12.1 Centrifugal & Positive Displacement Marine Pumps

12.1 Centrifugal & Positive Displacement Marine Pumps

Marine pumps are the mechanical workhorses of a merchant vessel's engine room and deck systems. They circulate sea water and fresh water for main engine cooling, transfer combustible fuel oils from bunker tanks to service systems, deliver pressurized lubricating oil to spinning journal bearings, pump out oily bilge accumulations, and transfer thousands of tonnes of ballast water to maintain ship stability. Trainee General Purpose (GP) Ratings must master the two fundamental classifications of marine pumps—Centrifugal (Roto-dynamic) and Positive Displacement (PD) pumps—understanding their distinct operating principles, internal construction, sealing methods, starting procedures, and critical safety rules.

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1. Centrifugal Marine Pumps

Centrifugal pumps belong to the roto-dynamic category. They are widely used aboard merchant vessels for high-volume, low-to-medium pressure applications such as main sea water cooling, low-temperature (LT) fresh water circulating, ballast handling, fire main pressurization, and general service.

                          +------------------------+
                          |  Discharge Flange      |
                          |  (High Static Pressure)|
                          +-----------+------------+
                                      ^
                                     / \  Spiral Volute Casing
                                    /   \ (Converts Velocity Head
                                   /  *  \ into Pressure Head)
                                  /  ***  \
       +--------------------+    |  *****  |
       | Suction Flange     |--->| (  O  ) | (Rotating Impeller)
       | (Low Pressure Eye) |    |  *****  |
       +--------------------+     \  ***  /
                                   \  *  /
                                    \   /
                                     \ /
                                      +

Operating Principle and Hydrodynamics

1. Kinetic Energy Impartation: Fluid enters the pump horizontally through the suction branch and reaches the central impeller eye. As the electric motor spins the impeller at high speed (typically 1,450 to 1,750 RPM or 2,950 RPM), the curved impeller vanes throw the liquid radially outwards by centrifugal force, imparting high velocity (kinetic energy).
2. Velocity to Pressure Conversion: The liquid discharges from the impeller tips into the surrounding spiral volute casing. The cross-sectional flow area of the volute gradually expands towards the discharge nozzle. According to Bernoulli's principle of fluid dynamics, as the cross-sectional area increases, the liquid flow velocity decreases, converting kinetic velocity head into static pressure head: $$H_v = \frac{v^2}{2g}$$ where $H_v$ is velocity head in metres, $v$ is the liquid flow velocity ($\text{m/s}$), and $g=9.81\text{ m/s}^2$. This is not the total head generated by the impeller; obtain developed pump head from its characteristic curve. For comparable inlet/outlet velocities and elevations, pressure rise is approximately $\Delta p=\rho gH$, where $\rho$ is fluid density.
3. Continuous Flow Generation: As liquid is thrown outwards from the center, a localized low-pressure zone (partial vacuum) forms at the impeller eye. Atmospheric pressure or liquid static head acting on the external suction reservoir continuously pushes more liquid into the eye, establishing uninterrupted flow.

Non-Positive Displacement Characteristics

• Flow Varies with Back-Pressure: Unlike positive displacement pumps, a centrifugal pump does not deliver a constant volume. If system discharge resistance (back-pressure) increases, flow rate drops. If the discharge valve is closed completely, the pump reaches its shut-off head—flow falls to zero while the impeller continues rotating. Do not leave the pump dead-headed: churning heats the trapped liquid and can damage the pump. Observe the approved minimum-flow and brief-starting limits.
• Lack of Inherent Self-Priming: Centrifugal pumps cannot pump air or gases. Because air has a density roughly $\frac{1}{800}\text{th}$ that of liquid water, an air-filled impeller spinning at normal speed generates an insignificant pressure differential (insufficient to draw liquid up the suction pipe). If air enters the suction casing, the pump suffers an air lock and immediately loses suction.

Priming Devices

To operate from a suction lift (e.g., pumping bilges or drawing ballast from double bottom tanks below pump level), centrifugal pumps require auxiliary priming devices:

• Positive Suction Head (Flooded Suction): When drawing sea water from a low sea chest located well below the vessel's waterline, hydrostatic sea pressure naturally floods the pump casing without mechanical assistance.
• Central Vacuum Priming System: Consists of an automated liquid-ring vacuum pump connected to the centrifugal pump casing via a float-operated suction valve. The vacuum pump exhausts air from the suction pipe and casing until liquid rises, lifts the float, and closes the vacuum line.
• Air Ejector Priming: Uses compressed air passing through a convergent-divergent Venturi nozzle to create a deep suction vacuum that evacuates air from the pump casing.

Major Constructional Components

Shaft Sealing: Stuffing Box Gland Packing vs. Mechanical Seal

Where the rotating drive shaft penetrates the stationary pump casing, high-integrity sealing is mandatory to prevent liquid leaking out (or air drawing in under suction lift).

   STUFFING BOX GLAND PACKING                     MECHANICAL SEAL
   +-------------------------------+             +-------------------------------+
   | Casing Wall                   |             | Casing Wall                   |
   |  [===] [===] [===] Gland      |             |     Stationary Seal Ring      |
   |  Packings (Cotton/Graphite)   |             |     (Silicon Carbide/Ceramic) |
   |  ---------------------------  |             |     || Rotating Ring (Carbon) |
   |  ===> Controlled cooling drip |             |     || [Spring Compression]   |
   |  Shaft Sleeve                 |             |  ---------------------------  |
   |===============================|             |  Zero Visible Leakage         |
   +-------------------------------+             +-------------------------------+

• Stuffing Box Gland Packing:
  • Construction: Annular chamber packed with braided square rings of cotton, flax, or synthetic fibers impregnated with graphite and PTFE lubricants. Rings are cut with ends beveled at $45^\circ$ and staggered at $90^\circ$ or $120^\circ$ offsets around the shaft.
  • Operational Rule: The gland follower must never be overtightened. A controlled, steady cooling drip of 10 to 20 drops per minute must leak past the packing while running. This fluid leakage lubricates the interface and carries away friction heat. Overtightening burns and glazes the packing, scores the shaft sleeve, and overloads the electric motor.
• Mechanical Seal:
  • Construction: Consists of two ultra-flat mating faces: a stationary face (silicon carbide or ceramic) mounted in the casing with an elastomeric O-ring, and a rotating face (carbon graphite) mounted on the shaft sleeve and pressed firmly against the stationary face by stainless steel coil springs.
  • Operational Rule: Provides complete sealing with zero visible leakage. However, mechanical seals are brittle and must never be run dry. Even a few seconds of dry running destroys the micro-thin liquid lubricating film between the faces, causing instantaneous friction overheating, thermal cracking, and catastrophic seal failure.

Operating Protocol for Centrifugal Pumps

• Pre-Starting Checks:
  1. Verify the pump shaft rotates freely by hand (turn the shaft coupling with motor power isolated).
  2. Check bearing lubrication oil level in the sight glass or verify grease cup replenishment.
  3. Fully open the pump suction valve.
  4. Open the top casing vent cock until air is completely expelled and a steady stream of liquid emerges; close the vent cock.
• Starting Sequence (Typical Radial-Flow Pump; Confirm Maker Instructions):
  1. Set the discharge valve to the approved starting position, commonly closed or throttled for a radial-flow pump. Maintain any required minimum-flow bypass.
  2. Press the motor start button on the starter panel. Monitor the starting current (ammeter needle will surge then settle down).
  3. Observe the discharge pressure gauge. The pressure will quickly rise to the pump's shut-off head.
  4. Slowly open the discharge valve to the required duty, checking operating pressure, flow and motor current. Do not assume fully open is the correct running position.
• Why use a throttled start? A typical radial-flow pump requires less shaft power at low flow, reducing hydraulic load while the motor accelerates. This does not remove the motor’s initial electrical inrush. Gradual valve adjustment controls the rise in flow; follow the approved procedure to avoid pressure surges and prolonged zero-flow operation.
• Stopping Sequence:
  1. Slowly close the discharge valve completely.
  2. Press the stop button to de-energize the electric motor.
  3. Close the suction valve only if the pump is being isolated for maintenance or inspection.

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2. Positive Displacement (PD) Marine Pumps

Positive displacement pumps operate on an entirely different mechanical principle. Instead of imparting velocity, they trap a fixed, discrete volume of fluid inside a contracting chamber and mechanically force it into the discharge pipe against whatever resistance exists.

                     ROTARY GEAR PUMP                  RECIPROCATING PLUNGER PUMP
                +-----------------------+              +-----------------------+
                |       Discharge       |              |       Discharge       |
                |           ^           |              |       (NRV Lifts)     |
                |      /----|----\      |              |           ^           |
                |    ( ####### )        |              |     [====]===]        |
                |    ( Drive   )        |              |      Cylinder         |
                |    ( Gear    )        |              |     [=========]       |
                |      \       /        |              |     ( Plunger )       |
                |    ( Idler   )        |              |          |            |
                |    ( Gear    )        |              |          V            |
                |      \-------/        |              |       Suction         |
                |           ^           |              |       (NRV Lifts)     |
                |        Suction        |              +-----------------------+
                +-----------------------+

Operating Characteristics

• Constant Volumetric Delivery: The discharge flow rate is directly proportional to rotational or reciprocating speed, remaining virtually constant regardless of whether discharge back-pressure is 2 bar or 50 bar.
• Self-Priming Capability: Because the moving elements (gears, screws, pistons, vanes) maintain tight mechanical tolerances within the casing, they effectively evacuate air and gases from the suction piping, creating a deep suction vacuum without requiring external priming devices.
• High Viscosity Handling: Exceptionally well-suited for high-viscosity liquids such as Heavy Fuel Oil (HFO), Marine Diesel Oil (MDO), cylinder lubricating oil, sludge, and hydraulic oil.

Major Classifications of Marine Positive Displacement Pumps

1. Gear Pumps:
   • Mechanism: Composed of two intermeshing precision gears—a motor-driven driver gear and an idler gear—enclosed inside a close-fitting casing. Liquid enters the suction branch, fills the cavities between the unmeshing gear teeth and the casing wall, is carried around the outer perimeter of both gears, and is squeezed out into the discharge branch as the teeth mesh together on the opposite side.
   • Applications: Fuel oil transfer pumps, auxiliary boiler fuel booster pumps, lubricating oil supply, and hydraulic steering power units.
2. Screw Pumps:
   • Mechanism: Intermeshing helical screws form sealed chambers that move liquid axially from suction to discharge. A common three-screw unit has one driven power rotor and two idler screws inside a close-fitting liner. Single-flow units move liquid along the screws in one direction; double-flow arrangements feed from both ends towards a central discharge.
   • Applications: Main engine lubricating oil supply pumps and fuel oil high-pressure supply systems. Screw pumps provide exceptionally smooth, non-pulsating, pulse-free flow, low noise levels, and high suction lift capability.
3. Reciprocating Pumps:
   • Mechanism: Employs a reciprocating piston or plunger driven back and forth inside a cylinder by an electric motor via a crankshaft or directly by steam. Fitted with spring-loaded suction and discharge non-return check valves. On the outward suction stroke, cylinder pressure drops, the suction valve lifts, and liquid enters. On the inward discharge stroke, the suction valve snaps shut, the discharge valve lifts, and liquid is forced into the system.
   • Applications: Marine auxiliary boiler feed water pumps, double-bottom tank stripping pumps, and high-pressure bilge stripping.
4. Rotary Vane Pumps:
   • Mechanism: An eccentrically mounted rotor containing radial slots fitted with sliding rectangular vanes rotates inside a circular cam ring. Centrifugal force and springs push the vanes outward against the casing wall. Fluid is drawn into expanding crescent-shaped pockets on the suction side and forced out through contracting pockets on the discharge side.
   • Applications: Hydraulic steering gear servo control pumps, lube oil priming, and fuel transfer units.

Centrifugal versus Positive Displacement Comparison

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3. Critical Safety Rule for Positive Displacement Pumps

Pressure Relief Protection

To safeguard machinery and crew against accidental overpressurization, provide a suitable internal or external pressure relief valve. Marine class rules require the valve on the pump casing or delivery pipe before the first stop valve where pump pressure can exceed the rated value.

• Relief Operation: The valve is calibrated to lift at a predetermined safe margin (typically 10% to 15% above normal working pressure).
• Recirculation Path: When the relief valve opens, excess liquid is safely bypassed directly back to the pump suction chamber or returns to the storage tank, relieving overpressure and preventing mechanical explosion.