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Eductor Pump on Ship - Working Principles & How to Operate

Dimitris Tsapis
January 8, 2022
Why do we need an eductor pump on ship? What is its working principle and how exactly is it operated? Read on to find out all you need to know.

In this article, we delve into the basic purpose, working principles, and functions of an eductor pump. While most often found in tankers and cargos, it also empowers yacht’s fresh water systems, toilet systems, and more. As such, learning how to effectively operate an eductor pump on ship will further enrich your skillset.

As such, and to help you get a good understanding of the basics, we delve into everything you need to know in the following chapters. Make sure to check out the attached video as well, to get a better idea of the process. Let’s delve in.

What is an eductor pump?

An eductor, often referred to as a liquid jet pump, is a pump that drives fluid (liquid, gas, air) from a specific area through an enclosed circulation. In simple words, it operates like a large-scale vacuum, fueled by driving liquid, operating on a principle known as the “Venturi effect”.

The pump itself has no moving parts or adjustable components as it only requires the motive fluid to operate effectively. As the liquid circulates within the eductor, pressure starts to develop, which in turn enables the pump to draw gas or liquid and extract it through the driving fluid recharge.

When looked at from a physics point of view, the eductor pump is a simple device that converts pressure to velocity when a fluid (liquid or gas) is forced through the restriction in an enclosed line (the size and pressure of which depends on the eductor pump’s model).

Eductor Working Principle - Delving into specifics

The way an eductor pump operates follows relates to what is known as Bernoulli principle. According to Bernoulli’s principle, a reduction in pressure happens parallel (simultaneously) with an increased speed of fluid circulation. The following equation explains Bernoulli’s principle in a simplified manner:

eductor pump on ship

In this instance the letters describe the following:

  • V  = Velocity of the fluid
  • p = Pressure of the fluid
  • ρ = Density of the fluid

Based on Bernoulli’s principle, it becomes clear that an increase in the fluid’s velocity reduces the pressure. The opposite is true as well. Overall, the equation portrays the conservation of energy. Knowing this information, we can now have a look at an arrangement that represents the inner function of an eductor pump on ship:

eductor pump on ships

The amount (F) of fluid passing through the pipe arrangement above results from the narrow section of the pipe (A) paired with the fluid’s velocity (v). The equation for this is summarized as follows: 

F = A v

Take a moment to understand how the fluid passes through the pipe in constant motion. The entry and exit point of the pipe (labeled as 1) is wider than the midpoint, here labeled as 2. Since the flow is constant and the discharge equal at all parts of the pipe, the acceleration of the fluid increases in 2 compared to 1. Put in an equation, this would look as follows:

A1 v1 = A2 v2

Therefore it can be concluded that, due to the built of the eductor’s pump, fluid accelerates when going through the narrow part of the pipe; point 2. Based on Bernoulli’s principle, when the velocity of fluid increases, the pressure decreases. Hence, this results into:

P2 < P1

When passing through point 2, the fluid will have reduced pressure (vacuum).

How does an eductor work?

To better understand all the components of an eductor pump and how these synergistically create the effect we previously described, take a look at the image below:

eductor pump basics

 As you can see from the picture above, the motive fluid(1), which in this case is seawater, enters through the nozzle(2). It then passes through the narrow part of the pipe, returns to the wider part of the pump, the diffuser(4), and exits through the outlet (5). When the fluid reaches a certain level of pressure, it starts absorbing fluid through the inlet (7).

Typically, every eductor pump on ship regulates the inlet, discharge, and the motive fluid’s speed by using valves, which the engineer or crew will need to operate. The following guidelines give a brief overview of the steps that need to be taken:

  • Open both the inlet and discharge valve of the eductor pump
  • Once both valves are opened, start the motive fluid pump and adjust its pressure to the needs of the eductor. The amount of pressure that is required to operate the eductor depends on the discharge head.
  • Once the desired pressure has been reached, open the suction valve to start the suction process from a previously designated area.
  • Keep in mind that the suction valve should only be opened once the motive fluid reached the desired pressure. If you release the valve before the pressure is attained, you may cause a liquid back flow through the suction process.
  • Likewise, before you finalize the process, first close the suction valve and only then close the motive fluid pump.

How to use an eductor pump on ship

The following video gives a good overview of the engineer’s point of view when walking through the series of steps discussed in the bullet points above.

Where is an eductor used on a ship?

The most common function of an eductor pump on ships is that of tank cleaning tasks, as well as COW operations, and its capacity differs based on the operation. The majority of ships that use eductors do so for stripping duty and clean ballast.

In essence, the eductors are used to remove solids from liquid or air in large quantities. Here are a couple of examples:

  • Naval ships can use an eductor to control damage that leads to contaminated water.
  • Chemical tankers use eductors for pumping bilges and stripping ballast tanks.
  • You may also come across portable eductors which are used in emergencies where pumping is required (e.g. flooding). These are used both onboard and at the shore.

Other applications of eductor on ships

  • Creating a vacuum in fresh water generator
  • Creating a vacuum in the ship’s toilet system
  • Foam pressure mechanism in case of a fire emergency

Difference between ejector and eductor pump

Due to their name similarity and with both pumps looking the same, ejector and eductor pumps are often mistaken for one another. Both pumps operate on the basis of the Bernoulli principle, also known as Ventauri effect, as we analyzed above. However, they have two fundamental differences:

  • Where the eductor uses water (liquid) as a motive medium, the ejector uses steam or air.
  • Due to this, the ejector’s velocity can go up to 2 mach (2000 feet/sec). The eductor’s velocity, on the other hand, is much lower, generally ranking up to 20 feet/sec.

Eductor performance limitations

When observing the pumping process seen in the image below, one can witness how the capacity of suction differs when making changes to the fluid pressure. The image below indicates that, when keeping a discharge head stable, suction capacity volumes are altered based on the driving pressure applied to the eductor. An increase in motive fluid that surpasses the recommended pressure does not lead to a better or larger suction volume. On the contrary, reducing the pressure below recommended levels leads to reduced suction volumes. In short, the higher the discharge head, the lower the suction volume.

suction flow capacity curve

Adding to that, check out the driving flow capacity curve illustrated below. It gives a good overview of the driving capacity for specific pressure levels.

driving flow capacity curve

Eductor pump maintenance

If you happen to face some trouble with the eductor pump, the issue is usually resulting from one of the following:

Clogged motive fluid nozzle

When it comes to eductor pumps, restriction of fluid is the most common issue of trouble. The nozzle is usually clogged with dirt and can easily be removed with a soft object, like a wooden stick. Avoid using sharp materials, like knives or picks, as they can cause unrepairable damage to the nozzle.

Decreased pressure of motive fluid

In some cases, you may come across occasions where the motive fluid does not reach the desired velocity, which in turn affects the suction capacity. In the worst case, this can lead to fluid back flow in the suction side. In this case, make sure you correctly follow all the steps described in the “How does an eductor work” chapter.

Increased back pressure

The strength of the eductor’s suction decreases when the back pressure of the motive fluid increases. To resolve this issue, simply increase the pressure of the motive fluid. That said, even if you do not come across this issue, it is important to always monitor the eductor during its operation.

Wrapping up

To learn how to operate and troubleshoot an eductor pump, you first need to understand its basic operational mechanisms and working principles. This is what we attempted to do in the chapters above. As this blog post is highly technical, make sure you bookmark it for future reference and make sure you review the specifications of your ship’s eductor.

If you wish to further enrich (or refresh) your knowledge, make sure you also read our guide on marine boiler water treatment.