Pumps—An Overview

[SailNet]

The variety of pumps on the market can make selecting the right pump for a job confusing. Some pumps can be used for multiple purposes while others are designed exclusively for one particular job.

TYPES
There are three types of pumps, electric, manual, and engine driven, and these have a variety of pumping mechanisms.

Electric pumps are typically 6, 12, 24, or 32 volts DC, or 110/120 volts AC. When used in bilges, electric pumps have waterproof housings.

• Flexible impeller pumps combine the priming features of rotary pumps with the general transfer ability of a centrifugal. The impeller draws fluid in, priming the pump, and as it rotates, it continues to draw water in while moving it toward the outlet.

• These pumps can handle either thin or viscous liquids and can handle more solids in suspension than other types of rotary pumps.
  
• Flexible impeller pumps operate efficiently at high or low speeds and can be mounted at any angle.
  
• They provide a continuous and consistent discharge of fluids.

• Centrifugal pumps use kinetic energy to move the water. Water enters the pump on the suction side at the center of an impeller, gains momentum by the rotation of the impeller, then moves to the outer edges and is forced out by its own momentum, or centrifugal force.

• Centrifugal pumps loose efficiency as viscosity increases. In other words, they’re best suited to water.
  
• The flow rate decreases as pressure decreases.
  
• Centrifugal pumps need to be primed before pumping.

• Rotary pumps, also known as positive displacement (PD) pumps, move a consistent amount of fluids with each rotation of the pump shaft by the motion of the pumping gears that expand volume within the pump, allowing fluids to enter. The gears then reduce the volume and force the fluids out.

• The flow rate is smooth regardless of pressure.
  
• The higher the fluid viscosity the more efficient the pump.
  
• Rotary pumps are self-priming because they can pump air as well as fluids.

• Multi-chamber diaphragm pumps, sometimes called "wobble plate" pumps, use a piston mechanism to draw in and force water out of the pump chambers. When the piston draws, the valve on the suction side opens, the exhaust-side valve closes, and fluids are pulled into the chamber. When the piston pushes, the fluids are forced out, the valve on the suction side closes, and the exhaust side opens. There may be two to five chambers, each with its own diaphragm and valves. The multiple-chamber pump is usually quiet and maintains a smooth flow of water.

• Diaphragm pumps operate by a shaft pushing a single diaphragm up and down, pulling water in through an intake valve and then pushing water out through an output valve.

• Diaphragm pumps are ideal for use with a variety of fluids, even fluids with abrasives, such as salt water.
  
   
  

  
  

  
  
  
  

  Pumps/3131_pump.jpg" width=200> Electric Single Diaphragm Pump

Manual pumps on boats are usually synonymous with diaphragm and piston style pumps. Most are used as backup to electric pumps and some styles of diaphragm pumps are used in conjunction with sanitation systems.

• Manual diaphragm pumps are either double or single action with single or double diaphragms. A lever is used to pump the diaphragm, which in turn primes the pump box by drawing water, exhausts air until the chamber is full of fluid, then, by continuing to pump, pulls the fluid in and push it out with each stroke. A "flapper" valve prevents the water from flowing back into the intake hose.
  
  
  
  • Diaphragm pumps can be used with most fluids, provided the diaphragm material is rated for that use.
    
  • The pumping chamber and the fluid being pumped are not in contact with any close fittings or moving parts. This makes diaphragm pumps ideal for use with a variety of fluids, even fluids with abrasives.
    
  • Diaphragm pumps can be run dry.
    
  • There are no electric motors or controls with manual pumps to cause fire or explosion hazards.
    
  • Diaphragm pumps are portable and easy to install.
    
     
    

    
    

    
    
    
    

    Pumps/4245_pump.jpg" width=125> Manual Piston Pump

• Manual piston pumps use a vertical, hand-operated handle to operate the piston, which provides suction from the intake end of the pump body and exhaust through an exhaust hose.

• These pumps are self-priming and clean easily.
  
• There is a minimal number of moving parts to maintain.
  
   
  

  
  

  
  
  
  

  Pumps/3620_pump.jpg" width=125> Manual  Diaphragm Pump

• Engine drive pumps are self-priming, high-volume, belt-driven impeller pumps.
  
  
  
  • These pumps run off the engine’s crankshaft pulley and are set into operation by either a manual or electric clutch.
    
  • Do not run these pumps dry.
    
  • Engine driven pumps, depending on the hose size and engine RPM at which the pump is driven, can move up to 3,600 GPH.
    
  • Because of the large volumes of water, these pumps can fight fires or be used for deck wash down if the intake is teed into a water source.

USES
See our Buying Guides on Bilge Pumps, Freshwater System Pumps, Saltwater Deck Washdown Pumps, and Sanitation Pumps.

TERMINOLOGY

• Check valves allow fluids to flow in one direction only.
  
• Continuous-duty motors are rated for continual 24-hour use. These motors can be run for extended periods without damage to the motor or reduction of its lifespan.
  
• Double action is a pumping action that moves fluids both in and out on the up stroke and the down stroke.
  
• Float switches are electrical devices that are actuated by a contact closing on the motor circuit. When the water reaches the "cut-in" point, or pre-set high level, the pump motor contact closes and turns on the pump. When the water drops to a "cutout", or pre-set low point, the contact opens on the pump motor and turns off.
  
• Flow rate, expressed in gallons per hour (GPH), is the velocity of water movement as determined by pipe size, pipe bends, interior pipe surface (ribbed or smooth), and pressure.
  
• Head is the discharge height that the pump is forced to pump against.
  
• Intermittent-duty motors are rated to run continuously for short periods of time, usually in increments of 15 minutes, up to 60 minutes. The load is then removed for a cool down period before restarting.
  
• Impellers resemble multi-bladed fans. They’re used to pull in and exhaust water by centrifugal force.
  
• Pressure is the amount of force applied by water that is either pump driven or gravity fed, and usually defined in terms of pounds per square inch (PSI).
  
• Pressure switches are activated by pressure dropping or rising to preset points. A low set point or cut-in level activates the switch to turn the pump on. A high set point or cutout level activates the switch to turn the pump off.
  
• Pressure tanks, also known as accumulator tanks, have an internal air bladder that as the pump fills the tank with water, the air space in the bladder is compressed creating pressure. The tank stores the water and after the pump has filled the tank, the water can be released by activating a faucet. The compressed air, under pressure, forces the water out. These tanks save wear and tear on intermittent duty motors by keeping run time to a minimum.
  
• Self-priming pumps, by use of an air-extracting device in the suction line, are able to draw some air in to prime themselves.
  
• Single action is a pumping action that moves fluids in one direction only on both the up stroke and down stroke.

INSTALLATION
The average-skilled sailor with basic tools and some electrical skill can install most pumps.

Toolbox
Electric pumps require:

• Tin-plated wire,
  
• Connectors
  
• Wire cutter/stripper/crimper
  
• Wire ties or clips
  
• Multi-meter
  
• Soldering iron if required
  
• Additional circuit breaker for electrical system if required
  
• Hose rated for the use and stainless steel hose clamps

Manual pumps require:

• Drills and bits
  
• Hose rated for the use and stainless steel hose clamps