calculation for Suction Lift Pvp = 0.256 psia ρ = 62.4 lb/ft3
=
=
+ − ×
144
+ + − − ℎ
144 14.7 − 0.256 × + 85 + 5 − 95 − 1.7 = 26.6 62.4 3
Solutions for a Cavitating Pump •
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Reduce pump flow rate to reduce NPSHr and increase NPSHa Raise the supply tank level or pressure Reduce fluid temperature Reduce head loss in suction line –
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Use low resistance isolation valves (ball, gate, or butterfly valves vs. globe valve) Use larger pipe size to reduce fluid velocity and head loss Minimize number of elbows and other fittings
Install an inducer on the impeller Use impeller and casing made of harder material Select a different pump with lower NPSHr If designing a new system: – –
Raise supply tank elevation Lower pump suction
Pump Affinity Rules •
Pump performance can be changed by changing the pump speed or impeller diameter –
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Changing pump speed Capacity
Q 1 /Q 2 =N1 / N2
Head
H1 /H2 = (N1/N2)2
Power
P1 /P2 = (N1/N2)3
Changing pump impeller diameter* Capacity
Q 1 /Q 2 = D1 /D2
Head
H1 /H2 = (D1/D2)2
Power
P1 /P2 = (D1/D2)3
* Use for small changes
Affinity Rules for Speed Are Very Accurate
Affinity Rules for Impeller Size Not as Accurate Due to Hydraulic Dissimilarities •
Best to interpolate between the manufacturer’s
catalog curves
Sizing a Centrifugal Pump •
A pump is selected to meet the hydraulic needs of the piping system based on design flow rates, system configuration, and end user requirements Static head: elevation and pressure differences between the end user and supply tank –
= ∆ + ∆ ∆ =
∆ –
−
−
Dynamic head: sum of head loss for pipelines, valves, fittings, and components at the desired flow rate •
Includes the “Control Head”, the desired amount of head available
for control of flow or pressure using a control valve –
Design Margin
Example: Sizing a Centrifugal Pump
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Static Head = Δ Elevation Head + Δ Pressure Head –
Elevation Head = (50 ft + 15 ft) – (0 ft + 25 ft) = 40 ft
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Pressure Head = (10 psi – 0 psi) x 144/ 62.4 = 23.1 ft
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Total Static Head = 40 ft + 23.1 ft = 63.1 ft
Dynamic Head = Individual Head Losses –
Dynamic head = (1.2 + 1.5 + 57.8 + 1.3 + 1.5) = 63.3 ft
Example: Sizing a Centrifugal Pump •
Control Head –
Varies depending on controllability required for the process
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Various thumb rules are used:
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30% of total dynamic head
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10 to 25 psi pressure drop at maximum flow rate
Head produced by the pump above that needed by the process will be dissipated across the control valve as additional control head
Design Margin –
Based on uncertainty in the amount of dynamic head
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10 to 30% of dynamic head
Total Head Needed = 170.5 ft –
Static + dynamic head: 126.4 ft
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Control head: 15 psi = 34.6 ft
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Design margin: 15% (of 63.3 ft of dynamic head) = 9.5 ft
Example: Sizing a Centrifugal Pump •
Range charts (family curves) –
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2,500 gpm and 170 ft TH falls in envelop of 3 or 4 potential pumps
Select a pump and impeller trim that has an operating point close to the BEP
Example: Sizing a Centrifugal Pump
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Pump and control valve sizing go hand-in-hand –
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With a pump selected and the Total Head determined, the actual control valve differential pressure can be calculated and the valve can be appropriately sized Additional pump head added because of the design margin will have to be dissipated across the control valve
Pump and System Interaction: Operating Point •
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Pump must operate somewhere on its curve Pump operates at the intersection of the Pump Curve and the System Resistance Curve
Effect of Throttling on the SRC •
Throttling can be viewed two ways on the Pump and System Resistance Curves –
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SRC becomes steeper and pivots as the operating point moves to the left on the pump curve SRC represents the system with all control valves fully open and stays the same, the amount of head loss across the throttled valve is the difference between the SRC and pump curve at the pump flow rate 40
Control Valve Regulates System Flow Rate •
System operating at 2500, 2000, 1500, and 1000 gpm for 2,000 hours/year each — Total power used:
725,978 kWh — Energy Costs at
$0.10/kWh: $72,598
Pump Speed Controls System Flow Rate •
System operating at 2500, 2000, 1500, and 1000 gpm for 2,000 hours/year each — Total power used:
388,600 kWh — Total Energy Costs at
$0.10/kWh: $38,860 — Savings: $33,740
(46%)
Parallel Pump Operation
Series Pump Operation
