3.6 Pumps

3.6 Pumps

1. Classification and Working Principles

1. Classification by Principle:

   • Dynamic/Roto-dynamic Pumps:

     • Centrifugal pumps (most common).

     • Axial flow (propeller) pumps.

     • Mixed flow pumps.

   • Positive Displacement Pumps:

     • Reciprocating pumps (piston, plunger, diaphragm).

     • Rotary pumps (gear, vane, screw, lobe).

2. Classification by Flow Direction:

   • Radial Flow (Centrifugal): Flow perpendicular to shaft.

   • Axial Flow: Flow parallel to shaft.

   • Mixed Flow: Combination of radial and axial.

3. Working Principles:

   • Centrifugal Pump:

     • Impeller imparts kinetic energy to fluid.

     • Volute casing converts kinetic energy to pressure.

     • Based on centrifugal force: $P \propto \rho \omega^2 r^2$

   • Reciprocating Pump:

     • Piston draws fluid in during suction stroke.

     • Piston pushes fluid out during delivery stroke.

     • Check valves control flow direction.

   • Axial Flow Pump:

     • Propeller-like impeller pushes fluid axially.

     • High flow rates, low head.

4. Specific Speed ($N_s$) for Pumps:

   • $N_s = \frac{N\sqrt{Q}}{H^{3/4}}$ (SI units)

   • Low $N_s$ (< 30): Radial flow (centrifugal).

   • Medium $N_s$ (30-80): Mixed flow.

   • High $N_s$ (> 80): Axial flow.

2. Components and Functions

1. Centrifugal Pump Components:

   • Impeller:

     • Rotating component with vanes.

     • Types: Open, semi-open, closed.

     • Transfers energy from motor to fluid.

   • Casing:

     • Volute Casing: Spiral shape, converts velocity to pressure.

     • Diffuser Casing: Guide vanes around impeller.

   • Shaft: Transmits torque from motor to impeller.

   • Seals:

     • Mechanical Seals: For high pressure/toxics.

     • Packed Gland: Simple, adjustable.

   • Bearings: Support shaft, reduce friction.

2. Reciprocating Pump Components:

   • Cylinder: Contains piston/plunger.

   • Piston/Plunger: Creates pressure difference.

   • Suction Valve: Allows inflow during suction stroke.

   • Delivery Valve: Allows outflow during delivery stroke.

   • Crank Mechanism: Converts rotary to reciprocating motion.

3. Priming Device Components (if separate):

   • Priming Chamber.

   • Foot Valve (retains prime).

   • Priming Pump.

3. Priming

1. Definition:

   • Filling pump casing and suction line with liquid.

   • Removing air/gas before starting pump.

2. Why Priming is Necessary:

   • Centrifugal pumps cannot pump air/vapor (low density).

   • Prevents dry running (damage to seals, bearings).

   • Ensures proper suction lift.

3. Methods of Priming:

   • Manual Priming: Pouring liquid into casing.

   • Vacuum Priming: Using vacuum pump to remove air.

   • Foot Valve Method: Valve retains liquid in suction line.

   • Priming Chamber: Separate chamber maintains liquid level.

   • Self-priming Pumps: Special design recirculates liquid to remove air.

4. Positive Displacement Pumps:

   • Generally self-priming (can handle air).

   • Can create sufficient vacuum to draw liquid.

4. Net Positive Suction Head (NPSH)

1. NPSH Available (NPSHₐ):

   • Total head at pump inlet minus vapor pressure head.

   • $NPSH_a = \frac{P_{inlet}}{\rho g} + \frac{V^2}{2g} - \frac{P_v}{\rho g}$

   • Alternatively: $NPSH_a = H_{atm} - H_{vap} - H_s - H_f$ Where: $H_s$ = suction lift (+ if pump above source, - if below).

2. NPSH Required (NPSHᵣ):

   • Minimum NPSH needed to prevent cavitation.

   • Specified by pump manufacturer.

   • Depends on pump design, speed, flow rate.

3. Cavitation Prevention:

   • Must satisfy: $NPSH_a > NPSH_r$ (with margin).

   • Typical margin: $NPSH_a > 1.1-1.5 \times NPSH_r$.

4. Factors Affecting NPSH:

   • Increasing NPSHₐ:

     • Lower pump installation (reduce suction lift).

     • Increase source pressure.

     • Larger suction pipe (reduce friction).

     • Cooler liquid (lower vapor pressure).

   • Increasing NPSHᵣ:

     • Higher pump speed.

     • Larger flow rate.

     • Impeller design.

5. Suction Specific Speed ($S$):

   • $S = \frac{N\sqrt{Q}}{(NPSH_r)^{3/4}}$

   • Indicator of cavitation susceptibility.

   • Lower S = better cavitation performance.

5. Performance Curves

1. Head-Capacity Curve (H-Q):

   • Shows head developed vs flow rate.

   • Centrifugal: H decreases with Q (typically).

   • Positive Displacement: Nearly constant H (with relief valve).

2. Efficiency Curve (η-Q):

   • Shows pump efficiency vs flow rate.

   • Peak at Best Efficiency Point (BEP).

   • Operate near BEP for energy savings.

3. Power Curve (P-Q):

   • Shows power consumption vs flow rate.

   • Centrifugal: Power increases with Q.

   • Positive Displacement: Nearly constant power.

4. NPSHᵣ Curve:

   • Shows required NPSH vs flow rate.

   • Typically increases with Q.

5. System Curve:

   • Represents system resistance: $H_{system} = H_{static} + KQ^2$

   • Intersection with pump curve = operating point.

6. Performance at Different Speeds:

   • Affinity Laws:

     • $Q \propto N$

     • $H \propto N^2$

     • $P \propto N^3$

   • Used for speed control and pump selection.

7. Multiple Pump Operation:

   • Series Operation: Heads add, same flow.

     • For high head systems.

   • Parallel Operation: Flows add, same head.

     • For high flow systems.

8. Operating Point:

   • Intersection of pump curve and system curve.

   • Should be near BEP for optimal efficiency.

   • Can be adjusted by valve throttling or speed control.

9. Performance Parameters:

   • Head (H): Energy per unit weight (m).

   • Capacity (Q): Flow rate (m³/s or L/s).

   • Power (P):

     • Hydraulic power: $P_h = \rho g Q H$

     • Shaft power: $P_s = \frac{P_h}{\eta}$

   • Efficiency (η):

     • Overall efficiency: $\eta = \frac{\rho g Q H}{P_{input}}$

     • Components: Mechanical + Hydraulic + Volumetric.