Pumps

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18 Terms

1

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.

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2

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).

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3

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.

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4

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.

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5

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.

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6
<p>Explain suction head, suction lift, discharge head, total static head, and total (dynamic) head.</p>

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.

<ul><li><p><strong>Suction Head vs. Suction Lift</strong></p><ul><li><p><em>Suction Head</em>: The pump inlet is <em>below</em> the liquid level, so liquid is pushed into the pump by its own weight + atmospheric pressure.</p></li><li><p><em>Suction Lift</em>: The pump inlet is <em>above</em> 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).</p></li></ul></li><li><p><strong>Total Suction Head (Dynamic)</strong></p><ul><li><p><em>Total Suction Head</em> = Static suction head − friction losses in the inlet line.</p></li><li><p>If it’s a <em>lift</em> scenario, <em>Total Suction Lift</em> = Static suction lift + friction losses.</p></li></ul></li><li><p><strong>Discharge Head</strong></p><ul><li><p><em>Static discharge head</em>: Vertical distance from pump centerline to the fluid surface in the discharge tank.</p></li><li><p><em>Total Discharge Head (Dynamic)</em> = Static discharge head + velocity head + friction losses.</p></li></ul></li><li><p><strong>Total Static Head</strong></p><ul><li><p><em>Suction Head scenario:</em> Total static head = static discharge head − static suction head.</p></li><li><p><em>Suction Lift scenario:</em> Total static head = static discharge head + static suction lift.</p></li></ul></li><li><p><strong>Total Head (Total Dynamic Head)</strong></p><ul><li><p>Measures the work done by the pump in lifting fluid from the suction reservoir to the discharge reservoir at a given flow.</p></li><li><p><em>Suction Head version:</em> Total head = total discharge head − total suction head.</p></li><li><p><em>Suction Lift version:</em> Total head = total discharge head + total suction lift.</p></li></ul></li></ul><p></p>
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7

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.

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8

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.

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9

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.

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10

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.

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11

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.

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12

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

False - some can.

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13

True or False - all PD pumps have pumping chambers

True

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14

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

False - return stroke is less volume discharge

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15

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

Cavitation or may become vapor locked

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16

Which pump component converts mechanical into kinetic energy?

Impeller

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17

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

NPSHR must be less than the NPSHA

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18

Are diffusers stationary?

Yes

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