The Steamboating Forum

Technology - Pumps

The definition of "Pump" is "A machine or device for raising, compressing, or transferring fluids".

In Steamboating terms, several pumps are required for different jobs. The main types being:

Feed Pump - to pump water into the boiler.

Air Pump - to pump condensate out of the condenser.

Bilge Pump - to pump water out of the boat.

Lubricating Pump - to pump oils around the engine.

Steering Pump - to operate a hydraulic steering system.

Feed Pumps

Feed pumps have the hardest job of all. They must pump water, ideally at the same rate as the boiler produces steam, into the boiler at whatever pressure the boiler is currently at.

Fortunately feed pump designs can be very straight forward. Most comprise of a cylinder, a piston and two valves. One valve lets water into the cylinder, the other lets it out again.

The above diagram shows the basic principle of a simple feed pump. The piston moves up, water is drawn into the pump (technically it is pushed in by air pressure filling the space left by the piston). The ball valve on the right allows water to flow into the pump while the ball valve on the left only allows water to flow out of the pump. When the piston moves down, the water in the piston is pushed out of the pump.

How much water is pumped? That's an easy question with this style of pump. The displacement of the cylinder is Pi x Radius Squared x distance the piston moves. E.g. if the piston is 2.5" diameter and the piston moves up and down by 6", the displacement is Pi (3.14159) x 1.25" x 1.25" x 6" = 29 Cubic Inch (0.48 Litres). This means that every time the piston is pushed down by 6", 29 Cubic Inches of water is pushed out of the pump.

How much force is required to push the piston? Again this is very easy to calculate. The surface area of the piston is Pi x Radius Squared. In the example above this is 4.91 Square Inch. If your boiler is running at 150 PSI, this is 150 Pounds per Square Inch. Pressure x by area = force. 150 / 4.91 = 736 Pounds of force required. This is a lot of force, but then a 2.5" Diameter piston is a very large pump. For a small steam launch, a 1" diameter piston with a surface area of 0.785" Sq requires 118 Lb, easily achievable with a lever, and wouldn't over-stress an engine to drive.

The big advantage with this style of feed pump is it doesn't matter how much water is within the Cylinder chamber, that is the space between each valve. As long as the water is being pushed and pulled, the pump will pump the water. Even if the pump cavitates, the water will catch-up and rarely impedes the performance of the pump.

The main disadvantage of this style of pump is that it cannot draw from any real height. I.e. if the pump is dry it will not effectively pump the air through to draw the water, and so the water level needs to be close or above the level of the pump. On most steam launches this is not an issue at all as a pump on the floor is typically lower than the water level outside. The only limitation to the pressure these pumps can work at is the strength of the pump its self.

The pump in the animation is single-acting. Although it is possible to make a double-acting version, the added complexity, the smallness of the cylinder and the importance of the pump means that the pump design is often kept very simple and rugged.

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Air Pump

*Do not confuse a Steam Launch "Air Pump" with a locomotive Air Pump. Locomotive air pumps power the brakes.

Note that for simplicity, the term "Vacuum" relates to a "lower than atmospheric pressure", and the term "sucking" relates to atmospheric pressure pushing water into a vacuum-occupied area.

The "Air Pump" is a very misleading term which originated from the very early days of steam engines. The purpose of the Air Pump on a steam launch is to remove the water from the condenser and push it into the Hotwell. In doing so a vacuum is created in the Condenser, often approaching 29" Hg (near perfect Vacuum). There are many air pump designs, some better than others depending on the condition.

The animation above shows a basic air pump design. The first thing to note is the air pump is much larger than the feed pump, even through the water through-put is the same overall. There are several reasons for this. One reason is the air pump may be required to empty the condenser at a reasonable rate. If the condenser is full, the air pump may need to move 20 or more litres in a short time. Also the principle of the Air Pumps is to operate as a scavenging pump. That is the pump is designed to pump more (around 10x) the amount of water as the feed pump, and so on a stable system the air pump will only be 10% full of water at each pump stroke. The rest would either be a void (vacuum) or steam, because water will boil at about 25C at a near vacuum. Also note that the outlet valve is at the top. This allows any air which was dissolved in the water and has since escaped to be pushed out first thing. This allows only water to remain in the pump area, allowing the best vacuum to form.

As a scavenging pump, the air pump has to grab what it can. The best way to do this is to ensure the pump is as empty as possible before it draws in condensate. The only way to achieve this is to design the pump with minimal volume left in the cylinder when the piston is fully in. Valves should also be designed to leave little space inside the cylinder area.

Calculating the forces on the piston of the air pump is almost as easy as the feed pump, but they will vary constantly and depend upon many variables. The first thing you need to know is the pressure differential. Vacuum is usually measured in Inches of Mercury (" Hg). On a nice day, with 1 Atmosphere of pressure (1013.25 mBar), a full vacuum is 29.92" Hg. On a bad day, the air pressure is reduced, a full vacuum may only be 29.9" Hg with 980 mBar. A good rule of thumb is to imagine you are lucky and can achieve 29" Hg from your air pump. This equates to about 14 PSI. Using the area of the piston Pi x Radius Squared, a 4" diameter piston is 12.57" Sq. 14 PSI x 12.57" = 176.0 Lb of force.

This may sound like a lot of force for the engine to give, however we have already established that in normal operation the pump would be around 10% full. This means that 90% of the effort given by the engine to pull the air pump is used to create a vacuum. When the piston moves back in, this vacuum returns energy to the engine. Imagine it is like an elastic band. You pull the elastic band and it gets hot, some heat escapes and you let it return, it gets cold. 90% of the energy was used to pull the elastic band, which was given back 10% was lost in the process.

Edward's Air Pump

The Edwards Air pump design is one of the more successful designs. It is a simple design with only two moving parts, it makes use of the fact that water contains momentum and at the same time compressing the water to prevent it from flashing into steam (remember at 29" Hg water will boil at about 25C).

The design at first looks like it wouldn't work, however once you understand the principle of the mechanism, it makes a lot of sense. The piston moves down, forming a vacuum. As it approaches the bottom it open up ports around the cylinder. This allows the water to fill the void (vacuum) in the cylinder. By doing this, water vapour and steam is generated because of the rapid expansion of the water, taking up valuable space in the cylinder which could otherwise be filled with water. To help fill the cylinder with water, when the piston has reached the bottom of its travel it forces a jet of water from the very bottom of the air pump into the cylinder. By compressing the water into the cylinder instead of "sucking" the water in, no steam or water vapour is created, and most which was created before is forced back to water.

The piston is then raised shutting off the inlet ports and forcing all air and water through the top valve. This top valve, often a simple flat ring over a series of holes, is held shut only due to the vacuum within the cylinder. The only downsides to an Edward's air pump design is they don't operate to their design at very low speeds, and they have a tendency to clank. These are often small factors compared to the benefits of having a decent vacuum.

Gear and Vane Air Pumps

Some people use a Vane air pump on their steam boat. Gear pumps have been included here as well because they work in a similar way.

A Vane pump, such as the Jabsco pump range use a Vane or Paddle to push the fluid through one side of a wheel, while reducing the volume on the other side forcing the fluid out one side. A Vane Pump animation can be seen below:

Gear pumps use two tight-fitting gears. As they run the fluid is pushed around the outside and can't return between the gears and is forced out. Both Vane pumps and Gear pumps can't run dry for very long. A Gear Pump animation can be seen below:

Vane and Gear pumps can deliver high volume and pressure, and are often used as hydraulic pumps, giving the characteristic whining hydraulic noise.