Pumps

What are the two main types of pumps, and how are they generally characterized?

• Positive Displacement Pumps

  • Definition: They physically displace or push a fixed volume of fluid out through the discharge port. The volume of the pump cavity is fixed and forces fluid movement.

  • Key Point: Fluid flow is directly related to the pumping element’s movement (e.g., a piston or gear).

• Variable (Dynamic) Displacement Pumps

  • Definition: They impart velocity (kinetic energy) to the fluid—often using centrifugal force—and then convert that velocity to pressure.

  • Key Point: Flow rate can change depending on discharge pressure and pump design; these are typically centrifugal-type pumps.

Positive Displacement Pumps: What are the general characteristics, and what are the major categories (with subtypes)?

General Characteristics

• They physically force liquid to move in discrete volumes.

• Flow is constant regardless of pressure, within mechanical limits.

• Old-fashioned well pumps work by moving a piston with check valves (foot valve at the bottom).

• Piston must move in order to displace fluid.

Major Categories

1. Reciprocating Pumps (Back-and-forth movement)

   • Advantages:

     • More constant efficiency over a range of capacities.

     • Can handle variable pressures without speed adjustment or throttling.

   • Subtypes:

     1. Plunger Pumps

        • Plunger moves back and forth in a cylinder.

        • Inlet valve and discharge valve control flow.

        • Stroke: distance the plunger travels.

        • Single-acting plunger has one discharge stroke per revolution → results in pulsations.

     2. Piston Pumps

        • May be double-acting (the back side of the piston can do the opposite phase, enhancing output).

        • Low cost, simple design, high pressure capability.

        • Cannot handle abrasives or crystallizing liquids; cannot run dry.

        • Positive suction head typically required.

     3. Diaphragm Pumps

        • Diaphragm flexes to displace fluid.

        • Excellent for chemical or hazardous fluids (no leakage).

        • Accurate metering of flow, can run dry, and can handle air.

        • Typically smaller capacity (around 20 gpm) and produce pulsating flow.

2. Rotary Pumps (Rotating element inside a tight casing)

   • Characteristics:

     • Fluid moves in pockets from inlet to discharge with minimal clearance.

     • Smoother flow than reciprocating pumps.

     • Good for metering, viscous fluids, hydraulic fluids, slurries, food processing.

   • Common Types:

     1. Gear Pumps (internal or external)

     2. Lobe Pumps (e.g., single-lobe, three-lobe)

     3. Circumferential Piston Pumps

     4. Rotary Vane Pumps

        • Off-center rotor with sliding vanes that create variable-volume pockets.

        • Commonly used in vacuum installations (e.g., two-stage vane pumps).

Variable (Dynamic) Displacement Pumps: How do they work, and what are their main subtypes?

General Principle

• Fluid enters the pump near the center of rotation (the “eye” of the impeller).

• The spinning impeller imparts velocity to the fluid (centrifugal force).

• That velocity is then converted into pressure energy in the pump casing (volute or diffuser).

Subtypes

1. Centrifugal (Radial Flow) Pumps

   • Most common type (especially in heating/cooling).

   • Discharge flow is perpendicular (radial) to the impeller’s axis.

   • Has two main parts:

     • Rotating element: impeller + shaft

     • Stationary element: casing + stuffing box + bearings (“collector”)

   • Collector can be a volute (spiral shape) or have a diffuser (stationary vanes).

   • Typically used for high head, moderate flows.

2. Axial Flow Pumps (“Propeller” pumps)

   • Fluid flows axially (parallel) through impeller.

   • Uses diffuser vanes instead of a volute casing.

   • Usually low discharge pressure but high flow capacity.

   • Often in large water-movement applications (e.g., water treatment, fire deluge systems).

   • Requires submergence in the fluid (poor suction lift).

3. Peripheral Flow Pumps

   • The pumping action occurs at the periphery of the casing.

   • Two types: High-speed and Regenerative turbine.

4. Mixed Flow Pumps

   • Combine radial (centrifugal) and axial flow.

   • Provide moderate discharge head with high flow capacities.

   • May use volute or diffuser-style casing.

Key Concepts in Pump Selection (Obj 2): Define force, density, pressure, head pressure, vapor pressure, and boiling point.

• Force

  • A measure of effort causing motion or change in motion.

  • Units: pounds (lb) or tons (imperial), Newtons (N) (SI).

• Density

  • Mass per unit volume of a substance.

  • Units: lb/ft³ (imperial), kg/m³ (SI).

• Pressure

  • Force per unit area.

  • Units: psi (imperial), Pascals (Pa) or kPa (SI).

• Head Pressure

  • Pressure expressed in terms of the height of a liquid column.

  • Often given in feet of head (especially for water).

• Vapor Pressure

  • The pressure at which a liquid’s molecules begin to vaporize at a given temperature.

  • Increases with temperature.

• Boiling Point

  • The temperature at which a liquid’s vapor pressure equals external pressure.

  • Liquid and vapor coexist in equilibrium at this point.

Define cavitation, aeration, laminar/turbulent flow, and friction head (pressure losses).

• Cavitation

  • Occurs when the local pressure of the fluid falls at or below its vapor pressure, causing vapor bubbles to form.

  • These bubbles implode in higher-pressure regions, damaging pump surfaces over time.

• Aeration

  • Air entry at the suction inlet (e.g., from a leak).

  • Air is compressible, reducing the effective liquid flow.

  • 1% air by volume can significantly affect performance; 6% air can stop the pump.

  • Can cause the pump to lose prime.

• Laminar & Turbulent Flow

  • Laminar: fluid layers move in parallel, minimal mixing.

  • Turbulent: chaotic, non-linear flow with mixing (often from bends, velocity changes, or sudden diameter changes).

• Friction Head (Pressure Losses)

  • Energy lost due to friction with pipe walls/fittings.

  • Higher fluid velocity = greater losses.

  • Halving pipe diameter quadruples velocity (for the same flow), leading to a 16-fold increase in friction losses.

Explain suction head, suction lift, discharge head, total static head, and total (dynamic) head.

• Suction Head vs. Suction Lift

  • Suction Head: The pump inlet is below the liquid level, so liquid is pushed into the pump by its own weight + atmospheric pressure.

  • Suction Lift: The pump inlet is above the liquid level, so the pump must create enough vacuum to “pull” liquid up. Practical limit is ~15–20 ft (though theoretical is ~34 ft).

• Total Suction Head (Dynamic)

  • Total Suction Head = Static suction head − friction losses in the inlet line.

  • If it’s a lift scenario, Total Suction Lift = Static suction lift + friction losses.

• Discharge Head

  • Static discharge head: Vertical distance from pump centerline to the fluid surface in the discharge tank.

  • Total Discharge Head (Dynamic) = Static discharge head + velocity head + friction losses.

• Total Static Head

  • Suction Head scenario: Total static head = static discharge head − static suction head.

  • Suction Lift scenario: Total static head = static discharge head + static suction lift.

• Total Head (Total Dynamic Head)

  • Measures the work done by the pump in lifting fluid from the suction reservoir to the discharge reservoir at a given flow.

  • Suction Head version: Total head = total discharge head − total suction head.

  • Suction Lift version: Total head = total discharge head + total suction lift.

What is NPSH (Net Positive Suction Head), why is it critical, and how do we calculate NPSHA vs. NPSHR?

• The energy (head) in the fluid at the pump suction above the fluid’s vapor pressure.

• NPSHR = Net Positive Suction Head Required by the pump (manufacturer-tested value).

• NPSHA = Net Positive Suction Head Available in the system (must be calculated).

• Importance

  • Prevents cavitation. If suction pressure = vapor pressure, vapor bubbles form.

  • Must ensure NPSHA ≥ NPSHR (with a safety margin ~10%).

• NPSHA Formulas

  • Suction Head (open system):
    → NPSHA = atm press + suction head - friction head - vapor pressure

  • Suction Lift (open system):

    → NPSHA = atm press - suction lift - friction head - vapor pressure

  • For closed systems, you’d incorporate system pressure instead of atmospheric.

Distinguish between open and closed piping systems, and explain how that impacts pump selection. Also, where should the expansion tank be located?

• Open Piping System

  • Suction and discharge sides both exposed to atmospheric pressure.

  • Pump must overcome static lift or static head plus friction losses.

• Closed Piping System

  • The discharge pipe eventually loops back to the suction.

  • The pump acts as a circulator, overcoming only friction losses (and minor static pressure differentials if expansion tank or other components are involved).

• Expansion Tank Placement

  • Should be on the suction side of the pump to avoid dropping pump inlet pressure below required NPSH.

  • If placed on the discharge side, it may reduce suction pressure, risk cavitation, and not maintain sufficient net positive suction head.

What is a pump curve, and how does it relate to typical applications and flow/head requirements?

• Pump Curve: Graph showing how pump flow capacity (x-axis) varies with discharge head (y-axis).

• Typically, as discharge head goes down, flow goes up (and vice versa).

• Curves also often show:

  1. Efficiency curves.

  2. Brake Horsepower (BHP) needed for specific flows/heads.

  3. NPSH Required at varying flow rates.

• Application:

  • For large volumes at moderate pressures, dynamic (centrifugal) pumps often used.

  • For high pressures at lower flows (or very precise flow/pressure), positive displacement pumps can be more suitable.

Installation Requirements: How should inline pumps and base-mounted pumps be installed, and what piping trim is needed?

• Inline Pumps

  • System fluid circulates between rotor and stator, providing cooling and lubrication.

  • Motor must be installed horizontally to keep bearings in proper contact with system water.

• Frame or Base-Mounted Pumps

  • If using rigid couplings, the frame and concrete base must be rigid and stable so it does not flex.

  • Misalignment leads to coupling/bearing failures.

• Piping Trim Requirements (Each pump should have):

  • Two Isolation Valves (one on suction, one on discharge).

  • Strainer (on the suction side).

  • Check Valve (on the discharge side) to prevent backflow and relieve head pressure on startup.

  • Pressure Taps (three typical):

    1. Upstream of the strainer,

    2. At pump suction,

    3. At pump discharge,

    • Used to measure pressure differentials and diagnose issues.

• No Excessive Piping Strain on pump flanges; do not force piping to align.

• Suction Line should be properly sized and free of air pockets.

  • Eccentric reducers (flat on top) help prevent air entrapment.

What are important considerations for pump replacement?

• Safety/Drainage

  • Do not drain fluids onto the floor (safety hazard).

  • Be mindful of potential hazardous gases from fluid lines.

• Isolation

  • Use “pancake” or slip blinds and verify valves are fully isolating the pump before opening.

  • Do not rely solely on a valve for complete isolation.

• Check for Venting

  • Relieve any trapped pressure before unbolting.

  • If fluid is hazardous or high-temperature, ensure proper protective measures.

True or False - Positive displacement pumps cannot handle entrained air?

False - some can.

True or False - all PD pumps have pumping chambers

True

True or False - Double acting piston pumps discharge equal amounts of liquid on all strokes

False - return stroke is less volume discharge

What occurs in a pump when liquid vaporizes in a closed system?

Cavitation or may become vapor locked

Which pump component converts mechanical into kinetic energy?

Impeller

What value must the NPSHR be in relation to NPSHA for proper operation

NPSHR must be less than the NPSHA

Are diffusers stationary?

Yes