The Steamboating Forum

Welcome to the Steamboating Forum

Pipe Theory

One major aspect of steam is that it must flow through a pipe at pressure to reach its destination. This is not quite as simple as it first sounds.

Pressure within the pipe is not a great concern, however when combined with the heat that the steam contains, the strength of pipe may be reduced to the point where the pressure rating is not enough for the temperature.

Standard house water copper pipe for example is rated to 10 Bar (150psi) cold. However at steam temperature, the rated pressure may only be 6 Bar (87psi), and therefore standard copper pipe is unsuitable for most steamboats. Thick-walled copper pipe rated to the working pressure and temperature should be used. Copper pipe work-hardens badly, and a vibrating pipe will eventually become very hard and brittle. Copper is also expensive.

Steel Gas pipe is often a good choice. It does not work-harden like Copper pipe and retains a reasonable amount of spring. For most steam lines, steel is an ideal choice for strength and cost, however it does corrode.

Stainless steel pipe is therefore the ideal choice. The main problem with Stainless is how to connect it to any fittings, as it is difficult to machine and not always feasible to weld. Cost is usually mid-way between steel and copper.

Water Hammer is always an issue when dealing with pumps. It can never be removed from pipes, however it can be reduced. The main issue within pipes is the instantaneous extreme pressure it causes. A single impulse of water within a pipe can cause the pressure to raise to many thousands of Bar in pressure along with the pipe having to deal with the momentum from the water causing large vibrations. Both pipes and fittings must be strong enough to withstand these forces. Fortunately pipes dealing with water hammer are rarely heated to high temperatures.

Pipe Design

There are two standard types of pipe, solid drawn and welded. Solid drawn is literally drawn from molten metal as a whole piece, welded pipe is rolled from a sheet and welded down the join. Solid drawn pipe is much stronger overall for the same size, welded is cheaper. Most Gas pipe is welded, most copper pipe is solid drawn.

The general rule of thumb is welded steel & stainless gas pipe of reasonable size would be perfectly strong enough for a vibrating steam pipe or a pipe with large water hammer within it, provided of course the pipe is rated for use. Solid drawn steel is generally used for the boiler pressure vessel where strength is priority over cost.

Standard house water pipe is very thin and is difficult to bend. Thick-walled (sometimes known as refrigerator pipe) is much nicer to work with. Standard house water pipe should not be used on steam or with water hammer. It should only be used on water suction or where the water or steam is under no real pressure, e.g. a tank filling pipe, or bilge ejector.

Plastic pipe is always sold drawn.

Joining Pipe to Pipe, Fitting and anything else:

The real art to pipe work, after bending the pipe, is being able to join it to a fitting, another pipe or a boss.

There are hundreds of different ways to join pipes to things, and variations within those different ways. The normal methods depend on the type of pipe, pressure & temperature, how much space you have and, more often than not, what you have.

Yorkshire Fittings

Treat Yorkshire Fittings the same as standard house pipe. They are copper fittings designed so the copper pipe fits inside, and then the fitting is soft-soldered onto the pipe. Very good for tap water, reasonably quick to fit and solder. Very cheap. Not to be used with steam under pressure.


A Coupling is a short section of pipe large enough for two pipes to connect to. These are used to join two pipes together of the same size and type, with the coupling usually being made of the same material. Couplings can be designed to screw onto the pipes, solder, weld, crimp, glue etc. The common type with copper is to solder or crimp. The common type with Steel and Stainless is to screw or weld. Glue is used with plastic pipe.


A Boss is often used to describe a fitting attached to a larger structure or another pipe. They are used to attach a pipe directly to the larger structure, even if that pipe is very short onto another fitting.


Threading a pipe, internally or externally, is not easy. Most copper pipe is too thin to thread. Steel gas pipe is often threaded with BSP, and the pipe diameter is usually made with BSP in mind. Tapping a pipe may mean having to bore out the pipe to the correct size. Threading a pipe, up to 1/2" diameter, is reasonably easy with a die as long as you can grip the pipe. After about 1/2" diameter you may need to think about a proper threading machine or screw cutting on a lathe. Stainless Steel pipe is much harder to thread and screw cutting is required.

Soldering & Brazing

There are two types of soldering; soft and hard. Soft solder is your common type of lead/tin alloy solder. It melts at around 400°C, depending on the grade. Hard solder, also known as Silver Solder, melts around 750°C depending on the grade, and is much stronger than soft solder.

Soft solder works wonders on Brass and Copper, but will not solder very well to Steel and not at all to Aluminium or Stainless Steel.

Silver Solder works fine on Brass and Copper, but you can reach the melting point of Brass and the job can sag. Silver solder also work on Steel though not as nice, and it will solder to Stainless Steel with some encouragement. Aluminium will have melted before the silver solder melts. Silver solder is much more expensive than soft solder.

General rule of thumb, if using pressurised steam, use silver solder.


An Olive, besides being a tasty fruit, is also a metal ring that is compressed onto a pipe to form a seal to a fitting. They work very well on standard house copper pipe, cheap and quick to use, and ideal for any pipe not requiring steam pressure. Not suitable for steam under pressure.


As daft as it may sound, glue one of the more common types of permanent fitting for plastic pipes. Not used much on metal. The "glue" is commonly solvent cement. It melts the plastic into its self. Not as cheap as other methods but it makes the pipes and fitting so strong they can be considered as one piece.


If you think glue is daft idea, try a fitting you simply push-on with reasonable ease, and that’s it. Honest. They seal with a built-in O ring and have a stainless metal washer inside with teeth pointing inwards that grip the pipe and prevent it from pulling off. These fittings are seriously strong, especially on plastic pipe, but are not suitable for higher temperatures. They are unaffected by water hammer, as the more you pull the pipe, the harder it grips. Ideal for all water pipes, and commonly used in house hold plumbing so they are quite cost effective.


A Flange is usually a large disc on the end of the pipe with bolt holes through it. The flange is either screwed or welded to the pipe, often via a Boss. The flange can be bolted to another flange, or to a flat surface. The flange its self can form the seal using a gasket, or the pipe can protrude through the flange and form the seal leaving a small gap between the flange and the other side. It is even possible to have an O ring on the outside of the protruding pipe and for that O ring to seal inside a hole, with the flange holding the pipe in. Flange fittings are commonly used on very large pipe and can take extreme pressure and temperature.

Compression Fitting

A new fitting that has appeared on the market recently is a copper compression fitting. This is much like a Yorkshire Fitting but instead of soldering the fitting on, the fitting has an O ring in each end and a special machine crimps the fitting into a hexagonal shape. This squashes the O ring to form a seal onto the pipe. This type of fitting is used mainly on solar heating and house radiator systems where the fitting will be at medium pressure (2 Bar) and could be very hot. Not suitable for steam pressures but they will take the temperature. They are also a one-time use.


You can of course simply weld pipes to fittings. This is considered to be very strong and gas tight, however removing the fitting afterwards may be slightly harder. A Coupling should be used to join two bits of smaller pipe together to provide strength.


Flaring a pipe is to make a cone shape outwards on the end of the pipe. A special nut is required to hold the pipe onto a special fitting. Very good for high-pressures and little pipe. Easy to do with a flaring tool. Ideal for hydraulic pipes using Copper, Steel or Cupronickel. Stainless steel is too brittle to flare properly. This method is used on car brake systems, often with a Double Flare.


SAE is a fitting standard. The system is an up-rated version of the Flare, sometimes with alternative seals. They are commonly found on large hydraulic systems and are not often found on a steam boat, except maybe on hydraulic steering systems.


An Elbow is a common name for any fitting that bends the pipe, usually 90° or 45°. Elbow's may be male, female or both. Elbow's that have one male and one female thread are known as Street Elbows. Elbow's may also be welded or flanged.


A Nipple is a fitting or short section of pipe with male ends.


This fitting is T shaped, with its three connections being any gender, though more often all female.


A Plug is a male-threaded bung that blocks a hole. Not so common on pipe work as most pipes are Male threaded, a coupler is often used to connect a pipe to plug it.


A Cap is a Female version of a Plug. They are not as common as a plug except on larger pipe. The cap screws onto a threaded pipe to block the end. They are also not as strong in terms of pressure as a plug.


A Reducer alters the size of input to output. E.g. they can convert 1" BSP to 3/4" BSP. They are the same gender in and out, and always the same thread type.


An adaptor changes thread types from one to the other. E.g. one side may be BSP and the other side may be M.E.

Internal Reducer

This device, like a reducer, changes thread size. However this type is Male on the outside and Female on the inside, meaning when the fitting is screwed into the larger pipe, and the smaller pipe is screwed into the reducer, there is only around 1/4" of the bushings visible.


Like the T, the Cross fitting is simply to connect multiple pipes, in this case 4 pipes. Typically all Female.


Perhaps the most useful and expensive fitting. A Union is like a plug and socket bolted together. You attach one side to one pipe, the other side to another pipe, and without requiring to twist the pipes, you can screw them together using the Union's large nut. These fittings are most useful when plumbing a steam boat together. They are suitable for steam and water hammer, available in all sizes and very easy to screw on to a pre-threaded pipe. They may also be attached direct to other fittings, valves, bosses etc.


Just like the T, only in Y shape.

U Bend

If you need a U bend on a steam boat you must be doing something wrong.

Problems with Pipes and Fittings

Nothing is ever quite as simple as you would think. Surely if you push water into one end of a pipe it should come out the other end? That may be true given you have water, but when it comes to steam at high pressures and temperatures, there are other things you need to think of, as steam is neither a liquid or a gas, it exists half way between the two.

Venturi Effect - This is where the flow of a gas or liquid induces a lower pressure due to pressure and speed change. E.g. the bilge ejector or water injector works on the same principle.

So imagine having a T fitting, steam or water at reasonable speed is flowing straight through the T fitting, but due to its design, the centre of the T section is smaller than the rest. This may be enough to cause a venturi effect within the T fitting.

Another possible problem is a bottle neck. Usually associated with valves, but many fittings are also an issue. If you have a large 1" pipe feeding an engine, with a large valve, and a nipple joining the valve to the engine, that nipple may be more like a 1/2" pipe. The nipple would cause a pressure reduction across it meaning the 1" pipe may as well be a 1/2" pipe. You only want one bottle neck in the whole system, and that should be the controlling valve, and even then when it is fully open it should be as unresisting as the rest of the system.

Work hardening of copper is an issue as mentioned at the top of this page. If copper is allowed to move or vibrate it will work harden and become brittle. This isn't an issue if the pipe can't move about, but accidentally bend a work-hardened pipe and it is likely to snap.

Rusting of steel is always a problem. The only way round is to either galvanise everything or use Stainless Steel.

Water Hammer, as mentioned before, causes huge pressures for a split second conjoined with vibrating pipes. Any pipes that experience water hammer should be strong enough to take the hammering.

Sealing Threads is another big issue. By their very nature, threads will leak steam, even tapered threads will leak. The best solution is to use PTFE tape or Loctite to seal the thread.

Spot an error?

Is you see an error on the website, no matter how tiny, please send me an E-mail. Errors can be spelling/grammar mistakes, broken links, incorrect descriptions etc.