Pumps and Pumping Basics

A Primer on Pumps and Pumping for Shore and Vessel Applications

Pumps are used extensively throughout the maritime industry. Vessels and shore installations utilize a wide range of pumps and pumping techniques to transfer and distribute fluids and slurries of all types.

Types of Pumps

All common types of pumps use a source of energy to move a liquid through a confined space. The energy may be applied by mechanical force directly on the liquid or by using an energy carrier like compressed air.

Pumps for general applications are mostly direct mechanical pumps. Specialized pumps use a variety of techniques to appropriately handle whatever material is being transferred. Liquids that are highly viscous or flammable need special equipment to be transferred quickly and safely.

Positive Displacement Pumps

Positive displacement pumps fill a chamber with fluid from an inlet then compress the fluid to eject it from the outlet. One example of this is the internal combustion engine which fills a cylinder with an air and fuel mixture and then empties the cylinder when the mixture has burned and expanded. Of course this is being done with gasses rather than liquid, a device that moves gasses is usually called a compressor.

The method of displacement can be achieved by several different means.

  • Piston and Cylinder – This is the same as our example of the internal combustion engine above. One variation on this method is called diaphragm pumping. In these types of displacement pumps a flexible diaphragm moves back and forth to change the displacement in the pump chamber in place of the usual piston.
  • Rotary Displacement – A spinning device pushes water using fins or lobes which fit tightly against the walls of the pump chamber. The operation is similar to old fashioned water wheels that ran industrial equipment before other forms of power became available. The difference is that the wheel is enclosed within a housing which allows it to operate at a much higher speed and efficiency.

    Centrifugal or Rotary Velocity Pumps

    This type of pump spins a device called an impeller inside a chamber which causes the liquid being transferred to be pushed to the outer edge of the chamber by centrifugal forces. An outlet along this outer edge gives the compressed liquid a place to escape.

    The action of the impeller on the liquid comes in two forms depending on the application. Centrifugal pumps designed for a variety of applications use a combination of these two methods to meet performance and efficiency goals.

    • Radial Flow – Fluids are transported around the pump chamber by the impeller. Energy is supplied to spin the impeller by a shaft or coupling which is at a right angle to the flow of the liquid.
    • Axial Flow – In this application the impeller moves the liquid along in a screw-like fashion. The rotating shaft that moves the impeller is parallel to the movement of the fluid. The impeller and shaft may be in the center of the pump chamber or offset to one side which exposes only one side of the impeller to the fluid being pumped.

    Pump Variations

    The two main types of pumps outlined above are the basis for a very large number of variations.

    Valves come in many configurations that add performance or reduce manufacturing or maintenance costs.

    The types of valves, or lack of valves, also depend on the liquid being transferred.

    Corrosive or flammable liquids need careful handling. Some pumps must move very hot or abrasive materials and operate without maintenance for extended periods of time. Valve design and material is essential to longevity of these pieces of equipment.

    Materials that make up the pump chamber and impeller are important to many aspects of pump operation. Iron, bronze, and stainless steel construction are common in many maritime applications on board ship and on shore. This is a well-tested and familiar group of materials with known long-term performance characteristics.

    Recently, pumps built from engineering composites have begun replacing the more traditional materials in the industry. Structural graphite pump construction offers high strength and fully corrosion resistant operation in corrosive environments.

    The use of composites throughout the maritime industry is now seen by a growing number of manufacturers and operators as one of the best ways to reduce maintenance and the associated overhead costs.

    Pump Design

    Overall the most important aspect of pump performance lies in the design. Efficiency is best in a pump that is exactly sized for the job it needs to do. Many pump solutions advise to use a pump within a performance range that can be quite wide. A pump that operates at 10 percent less efficiency than an exactly sized pump will consume much more energy over its lifetime than one that is sized exactly for the job.

    Factors influencing pump design are material being transferred, pump head, pump lift, volume of flow, amount of maintenance required, and many other variables that fill textbooks on fluid dynamics and engineering.

    If there were three basic concepts to understand about pump design and fluid dynamics it could be narrowed down to volume of flow, pump head, and pump lift.

    Volume of Flow is fairly simple; how big is the pipe that the pump is feeding and how fast must the fluid flow through the pipe to meet your goals for flow rate. This is where you need to brush up on your fluid dynamics to get the best possible balance between all the variables.

    Pump Lift is the ability of a pump to draw a liquid up to itself through the inlet pipe.

    The laws of physics show us that a liquid like water can only be drawn up to a certain height by a perfect vacuum. Since no pump draws a perfect vacuum it is only possible to draw liquids to a certain height with a pump positioned well above the liquid reservoir.

    This height is variable according to the density of the liquid being moved.

    Pump Head is the distance a pump can move a liquid vertically before the pressure of the liquid above the pump stops it from adding new liquid to the output side of the pump.

    Pumps come in a wide variety of head efficiencies. As the height of the output rises the volume of the pump is decreased proportionally until no liquid flows from the outlet.

    Large pumps can have thousands of feet of head even with a large diameter pipe which decreases head because of the larger volume of liquid the pump mechanism is supporting.

    What is the best type of pump?

    This is kind of a difficult question but one that is asked often. It is kind of like asking ‘what is the best kind of dog?’ or ‘what is the best kind of car?’. The answer lies in the use of the pump or dog or car.

    There are some pros and cons of centrifugal and positive displacement pumps in general but correct pump sizing is more important than pump type in most situations.