Pump Cavitation and NPSH Calculation

NPSH (Net Positive Suction Head)

• NPSH available must be greater than NPSH required to avoid pump cavitation
• NPSH = Hp - Hvap + Hst - Hsf
• Factors affecting NPSH: temperature, viscosity, suction pressure, and supply restrictions
• Solutions to cavitation: reduce Hp or RPM, increase diameter of suction line

Cavitation

• Occurs when suction pressure falls below NPSH
• Liquid vaporizes, creating gas pockets that slug against components
• Seriously harms internal components

Positive Displacement Pumps (PDP)

• Use pistons, diaphragms, screws, vanes, and gears to displace liquid
• Trap a fixed volume of water and then displace it out
• Mechanical method can be either rotary or reciprocating

Rotary Pumps

• Most common type of PDP
• Use various rotary methods to convey liquid: screws, gears, vanes, lobes

Screw Pumps

• Use a screw to draw fluids through the shaft and into the discharge
• Create very little turbulence in the process
• Used to draw heavy and viscous fluids

Gear Pumps

• Use two mating gears inside a pump casing
• Three categories: centrifugal, positive displacement, and jet pumps

Centrifugal Pumps

• Centrifugal force of inertia propels liquids in a circular rotation into a discharge chute (volute)
• Advantages: relatively affordable, use less space, require less maintenance
• Come in a variety of shapes and sizes by design and for different applications

Centrifugal Pump Designs

• Horizontal and vertical (depending on shaft position)
• Single and multiple stage (referring to the number of impellers)
• Single and multiple suction inlets
• Axial, radial, and mixed flow
• Impeller design

Impellers

• Attached to shaft or on a plate attached to shaft
• Liquid enters eye, and impeller forces liquid out
• Three design types: open, closed, and semi-open

Head Pressure

• Pressure required to push fluid into impeller
• Also referred to as suction head
• Net Positive Suction Head (NPSH) is the minimum absolute pressure at the suction nozzle at which the pump can operate

Cooling Towers

• Material balance: M = Make-up water, C = Circulating water, D = Draw-off water, E = Evaporated water, W = Windage loss of water, X = Concentration in ppmw
• Cycles of concentration: XC / XM (dimensionless)
• Fog forms when conditions are right, and plumes of water vapor can be seen rising from a cooling tower